The effect of dwarfing genes on sorghum grain filling from remobilized stem reserves, under stress

ELSEVIER

Field Crops Research 52 (1997) 43-54

Field Crops Research

The effect of dwarfing genes on sorghum grain filling from remobilized stem reserves, under stress A. Blum *, G. Golan, J. Mayer, B. Sinmena Institute of Field and Garden Crops, The Volcani Centre, PO Box 6, Bet Dagan, Israel

Received 29 June 1996; revised 2 December 1996; accepted 3 December 1996

Abstract Stem reserves are increasingly recognized as an important source for grain filling when current photosynthesis is inhibited by stress. However, information in this respect for sorghum is seriously lacking. This study was designed to test the hypothesis that tall (1-dwarf) sorghum has greater stem reserve storage than shorter (3-dwarf; 'combine height') sorghum and that under conditions of environmental stress during grain filling this storage becomes an important source for grain filling. Tall (SA1170) and short (SM100) isogenic lines of milo sorghum (Sorghum bicolor (L.) Moench.) were tested in the field for 3 years under 3 treatments: (a) full irrigation (control); (b) drought stress which intensified towards grain filling; and (c) chemical desiccation of the leaf canopy at 12 days after anthesis, which eliminated the main photosynthetic source during grain filling. Data were collected on grain dry matter, stem dry matter and stem water soluble carbohydrates (WSC) changes with time. The tall genotype was about 2 m and the short genotype was about 1 m tall, under non-stress conditions. It was found that grain weight per panicle was reduced by drought stress only in the short genotype. Both kernel number and kernel weight and their interaction were responsible for this reduction. Chemical desiccation was a more aggressive treatment than drought stress. It reduced kernel weight and grain weight per panicle much more in the short than in the tall genotype. Changes in stem dry matter reflected well the changes in stem WSC content. Maximum stem weight at the onset of grain filling and stem dry matter loss during grain filling were always greater in the tall than the short genotype. Stem weight loss was markedly promoted by drought and chemical desiccation only in the tall genotype or in the short genotype only in a year when it developed heavy stems. Stem weight loss as percent of grain weight per panicle under drought and chemical desiccation was always greater in the tall than in the short genotype. Since stem weight loss was promoted by stress, it could not have been fully accounted for by maintenance respiration. Irrespective of year, treatment and genotype, plants with greater stem weight at the onset of grain filling lost more reserves from the stems during grain filling (r = 0.89, p < 0.0001; n = 18). Consequently, stem weight loss as percent of grain weight per panicle increased with larger stem weight (r = 0.73, p = 0.0006; n = 18). It is concluded that large stem reserve storage at the onset of grain filling ascribe stable grain filling under any stress which depresses the photosynthetic source during grain filling. Keywords: Drought resistance; Sorghum; Grain filling; Plant height; Reserves Dwarf; Stem

* Corresponding author. 0378-4290/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. PII S 0 3 7 8 - 4 2 9 0 ( 9 6 ) 0 3 4 6 2 - 4

44

A. Blum et a L / Field Crops Research 52 (1997) 43-54

1. Introduction

When cereal crops are subjected to an increasing environmental or biotic stress after anthesis ('terminal stress') the most prominent result is the poor grain filling and the consequent loss in yield. Poor grain filling under stress is a result of a direct effect of stress on the growing grain a n d / o r the inhibition of the source activity by stress. The source of carbon for grain filling is current photosynthesis and remobilized reserves stored in the stem from pre-anthesis assimilation in the form of sugars, starch a n d / o r fructan (Schnyder, 1993). When current photosynthesis is inhibited by terminal stress, grain filling depends on stem reserves (e.g. Bidinger et al., 1977; Blum, 1996). The extent of grain filling from stem reserves is a function of the size of the reserve storage in the stem, the ability to remobilize this reserve into the grain and sink demand which is determined by its size (Kuhbauch and Thome, 1989; Connor and Sadras, 1992; Fortmeier and Schubert, 1995). The plant's capacity for supporting grain filling from stem reserves when current photosynthesis is inhibited by post-anthesis stress can be revealed by eliminating most of the current photosynthetic source, as first shown in wheat (Blum et al., 1983a; Blum et al., 1983b). When a chemical contact-desiccant, such as magnesium or sodium chlorate (Blum et al., 1983a; Hossain et al., 1990) or a senescing agent such as potassium iodide (Nicolas and Turner, 1993; Mahalakshmi et al., 1994) is sprayed on the canopy at the onset of grain filling, all exposed green tissues are incapacitated while the plant remains intact. Kernel growth is then supported exclusively by stem reserves. Kernel weight under this treatment, as compared with non-treated controls, is a good estimate of genotypic capacity for stem reserve utilization for grain filling under terminal stress. Compared with the small grains and millet, information in sorghum on the role of stem reserve in supporting grain filling under terminal stress is limited. Chemical desiccation of the canopy has never been tested in sorghum as a method for assessing the utilization of stem reserves for kernel growth. Early work with sorghum in Queensland showed that the relative amount of pre-anthesis stored carbon used

