Ear: Stem ratio in old and modern wheat varieties; relationship with improvement in number of grains per ear and yield

Field Crops Research, 21 (1989) 59-78

59

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

E a r : S t e m Ratio in Old and Modern Wheat Varieties; Relationship with I m p r o v e m e n t in N u m b e r of Grains per Ear and Yield K.H.M. SIDDIQUE 1, E.J.M. KIRBY 2 and M.W. PERRY 1

:Division of Plant Industry, Western Australian Department of Agriculture, Baron-Hay Court, South Perth, 6151 W.A. (Australia) 2Agronomy Group, School of Agriculture, University of Western Australia, Nedlands, 6009 W.A. (Australia) (Accepted 7 November 1988)

ABSTRACT Siddique, K.H.M., Kirby, E.J.M. and Perry, M.W., 1989. Ear: stem ratio in old and modern wheat varieties; relationship with improvement in number of grains per ear and yield. Field Crops Res., 21: 59-78. Three field experiments were conducted over two seasons at three sites in Western Australia, to investigate the physiological basis of increased grain number per ear in some old and modern wheat varieties and lines isogenic for Rht dwarfing genes. Growth rates and relative growth rates of the stem and ear varied between varieties, but there were no discernible trends with date of introduction or maturity grouping. The ratio of ear: stem growth rates was generally higher in modern varieties than in old. Among the Rht lines, the dwarf lines had a higher ratio of ear: stem growth rate than the tall lines. The ratio of ear: stem dry-matter increased from terminal spikelet stage to values ranging from about 20 to 50% at anthasis. The ratio of ear: stem dry-matter at anthesis was higher in modern than old varieties and in dwarf lines compared with tall. There was an allometric relationship between ear and stem dry-matter, indicating a constant ratio between relative growth rates of the ear and stem. This analysis showed that a difference between old and modern varieties was evident soon after the terminal spikelet stage, and that a greater ear: stem ratio was due mainly to a bigger intercept of the regression of In ear dry-matter versus In stem dry-matter. The allometric relationship showed that when ear weight was 1 mg there was a difference in stem dry-matter which ranged from about 50 mg in a modern variety to 200 rng in an old variety. There were from 6.7 to 9 florets initiated in the mid-ear spikelets, but only a proportion survived to form grain. Floret death started when the flag leaf was fully emerged, at the beginning of the rapid ear-growth and extension of the peduncle and was completed by ear-peep stage. Modern varieties initiated more florets and had a higher number of viable florets present at anthesis than did old varieties. The isogenic lines initiated similar numbers of florets, but the proportion of florets which survived was greater in the dwarf lines. The grain yield and harvest index were generally higher in modern than in old varieties. Among the isogenic lines, the dwarf lines had a greater harvest index than the tall lines. Grains per spikelet and per ear increased from old to modern varieties and from tall to dwarf lines. There was positive

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60 correlationbetweenear: stemratio at anthesisand harvestindexwithdifferentresponsesbetween sites and years. The results indicate that improvementsin grain number have come about becausethe stem competedless stronglythan the ear for dry-matter.This reducedcompetitionresultedin either the initiation of morefloretsand/or greatersurvival of floretsto formgrains. Selectionfor high ear: stem ratio at anthesis may leadto furtherimprovementin grain yield.

INTRODUCTION

Studies on yield improvement in wheat have analysed dry-matter production, its partition between grain and the rest of the plant, and the morphological components of yield. These studies have shown that the higher yield of modern varieties is related to higher harvest indices, and that there has been little change in total biomass (Jain and Kulshrestha, 1976; Austin et al., 1980; Perry and D'Antuono, 1989). In terms of yield components, per-grain drymatter has been found to be relatively stable and increases in yield have been associated with increase in number of grains per ear or per unit area. Grain number per ear depends upon floret production and survival of florets within each spikelet, and fertilization at anthesis. Previous studies have shown that only a proportion of the total number of florets initiated eventually sets grain (Kirby, 1974; Whingwiri and Stern, 1982). Comparisons between Rht isogenic lines suggest that there is greater partitioning of dry-matter to the ear during the early stages of development in the dwarf compared to the tall lines, and thus the competitive relationship between ear and stem is believed to determine floret survival and hence grain number per spikelet and ear (Brooking and Kirby, 1981; Fischer and Stockman, 1986). Experiments with old and modern wheat varieties in Western Australia have shown that increases in grain yield in modern varieties have been associated with increases in number of grains both per ear and per unit area (Perry and D'Antuono, 1989 ). However, the basis of the increase in grain numbers per ear in the modern varieties was not known. In this study, our aim was to investigate the physiological basis of increased grain number per ear in some of the old and modern wheat varieties used by Perry and D'Antuono (1989). The study also included some more recently released varieities and some experimental lines isogenic for Rht genes. The experiments were designed to study the relations between pre-anthesis development and growth of stem and ear, floret production and survival, and the number of grains per ear, and grain-yield. MATERIALSAND METHODS Two field experiments were done in 1986, one at the University of Western Australia Field Station, Shenton Park, Perth (31°57'S, 115°45'E) and the

