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O. longistaminata population
Field Crops Research 95 (2006) 39–48 www.elsevier.com/locate/fcr
Breeding for perennial growth and fertility in an Oryza sativa/O. longistaminata population E.J. Sacks a,*,1, M.P. Dhanapala a, D.Y. Tao b, M.T. Sta. Cruz a, R. Sallan a a
Plant Breeding Genetics and Biochemistry Division, International Rice Research Institute, DAPO Box 7777, Metro Manila, Philippines b Institute of Food Crops, Yunnan Academy of Agricultural Sciences, Kunming 650205, China Received 14 August 2004; received in revised form 29 January 2005; accepted 31 January 2005
Abstract The development of perennial cultivars (CVs) of upland rice would give farmers a new tool to reduce soil erosion from hilly fields, thereby mitigating a problem of regional concern in Southeast Asia. Oryza longistaminata is an undomesticated, perennial, rhizomatous relative of domesticated Asian rice (Oryza sativa). Using five sets of 4 2 factorial mating designs, we crossed rhizomatous interspecific genotypes (IGs) from an intermated O. sativa/O. longistaminata population with male-fertile IG selections from the intermated population, and with O. sativa CVs. Parents and progeny were planted in an upland field at IRRI using a randomized complete block design and evaluated for rhizome expression, survival after 1 year, vigor of the survivors, and yield. For the IG parents, rhizome expression was variable and penetrance of most genotypes was incomplete, but genotypes that demonstrated the potential for moderate rhizome expression had high penetrance (89% average). The CV parents yielded 11.0 g/plant on average but none produced rhizomes or survived 1 year. The IG parents averaged yields of 3.1 g/plant, 57% rhizomatous and 36% survival. The IG/IG progeny averaged yields of 4.2 g/plant, 32% rhizomatous and 37% survival. The IG/CV progeny averaged yields of 6.0 g/plant, 18% rhizomatous and 16% survival. Nine IG/IG progeny and six IG/CV progeny were rhizomatous, perennial, and yielded at least 5 g/plant, and five of these yielded more than 10 g/plant. For the IG parents and IG/IG progeny, rhizome presence and expression were positively associated with survival and vigor of the survivors. General combining ability effects were significant for percent survival and yield but not percent rhizomatous. Specific combining ability effects were significant for percent rhizomatous, percent survival and yield. By selecting female parents for long rhizomes and male parents for fertility, considerable gains in rhizome expression, survival and yield were made. The development of perennial upland rice CVs should be feasible via introgression of genes from O. longistaminata. # 2005 Elsevier B.V. All rights reserved. Keywords: Perennial upland rice; Oryza sativa; Oryza longistaminata; Interspecific crosses; Clonal performance; Rhizome; Fertility
* Corresponding author. Tel.: +1 650 7965719; fax: +1 662 6865218. E-mail address: [email protected] (E.J. Sacks). 1 Present address: USDA-ARS, P.O. Box 345, Stoneville, MS 38776, USA.
1. Introduction Upland (dryland) rice (Oryza sativa L.) is grown on about 3 million ha in Southeast Asia (IRRI, 2002;
0378-4290/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.fcr.2005.01.021
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E.J. Sacks et al. / Field Crops Research 95 (2006) 39–48
Table 30 at: www.irri.org/science/ricestat/index.asp). An annual crop, upland rice is typically planted anew at the beginning of each rainy season. In Southeast Asia, upland rice is grown primarily on hilly land, which causes soil erosion that is of local and regional concern (Brady, 1994; Pimentel et al., 1987). This study was part of an effort to develop a perennial form of upland rice that could be grown multiple years after planting and used to mitigate soil erosion (Schmit, 1996). This study may also be considered in the broader context of efforts to develop other perennial grain crops, including wheat, rye, sorghum and maize (Cox et al., 2002). O. longistaminata A. Chev. et Roehr. is a perennial relative of O. sativa that typically produces long (>8 cm) rhizomes. One potential strategy for developing perennial upland rice is to introgress genes for perennial growth and especially rhizome production, a trait that contributes to perennation, from O. longistaminata into current annual upland rice cultivars (CVs). Though both species share a common diploid genome (AA), introgression is typically hampered by embryo abortion in the initial cross and in early generation backcrosses (Oka, 1988). Nevertheless, useful genes have been introgressed from O. longistaminata into O. sativa (e.g. the disease resistance gene, Xa21). Recently, efforts have been made to determine the inheritance of rhizomes and to introgress genes for rhizome production from O. longistaminata into O. sativa. Studying an F2 O. sativa/O. longistaminata population, Maekawa et al. (1998) suggested that a single dominant gene conditioned rhizome presence and that additional genes modified rhizome length. In a collaborative project between IRRI and the Yunnan Academy of Agricultural Sciences, Hu et al. (2001) studied another O. sativa/O. longistaminata cross and reported that segregation of rhizome presence in F2 and backcross progeny was consistent with control by two dominant complementary genes. Both of the rhizome inheritance studies were conducted over single seasons (3–4 months) and in paddies, which limits inferences for developing perennial upland rice. In a previous study, we evaluated 94 unselected S1 families, from an intermated O. sativa/O. longistaminata population that is also the source of selected genotypes used in the present study. The unselected S1 families were grown under upland conditions for 1 year and we found that
survival for the families averaged about 10% with a maximum of 64%, but rhizomatous progeny accounted for less than 1% of the total (Sacks et al., 2003). In this study we evaluated selected genotypes and their progeny, for perennial growth traits and yield over the course of a year and under upland field conditions. The objectives were to (i) compare clonally propagated O. sativa/O. longistaminata interspecific genotypes (IGs) that were previously selected in pots primarily for rhizomatous growth; (ii) determine general and specific combining abilities (GCAs and SCAs, respectively) by using factorial mating designs to test IG selections with other IG selections and with rice CVs; and (iii) study interrelationships among the traits.
2. Materials and methods In a replicated field trial, we compared five upland rice CV parents, 24 clonally propagated IG parents, 18 IG/IG full-sib families and 20 IG/CV full-sib families (Tables 1 and 2). The full-sib families were produced by crossing, in each of five factorial design sets, four IGs as female parents to one IG and one CV as male parents (Tables 1 and 2). The IG parents in this study were chosen from a previously described intermated population (IRRI, 1998: http://www.irri. org/publications/program/prog1997.asp; Sacks et al., 2006). The intermated population was derived from an F1 of the O. sativa ssp. indica land-race BS125 crossed with the O. longistaminata genotype WL02-15. Because the F1 had insufficient pollen fertility to produce F2 progeny, it was crossed as a female back to 11 upland rice CVs and to five O. longistaminata genotypes. The F1/O. longistaminata progeny, which were all rhizomatous, were selfed. Rhizomatous F1/O. longistaminata F2 progeny were crossed with F1/O. sativa progeny, which lacked rhizomes, to form the intermated population. Subsequently, rhizomatous individuals were selected and intermated. For this study, the female IG parents were selected primarily for rhizomes >3 cm long, based on observations of plants grown in 16 l pots in a greenhouse (Table 1). The male IG parents were selected primarily for pollen fertility sufficient to produce progeny and secondarily for rhizome presence.
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Table 1 Mean rhizome production, survival and yield of parental controls, including O. sativa cultivars (CV) and interspecific genotypes (IG) from an intermated O. sativa/O. longistaminata population, grown for 1 year in a replicated upland field trial at IRRI Rhizomatous/ penetrance (%)
Rhizomatous with R scorea of 2 (%)
Mean R score (0–3 scale)
Rhizome length in pots (S–L scale)b
Survival (%)
Vigor of survivors (1–9 scale)c
Yield (g/plant)
N
Female/ male
CV parents Azucena Basmati370 IR 55423-01 IR 60080-46A IRAT104
0 0 0 0 0
0 0 0 0 0
0.0 0.0 0.0 0.0 0.0
None None None None None
0 0 0 0 0
– – – – –
10.9 10.4 13.1 13.1 7.3
47 48 48 47 47
M M M M M
IG parents UPR-0814-96-4 UPR-0930-96-1 UPR-1000-96-1 UPR-1062-96-1 UPR-1250-96-1 UPR-1355-96-1 UPR-1554-97-3 UPR-1589-97-2 UPR-1645-97-1 UPR-1735-97-1 UPR-1852-97-1 UPR-2860-97-1 UPR-2963-97-4 UPR-2979-97-1 UPR-3046-97-2 UPR-3254-97-1 UPR-3417-97-1 UPR-3418-97-1 UPR-3477-97-2 UPR-3572-97-2 UPR-3901-98-1 UPR-3901-98-2 UPR-4112-98-1 UPR-4499-98-1
90 100 44 58 98 92 100 83 13 100 86 26 48 35 8 4 98 56 75 46 25 25 0 64
4 42 2 0 41 63 58 16 0 21 0 0 0 0 0 0 42 0 0 0 0 0 0 0
0.9 1.4 0.5 0.6 1.4 1.5 1.6 1.0 0.1 1.2 0.9 0.3 0.5 0.4 0.1 0.0 1.4 0.6 0.8 0.5 0.3 0.3 0.0 0.6
M L L M L L L L M L L M M M None M L L M S M M S L
96 90 32 4 94 91 54 5 19 44 34 6 71 8 5 0 55 19 77 28 0 22 0 16
6.8 7.0 7.6 8.1 6.7 6.1 8.3 5.9 8.8 7.8 8.5 9.1 8.2 8.6 8.8 – 7.1 7.8 7.4 8.1 – 8.5 – 9.0
5.3 2.7 1.1 0.8 3.4 2.6 2.2 0.4 2.4 1.9 0.2 0.8 5.0 5.5 4.7 16.7 1.8 0.3 0.9 5.6 1.8 3.6 0.7 2.9
48 48 44 48 47 47 48 43 47 48 23 42 48 48 43 16 47 48 48 40 35 47 9 44
F F F F F F F F F F F F F F, M M F F F F M M F M F
11
3
0.1
8
0.4
1.6
Genotype
S.E. a
0: no rhizomes; 1: low with development of short rhizomes and much fewer rhizomes than tillers; 2: moderate with rhizomes contributing about 25–50% of the vegetative biomass; 3: high with development of the plant primarily by rhizomes and practically no tillering. b Data from a prior greenhouse trial; S = 1–3 cm, M = 3–8 cm, L > 8 cm. c 1: extra vigorous, 3: vigorous, 5: normal, 7: weak, 9: very weak.