for grain filling under relatively stress-free conditions was 12% (Fischer and Wilson, 1971). For two combine-height grain sorghum hybrids manipulated by shading (Kiniry et al., 1992) the importance of pre-anthesis carbon storage towards grain filling appeared to be 'minimal' in these materials. In the small grains the importance of stem reserve utilization for grain filling under terminal stress is derived, at least partially, from stem storage capacity. Storage increases with longer stems and greater specific stem weight. The contribution of stem reserves to grain filling in sunflower was partially dependant on plant height (Sadras et al., 1993). Sorghum stem length and plant height are subjected to very large genetic variation, which is under relatively simple genetic control. Four major genes determine most of the genetic variation in sorghum plant height (Quinby, 1974). This study was therefore designed to test the hypothesis that tall sorghum (1-dwarf) have greater stem reserve storage than shorter (3-dwarf, 'combine height') sorghum and that under conditions of environmental stress during grain filling this storage becomes an important source for grain filling.

2. Material and methods

The experiments were performed on well-fertilized deep vertisol at Bet Dagan in the Coastal region of Israel, in 1989, 1991 and 1992. Two isogenic lines of milo sorghum were used. SA1170 was a tall 1-dwarf (DWlDW2Dw3dw4) genotype and S M 1 0 0 was a short 3 - d w a r f (dWldWzDw3dw4) genotype of 'combine-height' (Quinby, 1974). Judging by their phenotype during these three years of study the two lines were indeed highly homozygous and isogenic. In each year the two genotypes were planted during the last week of March or the first week of April, which is the standard planting time. Three experimental treatments were imposed on both genotypes in each year: 'irrigation', 'drought' and 'chemical desiccation'. In the 'drought' treatment plants were grown under the prevailing stored soil moisture conditions with no additional seasonal rainfall or irrigation. Total available stored soil moisture at

A. Blum et al. / Field Crops Research 52 (1997) 43-54

planting, as estimated from gravimetric soil moisture determination to a depth of 1.8 m, was 234, 264 and 290 mm, in 1989, 1991 and 1992, respectively. This moisture is normally fully extracted by sorghum and plants experience an increasing seasonal water deficit towards grain filling (e.g. Blum et al., 1989). In the 'irrigation' treatment, plots were sprinkler-irrigated three times during the season with a total effective amounts of 240, 220 and 220 mm, in the three years, respectively. The last irrigation was always applied just before anthesis. Total available moisture for seasonal crop evapotranspiration was therefore at least 474, 484 and 510 mm, in 1989, 1991 and 1992, respectively. The 'chemical desiccation' treatment was applied to irrigated plots. The canopy of the plants (including leaf sheaths and excluding panicles) was sprayed to full wetting with 0.4% magnesium chlorate at 12 days after anthesis. The experimental unit was a single plot consisting of 4 rows, 5 m in length and spaced 1 m apart. The two central rows in each plot were sprayed in the desiccation treatment. The experimental layout was split plots in 4 replicates, with treatments in main plots and genotypes in sub-plots. A sufficient number of panicles of a common date of anthesis (pollen shed at the middle of the panicle) were tagged. On several dates (depending on year and treatment) between anthesis and physiological maturity, five tagged main stems were sampled from each plot. Stems were harvested at the ground level. Panicles were detached from the stems just under the lowest panicle branch (the peduncle was considered as part of the stem). Leaf laminae were removed, so that 'stems' included the leaf sheaths. A preliminary study performed in 1988 showed that dry weight of leaf laminae hardly changed during grain filling under irrigation and drought stress treatments. Therefore, leaf laminae were considered as relatively unimportant for reserve storage and remobilization. Stems were measured for length (in 1989 and 1992), after which they were dried (80°C for 48 h) and weighed. Stem density was calculated by dividing stem dry matter weight by stem length. Thirty kernels were sampled from the middle section of each panicle, dried and weighed. The number of kernels per panicle was estimated by counting the number of 'plump' spikelets in half (longitudinally)