61

second at Wongan Hills, in the central wheat belt (30°54'S, 116°43'E). A third experiment was done in 1987 at Merredin (31 ° 29'S, 118 °E) in the eastern wheat belt. Full details of treatments, design, soil, cultural and management practices for the 1986 experiments, given in Kirby et al. (1989), are summarised here: in 1986, each experiment included 14 old and modern varieties (Table 1) plus 6 near-isogenic lines differing in Rht dwarfing genes (Table 2 ). The near-isogenic lines were obtained from Dr. R.A. Richards (Division of Plant Industry, CSIRO, Canberra). The KCD and APD lines (Table 3) are from the crosses Kalyansona ( R h t l ) × C h e n a b 70 (Rht2) and Arz ( R h t l ) × Pato Argentino (Rht2), respectively; 0, 1 and 2 refer to whether they have none, one or two of the major dwarfing genes. To develop these lines, a single plant heterozygous at the R h t l and Rht2 locus was selected at each generation until the F7 in the case of KCD and the F6 in APD. Then progeny from a single F7 or F6 plant in each cross was grown out and plants homozygous for none, one or two dwarfing genes were bulked. The seed used in this study was derived from field plots. Most of the varieties of the 1986 experiments were included in the 1987 TABLE 1 Growth rate a a n d relative growth rate of the stem a n d ear among adapted varieties at P e r t h in 1986 Variety

Year of release

Growth rate (mg ( ° Cd ) - 1 ) Stem

Purple Straw b Gluyas Early Nabawa Bencubbin Gabo Insignia Gamenya Condor Tincurrin Miling Aroona Bodallin Gutha Kulin Mean s t a n d a r d error (%)

1860 1894 1915 1929 1945 1946 1960 1961 1978 1979 1981 1982 1982 1986

3.273 2.426 3.203 3.073 2.826 1.745 2.619 2.575 2.251 2.318 2.345 2.671 2.717 2.415 4.9

Relative growth rate ( ° Cd ) - 1

Ear 1.275 0.905 2.189 1.674 1.778 1.166 1.373 2.094 1.971 1.964 1.505 1.554 1.283 1.687 10.3

Stem

Ear

0.0063 0.0049 0.0067 0.0065 0.0050 0.0060 0.0052 0.0060 0.0079 0.0058 0.0068 0.0070 0.0067 0.0064

0.0183 0.0146 0.0165 0.0147 0.0156 0.0149 0.0188 0.0160 0.0144 0.0176 0.0212 0.0167 0.0207 0.0174

5.1

6.2

aRates were calculated by linear regression (see text for details); s t a n d a r d errors (as a percentage of the m e a n ) for any character were generally similar amongst varieties. bAccession AUS 888.

62 TABLE 2 Relative growth rates ( ( ° Cd) - 1X 103) of (a) ear and (b) stem for the Rht isogenic lines at Perth, Wongan Hills in 1986 and from Fischer and Stockman (1986; F&S )a Perth

Wongan Hills

F &S

14.9 14.8 15.7 19.5

15.4 12.8 16.1 15.4

Earrel~ivegrowthrate

APD0 APD1 KCD0 KCD1

14.1 13.8 18.0 17.3

Stemrelative

APD0 APD1 KCD0 KCD1

~owthr~e

6.4 6.5 7.5 7.4

4.6 4.7 2.7 4.6

4.2 3.3 4.5 3.8

aThese rates have been converted to thermal time, using the constant day and night temperatures quoted in their paper.

TABLE 3 Ratio of ear: stem dry-matter growth rate at Perth and at Wongan Hills in 1986 Variety

Perth

Wongan Hills

Purple Straw Gluyas Early Nabawa Bencubbin Gabo Insignia Gamenya Condor Tincurrin Miling Aroona Bodallin Gutha Kulin APD0 APD1 APD2 KCD0 KCD1 KCD2

0.390 0.373 0.683 0.545 0.629 0.668 0.524 0.813 0.876 0.847 0.642 0.582 0.472 0.699 0.980 1.228 -0.443 0.597

0.253 0.396 0.343 0.459 0.501 0.445 0.565 -0.649 0.532 0.578 0.534 0.444 0.596 0.641 0.873 1.546 0.385 0.468 0.684

--

63

experiments, but the 1987 experiment included some additional modern varieties and some high-yielding advanced lines (Table 4). The plots at Perth and Wongan Hills were sown on 28 May; the plot size was 8 rows, 18 cm apart, 10 m long. In 1987, the plots were sown on 27 May and the plot size was 2.16 m wide (12 rows, 18 cm apart) and 15 m long. Soil details, cultural and management practices for the 1987 experiment are similar to that of Siddique et al. {1988). All three experiments used a randomized block experimental design with five replicates for Perth and Wongan Hills experiments and three replicates for the Merredin experiment. All three experiments were sown 3-4 cm TABLE4 Ear: stem ratio (%) at anthesis for old and m o d e m wheat varieties and the isogenic lines at Perth, Wongan Hills in 1986 and Merredin 1987 Variety