Seeds were incubated at 50 8C for 96 h before planting to break dormancy, then dehulled and treated with fungicide (Benlate). Seeds of the CV parents, IG/ IG progeny and IG/CV progeny were sown in flats on 6 June 2001 and grown in a glasshouse. IG parents were vegetatively propagated by removing tillers with three leaves from mother plants that had been grown in a greenhouse in 16 l pots. The tillers were taken on 6 June 2001 and individually cultivated in small polystyrene foam drinking cups (60 mm 100 mm)
with puddled soil. Seedlings and cuttings were transplanted to an upland field at IRRI headquarters on 3 July 2001. The experiment was a randomized complete block design with four blocks. Each plot was one 3 m row of up to 12 plants, with 25 cm between plants and 60 cm between rows. The field soil was a silty clay-loam with a pH of 5.9 0.2. Fertilizer (15 kg ha 1 of N, P and K) was applied at planting (July 2001), and after harvest (November 2001). During the dry season, supplemental sprinkler irriga-
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Table 2 Mean rhizome production, survival and yield for full-sib families from five sets of 4 2 factorial mating designs, where female parents were interspecific genotypes from an intermated O. sativa/O. longistaminata population and male parents were one interspecific genotype (UPR numbers) and one O. sativa cultivar per set, grown for 1 year in a replicated upland field trial at IRRI Vigor of survivors (1–9 scale)b
Yield (g/plant)
N
71 11 33 15 21 0 4 2
7.7 6.3 8.3 8.9 8.3 – 7.0 9.1
1.3 1.9 2.2 2.3 13.2 0.9 14.8 3.3
48 40 48 48 48 48 48 48
0 0 0 0 0 0 0 0
20 18 8 35 30 15 2 17
8.5 8.8 9.1 7.9 8.3 8.4 7.0 8.6
3.4 2.6 5.2 4.0 6.3 4.0 8.4 5.0
45 41 21 25 47 47 48 41
60 44 51 30 0 4 25 29
6 6 2 0 0 0 0 0
54 69 85 77 36 21 67 46
7.4 7.1 7.2 7.3 8.2 8.5 7.5 7.7
0.7 1.1 2.8 1.5 1.4 3.4 2.1 2.4
48 48 47 47 47 48 12 48
UPR-1589-97-2/UPR-3901-98-1 UPR-1645-97-1/UPR-3901-98-1 UPR-2963-97-4/UPR-3901-98-1 UPR-3477-97-2/UPR-3901-98-1 UPR-1589-97-2/IR 60080-46A UPR-1645-97-1/IR 60080-46A UPR-2963-97-4/IR 60080-46A UPR-3477-97-2/IR 60080-46A
– 25 17 10 23 0 21 31
– 0 0 0 0 0 0 0
– 25 10 43 16 8 0 2
– 8.8 8.8 8.3 8.7 8.1 – 7.0
– 0.3 3.8 3.8 4.8 23.3 5.1 6.0
– 4 47 46 44 48 48 46
UPR-0814-96-4/UPR-4112-98-1 UPR-1852-97-1/UPR-4112-98-1 UPR-2860-97-1/UPR-4112-98-1 UPR-3418-97-1/UPR-4112-98-1 UPR-0814-96-4/IRAT104 UPR-1852-97-1/IRAT104 UPR-2860-97-1/IRAT104 UPR-3418-97-1/IRAT104
72 43 33 – 13 17 25 21
0 0 0 – 0 0 0 0
100 0 0 – 29 4 0 8
8.8 – – – 8.1 7.1 – 7.4
34.5 2.6 1.8 – 6.9 1.5 3.8 3.2
1 42 18 – 48 47 46 48
S.E.
11
3
8
0.4
1.6
Set
Cross
Rhizomatous (%)
Rhizomatous with R scorea of 2 (%)
1
UPR-0930-96-1/UPR-2979-97-1 UPR-1062-96-1/UPR-2979-97-1 UPR-1735-97-1/UPR-2979-97-1 UPR-3901-98-2/UPR-2979-97-1 UPR-0930-96-1/IR 55423-01 UPR-1062-96-1/IR 55423-01 UPR-1735-97-1/IR 55423-01 UPR-3901-98-2/IR 55423-01
48 31 19 4 13 40 8 23
4 2 0 0 0 0 0 0
2
UPR-1554-97-3/UPR-3046-97-2 UPR-2979-97-1/UPR-3046-97-2 UPR-3254-97-1/UPR-3046-97-2 UPR-4499-98-1/UPR-3046-97-2 UPR-1554-97-3/Basmati370 UPR-2979-97-1/Basmati370 UPR-3254-97-1/Basmati370 UPR-4499-98-1/Basmati370
29 31 0 37 4 0 23 45
3
UPR-1000-96-1/UPR-3572-97-2 UPR-1250-96-1/UPR-3572-97-2 UPR-1355-96-1/UPR-3572-97-2 UPR-3417-97-1/UPR-3572-97-2 UPR-1000-96-1/Azucena UPR-1250-96-1/Azucena UPR-1355-96-1/Azucena UPR-3417-97-1/Azucena
4
5
a
Survival (%)
0: no rhizomes; 1: low, with development of short rhizomes and much fewer rhizomes than tillers; 2: moderate, with rhizomes contributing about 25–50% of the vegetative biomass; 3: high, with development of the plant primarily by rhizomes and practically no tillering. b 1: extra vigorous; 3: vigorous, 5: normal, 7: weak, 9: very weak.
E.J. Sacks et al. / Field Crops Research 95 (2006) 39–48
tion was supplied in the third week of March for about 30 min and then repeated 1 week later for 4 h. Data on rhizome expression, plant survival, vegetative vigor and yield were recorded. Individual plants in plots were measured. Rhizome presence and expression (R) was measured on 26 September 2001 by visual observations at the soil surface using a 0–3 scale that was developed by Ghesquie`re (1988): 0 = no rhizomes, 1 = low, with development of short rhizomes and much fewer rhizomes than tillers, 2 = moderate, with rhizomes contributing about 25– 50% of the vegetative biomass, 3 = high, with development of the plant primarily by rhizomes and practically no tillering. IRRI’s 1–9 scale for vegetative vigor (IRRI, 1996) was modified to include a value for dead plants: 1 = extra vigorous, 3 = vigorous, 5 = normal, 7 = weak, 9 = very weak, 10 = dead. Initial vigor was recorded on 17 September 2001, about 2–4 weeks prior to harvest. About 1 year after planting, vigor and survival were recorded on 25 June 2002. Plants were considered perennial under upland conditions if they survived 1 year. Analyses of variance were conducted on plot means, using type III sums of squares to test for differences among all entries, among entries within entry types, and for GCA and SCA effects. Single degree of freedom contrasts were used to compare groups of genotypes belonging to different entry types. Chi-square tests were used to evaluate associations between survival frequency and classes of R. Linear regression was used to test associations between traits, and associations between progeny and midparent values.
3. Results and discussion 3.1. Rhizomes The CV parents did not produce rhizomes (Table 1). All but one of the IG parents were previously observed to produce rhizomes when grown in 16 l pots in a greenhouse (Table 1). However, among the IG parents identified as rhizomatous in pots, rhizome presence in the field averaged 57% and ranged from 0 to 100% (Table 1). Rhizome presence in the field averaged 65% for the female IG parents and 22% for the male IG parents. Thus, the IG parent
43
controls in the field were either incompletely penetrant for rhizome presence or produced rhizomes that were sometimes too short to be detected by visual observations at the soil surface. The apparent incomplete penetrance among parental controls suggests that the field assessments underestimated the number of rhizomatous progeny produced. Rhizome penetrance was typically high for IG parents that demonstrated the potential for moderate rhizome expression (>80% penetrance in eight out of nine cases in which R = 2, 44% minimum, Table 1). In contrast, the IG parents with low rhizome expression ranged in penetrance from low to high (4–86%), with >80% in only one out of 14 cases. Our assessments of rhizome length for IG parents grown in pots were moderate to strong predictors of their mean R scores (r = 0.70***) and penetrance (r = 0.72***) in the replicated field plots (Table 1). Thus, selection for moderate rhizome expression in pots or in the field would be expected to facilitate selection for high penetrance. Rhizomes were produced by about 18% of the IG/ CV progeny and this was the first time that many rhizomatous progeny were obtained from this population in crosses with CVs. In a previous greenhouse study, we observed that the BS125/WL02-2 F1 progenitor produced very short rhizomes (<3 cm) that were apparent only after soil and roots were removed, and all 286 of its progeny with CVs either lacked rhizomes or had such short rhizomes that they escaped detection (Sacks et al., 2006). Thus, the rhizomatous progeny produced in the current study represent genetic gains from selecting parents for the production of moderate (3–8 cm) to long (>8 cm) rhizomes in pots (Table 1). Rhizomes were produced by 32% of the IG/IG progeny and this was significantly greater (P 0.001) than the frequency observed for the IG/CV progeny. Though rhizomatous individuals were generally more frequent in IG/IG families than in IG/CV families, this trend was not uniform (Tables 2 and 3). For example, UPR-1062-96-1 and UPR-4499-98-1 produced IG/CV families with 40 and 45% rhizomatous individuals, respectively, and these frequencies were not significantly different from those observed for their IG/IG families (31 and 37%, respectively; Table 2). Moderate rhizome expression (R = 2) was observed for 10 out of 664 IG/IG progeny but for none of the IG/
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Table 3 Analyses of variance for rhizome production, survival and yield of parents, which included interspecific genotypes (IG) from an intermated O. sativa/O. longistaminata population and O. sativa cultivars (CV), and progeny from five sets of 4 2 factorial mating designs, where female parents were interspecific genotypes and male parents were one interspecific genotype and one O. sativa cultivar per set, grown for 1 year in a replicated upland field trial at IRRI Source
Rhizomatous (%)
Survival (%)
Vigor of survivors (1–9 scale)
Yield (g/plant)
d.f.
d.f.
d.f.
d.f.