45

of the panicle. On the last sampling date (at maturity) kernel number per panicle was estimated from mean kernel weight (in a 200 kernel sample) and grain weight per panicle after threshing. The developing sorghum grain constitutes a major demand for remobilized stem reserves (Kuhbauch and Thome, 1989; Sadras et al., 1993; Fortmeier and Schubert, 1995). However, there are several additional potential sinks for remobilized stem reserves to consider, such as maintenance respiration, tiller and root growth. Since no tillers develop after anthesis in the tested genotypes, reserves are not used for tiller growth. It can also be safely assumed that roots are not a significant sink for remobilized stem reserves since roots do not grow and they do senesce during grain filling (Zartman and Woyewodzic, 1979; Blum and Arkin, 1984). Therefore, the only major active sinks for remobilized stem reserves in this study are the growing kernels and maintenance respiration. Under given conditions, the reduction in stem weight during grain filling is a widely adopted estimate of stem reserve utilization for grain filling and maintenance respiration. Stem weight loss was calculated as the total reduction in stem weight from its maximum during the time when kernels were growing (e.g. Fig. 1). Stem weight loss as percent of final grain weight per panicle (SWGW) was calculated to represent the relative contribution of stem reserves to grain mass per panicle, not accounting for maintenance respiration. To ascertain that dry matter weight changes during grain filing were representative of water soluble carbohydrate (WSC) changes in sorghum, WSC were analyzed in stems sampled during the 1989 experiment. One stem was sampled for each date X treatment Xgenotype combination. Each sampled oven-dried stem was chopped and then two subsampies of 5 g each were ground in a Wiley mill. A 250 mg sub-sample was then taken for analysis. WSC was determined according to Karkalas (1985). Briefly, the sample was suspended in distilled water and autoclaved. After cooling it was incubated in a buffered (acetate buffer pH = 4.45) amyloclucosidase enzyme (Sigma Chemicals Co.) solution for 44 h at 38°C. Hexose was then quantified by the phenolosulfuric acid procedure of Dubois et al. (1956).

46

A. Blum et al. / Field Crops Research 52 (1997) 43-54

3. Results A

40

3.1. 1989 experiment

1- d w a r f ~ _ 30-

20-

/f

1o-

$ t 4o-

I-o- 'r"gati°n

//

v

I" I

C/

a

o,ou0.t

3-dwarf

E (D

3E

30-

20-

lO-

04 o

a

10

20

30

40

50

60

60.

1-dwarf 5040. 30. v

20-

,'1'

10 C~

E .~.

co

3-dwarf

S

Irrigation Drought

50

Desiccation 40-

Stem length under irrigation (controls) was reduced by two dwarfing genes from 185 cm in the tall (1-dwarf) to 99 cm in the short (3-dwarf) (Table 1). In both genotypes stem length was reduced by drought stress compared with controls, indicating that stress was affecting growth during stem elongation. The rate of reduction in stem length under drought stress compared with controls was less in the tall (26%) than in the short (34%) genotype. Average (over treatments) grain weight per panicle was significantly greater in the tall than in the short genotype, as a result of a respective difference in average kernel weight (Table 1). Average kernel number per panicle was the same in both genotypes. The drought treatment significantly reduced kernel number per panicle only in the short genotype. Kernel weight was not affected significantly by drought stress in either genotype. Consequently, grain weight per panicle was not significantly affected by drought stress as compared with controls. The reduction in kernel number per panicle under drought stress in the short genotype was partially compensated for by a slight increase (non-significan0 in kernel weight (Fig. 1A). Chemical desiccation by magnesium chlorate caused total bleaching and senescence of the treated leaves within 2 to 3 days after the spray application. Panicles were unaffected. Compared with the controis this treatment did not affect plant height or kernel number per panicle (Table 1), evidently because it was applied after stem and panicle growth terminated. Grain weight per panicle was reduced by desiccation due to a reduction in kernel weight. (Table 1; Fig. 1).The relative reduction in kernel weight by desiccation compared with the controls

30. 20-

,I"

10 0

10

20

30

4'0

Days After Anthesis

5~0

60

Fig. 1. Kernel dry matter weight (A) and stem dry matter weight (B) in 1-dwarf (SAll70; tall)) and 3-dwarf (SM100; short) isogenic lines of sorghum subjected to three treatments: full irrigation (controls); drought stress; and chemical desiccation of the leaf canopy applied under irrigated conditions at 12 days after anthesis (arrows). Vertical bars are one S.E. of means. Arrows denote the date of chemical desiccation spray application. 1989 experiment.

A. Blum et al. / Field Crops Research 52 (1997) 43-54

was greater in the short (51%) than in the tall (38%) genotype. Maximum stem weight of the short genotype was less than half that of the tall one, in parallel to the respective differences in stem length (Table 1). In the controls, stem weight decreased to a minimum between 30 and 35 days after anthesis and then it increased by about 9 to 10 g in both genotypes (Fig. 1B). Under the drought and desiccation treatments stem weight decreased throughout the period of grain filling, although there was a tendency for a lag in the rate of decrease at around 35 days after anthesis in the tall genotype. Changes in stem weight in the short genotype were relatively very small under the two stress treatments. Stem weight density did not differ significantly between genotypes and treatments, although it tended to be relatively smaller in the short genotype (Table 1). Therefore, variations in maximum stem weight between genotypes and treatments could be ascribed mainly to variations in stem length. In the tall genotype, stem weight loss (Table 1) ranged from 9 g in the irrigated control to 18.2 g in the drought treatment. Stem weight loss was relatively smaller in the short genotype, ranging from 4.4 g to 5.8 g. A significant treatment X genotype

47

interaction for stem weight loss was revealed. While stem weight loss was unaffected by the treatments in the short genotype, it increased appreciably relative to the control under drought and desiccation stress in the tall genotype (Fig. 1B). Compared with the irrigated control, Stem weight loss as percent of grain weight per panicle (SWGW) significantly increased under drought and desiccation in the tall genotype (Table 1). Compared with the irrigated control, SWGW significantly increased under chemical desiccation treatment also in the short genotype. On average, SWGW was higher in the tall than in the short genotype, although kernels were relatively heavier in the former. Water-soluble carbohydrates content per stem (WSC) over sampling dates, treatments and genotypes correlated very highly with stem dry matter weight (Fig. 2). Eighty eight percent of the variation in stem dry matter weight in this experiment could be attributed to variations in WSC.