Perth

Wongan Hills

Merredin

Purple Straw Gluyas Early Nabawa Bencubbin Gabo Insignia Gamenya Condor Tincurrin Miling Minewa (1979)a Aroona Bodallin Bass (1983) Gutha Cranbrook (1985) Kulin 77W677 b 79W783 b APD0 APD1 APD2 KCD0 KCD1 KCD2 Genero F8V

17.7 18.1 29.6 24.6 30.5 40.2 31.2 39.5 44.8 42.1 -35.7 33.4 -30.2 -44.0 --50.0 65.0 -25.0 32.0 ---

15.9 22.1 24.1 24.9 27.0 27.4 30.3 -36.1 32.9 -33.5 31.8 -26.9 -35.4 --38.8 57.0 65.7 21.5 28.9 30.1 --

24.2 38.5 26.2 37.0 40.2 41.6 41.5 40.6 60.3 51.3 47.1 47.9 46.9 52.9 34.3 46.0 44.7 49.8 48.1 ---36.9 34.1 63.5 50.6

7.2

6.9

10.7

LSD (P---0.05)

aYear of release of additional varieties used in 1987 experiment, b77W677 and 79W783 are advanced breeding lines from Western Australian D e p a r t m e n t of Agriculture. CGenaro F81 is an advanced breeding line from C I M M Y T

64 deep at a seed rate of 50 kg ha -1, using a precision cone-seeder drill. The experiments were surrounded by buffer plots to minimize edge effects. The experiment at Perth was set up as a site with minimal moisture stress. Plots at Perth were irrigated to avoid moisture stress, starting from the 2nd week of September, using overhead sprinklers. Some bird damage occurred at Perth in Purple Straw and Nabawa plots, so grain-yield and yield-component data were not included for these varieties.

Sampling procedures and measurements At Perth, 50-60 plants per plot were labelled for uniformity of growth and spacing before the terminal spikelet stage in an undisturbed area, adjacent to an area sampled for leaf and spikelet primordia measurements (Kirby et al., 1989 ). After each variety reached terminal spikelet stage, starting from 51 days after sowing (DAS) 1 plant per plot (i.e. 5 per variety) was sampled every Monday and Friday from the population of marked plants. Samples were taken every Thursday at Wongan Hills. Six plants per plot were collected, from which the median plant was selected for measurements (i.e. 5 per variety). On all selected plants the main shoot was identified, and the number of emerged leaves and the total number of leaves were counted. The leaves (including the leaf sheath) were removed and the lengths of the internodes, peduncle and ear were recorded. The number of spikelets and the number of living florets in spikelet 8 (mid-ear for most varieties) were then counted, using a stereoscopic dissecting microscope. After floret death had started and before anthesis, living or competent florets were recognized by their large turgid anthers and turgid carpels with well-developed stigmas. Leaves, peduncle, the rest of the stem and the ear were weighed separately after drying in an oven at 70°C for 48 h. Plant sampling ceased at anthesis for each variety. In 1987 at Merredin, 3 uniform plants per plot were sampled at anthesis. The main shoot was identified and stem, leaf (lamina and sheath ) and ear drymatter were recorded separately for each individual plant. For all three experiments, 1 m 2 of mature crop was harvested at ground level. The number of ears per m 2 was counted and the samples were dried in an oven at 70 ° C for 48 h and weighed. Grain was then threshed in an electric thresher, redried and weighed. Harvest index was calculated as the ratio of grain-yield to total above-ground biomass. Fifty main-stem ears were collected at final harvest to determine yield components. Twenty-five uniform ears were selected and numbers of spikelets and grains per ear were counted, and the mean number of grains per spikelet was calculated. The number of grains in spikelet 8 was determined. Maximum and minimum temperatures were obtained from weather stations close to the experimental sites, and thermal time (accumulated temperature, base temperature 0 °C ) was calculated as described in Weir et al. (1984).

65 RESULTS Weather conditions

In 1986 at Perth, the growing season (May-November) rainfall was 780 mm, of which 594 mm fell between sowing and anthesis for Purple Straw. Irrigation from the 2nd week in September prevented any soil moisture deficits in the anthesis and post-anthesis period of wheat varieties at Perth. The rainfall for the growing season (May-October) at Wongan Hills was 280 mm, of which 237 mm fell between sowing and anthesis for Purple Straw. No major water stress was observed for the crop in the pre-anthesis period. In 1987 at Merredin, the rainfall for the May-October growing season was 185 mm, 25 mm less than the 77-year average. Very little rainfall (12 mm) occurred during the period 23 June-27 July (27-61 DAS), when the majority of the varieties were in the ear-initiation stage. Leaf water-potential measurements on an adjacent experiment with similar wheat varieties (unpublished data) sown on the same day showed that at 61 DAS the midday leaf waterpotentials reached - 2.0 MPa. Thus the crop was severely water-stressed during this period. The spring was hot and dry, and only 12 mm of rain occurred in September compared with the long-term average of 22 mm. Thus, the anthesis period ( 100-125 DAS over all varieties) occurred under severe soil-moisture deficits and rising air temperatures. The weather experienced by the crops in the three experiments reported in this paper is typical of the conditions found in the eastern and central wheat belt in Western Australia. S t e m and ear growth-rates