MS
MS
MS
MS
Rep Entry Entry type IG parents CV parents IG/IG progeny IG/CV progeny All progeny
3 66 3 23 4 17 19 37
24491*** 3588*** 30350*** 4654*** 0NS 1130** 663NS
3 66 3 23 4 17 19 37
714* 3312*** 12489*** 4530*** 0NS 3142*** 1263***
3 53 2 20 – 15 16 32
0.6NS 1.7*** 0.7NS 2.3*** – 1.8** 0.8NS
3 66 3 23 4 17 19 37
58.6*** 85.0*** 367.0*** 35.2*** 22.7NS 66.4*** 118.4***
Combining ability analysisa Set Female/set (GCA) Male/set (GCA) Female male/set (SCA)
4 15 5 13
1302* 684NS 785NS 1119*
4 15 5 13
606* 1132*** 2019*** 649**
4 14 5 9
1.2NS 1.2NS 0.6NS 0.8NS
4 15 5 13
147.6*** 7.0NS 129.7*** 100.9***
194
521
194
186
9.8
Error
249
104
0.6
NS
: non-significant. An alternative subdivision of the progeny degrees of freedom. * Significant at P 0.05. ** Significant at P 0.01. *** Significant at P 0.001. a
CV progeny. All of the other 137 individuals that had moderate rhizome expression in this study were from the IG parents (Table 4). None of the parents or progeny had high rhizome expression (R = 3). Female and male GCA mean squares for rhizomatous growth were not significant (Table 3). Similarly, the correlation between midparent and mean progeny values was low, though significant (r = 0.35*). The significant SCA (Table 3) was consistent with the results of Hu et al. (2001, 2003), which indicated that two dominant complementary genes controlled rhizome presence in an F2 population of the O. sativa CV RD23 crossed with O. longistaminata. Incomplete penetrance and/or low expressivity likely reduced the number of progeny scored as rhizomatous in this study relative to the number expected given the two gene model (minimum of 56% rhizomatous progeny expected in crosses between two rhizomatous IG parents, Table 2). Additionally, deviations from the two gene model may have resulted from additional loci that previous studies have found to affect rhizome expression (Hu et al., 2003; Maekawa et al., 1998); this explanation is especially
compelling given the lower penetrance we typically observed for genotypes with shorter rhizomes in contrast to those with longer rhizomes (Table 1). Thus, environmental variation, as indicated by the relatively large error variance and the incomplete penetrance of the IG parent clones, was a partial impediment to selecting for rhizome production in this study. Based on our observations of the IG parents, as selection increases the frequency of genes for moderate rhizome expression in the population, environmental variation for rhizome presence is expected to decrease. Rhizome expression was generally greater in the RD23/O. longistaminata population (Hu et al., 2001, 2003) than in this study’s population. For example, Tao et al. (2001) reported that an RD23/O. longistaminata BC1 (backcrossed to RD23) had long rhizomes, and several BC2 had moderate rhizome expression. However, in the current study, none of the crosses back to CVs produced moderate rhizomes. This suggests that the RD23/O. longistaminata population is a better source of genes for moderate rhizome expression than the population used in the current study. This view is also consistent with our observations that the BS125/
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Table 4 Associations between class of rhizome scale and frequency of survival, and vigor of survivors for three entry types, including O. sativa/O. longistaminata interspecific genotype (IG) parents, progeny from crosses between IG parents and progeny from crosses between IG parents and O. sativa cultivars (CV), grown for 1 year in a replicated upland field trial at IRRI Entry type
IG parents
IG/IG progeny
IG/CV progeny
Data
Rhizome scale (R)a
Significance
0 (none)
1 (low)
2 (moderate)
Survival (%) Vigor of survivors (1–9 scale) N
19 8.2 390
47 7.3 479
77 6.8 137
***
Survival (%) Vigor of survivors (1–9 scale) N
33 7.8 453
47 7.5 201
70 6.1 10
***
Survival (%) Vigor of survivors (1–9 scale) N
15 8.1 745
12 7.9 160
***
***
NS NS
NS
: non-significant. 0: no rhizomes; 1: development of short rhizomes and much fewer rhizomes than tillers; 2: rhizomes contributing about 25–50% of the vegetative biomass. *** Significant at P 0.001. a
WL02-2 F1 had very short rhizomes, whereas the RD23/ O. longistaminata F1 had long rhizomes (Tao et al., 2001). The large difference in rhizome expression between the two O. sativa/O. longistaminata F1s was primarily of genetic origin, as these differences were also observed when 16 l pots of both genotypes were grown side by side in a greenhouse at IRRI. Even though the O. longistaminata parents of both populations had long rhizomes, their interspecific progeny differed in rhizome expression. Using simple sequence-repeat markers, Hu et al. (2003) observed that at least seven loci, including one of the loci for rhizome presence, affected rhizome length. Thus, there are likely allelic differences among O. longistaminata parents for genes determining rhizome length that can affect selection efficiency for long rhizomes in O. sativa/O. longistaminata progeny. 3.2. Survival None of the CV parents were perennial (Table 1). In contrast to the CVs, many of the IG parents, IG/IG progeny and IG/CV progeny were perennial (Table 2). Overall, the female IG parents and IG/IG progeny had similar survival frequencies that were not significantly different from each other (42 and 37%, respectively) but they were significantly higher (P 0.001) than the survival frequency of the male IG parents (8%). Survival was more than twice as frequent for the
female IG parents and IG/IG progeny than the IG/CV progeny (16%) and these differences were significant (P 0.001). In a previous field trial of unselected S1 progeny derived from the intermated source population for this study, average and maximum survival were similar to the values observed for the IG/CV families in this study (Sacks et al., 2003; Table 2). Survival was high for some of the IG parents (e.g. 96% for UPR-0814-96-4), IG/IG families (e.g. 85% for UPR-1355-96-1/UPR-3572-97-2) and IG/CV families (e.g. 67% for UPR-1355-96-1/Azucena), indicating that introgression of perenniality into upland rice is feasible (Tables 1 and 2). Though vigor of the survivors was generally poor, some individual IG parents (esp. UPR-1589-97-2, Table 1) and progenies (esp. in set 3, Table 2) had acceptable or nearly acceptable vigor. Lower than expected rainfall at the study’s end in June 2002 likely kept vigor poor at the time of measurement. Considering the strong drought pressure during the dry season (January through June, total natural water deficit was 898 mm), the results for survival and, to a lesser extent, for vigor of the survivors were encouraging. Drought pressure during the dry season is generally greater at IRRI headquarters than at typical sites of upland rice cultivation in Southeast Asia. For example, Luang Prabang in Lao PDR, which is representative of upland rice sites in mainland Southeast Asia, typically has a dry season water deficit of about
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one third the natural water deficit observed at IRRI during this experiment. Lower temperatures during the dry season at the typically hilly upland rice sites of mainland Southeast Asia relative to temperatures at IRRI headquarters largely account for the difference in water deficit. Additionally, large areas of upland rice are cultivated in Indonesia and in the Southern Philippines (IRRI, 2002) at sites where the dry season is even shorter and milder than on the mainland (Huke, 1982). The supplemental irrigation at the study site in April 2002 only partially compensated for the difference in natural water deficit between IRRI headquarters and typical upland rice sites in Southeast Asia. GCAs and SCA for survival were highly significant (Table 3). The male GCA mean square was nearly twice as large as the female GCA mean square (Table 3). Midparent values for survival were moderately good predictors of progeny survival (r = 0.68***). Survival was more heritable than rhizomatous growth. 3.3. Rhizome-survival association Among the IG parents, there was a significant positive linear correlation (r = 0.72***) between average rhizome penetrance and average percent survival. For the IG parents and the IG/IG progeny, rhizomatous individuals had a greater frequency of survival than those without rhizomes (Table 4). Individuals with moderate rhizome expression (R = 2) had a greater frequency of survival and the survivors were more vigorous (lower score) than those with low rhizome expression (R = 1, Table 4). Survivors of the IG parents within the R = 1 class were more vigorous than those that lacked rhizomes (Table 4). For the nine IG genotypes that demonstrated the potential for moderate rhizome expression, survival was significantly greater for the R = 2 class (77%) than for the R = 1 class (62%), indicating that the moderate rhizomes, and not just associated genes, were directly beneficial. In contrast to the IG parents and the IG/IG progeny, there were no differences in survival or vigor of the survivors between the R = 1 and non-rhizomatous classes of the IG/CV progeny (Table 4). More limited rhizome expression within the R = 1 class of the IG/ CV progeny than in the IG parents and the IG/IG progeny may account for the differences in survival. A revised scale that subdivides the R = 1 class by