3.2. 1991 Experiment Average grain weight per panicle did not differ significantly between genotypes (Table 2). On average, kernel number per panicle was smaller in the

Table 1 The effect of treatment (full irrigation, drought conditions and chemical desiccation of the leaf canopy) on panicle weight, panicle weight components, stem weight and estimated stem reserve contribution to panicle weight in two height isogenic lines of sorghum (1-dwarf: tall; 3-dwarf: short) tested in 1989 Genotype

Treatment

Grain weight per panicle (g)

1-dwarf 1-dwarf 1-dwarf

Irrigation (controls) Drought Desiccation

31.3 29.5 17.7

3-dwarf 3-dwarf 3-dwarf

Irrigation (controls) Drought Desiccation

Differences between genotype means Treatment × genotype interaction

Number of kernels per panicle

Kernel weight (mg)

Maximum stem length (cm)

Maximum stem weight (g)

Stem weight density (g/cm)

Stem weight loss (g)

Stem weight loss as % of grain weight per panicle

873 855 844

35.1 33.9 21.5 a

185 137 a 189

52.6 52.0 50.6

0.28 0.38 0.28

9.3 18.2 a 13.4 a

30.0 62.4 a 73.3 a

22.3 19.1 11.1 a

825 680 a 849

27.7 28.4 13.7 a

99 65 a 100

18.5 18.6 19.8

0.20 0.29 0.21

5.8 4.5 4.4

26.0 23.3 37.5 a

p < 0.05 n.s.

n.s. n.s.

p < 0.05 n.s.

p < 0.05 n.s.

n.s. n.s.

a

p < 0.05 n.s.

a Significantly ( p < 0.05) different from the irrigated treatment, within a genotype.

p < 0.05

p < 0.05

A. Blum et al. // Field Crops Research 52 (1997) 43-54

48 12-

Ioc.~

.~=

y = 0.014 + 0.180x R2= 0.88; n--42

8-

•

J l-dwa] J [] 3-dwarf o

o

lb

2b

ab

4b

sb

60

Stem Dry Matter (g) Fig. 2. The linear regression of water soluble carbohydrate (WSC) content in stems on stem dry matter weight as determined on seven dates between anthesis and maturity in two isogenic lines of sorghum subjected to three treatments (see Fig. 1). 1989 experiment.

tall than in the short genotype while average kernel weight was larger in the tall than in the short genttype. As in 1989, drought stress significantly decreased grain weight per panicle only in the short genotype and this reduction could be fully accounted for by a respective reduction in kernel number per panicle (Table 2). There was a slight tendency for an initial acceleration in grain growth under drought stress

compared with the control only in the short genotype (Fig. 3A). This acceleration diminished after about 5 to 10 days. Chemical desiccation reduced grain weight per panicle in both genotypes, but the relative rate of reduction was distinctly greater in the short (48%) than in the tall (13%) genotype. The relatively greater reduction in grain weight per panicle in the short genotype under desiccation could be fully accounted for by the respective reductions in kernel weight, which were 47% and 16% in short and the tall genotypes, respectively (Fig. 3A). Under the desiccation treatment kernel weight in the tall genotype increased slowly until about 25-30 days after anthesis while in the short genotype kernels stopped growing at about 15-20 days after anthesis (Fig. 3A). Maximum stem weight was greater in the tail than the short genotype, as expected. It was significantly reduced by drought stress as compared with the controls in both genotypes. Similar to 1989 results, stem weight in the controls decreased to a minimum at about 25 to 30 days after anthesis whence it increased by about 5 to 10 g in both genotypes (Fig. 3B). The onset of stem weight increase corresponded with the time when kernels attained near final weight. Such changes were also observed in the tall genotype

Table 2 The effect of treatment (full irrigation, drought conditions and chemical desiccation of the leaf canopy) on panicle weight, panicle weight components, stem weight and estimated stem reserve contribution to panicle weight in two height isogenic lines of sorghum (1-dwarf: tall; 3-dwarf: short) tested in 1991 Genotype

Treatment

Grain weight per panicle (g)

Number of kernels per panicle

Kernel weight (mg)

Maximum stem weight (g)

Stem weight loss (g)

Stem weight loss as % of grain weight per panicle

1-dwarf I-dwarf I-dwarf

Irrigation (controls) Drought Desiccation

34.5 35.5 30.1 a

1111 1183 1155

31.1 29.9 26.0 a

68.1 58.0 a 67.0

9.1 25.4 " 29.1 a

26.6 71.6 a 86.4 a

3-dwarf 3-dwarf 3-dwarf

Irrigation (controls) Drought Desiccation

38.5 29.9 a 20.0 a

1434 1146 a 1399

26.9 26.0 14.3 a

22.9 16.0 a 21.9

6.4 4.6 6.7

16.4 15.6 33.3 a

n.s. n.s.

p < 0.05 n.s.

p < 0.05 n.s.

p _< 0.05 n.s.