At terminal spikelet stage, stem dry-matter was about 50 mg and ear drymatter less than 1 mg. The growth-rate increased throughout, rapidly at the beginning and, in some varieties, at the end of the period of measurement (Fig. 1 ). Rapid ear-growth did not start until approximately 200 ° Cd after measurement started. Ear growth-rate then increased to a more or less constant rate (Fig. 1 ). Growth-rate and relative growth-rate were estimated by linear regression of dry-matter or the natural logarithm (ln) of dry-matter, (d.m.), respectively, versus thermal time. For stems, the whole sampling period was included. In the case of ear growth-rate, regression was restricted to the grand period of growth. For ear relative growth-rate, later samples were not included. Selection of the period for regression analysis was made by inspection of graphical data. Relative growth-rate of the stem with respect to thermal time was almost constant. In the ears, particularly where sampling started when the ear dry-matter was less than 1 mg (mainly at Perth), the relative growth-rate increased until the ear was about 4 mg; there was then a period of exponential growth, followed

66

1750

,

,

,

,

,

,

1250

750

25O 0 1300

1500

1700

Thermal

700

900

1100

1300

time (°Cd}

Fig. I.Leaf (•), stem ([]) and ear (•) dry-matter vs.thermal time. (a) Purple Straw (b) Kulin. Data for Perth.

by a marked decline in the relative growth-rate towards anthesis. Similar trends for growth-rate were observed both at Perth and at Wongan Hills with other varieties. The range of variation in growth-rate and relative growth-rate amongst the adapted varieties is shown in Table 1. There were no obvious trends with either date of introduction or maturity grouping. Stem and ear relative growth-rates of tall (APD0, KCD0) and dwarf (APD1, KCD1 ) isogenic lines were almost the same at both Perth and Wongan Hills (Table 2 ). Only at Wongan Hills, where KCD1 had a considerably greater ear relative growth-rate than KCD0, was the difference significant. This contrasts with the results of Fischer and Stockman (1986) who, using the same isogenic lines, found that the relative growth-rate of the tails (APD0, KCD0) was greater than that of the dwarfs (APD1, KCD1) for both ear and stem (Table 2). In the experiments reported here, only in the double dwarf lines (APD2, KCD2 at Wongan Hills, data not presented) was the relative growth rate less than that of tall lines. The ratio of ear growth-rate to stem growth-rate over the two sites varied from 0.25 for Purple Straw to 1.54 for APD2 {0.25 to 0.88 (Tincurrin) amongst adapted varieties; Table 3). The greatest values were found in the modern varieties, the least in the old varieties. Amongst the Rht isogenic lines there was a consistent trend, with the smallest values in the tall lines and the largest values in the APD2, KCD2 lines (at Wongan Hills). Overall, the ratios were higher at Perth (mean=0.65) than at Wongan Hills (mean=0.57) but the trends were similar (r=0.84; n--18). APD1 at Perth and APD2 at Wongan Hills were noteworthy, both having greater ear growth rates than stem {Table 3).

67

Ratio of ear: stem dry-matter The ear: stem ratio (the ratio of ear d.m.: stem d.m., expressed as a percentage) was greater in the modern than in the old varieties (Table 4 ). In 1986, the ear: stem ratio at anthesis at Perth tended to be greater than at Wongan Hills. In 1987 at Merredin, the ear: stem ratio at anthesis was generally greater than that in the 1986 experiments. The incorporation of R h t dwarfing genes in the near-isogenic lines generally increased the e a r : s t e m ratio at anthesis. During development, the ear: stem ratio of varieties increased from very low values at the terminal spikelet stage to values ranging from about 20 to about 50% at anthesis (Table 4). W h e n expressed as a function of stem d.m., there were clear differences among varieties, the extremes of which were represented by Kulin and Purple Straw (Fig. 2). In Kulin, the e a r : s t e m ratio increased from a very low value when the stem d.m. was about 50 mg to a maximum of more than 40% at about 1250 mg stem d.m. and then remained constant until about 2200 mg (anthesis). In Purple Straw, the e a r : s t e m ratio remained at a low value until about 250 mg stem d.m. It then increased, at a much slower rate than Kulin, reaching a value of about 20% when stem d.m. was about 2000 mg, at anthesis (Fig. 2). Plots of In ear d.m. vs In stem d.m. for Perth data showed that their relative growth rates were not related by a simple function. This was most clearly seen when d.m. measurements started when stem d.m. was low (e.g. Insignia, Fig. 3 ). In varieties where the first sample was not taken until stem d.m. was relatively greater or when ear d.m. was approaching 1 mg, the initial shallow slope was less apparent. W h e n the ear was very small, the slope of the line was comparitively shallow; as stem and ear increased in d.m., there was a marked inI

I

60

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n

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E 20

Z~

A

A

• •

i•

~

•

W

i

0

500

I

1000 Stem cl.m. (rag)

I

1500

Fig. 2. Ear: stem dry-matter vs stem for Purple Straw ( • ) and Kulin ( A ). The lines were fitted by eye. Data for Perth.