characteristics such as rhizome number and length may be useful for breeding perennial rice. Rhizomes may promote survival during the dry season by being directly tolerant of dry conditions or by exploiting water deep in the soil profile. We would expect long and deep rhizomes to more effectively avoid drought stress than short and shallow rhizomes. Rhizomes of cogongrass, torpedograss, and johnsongrass have been found to be tolerant of desiccation (Wilcut et al., 1988). Paterson et al. (1995) observed F2 and BC1 progeny of Sorghum bicolor/S. propinquum grown in Texas, USA and found that rhizomes were associated with perennial regrowth, after the plants over-wintered two frosts sufficient to kill aboveground growth. To survive the colder winter season in Kansas, USA, F2 progeny of S. bicolor/S. hapelense needed long and deep rhizomes (Kulakow and Ennis, 1988). Several good predictors of progeny survival were midparent values for: rhizome length of plants grown in pots in an earlier greenhouse trial (r = 0.56***), percent rhizomatous in the field trial (r = 0.69***), and mean rhizome scores in the field trial (r = 0.74***). Given the low association between progeny and midparent for percent rhizomes, the survival-rhizome associations could be accounted for if small undetected rhizomes were frequently produced by progeny and these conferred advantages for survival, or if genes for rhizome expression pleiotropically enhanced survival even when rhizomes were not produced, or if genes for survival were linked to genes for rhizome expression. In any case, the association between progeny survival and parental rhizome expression will be beneficial for breeding perennial rice. 3.4. Yield Yield for the CV parents averaged 11 g/plant. Average yields for the IG parents (3.1 g/plant), IG/IG progeny (4.2 g/plant) and IG/CV progeny (6.0 g/plant) were significantly less (P 0.001) than yield for the CV parents. Yield averaged 3.0 g/plant for the female IG parents and 3.7 g/plant for the male IG parents. Reduced fertility is common in early generation progeny of O. sativa/O. longistaminata (Oka, 1988) and was observed previously in the intermated population from which the IG parents of this study were selected. Average yield for the IG/CV progeny
E.J. Sacks et al. / Field Crops Research 95 (2006) 39–48
was significantly greater (P 0.01) than the yield for the IG/IG progeny. Within each of the entry types, except for CV parents, there were significant differences among entries for yield (Table 3). Among the IG/CV families, six had average yields that were not significantly different from the CV parents and one family had significantly greater (about double) yield (Table 2). Genes for high yield have been found in O. rufipogon (Xiao et al., 1998), another wild relative of domesticated rice, and our results suggest that O. longistaminata might also be a source of such beneficial genes once the incompatibilities that lead to infertility are overcome. The female GCA mean square for yield was small and non-significant but the male GCA and SCA mean squares were highly significant (Table 3). Midparent yield was a moderately good predictor of progeny yield (r = 0.46**). Thus, additive and non-additive gene effects were important for yield. 3.5. Multiple trait selection To develop a perennial CV of upland rice from this population, genotypes that have moderate rhizome expression, high percent survival, and yield at least as much as current annual upland CVs should be identified. Thus, concurrent gains in rhizome expression, survival and yield are necessary. In this experiment, nine IG/IG progeny and six IG/CV progeny were rhizomatous (R = 1), perennial, and yielded at least 5 g/plant, and five of these yielded more than 10 g/plant. Of the 201 rhizomatous (R = 1) IG/IG progeny, 23 yielded at least 5 g/plant, and eight of these yielded more than 10 g/plant. Of the 160 rhizomatous (R = 1) IG/CV progeny, 55 yielded at least 5 g/plant, and 22 of these yielded more than 10 g/ plant. On average, the IG/IG progeny were less rhizomatous (P 0.001) than the female IG parents but they were not significantly different for survival and they had significantly higher yield (P 0.05). The IG/CV progeny on average were less rhizomatous and had lower survival than the female IG parents but they had significantly higher yield (P 0.001 for all comparisons). In comparison to the male IG parents, the IG/IG progeny on average had greater survival (P 0.001) but they were not significantly different for percent rhizomatous and yield. However, the IG/ CV progeny on average were about as rhizomatous as
47
the male IG parents and had significantly greater survival and yield (P 0.05). Thus, the production of rhizomatous IG/CV progeny and the restored fertility observed for many of these progeny demonstrated that the fertility barriers typical of early generation O. sativa/O. longistaminata populations do not preclude the introgression of rhizomes into a fertile O. sativa genetic background. This conclusion is also consistent with observations from two prior studies (Sacks et al., 2006; Tao et al., 2001). Though progress has been made, further effort is needed to increase the rhizome expression and yield of perennial progenies. For example, of the 10 IG/IG progeny with R = 2, none yielded more than 2 g/plant. Replicated testing of ramets helped clarify which IG parents were best for multiple traits. The field trial demonstrated that four of the IG parents (UPR-081496-4, UPR-0930-96-1, UPR-1250-96-1 and UPR1355-96-1) combined moderate rhizome expression, high rhizome penetrance and high percent survival with partially restored fertility (2.6–5.3 g/plant, Table 1). Replicated testing using ramets of selected progeny seedlings would be costly in time but it would ensure that selections are well characterized and that potentially useful parents and cross combinations are not overlooked.
4. Conclusions Though low fertility in O. sativa/O. longistaminata progeny can be a significant barrier to breeding progress, we have made considerable gains in rhizome expression, survival and yield. Moreover, further gains from selection can be expected for all of the key traits. Selecting female IG parents with medium to long rhizomes in pots, enabled us to obtain many perennial and rhizomatous progeny in the field, even from crosses with annual CVs. Vigor of survivors may need much improvement but further testing at typical Southeast Asian sites of upland rice production would clarify this point. Recovery of fertility was, as expected, greater in crosses with male CV parents than male IG parents but perennial, rhizomatous progenies with adequate fertility were obtained from both male parent types. This study demonstrated that an important breeding objective for perennial rice
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should be moderate rhizome expression (R = 2) because it confers the advantages of high rhizome penetrance, survival, and vigor. Previously identified backcross progeny of RD23/O. longistaminata that have moderate to long rhizomes may be a useful source of genes for developing perennial upland rice with moderate rhizome expression. Acknowledgements Financial support from Bundesministerium fu¨r Wirtschaftliche Zusammenarbeit und Entwicklung (BMZ, project no. 97.7860.6-001.00) is gratefully acknowledged. References Brady, N.C., 1994. Alternatives to slash-and burn: a global imperative. In: Sanchez, P.A., van Houten, H. (Eds.), Alternatives to slash-and burn agriculture. Symposium ID-6, 15th International Soil Science Congress, Acapulco, Mexico, 1994. International Center for Research in Agroforestry and International Society of Soil Science, Nairobi, Kenya, pp. 3–14. Cox, T.S., Bender, M., Picone, C., Van Tassel, D.L., Holland, J.B., Brummer, E.C., Zoeller, B.E., Paterson, A.H., Jackson, W., 2002. Breeding perennial grain crops. Crit. Rev. Plant Sci. 21, 59–91. Ghesquie`re, A, 1988. Diversite´ ge´ne´tique de l’espe`ce sauvage de riz Oryza longistaminata A. Chev. et Roehr et dynamique des flux ge´niques au sein du groupe Sativa en Afrique. The`se Doc. Etat. Universite Paris XI Orsay, 228 pp. Hu, F.-Y., Tao, D.-Y., Sacks, E., Fu, B.Y., Xu, P., Li, J., Yang, Y., McNally, K., Khush, G.S., Paterson, A.H., Li, Z.-K., 2003. Convergent evolution of perenniality in rice and sorghum. Proc. Natl. Acad. Sci. U.S.A. 100, 4050–4054. Hu, F.-Y., Tao, D.-Y., Xu, P., Li, J., Yang, Y., Sacks, E., McNally, K., Cruz, T.S., Zhou, J., Li, Z., 2001. Two dominant complementary genes controlling rhizomatous expression in Oryza longistaminata. Rice Genet. Newsl. 18, 34–36. Huke, R.E., 1982. Agroclimatic and dry-season maps of South. In: Southeast and East Asia, IRRI, Los Banos, Philippines. IRRI, 1996. Standard evaluation system for rice, fourth ed. Manila, Philippines, 52 pp.