Differences between genotype means Treatment x genotype interaction

" Significantly ( p < 0.05) different from the irrigated treatment, within a genotype.

p < 0.05

p < 0.05

49

A. Blum et al. // Field Crops Research 52 (1997) 4 3 - 5 4

A

40.

1-dwarf A

30.

1-dwarfy

40 30.

20-

20. 10.

:~

a ~"

E ,o,)

/

/

.,.

f, 40

'1',

i • Drought I I= Desicca'i°°l ',

,/~

10.

,

/7

(

[ 3-dwarf

~,~ 40a

30.

E ~)

I '''£3-" Irrigati°n

/j,

i• ,'1"

Drought

[ ----0--- Desiccation

,

~ ,

,

20

30

40

3-dwarf

30-

20. 2010 10O(

B

70 60

5

15

25

35

45

1-dw~

0~ 0

100-

40

80-

30

60-

20

,.~

1',

E

60-

3-dwarf

50-

40-

t~

70I

Q

I S

50

B 12o

50

:I=

10

Irrigation ] Drought Desiccation

;~, 120 a E 100 cO 80-

l - d w ~

,I" 3-dwarf

•

Irrigation Drought Desiccation

4060

:30-

40

2010

20 5

15 25 35 Days After Anthesis

Fig. 3. As Fig. l; 1991 experiment.

45

10

20

30

40

Days After Anthesis

Fig. 4. As Fig. 1; 1992 experiment.

50

50

A. Blum et al. // Field Crops Research 52 (1997) 43-54

under drought stress. Only under chemical desiccation stems did not increase in weight after reaching their m i n i m u m weight. Stem weight loss in the short genotype was small and about the same in all treatments, between 4.6 and 6.7 g per stem (Table 2). Stem weight loss was also relatively low (9.1 g) in the tall genotype in the control. A significant treatment X genotype interaction was revealed for stem weight loss. When subjected to drought or desiccation stress stem weight loss increased significantly only in the tall genotype (Fig. 3B). Consequently, the same interaction was expressed also for stem weight loss as percent of grain weight per panicle (SWGW). Overall, S W G W tended to be greater in the tall than in the short genotype. In the tall genotype it increased from 26.6% in the control to 71.6% under drought and 96.4% under desiccation. In the short genotype S W G W did not increase under drought stress and it increased only to 33.3% under desiccation. 3.3. 1 9 9 2 e x p e r i m e n t

On average, grain weight per panicle was significantly greater and kernel number per panicle was significantly smaller in the tall than in the short

genotype (Table 3). Kernel weight tended to be greater in the tall than in the short genotype but the difference was not significant. Grain weight per panicle of the tail genotype was not reduced by drought or chemical desiccation compared with the control. It was significantly reduced by both stress treatments in the short genotype (Table 3). The reduction in grain weight per panicle in the short genotype under drought stress could be accounted for by a reduction in kernel number per panicle and a small (non-significant) reduction in kernel weight. Under chemical desiccation the reduction in grain weight per panicle could be fully accounted for by the reduction in kernel weight. In the tall genotype kernel growth was practically the same under all three treatments (Fig. 4A). In the short genotype kernel growth was initially accelerated under drought stress compared with the control but this accelerated rate was not sustained to maturity. Compared with the tall genotype, chemical desiccation in the short genotype reduced kernel growth rate appreciably. However, compared with the two previous years, kernel growth in the short genotype under chemical desiccation was still proceeding to maturity. In both genotypes, m a x i m u m stem weight was

Table 3 The effect of treatment (full irrigation, drought conditionsand chemical desiccationof the leaf canopy) on panicle weight, panicle weight components, stem weight and estimated stem reserve contributionto panicle weight in two height isogenic lines of sorghum (1-dwarf: tall; 3-dwarf: short) tested in 1992 Genotype Treatment

Grain weight per panicle (g)

1-dwarf 1-dwarf 1-dwarf

Irrigation(controls) Drought Desiccation

52.0 50.0 51.1

3-dwarf 3-dwarf 3-dwarf

Irrigation(controls) Drought Desiccation

55.3 44.0 a 40.2 a

Differences betweengenotype means p < 0.05 Treatment × genotypeinteraction n.s.

N u m b e r Kernel Maximum of weight stem kernels (mg) length per (cm) panicle

1316 1320 1331

39.8 37.9 38.4

1670 1409 a 1669

33.0 30.9 24.0 a

p < 0.01 n.s.

n.s. n.s.