68 !

I

6

I

I

I

"_

v

-6 2 ~0 B

•

-2 I

0

3

I

I

I

4 5 6 Ln stem d.m. (mg)

7

Fig. 3. Ln ear dry-matter vs In stem for Insignia. The fitted line was estimated by linear regression for all data where ear dry-matter exceeded In 0 (broken line). Data for Perth.

crease in slope. Inspection of the plots for all varieties indicated that the change in slope occurred when In (ear d.m. ) was 0 (1 mg). Up to that point, the slope was 1 or less, but then changed to about 2-3 (In ear d.m./ln stem d.m. ). Linear regression o f l n ear d.m. ( > 0 ) vs. In stem d.m. accounted for more than 90% of the variance, and the deviations from the fitted line were randomly distributed. Therefore over the increment of stem d.m. from 150 to 2000 mg, ear and stem d.m. were allometrically related, indicating a constant ratio between ear and stem relative growth-rates. The different relationship at low d.m. may have been because of the lower relative growth-rate which was found when the ear d.m. was < 1 mg. Fischer and Stockman (1986), working with Norin-10derived isogenic lines, also found a typical allometric relationship between ear and stem d.m. The parameters for linear regression and stem and ear d.m. for each variety at Perth are shown in Table 5. There was relatively little variation in the slope (b); expressed as a coefficient of variation ( c v ) it was 3.6% (ln units). The increment of In stem d.m. from In ear d.m. = 0 to anthesis (d) was more variable (cv 4.7% ), considerably more t h a n in stem d.m. at anthesis (cv 0.9% ). The intercept (a) was the most variable parameter of the regression ( - 7 . 4 to - 16.6 ) with a c v of 5.9%. There was good correspondence between the parameters for Perth and Wongan Hills ( r > 0.7, n--17).

Floret initiation, death and grain set Observations on floret production from the terminal-spikelet stage to anthesis showed that higher numbers of florets per spikelet were initiated in the modern varieties at both Perth and Wongan Hills (Table 6). The number of viable florets present at anthesis was also higher for modern varieties and the percentage of floret survival was similar in old and modern varieties. At final harvest, the number of grains per spikelet closely approximated the number of

69 TABLE5 Intercept (a) and slope (b) of linear regression In ear dry-matter vs. In stem dry-matter; predicted stem dry-matter at ear w e i g h t = 1 mg (e); increment of In stem from In e a r = 0 to anthesis (d) and expected ear: stem ratio (%) at anthesis (r). aData for P e r t h in 1986. Regression equations showing the relationships between b and a and b and d for P e r t h and Wongan Hills. a Purple Straw Gluyas Early Nabawa Bencubbin Gabo Insignia Gamenya Condor Tincurrin Miling Aroona Bodallin Gutha Kulin APD0 APD1 KCD0 KCD1

-

15.05 16.58 15.11 14.49 13.17 13.18 12.12 12.57 - 7.38 - 13.83 - 8.94 - 8.09 - 8.45 - 9.91 - 9.70 -8.31 - 9.53 -9.21

Perth (1986) b = 0 . 9 8 5 - 0 . 1 2 7 a (r--0.987; n = 18) b=4.21 - 0.680d (r=0.923; n = 18)

b

e

d

r

2.84 3.06 2.87 2.76 2.68 2.70 2.55 2.62 1.88 2.87 2.16 1.95 2.03 2.28 2.26 2.12 2.09 2.12

200.20 225.40 193.41 190.57 136.21 131.83 115.92 121.23 50.68 123.82 62.73 63.35 64.24 77.21 73.00 50.30 95.05 76.02

1.99 1.78 2.34 2.26 2.27 1.85 2.37 2.41 3.34 2.11 3.09 3.19 3.16 2.85 2.68 2.83 3.21 3.23

19 17 41 28 33 18 34 41 37 42 58 33 40 50 40 48 35 50

Equation ( 1 ) Equation (2)

Wongan Hills (1986) b = 1.25-0.109a (r--0.944; n - - 19) b=4.26-O.658d

(r=0.868; n = 19)

Equation (3) Equation (4)

viable florets, and thus the number of grains per spikelet was also higher in modern varieties. Similar numbers of florets per spikelets were initiated in the isogenic lines in each background (Table 6). However, a higher percentage of floret survival occurred with the addition of Rht dwarfing genes which, in turn, resulted in higher grain number per spikelet, except in the case of KCD lines at Wongan Hills. Floret numbers increased to maximums of 6.6-9.6 in spikelet 8 (Table 6). Maximum floret number was maintained for a short period, after which the number of living florets declined to between 2.2 and 4.8 (Table 6) within about 100 ° Cd (Fig. 4 ). Floret death started at the time when the flag leaf was almost fully emerged and boot stage was first noted. No further death of florets occurred after ear-peep stage (Fig. 4). The start of floret death coincided with the beginning of the grand period of growth of the ear and rapid extension of the peduncle (Fig. 4).