IRRI, 1998. Program report for 1997. Manila, Philippines, 175 pp. IRRI, 2002. Maclean, J.L., Dawe, D.C., Hardy, B., Hettel, G.P. (Eds.), Rice Almanac: Source Book for the Most Important Economic Activity on Earth, third ed. CABI Publication, New York, 253 pp. Kulakow, P., Ennis, J., 1988. Variation in the F1 and F2 generations of crosses between tetraploid Sorghum bicolor and S. hapelense, Land Institutional Research Report Number 5. The Land Institution, Salina, KS. Maekawa, M., Inukai, T., Rikiishi, K., Matsuura, T., Noda, K., 1998. Inheritance of the rhizomatous trait in hybrids of Oryza longistaminata Chev. et Roehr. and O. sativa L. SABRAO J. 30, 69– 72. Oka, H.I., 1988. Origin of Cultivated Rice. Jap. Sci. Soc. Tokyo. Paterson, A.H., Schertz, K.F., Lin, Y.A., Liu, S.C., Chang, Y.L., 1995. The weediness of wild plants: Molecular analysis of genes influencing dispersal and persistence of johnsongrass Sorghum halepense (L.) Pers. Proc. Natl. Acad. Sci. U.S.A. 92, 6127– 6131. Pimentel, D., Allen, J., Beers, A., Guinand, L., Linder, R., McLaughlin, P., Meer, B., Musonda, D., Perdue, D., Poisson, S., Siebert, S., Stoner, K., Salazar, R., Hawkins, A., 1987. World agriculture and soil erosion. BioScience 37, 277–283. Sacks, E.J., Roxas, J.P., Sta. Cruz, M.T., 2003. Developing perennial upland rice II: field performance of S1 families from an intermated Oryza sativa/O. longistaminata population. Crop Sci. 43, 129–134. Sacks, E.J., Schmit, V., McNally, K.L., Sta. Cruz, M.T., 2006. Fertility in an interspecific rice population and its effect on selection for rhizome length. Field Crop Res. 95, 30–38. Schmit V., 1996. Improving sustainability in the uplands through the development of a perennial upland rice. In: Piggin, C., Courtois, B., Schmit, V. (Eds.), Upland Rice Research in Partnership, Proceedings of the Upland Rice Consortium Workshop, pp. 4–13 January 1996. Padang, Indonesia, pp. 265–273, IRRI Discussion Paper Series, no. 16). Tao, D., Hu, F., Yang, Y., Xu, P., Li, J., Wen, G., Sacks, E., McNally, K., Sripichitt, P., 2001. Rhizomatous individual was obtained from interspecific BC2F1 progenies between Oryza sativa and Oryza longistaminata. Rice Genet. Newsl. 18, 11–13. Wilcut, J.W., Dute, R.R., Truelove, B., Davis, D.E., 1988. Factors limiting the distribution of cogongrass, Imperata cylindrica, and torpedograss Panicum repens. Weed Sci. 36, 577– 582. Xiao, J., Li, J., Grandill, S., Ahn, S.N., Yuan, L., Tanksley, S.D., McCouch, S.R., 1998. Identification of trait improving quantitative trait loci alleles from a wild rice relative Oryza rufipogon. Genetics 150, 899–909.
Breeding for perennial growth and fertility in an Oryza sativa/O. longistaminata population E.J. Sacks a,*,1, M.P. Dhanapala a, D.Y. Tao b, M.T. Sta. Cruz a, R. Sallan a a
Plant Breeding Genetics and Biochemistry Division, International Rice Research Institute, DAPO Box 7777, Metro Manila, Philippines b Institute of Food Crops, Yunnan Academy of Agricultural Sciences, Kunming 650205, China Received 14 August 2004; received in revised form 29 January 2005; accepted 31 January 2005
Abstract The development of perennial cultivars (CVs) of upland rice would give farmers a new tool to reduce soil erosion from hilly fields, thereby mitigating a problem of regional concern in Southeast Asia. Oryza longistaminata is an undomesticated, perennial, rhizomatous relative of domesticated Asian rice (Oryza sativa). Using five sets of 4 2 factorial mating designs, we crossed rhizomatous interspecific genotypes (IGs) from an intermated O. sativa/O. longistaminata population with male-fertile IG selections from the intermated population, and with O. sativa CVs. Parents and progeny were planted in an upland field at IRRI using a randomized complete block design and evaluated for rhizome expression, survival after 1 year, vigor of the survivors, and yield. For the IG parents, rhizome expression was variable and penetrance of most genotypes was incomplete, but genotypes that demonstrated the potential for moderate rhizome expression had high penetrance (89% average). The CV parents yielded 11.0 g/plant on average but none produced rhizomes or survived 1 year. The IG parents averaged yields of 3.1 g/plant, 57% rhizomatous and 36% survival. The IG/IG progeny averaged yields of 4.2 g/plant, 32% rhizomatous and 37% survival. The IG/CV progeny averaged yields of 6.0 g/plant, 18% rhizomatous and 16% survival. Nine IG/IG progeny and six IG/CV progeny were rhizomatous, perennial, and yielded at least 5 g/plant, and five of these yielded more than 10 g/plant. For the IG parents and IG/IG progeny, rhizome presence and expression were positively associated with survival and vigor of the survivors. General combining ability effects were significant for percent survival and yield but not percent rhizomatous. Specific combining ability effects were significant for percent rhizomatous, percent survival and yield. By selecting female parents for long rhizomes and male parents for fertility, considerable gains in rhizome expression, survival and yield were made. The development of perennial upland rice CVs should be feasible via introgression of genes from O. longistaminata. # 2005 Elsevier B.V. All rights reserved. Keywords: Perennial upland rice; Oryza sativa; Oryza longistaminata; Interspecific crosses; Clonal performance; Rhizome; Fertility
* Corresponding author. Tel.: +1 650 7965719; fax: +1 662 6865218. E-mail address: [email protected] (E.J. Sacks). 1 Present address: USDA-ARS, P.O. Box 345, Stoneville, MS 38776, USA.
1. Introduction Upland (dryland) rice (Oryza sativa L.) is grown on about 3 million ha in Southeast Asia (IRRI, 2002;
0378-4290/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.fcr.2005.01.021
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Table 30 at: www.irri.org/science/ricestat/index.asp). An annual crop, upland rice is typically planted anew at the beginning of each rainy season. In Southeast Asia, upland rice is grown primarily on hilly land, which causes soil erosion that is of local and regional concern (Brady, 1994; Pimentel et al., 1987). This study was part of an effort to develop a perennial form of upland rice that could be grown multiple years after planting and used to mitigate soil erosion (Schmit, 1996). This study may also be considered in the broader context of efforts to develop other perennial grain crops, including wheat, rye, sorghum and maize (Cox et al., 2002). O. longistaminata A. Chev. et Roehr. is a perennial relative of O. sativa that typically produces long (>8 cm) rhizomes. One potential strategy for developing perennial upland rice is to introgress genes for perennial growth and especially rhizome production, a trait that contributes to perennation, from O. longistaminata into current annual upland rice cultivars (CVs). Though both species share a common diploid genome (AA), introgression is typically hampered by embryo abortion in the initial cross and in early generation backcrosses (Oka, 1988). Nevertheless, useful genes have been introgressed from O. longistaminata into O. sativa (e.g. the disease resistance gene, Xa21). Recently, efforts have been made to determine the inheritance of rhizomes and to introgress genes for rhizome production from O. longistaminata into O. sativa. Studying an F2 O. sativa/O. longistaminata population, Maekawa et al. (1998) suggested that a single dominant gene conditioned rhizome presence and that additional genes modified rhizome length. In a collaborative project between IRRI and the Yunnan Academy of Agricultural Sciences, Hu et al. (2001) studied another O. sativa/O. longistaminata cross and reported that segregation of rhizome presence in F2 and backcross progeny was consistent with control by two dominant complementary genes. Both of the rhizome inheritance studies were conducted over single seasons (3–4 months) and in paddies, which limits inferences for developing perennial upland rice. In a previous study, we evaluated 94 unselected S1 families, from an intermated O. sativa/O. longistaminata population that is also the source of selected genotypes used in the present study. The unselected S1 families were grown under upland conditions for 1 year and we found that
survival for the families averaged about 10% with a maximum of 64%, but rhizomatous progeny accounted for less than 1% of the total (Sacks et al., 2003). In this study we evaluated selected genotypes and their progeny, for perennial growth traits and yield over the course of a year and under upland field conditions. The objectives were to (i) compare clonally propagated O. sativa/O. longistaminata interspecific genotypes (IGs) that were previously selected in pots primarily for rhizomatous growth; (ii) determine general and specific combining abilities (GCAs and SCAs, respectively) by using factorial mating designs to test IG selections with other IG selections and with rice CVs; and (iii) study interrelationships among the traits.
2. Materials and methods In a replicated field trial, we compared five upland rice CV parents, 24 clonally propagated IG parents, 18 IG/IG full-sib families and 20 IG/CV full-sib families (Tables 1 and 2). The full-sib families were produced by crossing, in each of five factorial design sets, four IGs as female parents to one IG and one CV as male parents (Tables 1 and 2). The IG parents in this study were chosen from a previously described intermated population (IRRI, 1998: http://www.irri. org/publications/program/prog1997.asp; Sacks et al., 2006). The intermated population was derived from an F1 of the O. sativa ssp. indica land-race BS125 crossed with the O. longistaminata genotype WL02-15. Because the F1 had insufficient pollen fertility to produce F2 progeny, it was crossed as a female back to 11 upland rice CVs and to five O. longistaminata genotypes. The F1/O. longistaminata progeny, which were all rhizomatous, were selfed. Rhizomatous F1/O. longistaminata F2 progeny were crossed with F1/O. sativa progeny, which lacked rhizomes, to form the intermated population. Subsequently, rhizomatous individuals were selected and intermated. For this study, the female IG parents were selected primarily for rhizomes >3 cm long, based on observations of plants grown in 16 l pots in a greenhouse (Table 1). The male IG parents were selected primarily for pollen fertility sufficient to produce progeny and secondarily for rhizome presence.