Maximum stem weight (g)

Stem weight density (g/cm)

Stem weight loss (g)

202 149 a 199

108.4 76.4 a 111.2

0.54 0.52 0.56

37.7 22.9 49.2

a

72.5 45.7 a 96.3 ~

107 70 a 110

55.0 33.3 a 52.1

0.50 0.47 0.47

28.3 11.1 a 22.1

51.2 25.2 a 54.6

p < 0.01 n.s.

n.s. n.s.

p < 0.01

n.s.

p < 0.01 n.s.

a Significantly(p < 0.05) different from the irrigated treatment, within a genotype.

Stem

weight loss as % of grain weight per panicle

p < 0.01

A. Blum et a l . / Field Crops Research 52 (1997) 43-54

notably greater this year than in the previous years studied. Maximum stem weight and stem length were reduced by drought stress compared with controis in both genotypes. The relative reduction in maximum stem weight under drought stress was somewhat greater in the short (40%) than in the tall (30%) genotype. Compared with the previous experiments, no increases in stem weight were observed here during grain filling or upon its termination. Stem weight density did not differ significantly between genotypes and treatments, although it tended perhaps to be relatively smaller in the short genotype (Table 3). Therefore, as in 1989, variations in maximum stem weight between genotypes and treatments could be ascribed mainly to variations in stem length. Stem weight loss in both genotypes was greater in this experiment (Table 3; Fig. 4B) than in the previous two. Compared with the previous experiments, appreciable stem weight loss was observed even in the short genotype. A significant treatment × genotype interaction was revealed for stem weight loss. Compared with the controls, stem weight loss decreased under drought stress (significant in the short genotype) probably because of the reduced stem weight under drought stress. Under desiccation treatment, stem weight loss significantly increased compared with the controls only in the tall genotype. On average, stem weight loss as percent of grain weight per panicle (SWGW) was significantly greater in the tall than in the short genotype. It was reduced by drought stress compared with the controls in both genotypes. It increased under chemical desiccation compared with the control only in the tall genotype.

4. Discussion The use of isogenic lines for different plant height in this study allowed to evaluate the effect of stem reserve storage size at the onset of grain filling on the contribution of stored reserves to grain filling. Indeed, the tall genotype had more than double the stem dry matter weight and WSC content per stem than the short one. This difference did not involve stem weight density and it was fully accounted for by stem length. WSC content in sorghum stems after anthesis was within the range of values observed by others in non-sweet sorghum types (Vietor and

51

Miller, 1990; Vietor et al., 1990). There was a very significant association between stem dry matter weight and WSC content per stem across treatments, genotypes and sampling dates (R 2 = 0.88) to the extent that variations in stem dry matter weight after anthesis reflected well the variations in WSC content per stem. Such an association has also been observed in wheat (Blum et al., 1994). There was a very high and significant correlation across years, treatments, and genotypes between maximum stem weight and total stem weight loss during grain filling ( r = 0.89, p < 0.0001; n = 18). Plants with larger maximum stem weight at the onset of grain filling lost more reserves from the stems during grain filling. Consequently, stem weight loss as percent of grain weight per panicle was highly and significantly correlated with maximum stem weight ( r = 0.73, p = 0.0006; n = 18). The unexpected increase in stem weight towards the end of grain filling, especially in the controls but never under chemical desiccation, could be an expression of the perennial habit of sorghum. It was maintained as long as plants were not completely senesced and canopy assimilation was still active. The increase in stem dry matter towards maturity was not observed in 1992 when stem weight was much larger than in previous years. Probably, in this year some reserves remained in the stem despite large remobilization during grain filling and this storage obliterated the demand for renewed reserve storage towards maturity. The absolute values of stem reserve contributions to grain filling in the various treatments and genotypes cannot be estimated here because it is impossible to determine the proportion of stem weight loss directly attributed to maintenance respiration. Kiniry et al. (1992) concluded from their shading experiment that a large part of stem reserves was used for respiration, at a maintenance coefficient (m) of 13 m g / g / d . However, they worked with combineheight sorghum hybrids, comparable to the 3-dwarf genotype tested here, which had relatively little stem reserves to begin with. Kiniry et al. (1992) also conceded that the determination of the maintenance coefficient was difficult. This is underlined by results the of Stahl and McCree (1988) who conclude that m declined with plant age from around 13 at anthesis to 6 or less at maturity and that it was proportional to

52

A. Blum et al. / Field Crops Research 52 (1997) 43-54

protein turnover processes in the specific plant tissue (Thornley, 1977). It is also impossible to determine the partitioning of whole-plant m between various plant compartments and organs, especially when different organs differ in longevity during grain filling as well as their capacity for carbon assimilation and protein turnover. It can only be assumed that most protein turnover processes during grain filling take place in the grain and the green leaves. When leaves are desiccated, as under the chemical desiccation treatment here, most protein turnover processes and the associated maintenance costs are limited to the growing kernels. It is therefore safe to assume that most WSC lost from the stem was remobilized to the panicle, to provide for the demand of this sink for endosperm synthesis and the associated maintenance cost.