70 TABLE 6 Maximum number of florets, number of florets at anthesis and grains per spikelet 8 at final harvest for old and modern wheat varieties and isogenic lines at Perth and Wongan Hills in 1986 Variety

Perth

Wongan Hills

Florets

Grains

Max.

Anthesis

Purple Straw Gluyas Early Nabawa Bencubbin Gabo Insignia Gamenya Condor Tincurrin Miling Aroona Bodallin Gutha Kulin APD0 APD1 APD2 KCD0 KCD1 KCD2

6.7 7.8 7.8 7.2 8.4 7.6 8.0 7.2 8.6 8.0 8.0 8.6 8.4 8.6 9.0 9.0 -8.5 8.4 --

3.0 3.0 3.8 3.4 3.4 3.8 4.0 3.8 4.6 3.6 3.3 4.0 4.2 4.4 3.8 4.0 -3.8 4.4 --

LSD (P=0.05)

0.68

0.44

•

Florets

Grains

Max.

Anthesis

2.2 2.9 2.7 2.9 2.8 3.0 3.3 3.1 4.2 3.2 3.2 4.0 3.8 4.1 3.5 3.4 -3.2 3.9 --

6.6 7.8 7.0 7.2 7.4 7.6 8.2 -8.6 6.3 7.4 8.4 8.4 8.8 9.0 9.6 9.0 8.8 8.0 8.2

2.2 3.6 3.0 3.6 3.8 3.2 3.8 -4.8 3.0 3.8 3.8 4.2 4.5 3.4 4.6 4.6 4.0 3.9 4.2

2.2 3.4 2.5 2.7 2.8 2.5 3.0 -4.5 3.0 3.4 3.5 3.4 4.2 3.2 3.5 3.7 3.8 3.8 3.8

0.68

0.68

0.44

0.61

Grain-yield and yield components I n 1986, h a r v e s t index a n d g r a i n - y i e l d at b o t h l o c a t i o n s were generally low in old c o m p a r e d with m o d e r n varieties (Table 7 ). T h e a d d i t i o n of R h t d w a r f i n g genes in b o t h b a c k g r o u n d s ( A P D a n d K C D ) i n c r e a s e d t h e h a r v e s t index a n d grain-yield c o m p a r e d with the tall lines at b o t h locations. T h e r e were significant differences a m o n g t h e varieties in spikelets p e r ear at final h a r v e s t ( d a t a n o t p r e s e n t e d ) , as o b s e r v e d at t e r m i n a l spikelet stage ( K i r b y et al., 1989), b u t n u m b e r o f spikelets per ear s h o w e d no t r e n d with b r e e d i n g progress. Varietal differences were also o b s e r v e d for ear n u m b e r per m 2, b u t s h o w e d no t r e n d f r o m old to m o d e r n varieties ( d a t a n o t p r e s e n t e d ) . O t h e r yield c o m p o n e n t s , such as grains per spikelet a n d grains p e r ear, increased f r o m old to m o d e r n varieties (Table 8). A m o n g s t t h e isogenic lines t h e

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1300

Thermal time (°Cd)

Fig. 4. Number of living florets ( A ) in spikelet 8 and peduncle length (•) vs thermal time. The arrows indicate when the stem (S) and the ear (E) started to grow at a most rapid rate. The stage of development of the shoot is shown in the bar diagram: f, flag leaf emerged; b, boot stage; e, ear emergence; p, peduncle (ear completely emerged from the boot), a, anthesis. Data for Kulin, Perth. TABLE 7 Grain-yield ( g m -2 ) and harvest index (%) for old and m o d e m wheat varietiesand the isogenic lines at Perth, Wongan Hills in 1986 and Merredin in 1987 Variety

Grain-yield

Harvest index

Perth

Wongan

Merredin

Perth

Wongan

Merredin

Purple Straw Gluyas Early Nabawa Bencubbin Gabo Insignia Gamenya Condor Tincurrin

-249 -303 321 258 385 315 405

233 250 279 266 281 327 327 -394

102 137 135 153 178 139 198 166 204

-26 -34 38 36 41 37 44

24 24 28 28 31 36 34 -42

22 29 31 31 34 36 37 36 39

Miling Millewa Aroona Bodallin Bass Gutha Cranbrook Kulin 77W677 79W783 APD0 APD1 APD2 KCD0 KCD1 KCD2 Genaro F81

357 -415 334 -374 -342 --262 210 -280 350 ---

372 -420 367 -327 -435 --256 263 267 209 316 323 --

161 177 197 184 147 178 196 196 192 212 ---138 176 183 143

39 -44 43 -43 -47 --37 39 -41 44 ---

39 -39 34 -34 -43 --32 36 38 23 26 40 --

34 38 37 35 36 35 37 39 42 39 ---33 38 40 33

70

44

33

3

4

5

LSD (P=0.05)