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Table 1 Mean rhizome production, survival and yield of parental controls, including O. sativa cultivars (CV) and interspecific genotypes (IG) from an intermated O. sativa/O. longistaminata population, grown for 1 year in a replicated upland field trial at IRRI Rhizomatous/ penetrance (%)
Rhizomatous with R scorea of 2 (%)
Mean R score (0–3 scale)
Rhizome length in pots (S–L scale)b
Survival (%)
Vigor of survivors (1–9 scale)c
Yield (g/plant)
N
Female/ male
CV parents Azucena Basmati370 IR 55423-01 IR 60080-46A IRAT104
0 0 0 0 0
0 0 0 0 0
0.0 0.0 0.0 0.0 0.0
None None None None None
0 0 0 0 0
– – – – –
10.9 10.4 13.1 13.1 7.3
47 48 48 47 47
M M M M M
IG parents UPR-0814-96-4 UPR-0930-96-1 UPR-1000-96-1 UPR-1062-96-1 UPR-1250-96-1 UPR-1355-96-1 UPR-1554-97-3 UPR-1589-97-2 UPR-1645-97-1 UPR-1735-97-1 UPR-1852-97-1 UPR-2860-97-1 UPR-2963-97-4 UPR-2979-97-1 UPR-3046-97-2 UPR-3254-97-1 UPR-3417-97-1 UPR-3418-97-1 UPR-3477-97-2 UPR-3572-97-2 UPR-3901-98-1 UPR-3901-98-2 UPR-4112-98-1 UPR-4499-98-1
90 100 44 58 98 92 100 83 13 100 86 26 48 35 8 4 98 56 75 46 25 25 0 64
4 42 2 0 41 63 58 16 0 21 0 0 0 0 0 0 42 0 0 0 0 0 0 0
0.9 1.4 0.5 0.6 1.4 1.5 1.6 1.0 0.1 1.2 0.9 0.3 0.5 0.4 0.1 0.0 1.4 0.6 0.8 0.5 0.3 0.3 0.0 0.6
M L L M L L L L M L L M M M None M L L M S M M S L
96 90 32 4 94 91 54 5 19 44 34 6 71 8 5 0 55 19 77 28 0 22 0 16
6.8 7.0 7.6 8.1 6.7 6.1 8.3 5.9 8.8 7.8 8.5 9.1 8.2 8.6 8.8 – 7.1 7.8 7.4 8.1 – 8.5 – 9.0
5.3 2.7 1.1 0.8 3.4 2.6 2.2 0.4 2.4 1.9 0.2 0.8 5.0 5.5 4.7 16.7 1.8 0.3 0.9 5.6 1.8 3.6 0.7 2.9
48 48 44 48 47 47 48 43 47 48 23 42 48 48 43 16 47 48 48 40 35 47 9 44
F F F F F F F F F F F F F F, M M F F F F M M F M F
11
3
0.1
8
0.4
1.6
Genotype
S.E. a
0: no rhizomes; 1: low with development of short rhizomes and much fewer rhizomes than tillers; 2: moderate with rhizomes contributing about 25–50% of the vegetative biomass; 3: high with development of the plant primarily by rhizomes and practically no tillering. b Data from a prior greenhouse trial; S = 1–3 cm, M = 3–8 cm, L > 8 cm. c 1: extra vigorous, 3: vigorous, 5: normal, 7: weak, 9: very weak.
Seeds were incubated at 50 8C for 96 h before planting to break dormancy, then dehulled and treated with fungicide (Benlate). Seeds of the CV parents, IG/ IG progeny and IG/CV progeny were sown in flats on 6 June 2001 and grown in a glasshouse. IG parents were vegetatively propagated by removing tillers with three leaves from mother plants that had been grown in a greenhouse in 16 l pots. The tillers were taken on 6 June 2001 and individually cultivated in small polystyrene foam drinking cups (60 mm 100 mm)
with puddled soil. Seedlings and cuttings were transplanted to an upland field at IRRI headquarters on 3 July 2001. The experiment was a randomized complete block design with four blocks. Each plot was one 3 m row of up to 12 plants, with 25 cm between plants and 60 cm between rows. The field soil was a silty clay-loam with a pH of 5.9 0.2. Fertilizer (15 kg ha 1 of N, P and K) was applied at planting (July 2001), and after harvest (November 2001). During the dry season, supplemental sprinkler irriga-
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Table 2 Mean rhizome production, survival and yield for full-sib families from five sets of 4 2 factorial mating designs, where female parents were interspecific genotypes from an intermated O. sativa/O. longistaminata population and male parents were one interspecific genotype (UPR numbers) and one O. sativa cultivar per set, grown for 1 year in a replicated upland field trial at IRRI Vigor of survivors (1–9 scale)b
Yield (g/plant)
N
71 11 33 15 21 0 4 2
7.7 6.3 8.3 8.9 8.3 – 7.0 9.1
1.3 1.9 2.2 2.3 13.2 0.9 14.8 3.3
48 40 48 48 48 48 48 48
0 0 0 0 0 0 0 0
20 18 8 35 30 15 2 17
8.5 8.8 9.1 7.9 8.3 8.4 7.0 8.6
3.4 2.6 5.2 4.0 6.3 4.0 8.4 5.0
45 41 21 25 47 47 48 41
60 44 51 30 0 4 25 29
6 6 2 0 0 0 0 0
54 69 85 77 36 21 67 46
7.4 7.1 7.2 7.3 8.2 8.5 7.5 7.7
0.7 1.1 2.8 1.5 1.4 3.4 2.1 2.4
48 48 47 47 47 48 12 48
UPR-1589-97-2/UPR-3901-98-1 UPR-1645-97-1/UPR-3901-98-1 UPR-2963-97-4/UPR-3901-98-1 UPR-3477-97-2/UPR-3901-98-1 UPR-1589-97-2/IR 60080-46A UPR-1645-97-1/IR 60080-46A UPR-2963-97-4/IR 60080-46A UPR-3477-97-2/IR 60080-46A
– 25 17 10 23 0 21 31
– 0 0 0 0 0 0 0
– 25 10 43 16 8 0 2
– 8.8 8.8 8.3 8.7 8.1 – 7.0
– 0.3 3.8 3.8 4.8 23.3 5.1 6.0
– 4 47 46 44 48 48 46
UPR-0814-96-4/UPR-4112-98-1 UPR-1852-97-1/UPR-4112-98-1 UPR-2860-97-1/UPR-4112-98-1 UPR-3418-97-1/UPR-4112-98-1 UPR-0814-96-4/IRAT104 UPR-1852-97-1/IRAT104 UPR-2860-97-1/IRAT104 UPR-3418-97-1/IRAT104
72 43 33 – 13 17 25 21
0 0 0 – 0 0 0 0
100 0 0 – 29 4 0 8
8.8 – – – 8.1 7.1 – 7.4
34.5 2.6 1.8 – 6.9 1.5 3.8 3.2
1 42 18 – 48 47 46 48
S.E.
11
3
8
0.4
1.6
Set
Cross
Rhizomatous (%)
Rhizomatous with R scorea of 2 (%)
1
UPR-0930-96-1/UPR-2979-97-1 UPR-1062-96-1/UPR-2979-97-1 UPR-1735-97-1/UPR-2979-97-1 UPR-3901-98-2/UPR-2979-97-1 UPR-0930-96-1/IR 55423-01 UPR-1062-96-1/IR 55423-01 UPR-1735-97-1/IR 55423-01 UPR-3901-98-2/IR 55423-01
48 31 19 4 13 40 8 23
4 2 0 0 0 0 0 0
2
UPR-1554-97-3/UPR-3046-97-2 UPR-2979-97-1/UPR-3046-97-2 UPR-3254-97-1/UPR-3046-97-2 UPR-4499-98-1/UPR-3046-97-2 UPR-1554-97-3/Basmati370 UPR-2979-97-1/Basmati370 UPR-3254-97-1/Basmati370 UPR-4499-98-1/Basmati370
29 31 0 37 4 0 23 45
3
UPR-1000-96-1/UPR-3572-97-2 UPR-1250-96-1/UPR-3572-97-2 UPR-1355-96-1/UPR-3572-97-2 UPR-3417-97-1/UPR-3572-97-2 UPR-1000-96-1/Azucena UPR-1250-96-1/Azucena UPR-1355-96-1/Azucena UPR-3417-97-1/Azucena
4
5
a
Survival (%)
0: no rhizomes; 1: low, with development of short rhizomes and much fewer rhizomes than tillers; 2: moderate, with rhizomes contributing about 25–50% of the vegetative biomass; 3: high, with development of the plant primarily by rhizomes and practically no tillering. b 1: extra vigorous; 3: vigorous, 5: normal, 7: weak, 9: very weak.
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tion was supplied in the third week of March for about 30 min and then repeated 1 week later for 4 h. Data on rhizome expression, plant survival, vegetative vigor and yield were recorded. Individual plants in plots were measured. Rhizome presence and expression (R) was measured on 26 September 2001 by visual observations at the soil surface using a 0–3 scale that was developed by Ghesquie`re (1988): 0 = no rhizomes, 1 = low, with development of short rhizomes and much fewer rhizomes than tillers, 2 = moderate, with rhizomes contributing about 25– 50% of the vegetative biomass, 3 = high, with development of the plant primarily by rhizomes and practically no tillering. IRRI’s 1–9 scale for vegetative vigor (IRRI, 1996) was modified to include a value for dead plants: 1 = extra vigorous, 3 = vigorous, 5 = normal, 7 = weak, 9 = very weak, 10 = dead. Initial vigor was recorded on 17 September 2001, about 2–4 weeks prior to harvest. About 1 year after planting, vigor and survival were recorded on 25 June 2002. Plants were considered perennial under upland conditions if they survived 1 year. Analyses of variance were conducted on plot means, using type III sums of squares to test for differences among all entries, among entries within entry types, and for GCA and SCA effects. Single degree of freedom contrasts were used to compare groups of genotypes belonging to different entry types. Chi-square tests were used to evaluate associations between survival frequency and classes of R. Linear regression was used to test associations between traits, and associations between progeny and midparent values.