Beyond the above arguments, it must be realized that the prominent enhancement of stem weight loss under the stress treatments as compared with the controls as seen in all 3 years cannot be explained by maintenance respiration. Even if one assumes that under irrigation (controls) all or most of the stem weight loss could be accounted for by maintenance respiration, most certainly the enhancement of stem weight loss under the stress treatments cannot be explained by greater maintenance costs in these plants, especially when plants were relatively smaller under drought stress than under irrigation or they lacked any live leaves as under chemical desiccation. Therefore, while the exact net contribution of stem reserves to grain filling cannot be quantified because of difficulty in estimating the proportion of reserves lost to respiration, strong circumstantial evidence support the conclusion that stem reserves are an important source for grain filling in sorghum when subjected to drought and desiccation stress. The drought stress treatment has been imposed as a given amount of stored soil moisture at planting, which has been depleted as the season progressed. This stress was effective in reducing plant height and stem weight of both genotypes. Indeed, sorghum plant height is very sensitive to drought stress (Blum et al., 1989). Kernel number per panicle was consistently reduced by drought stress only in the short genotype. Whereas the two genotypes were highly isogenic, only plant height and associated attributes could be responsible for this difference. It is possible

that the taller genotype had deeper roots than the shorter one and therefore had a relatively better plant water status. However, in a study by Wright et al. (1983) with the same isogenic lines used here it was found that the tall genotype did not differ from the short one in total root length. On the other hand, the taller genotype had a relatively larger leaf area. If these attributes were expressed in these experiments, the tall genotype would not have been in a better plant water status than the short one. The other possibility is that a higher WSC content in stems of the tall genotypes helped to sustain kernel set under drought stress, as proposed for maize (Boyle et al., 1991). However, this is a controversial issue (Schussler and Westgate, 1994). Kernel weight was not significantly affected by drought stress, not even in the short genotype. The known negative interaction between kernel number and kernel weight in the sorghum panicle (e.g. Blum, 1973) may have been behind the maintenance of kernel weight under drought stress in the short genotype, whereas the short genotype had reduced kernel number per panicle under drought stress compared with controls. Drought stress did not reduce kernel weight in the tall genotype while kernel number per panicle was as high as in the controls. This could be attributed to the large capacity for stem reserve remobilization in this genotype. Chemical desiccation was a more controlled and aggressive stress which was applied to the leaf canopy at the onset of the exponential phase of kernel growth. It effectively destroyed the major current source of carbon to the grain. The remaining green panicle could have provided some assimilates. The relative contribution of the panicle to total canopy assimilation was estimated by Fischer and Wilson (1971) to be only 14%. In all years, chemical desiccation caused a reduction in kernel weight and grain weight per panicle. However, the rate of reduction in both was consistently smaller in the tall than in the short genotype. In 1992 no reduction at all occurred in kernel weight in the tall genotype and in this genotype kernels grew under chemical desiccation as well as in the control. In that year maximum stem weight and stem weight loss were greater than in the other years. Hence, the comparison of results between 1992 and the previous year also support the conclusion that

A. Blum et al. / Field Crops Research 52 (1997) 43-54

when stem reserve storage is large, kernel growth is less affected by stress during grain filling. The effect of sink demand on stem-reserve utilization was seen in experiments where sink size has been manipulated (e.g. Kuhbauch and Thome, 1989; Fortmeier and Schubert, 1995). In our experiments this effect was not isolated, although it may have been implicated. For example, the short genotype had lower SWGW under drought than under chemical desiccation. While this difference could have been attributed to lower reserve storage in droughted plants it could also be attributed to their smaller sink demand. Thus, while the importance of sink demand in controlling stem reserve mobilization is recognized, the results presented here underline the dominant role of reserve availability in supporting grain filling. Both drought and chemical desiccation stress induced greater stem reserve remobilization as compared with the controls, when reserves were available. As discussed above, this promoted increase in stem weight loss cannot be explained by greater maintenance respiration. This is a stress responsive characteristic which seems to be non-specific to the agent of stress, drought or chemical desiccation. The common elicitor of reserve remobilization under both stresses may be the inhibition of current assimilation of carbon when the demand by the sink is still strong. A large stem reserve storage may therefore ascribe stable grain filing under any stress which depresses the photosynthetic source during grain filling, possibly including heat and leaf diseases.