72 TABLE 8 Grains per ear and grains per spikelet of old and modern wheat varieties and the isogenic lines at Perth, Wongan Hills in 1986 and Merredin in 1987 Variety

Grains e a r - 1

Grains spikelet - '

Perth

Wongan

Merredin

Perth

Wongan

Merredin

Purple Straw Gluyas Early Nabawa Bencubbin Gabo Insignia Gamenya Condor Tincurrin Miling Millewa Aroona Bodallin Bass Gutha Cranbrook Kulin 77W677 79W783 APD0 APD1 APD2 KCD0 KCD1 KCD2 Genaro F 81

-36.1 -41.9 44.2 35.5 50.7 55.5 61.5 53.6 -41.9 53.1 -51.8 -54.3 --55.8 53.5 -46.5 58.1 ---

36.2 39.3 40.1 34.7 39.0 29.0 45.6 -59.8 46.9 -44.8 51.4 -50.0 -59.8 --53.4 56.2 57.7 54.2 59.7 58.8 --

27.4 30.8 38.6 35.7 38.1 30.4 38.0 53.1 59.4 46.0 48.5 45.1 43.4 39.8 41.4 50.5 57.1 54.0 44.7 ---44.7 44.6 45.3 51.7

-2.07 -2.20 2.30 2.09 2.58 2.45 3.38 2.67 -2.47 2.90 -2.68 -2.97 --2.57 2.49 -3.16 3.26 ---

1.78 2.29 1.94 1.97 2.12 1.79 2.38 -3.26 2.27 -2.28 2.73 -2.68 -3.22 --2.38 2.44 2.85 2.79 3.14 3.08 --

1.46 2.00 2.14 2.12 2.14 2.02 2.07 2.65 3.34 2.43 2.77 2.69 2.48 2.27 2.28 2.82 3.10 3.35 2.55 ---2.50 2.52 2.51 2.66

LSD (P=0.05)

10.5

9.6

5.2

0.34

0.44

0.29

dwarf lines generally had more grains per spikelet than

the tall lines (Table

8). In 1987 at Merredin, the trends amongst the varieties in grain-yield and yield components

were similar to those of the 1986 experiments

(Tables

7 and 8).

However, grain-yield was generally lower than in the 1986 experiments,

prob-

ably due to less rainfall at this side. DISCUSSION The harvest indices (Table 7) followed the trends previously reported. They were higher in modern

varieties

compared

with old (Perry

and D'Antuono,

73 1989), and higher in dwarf compared with tall isolines (Brooking and Kirby, 1981; Fischer and Stockman, 1986). How far the differences in harvest index were due to differences which emerged during the grain-growth phase, or to differences already established at anthesis, was assessed by analysing the relationships between harvest index and ear: stem ratio at anthesis. Overall there was a significant relationship between harvest index and ear: stem ratio at anthesis (P < 0.001 ), but the response differed between sites and years (P < 0.001; Fig. 5) The relationship was strongest at Wongan Hills. At Perth and Merredin, it appeared that some of the varieties consistently deviated from the general trend. At all three sites, Gutha had a higher harvest index than was indicated by reference to the ear:stem ratio. In contrast, at both Perth and Merredin, Miling had a considerably lower harvest index than would have been expected from the ear: stem ratio. This analysis may identify both the potential of a variety to yield well (ear: stem ratio), assuming no differences in biomass, and the degree to which it can respond to stress during grain-filling (harvest index minus predicted harvest index). Because the harvest index is strongly correlated with the ear: stem ratio at anthesis, the differences in harvest index between varieties may result from differences which occur before anthesis. Analysis of the growth-rates of ear and stem showed that, while there were no trends in absolute rates, the ratio of ear:stem growth-rate was greater in the more modern varieties and was strongly correlated with ear: stem ratio at anthesis. However, because of the complexity of the curve of d.m. vs thermal time (Fig. 1 ), and the difficulty of estimating instantaneous growth rate, further analysis of differences in growthrates between varieties was not attempted. Analysis of either the ear:stem ratio vs stem d.m. or the application of the allometric function of ear d.m. and 50

,

,

i

i

i

A

~40

•.o

"2

0°

0

i

J

~3o I 2O

o~ 0

i

0

2O 3 4O 5O 6 Ear: stem d.m. at anthesis (%)

7O

Fig. 5. Harvest index vs ear: stem ratio at anthesis: Perth (• ), Wongan Hills (• ) and Merredin ([]). The lines were fittedby linear regression. The isogenic lines were not included in the graph.