3. Results and discussion 3.1. Rhizomes The CV parents did not produce rhizomes (Table 1). All but one of the IG parents were previously observed to produce rhizomes when grown in 16 l pots in a greenhouse (Table 1). However, among the IG parents identified as rhizomatous in pots, rhizome presence in the field averaged 57% and ranged from 0 to 100% (Table 1). Rhizome presence in the field averaged 65% for the female IG parents and 22% for the male IG parents. Thus, the IG parent
43
controls in the field were either incompletely penetrant for rhizome presence or produced rhizomes that were sometimes too short to be detected by visual observations at the soil surface. The apparent incomplete penetrance among parental controls suggests that the field assessments underestimated the number of rhizomatous progeny produced. Rhizome penetrance was typically high for IG parents that demonstrated the potential for moderate rhizome expression (>80% penetrance in eight out of nine cases in which R = 2, 44% minimum, Table 1). In contrast, the IG parents with low rhizome expression ranged in penetrance from low to high (4–86%), with >80% in only one out of 14 cases. Our assessments of rhizome length for IG parents grown in pots were moderate to strong predictors of their mean R scores (r = 0.70***) and penetrance (r = 0.72***) in the replicated field plots (Table 1). Thus, selection for moderate rhizome expression in pots or in the field would be expected to facilitate selection for high penetrance. Rhizomes were produced by about 18% of the IG/ CV progeny and this was the first time that many rhizomatous progeny were obtained from this population in crosses with CVs. In a previous greenhouse study, we observed that the BS125/WL02-2 F1 progenitor produced very short rhizomes (<3 cm) that were apparent only after soil and roots were removed, and all 286 of its progeny with CVs either lacked rhizomes or had such short rhizomes that they escaped detection (Sacks et al., 2006). Thus, the rhizomatous progeny produced in the current study represent genetic gains from selecting parents for the production of moderate (3–8 cm) to long (>8 cm) rhizomes in pots (Table 1). Rhizomes were produced by 32% of the IG/IG progeny and this was significantly greater (P 0.001) than the frequency observed for the IG/CV progeny. Though rhizomatous individuals were generally more frequent in IG/IG families than in IG/CV families, this trend was not uniform (Tables 2 and 3). For example, UPR-1062-96-1 and UPR-4499-98-1 produced IG/CV families with 40 and 45% rhizomatous individuals, respectively, and these frequencies were not significantly different from those observed for their IG/IG families (31 and 37%, respectively; Table 2). Moderate rhizome expression (R = 2) was observed for 10 out of 664 IG/IG progeny but for none of the IG/
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Table 3 Analyses of variance for rhizome production, survival and yield of parents, which included interspecific genotypes (IG) from an intermated O. sativa/O. longistaminata population and O. sativa cultivars (CV), and progeny from five sets of 4 2 factorial mating designs, where female parents were interspecific genotypes and male parents were one interspecific genotype and one O. sativa cultivar per set, grown for 1 year in a replicated upland field trial at IRRI Source
Rhizomatous (%)
Survival (%)
Vigor of survivors (1–9 scale)
Yield (g/plant)
d.f.
d.f.
d.f.
d.f.
MS
MS
MS
MS
Rep Entry Entry type IG parents CV parents IG/IG progeny IG/CV progeny All progeny
3 66 3 23 4 17 19 37
24491*** 3588*** 30350*** 4654*** 0NS 1130** 663NS
3 66 3 23 4 17 19 37
714* 3312*** 12489*** 4530*** 0NS 3142*** 1263***
3 53 2 20 – 15 16 32
0.6NS 1.7*** 0.7NS 2.3*** – 1.8** 0.8NS
3 66 3 23 4 17 19 37
58.6*** 85.0*** 367.0*** 35.2*** 22.7NS 66.4*** 118.4***
Combining ability analysisa Set Female/set (GCA) Male/set (GCA) Female male/set (SCA)
4 15 5 13
1302* 684NS 785NS 1119*
4 15 5 13
606* 1132*** 2019*** 649**
4 14 5 9
1.2NS 1.2NS 0.6NS 0.8NS
4 15 5 13
147.6*** 7.0NS 129.7*** 100.9***
194
521
194
186
9.8
Error
249
104
0.6
NS
: non-significant. An alternative subdivision of the progeny degrees of freedom. * Significant at P 0.05. ** Significant at P 0.01. *** Significant at P 0.001. a
CV progeny. All of the other 137 individuals that had moderate rhizome expression in this study were from the IG parents (Table 4). None of the parents or progeny had high rhizome expression (R = 3). Female and male GCA mean squares for rhizomatous growth were not significant (Table 3). Similarly, the correlation between midparent and mean progeny values was low, though significant (r = 0.35*). The significant SCA (Table 3) was consistent with the results of Hu et al. (2001, 2003), which indicated that two dominant complementary genes controlled rhizome presence in an F2 population of the O. sativa CV RD23 crossed with O. longistaminata. Incomplete penetrance and/or low expressivity likely reduced the number of progeny scored as rhizomatous in this study relative to the number expected given the two gene model (minimum of 56% rhizomatous progeny expected in crosses between two rhizomatous IG parents, Table 2). Additionally, deviations from the two gene model may have resulted from additional loci that previous studies have found to affect rhizome expression (Hu et al., 2003; Maekawa et al., 1998); this explanation is especially
compelling given the lower penetrance we typically observed for genotypes with shorter rhizomes in contrast to those with longer rhizomes (Table 1). Thus, environmental variation, as indicated by the relatively large error variance and the incomplete penetrance of the IG parent clones, was a partial impediment to selecting for rhizome production in this study. Based on our observations of the IG parents, as selection increases the frequency of genes for moderate rhizome expression in the population, environmental variation for rhizome presence is expected to decrease. Rhizome expression was generally greater in the RD23/O. longistaminata population (Hu et al., 2001, 2003) than in this study’s population. For example, Tao et al. (2001) reported that an RD23/O. longistaminata BC1 (backcrossed to RD23) had long rhizomes, and several BC2 had moderate rhizome expression. However, in the current study, none of the crosses back to CVs produced moderate rhizomes. This suggests that the RD23/O. longistaminata population is a better source of genes for moderate rhizome expression than the population used in the current study. This view is also consistent with our observations that the BS125/
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Table 4 Associations between class of rhizome scale and frequency of survival, and vigor of survivors for three entry types, including O. sativa/O. longistaminata interspecific genotype (IG) parents, progeny from crosses between IG parents and progeny from crosses between IG parents and O. sativa cultivars (CV), grown for 1 year in a replicated upland field trial at IRRI Entry type
IG parents
IG/IG progeny
IG/CV progeny
Data
Rhizome scale (R)a
Significance
0 (none)
1 (low)
2 (moderate)
Survival (%) Vigor of survivors (1–9 scale) N
19 8.2 390
47 7.3 479
77 6.8 137
***
Survival (%) Vigor of survivors (1–9 scale) N
33 7.8 453
47 7.5 201
70 6.1 10
***
Survival (%) Vigor of survivors (1–9 scale) N
15 8.1 745
12 7.9 160
***
***
NS NS
NS
: non-significant. 0: no rhizomes; 1: development of short rhizomes and much fewer rhizomes than tillers; 2: rhizomes contributing about 25–50% of the vegetative biomass. *** Significant at P 0.001. a
WL02-2 F1 had very short rhizomes, whereas the RD23/ O. longistaminata F1 had long rhizomes (Tao et al., 2001). The large difference in rhizome expression between the two O. sativa/O. longistaminata F1s was primarily of genetic origin, as these differences were also observed when 16 l pots of both genotypes were grown side by side in a greenhouse at IRRI. Even though the O. longistaminata parents of both populations had long rhizomes, their interspecific progeny differed in rhizome expression. Using simple sequence-repeat markers, Hu et al. (2003) observed that at least seven loci, including one of the loci for rhizome presence, affected rhizome length. Thus, there are likely allelic differences among O. longistaminata parents for genes determining rhizome length that can affect selection efficiency for long rhizomes in O. sativa/O. longistaminata progeny. 3.2. Survival None of the CV parents were perennial (Table 1). In contrast to the CVs, many of the IG parents, IG/IG progeny and IG/CV progeny were perennial (Table 2). Overall, the female IG parents and IG/IG progeny had similar survival frequencies that were not significantly different from each other (42 and 37%, respectively) but they were significantly higher (P 0.001) than the survival frequency of the male IG parents (8%). Survival was more than twice as frequent for the
female IG parents and IG/IG progeny than the IG/CV progeny (16%) and these differences were significant (P 0.001). In a previous field trial of unselected S1 progeny derived from the intermated source population for this study, average and maximum survival were similar to the values observed for the IG/CV families in this study (Sacks et al., 2003; Table 2). Survival was high for some of the IG parents (e.g. 96% for UPR-0814-96-4), IG/IG families (e.g. 85% for UPR-1355-96-1/UPR-3572-97-2) and IG/CV families (e.g. 67% for UPR-1355-96-1/Azucena), indicating that introgression of perenniality into upland rice is feasible (Tables 1 and 2). Though vigor of the survivors was generally poor, some individual IG parents (esp. UPR-1589-97-2, Table 1) and progenies (esp. in set 3, Table 2) had acceptable or nearly acceptable vigor. Lower than expected rainfall at the study’s end in June 2002 likely kept vigor poor at the time of measurement. Considering the strong drought pressure during the dry season (January through June, total natural water deficit was 898 mm), the results for survival and, to a lesser extent, for vigor of the survivors were encouraging. Drought pressure during the dry season is generally greater at IRRI headquarters than at typical sites of upland rice cultivation in Southeast Asia. For example, Luang Prabang in Lao PDR, which is representative of upland rice sites in mainland Southeast Asia, typically has a dry season water deficit of about