References Bidinger, F.R., Musgrave, R.B. and Fischer, R.A., 1977. Contribution of stored preanthesis assimilates to grain yield in wheat and barley. Nature, 27: 431-433. Blum, A., 1973. Component analysis of yield response to drought of sorghum hybrids. Exp. Agric., 9: 155-167. Bhim, A., 1996. The role of mobilized stem reserves in stress tolerance. In: G. Scoles and B. Rossnagel (Eds.), Proc. of the 5th Int. Oat Conf. and the 7th Int. Barley Genetics Symp., Saskatchewan, Canada, pp. 267-275. Blum, A. and Arkin, G.F., 1984. Sorghum root growth and water-use as affected by water supply and growth duration. Field Crops Res., 9: 131-142. Blum, A., Mayer, J. and Golan, G., 1983b. Chemical desiccation

53

of wheat plants as a simulator of post-anthesis stress. II. Relations to drought stress. Field Crops Res., 6: 149-155. Blum, A., Mayer, J. and Golan, G., 1989. Agronomic and physiological assessments genotypic variation for drought resistance in sorghum. Aust. J. Agric. Res., 40: 49-61. Blum, A., Poyarkova, H., Golan, G. and Mayer, J., 1983a. Chemical desiccation of wheat plants as a simulator of post-anthesis stress. I. Effects on translocation and kernel growth. Field Crops Res., 6: 51-58. Blum, A., Sinmena, B., Mayer, J., Golan, G. and Shpiler, L., 1994. Stem reserve mobilization supports wheat grain filling under heat stress. Aust. J. Plant Physiol., 21: 771-781. Boyle, M.G., Boyer, J.S. and Morgan, P.W., 1991. Stem infusion of liquid culture medium prevents reproductive failure of maize at low water potential. Crop Sci., 31: 1246-1252. Connor, D.J. and Sadras, V.O., 1992. Physiology of yield expression in sunflower. Field Crops Res., 30: 333-389. Dubois, M., Gilles, K.A., Hamilton, J.K., Rebers, P.A. and Smith, F., 1956. Colorimetric method for determination of sugars and related substances. Anal. Chem., 28: 350-356. Fischer, K.S. and Wilson, G.L., 1971. Studies of grain production in Sorghum bicolor (L. Moench). I. The contribution of preflowering photosynthesis to grain yield. Aust. J. Agric. Res., 22: 33-37. Fortmeier, R. and Schubert, S., 1995. Storage of non-structural carbohydrates in sweet sorghum [Sorghum bicolor (L.) Moench]: comparison of sterile and fertile lines. J. Agron. Crop Sci., 175: 189-193. Hossain, A.B.S., Sears, R.G., Cox, T.S. and Panlsen, G.M., 1990. Desiccation tolerance and its relationship to assimilate partitioning in winter wheat. Crop Sci., 30: 622-627. Karkalas, J., 1985. An improved enzymatic method for the determination of native and modified starch. J. Sci. Food Agric., 36: 1019-1027. Kiniry, J.R., Tischler, C.R., Rosenthal, W.D. and Gerik, T.J., 1992. Nonstructural carbohydrate utilization by sorghum and maize shaded during grain growth. Crop Sci., 32: 131-137. Kuhbauch, W. and Thome, U., 1989. Nonstructural carbohydrates of wheat stems as influenced by sink-source manipulations. J. Plant Physiol., 134: 243-250. Mahalakshmi, V., Bidinger, F.R., Rao, K.P. and Wani, S.P., 1994. Use of the senescing agent potassium iodide to simulate water deficit during flowering and grainfilling in pearl millet. Field Crops Res., 36: 103-111. Nicolas, M.E. and Turner, N.C., 1993. Use of chemical desiccants and senescing agents to select wheat lines maintaining stable grain size during post-anthesis drought. Field Crops Res., 31: 155-171. Quinby, J.R., 1974. Sorghum Improvement and the Genetics of Growth. Texas A and M University Press, College Station, 108 pp. Sadras, V.O., Connor, D.J. and Whitfield, D.M., 1993. Yield, yield components and source-sink relationships in waterstressed sunflower. Field Crops Res., 31: 27-39. Schnyder, H., 1993. The role of carbohydrate storage and redistribution in the source-sink relations of wheat and barley during grain filling - a review. New Phytol., 123: 233-245.

54

A. Blum et al. / Field Crops Research 52 (1997) 43-54

Schussler, J.R. and Westgate, M.E., 1994. Increasing assimilate reserves does not prevent kernel abortion at low water potential in maize. Crop Sci., 34: 1569-1576. Stahl, R.S. and McCree, K.J., 1988. Ontogenetic changes in respiration coefficients of grain sorghum. Crop Sci., 2 8 : 1 1 1 114. Thornley, J.H.M., 1977. Growth, maintenance and respiration: a re-interpretation. Ann. Bot., 41: 1191-1203. Vietor, D.M. and Miller, F.R., 1990. Assimilation, partitioning, and nonstructural carbohydrates in sweet compared with grain sorghum. Crop Sci., 30:1109-1115.

Vietor, D.M., Miller, F.R. and CraUe, H.T., 1990. Nonstructural carbohydrates in axillary branches and main stem of senescent and nonsenescent sorghum types Crop Sci., 30: 97-100. Wright, S.A., Jordan, W.R., Morgan, P.W. and Miller, F.R., 1983. Genetic and hormonal control of shoot and root growth in sorghum. Agron. J., 75: 682-686. Zartman, R.E. and Woyewodzic, R.T., 1979. Root distribution patterns of two hybrid grain sorghums under field conditions. Agron. J., 71: 325-328.