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one third the natural water deficit observed at IRRI during this experiment. Lower temperatures during the dry season at the typically hilly upland rice sites of mainland Southeast Asia relative to temperatures at IRRI headquarters largely account for the difference in water deficit. Additionally, large areas of upland rice are cultivated in Indonesia and in the Southern Philippines (IRRI, 2002) at sites where the dry season is even shorter and milder than on the mainland (Huke, 1982). The supplemental irrigation at the study site in April 2002 only partially compensated for the difference in natural water deficit between IRRI headquarters and typical upland rice sites in Southeast Asia. GCAs and SCA for survival were highly significant (Table 3). The male GCA mean square was nearly twice as large as the female GCA mean square (Table 3). Midparent values for survival were moderately good predictors of progeny survival (r = 0.68***). Survival was more heritable than rhizomatous growth. 3.3. Rhizome-survival association Among the IG parents, there was a significant positive linear correlation (r = 0.72***) between average rhizome penetrance and average percent survival. For the IG parents and the IG/IG progeny, rhizomatous individuals had a greater frequency of survival than those without rhizomes (Table 4). Individuals with moderate rhizome expression (R = 2) had a greater frequency of survival and the survivors were more vigorous (lower score) than those with low rhizome expression (R = 1, Table 4). Survivors of the IG parents within the R = 1 class were more vigorous than those that lacked rhizomes (Table 4). For the nine IG genotypes that demonstrated the potential for moderate rhizome expression, survival was significantly greater for the R = 2 class (77%) than for the R = 1 class (62%), indicating that the moderate rhizomes, and not just associated genes, were directly beneficial. In contrast to the IG parents and the IG/IG progeny, there were no differences in survival or vigor of the survivors between the R = 1 and non-rhizomatous classes of the IG/CV progeny (Table 4). More limited rhizome expression within the R = 1 class of the IG/ CV progeny than in the IG parents and the IG/IG progeny may account for the differences in survival. A revised scale that subdivides the R = 1 class by
characteristics such as rhizome number and length may be useful for breeding perennial rice. Rhizomes may promote survival during the dry season by being directly tolerant of dry conditions or by exploiting water deep in the soil profile. We would expect long and deep rhizomes to more effectively avoid drought stress than short and shallow rhizomes. Rhizomes of cogongrass, torpedograss, and johnsongrass have been found to be tolerant of desiccation (Wilcut et al., 1988). Paterson et al. (1995) observed F2 and BC1 progeny of Sorghum bicolor/S. propinquum grown in Texas, USA and found that rhizomes were associated with perennial regrowth, after the plants over-wintered two frosts sufficient to kill aboveground growth. To survive the colder winter season in Kansas, USA, F2 progeny of S. bicolor/S. hapelense needed long and deep rhizomes (Kulakow and Ennis, 1988). Several good predictors of progeny survival were midparent values for: rhizome length of plants grown in pots in an earlier greenhouse trial (r = 0.56***), percent rhizomatous in the field trial (r = 0.69***), and mean rhizome scores in the field trial (r = 0.74***). Given the low association between progeny and midparent for percent rhizomes, the survival-rhizome associations could be accounted for if small undetected rhizomes were frequently produced by progeny and these conferred advantages for survival, or if genes for rhizome expression pleiotropically enhanced survival even when rhizomes were not produced, or if genes for survival were linked to genes for rhizome expression. In any case, the association between progeny survival and parental rhizome expression will be beneficial for breeding perennial rice. 3.4. Yield Yield for the CV parents averaged 11 g/plant. Average yields for the IG parents (3.1 g/plant), IG/IG progeny (4.2 g/plant) and IG/CV progeny (6.0 g/plant) were significantly less (P 0.001) than yield for the CV parents. Yield averaged 3.0 g/plant for the female IG parents and 3.7 g/plant for the male IG parents. Reduced fertility is common in early generation progeny of O. sativa/O. longistaminata (Oka, 1988) and was observed previously in the intermated population from which the IG parents of this study were selected. Average yield for the IG/CV progeny
E.J. Sacks et al. / Field Crops Research 95 (2006) 39–48
was significantly greater (P 0.01) than the yield for the IG/IG progeny. Within each of the entry types, except for CV parents, there were significant differences among entries for yield (Table 3). Among the IG/CV families, six had average yields that were not significantly different from the CV parents and one family had significantly greater (about double) yield (Table 2). Genes for high yield have been found in O. rufipogon (Xiao et al., 1998), another wild relative of domesticated rice, and our results suggest that O. longistaminata might also be a source of such beneficial genes once the incompatibilities that lead to infertility are overcome. The female GCA mean square for yield was small and non-significant but the male GCA and SCA mean squares were highly significant (Table 3). Midparent yield was a moderately good predictor of progeny yield (r = 0.46**). Thus, additive and non-additive gene effects were important for yield. 3.5. Multiple trait selection To develop a perennial CV of upland rice from this population, genotypes that have moderate rhizome expression, high percent survival, and yield at least as much as current annual upland CVs should be identified. Thus, concurrent gains in rhizome expression, survival and yield are necessary. In this experiment, nine IG/IG progeny and six IG/CV progeny were rhizomatous (R = 1), perennial, and yielded at least 5 g/plant, and five of these yielded more than 10 g/plant. Of the 201 rhizomatous (R = 1) IG/IG progeny, 23 yielded at least 5 g/plant, and eight of these yielded more than 10 g/plant. Of the 160 rhizomatous (R = 1) IG/CV progeny, 55 yielded at least 5 g/plant, and 22 of these yielded more than 10 g/ plant. On average, the IG/IG progeny were less rhizomatous (P 0.001) than the female IG parents but they were not significantly different for survival and they had significantly higher yield (P 0.05). The IG/CV progeny on average were less rhizomatous and had lower survival than the female IG parents but they had significantly higher yield (P 0.001 for all comparisons). In comparison to the male IG parents, the IG/IG progeny on average had greater survival (P 0.001) but they were not significantly different for percent rhizomatous and yield. However, the IG/ CV progeny on average were about as rhizomatous as
47
the male IG parents and had significantly greater survival and yield (P 0.05). Thus, the production of rhizomatous IG/CV progeny and the restored fertility observed for many of these progeny demonstrated that the fertility barriers typical of early generation O. sativa/O. longistaminata populations do not preclude the introgression of rhizomes into a fertile O. sativa genetic background. This conclusion is also consistent with observations from two prior studies (Sacks et al., 2006; Tao et al., 2001). Though progress has been made, further effort is needed to increase the rhizome expression and yield of perennial progenies. For example, of the 10 IG/IG progeny with R = 2, none yielded more than 2 g/plant. Replicated testing of ramets helped clarify which IG parents were best for multiple traits. The field trial demonstrated that four of the IG parents (UPR-081496-4, UPR-0930-96-1, UPR-1250-96-1 and UPR1355-96-1) combined moderate rhizome expression, high rhizome penetrance and high percent survival with partially restored fertility (2.6–5.3 g/plant, Table 1). Replicated testing using ramets of selected progeny seedlings would be costly in time but it would ensure that selections are well characterized and that potentially useful parents and cross combinations are not overlooked.
4. Conclusions Though low fertility in O. sativa/O. longistaminata progeny can be a significant barrier to breeding progress, we have made considerable gains in rhizome expression, survival and yield. Moreover, further gains from selection can be expected for all of the key traits. Selecting female IG parents with medium to long rhizomes in pots, enabled us to obtain many perennial and rhizomatous progeny in the field, even from crosses with annual CVs. Vigor of survivors may need much improvement but further testing at typical Southeast Asian sites of upland rice production would clarify this point. Recovery of fertility was, as expected, greater in crosses with male CV parents than male IG parents but perennial, rhizomatous progenies with adequate fertility were obtained from both male parent types. This study demonstrated that an important breeding objective for perennial rice
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should be moderate rhizome expression (R = 2) because it confers the advantages of high rhizome penetrance, survival, and vigor. Previously identified backcross progeny of RD23/O. longistaminata that have moderate to long rhizomes may be a useful source of genes for developing perennial upland rice with moderate rhizome expression. Acknowledgements Financial support from Bundesministerium fu¨r Wirtschaftliche Zusammenarbeit und Entwicklung (BMZ, project no. 97.7860.6-001.00) is gratefully acknowledged. References Brady, N.C., 1994. Alternatives to slash-and burn: a global imperative. In: Sanchez, P.A., van Houten, H. (Eds.), Alternatives to slash-and burn agriculture. Symposium ID-6, 15th International Soil Science Congress, Acapulco, Mexico, 1994. International Center for Research in Agroforestry and International Society of Soil Science, Nairobi, Kenya, pp. 3–14. Cox, T.S., Bender, M., Picone, C., Van Tassel, D.L., Holland, J.B., Brummer, E.C., Zoeller, B.E., Paterson, A.H., Jackson, W., 2002. Breeding perennial grain crops. Crit. Rev. Plant Sci. 21, 59–91. Ghesquie`re, A, 1988. Diversite´ ge´ne´tique de l’espe`ce sauvage de riz Oryza longistaminata A. Chev. et Roehr et dynamique des flux ge´niques au sein du groupe Sativa en Afrique. The`se Doc. Etat. Universite Paris XI Orsay, 228 pp. Hu, F.-Y., Tao, D.-Y., Sacks, E., Fu, B.Y., Xu, P., Li, J., Yang, Y., McNally, K., Khush, G.S., Paterson, A.H., Li, Z.-K., 2003. Convergent evolution of perenniality in rice and sorghum. Proc. Natl. Acad. Sci. U.S.A. 100, 4050–4054. Hu, F.-Y., Tao, D.-Y., Xu, P., Li, J., Yang, Y., Sacks, E., McNally, K., Cruz, T.S., Zhou, J., Li, Z., 2001. Two dominant complementary genes controlling rhizomatous expression in Oryza longistaminata. Rice Genet. Newsl. 18, 34–36. Huke, R.E., 1982. Agroclimatic and dry-season maps of South. In: Southeast and East Asia, IRRI, Los Banos, Philippines. IRRI, 1996. Standard evaluation system for rice, fourth ed. Manila, Philippines, 52 pp.
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