74

stem d.m. gave more insight into the differences between varieties, and eliminated thermal time and the compounding factor of life-cycle differences. The plot of ear: stem ratio vs stem d.m. showed the importance of the size of the stem when the ear:stem ratio started to increase (Fig. 2). However, the relation between ear: stem ratio and stem d.m. was also complex, and the analysis using the allometric relationship appeared to give most information on varietal differences. In terms of the relationship between In ear d.m. and In stem d.m., the ear: stem ratio at anthesis was determined by slope, intercept and increment of growth from In ear d.m. = 0 to anthesis. However, as for many other characters which may affect yield, e.g. yield components, the parameters of the regression equation were highly correlated ( P < 0.001; Table 5 ). The equations given in Table 5 indicate that, generally, the smaller the intercept (and the bigger the stem at ear l n = 0 ) , the steeper the slope, and the steeper the slope the smaller the increment of stem growth (d; Fig. 6). These parameters combine in such a manner that, for example, the bigger In stem d.m. at In ear d.m. = 0, the smaller the ear:stem ratio at anthesis (Fig. 6). In relation to the date of introduction, there was generally an increase in intercept and a decrease in slope with the progression from old to modern varieties, with the attendant increase in ear: stem ratio. This may have been a function of breeding improvement for yield, or selection fro a shorter, more adapted life cycle. There were highly significant correlations between the parameters of equations (1)- (4) and life-cycle length ( ° Cd, sowing to anthesis); e.g. intercept vs life-cycle - Perth, r = - 0 . 7 9 4 , P<0.001; Wongan Hills, r = - 0.683, P < 0.001 - and the number of leaves on the main shoot, e.g. inter-

~' t 6

I

I

|

I /40 50 ~ ~ /"~30

~2

°

0

4

5 6 Ln stern d.m. (mg)

7

i

Fig. 6. Predicted relationships derived from equations (1) and (2), Table 5. The line A is computed for slope 1.9 and line B for slope 3.1, which represented the range of variation found in Tincurrin and Gluyas Early respectively, at Perth. The broken lines represent the 10, 20.....50% levels of the ear: stem ratio.

75

cept vs number of leaves - Perth, r = - 0 . 8 0 5 , P<0.001; Wongan Hills, r = -0.618, P < 0.01. Some varieties merit special attention. Miling, a modern variety, departed from the trend to shorter life-cycle and fewer leaves on the main shoot. Though a relatively recent introduction, Miling had a long life-cycle and had 11 leaves on the main shoot (Kirby et al., 1989 ). In terms of the intercept, it was similar to Gabo and Insignia, but because of a bigger slope, it achieved an ear: stem ratio comparable with that of Gutha (Table 5; Fig. 7a). In the case of the APD and KCD isogenic lines, the differences in the ear: stem ratio were particularly clear (Table 4) and could be ascribed directly to the Rht gene, as there are no differences in development or number of leaves on the main shoot (Brooking and Kirby, 1981; Fischer and Stockman, 1986; Youssefian, 1987; and unpublished results from the experiment described in this paper). The differences among the isogenic lines in terms of the allometric analysis were due chiefly to differences in the intercept rather than in the slope (Table 5; Fig. 7b). At ear w e i g h t = l mg, there were differences in ear:stem ratio which persisted until anthesis (Fig. 7b ). These results are in accord with those of Brooking and Kirby (1981) and Fischer and Stockman (1986), and the weight of evidence suggests that it is the stem dry-matter at which the ear starts to grow rapidly which is the important factor in determining the ear: stem ratio at anthesis. Maximum number of florets and the beginning of floret death occurred after leaf d.m. had reached maximum and stem d.m. growth was almost at its maximum, and when the ear and peduncle were starting to grow vigorously (Fig. 4). This situation is comparable to that reported by Kirby (1988) although floret death occurred somewhat earlier in this study relative to peduncle expansion. This may have been due to the greater environmental stress in these experiments compared to that in a well-watered cool British glasshouse. This I

I

I

I

(a)

I

5o

I

l

(b)

I

I

I .::Io °

. . ~ 2 ~

~. 30

"~6 E

~4

-0

SS

// _Jc 2

// I

4

I

I

I

5 6 7 Ln stem d.m. (mg)

I

i

I

8

4

5 6 7 Ln stem d.m. (mg)

I

I

I

8

Fig. 7. Fitted regression lines for In ear dry-matter vs. stem for: (a) Miling (--) and Gutha (--) and (b) A P D 0 ( o---o ), A P D 1 ( o--o ), K C D 0 (/x ---A ) and K C D 1 (/k--Z~ ). Data for Perth.

76 i

i

I

i

i

{a)

5 m4

~4

E z

z

i

i

!

i

I

I

i

I

(b)

5

5 2 O'~.4j

0

I

I

I

I

2

I

6 7 8 9 10 Maximum number of florets

)'r._~ 0

30 40 50 60 Floret survival (%)

Fig. 8. (a) Number of grains vs maximum number of florets and (b) number of grains vs percentage floret survival, in spikelet 8; old and modern wheat varieties at Wongan Hills. The lines were fitted by linear regression.

is consistent with the hypothesis that growth of the ear is restricted by competition for resources between ear and stem. If, therefore, size of the ear relative to that of the stem is an index of its competitive strength, then fewer florets or more floret death would be expected in varieties with a low ear: stem ratio, as was the case in both the adapted old varieties and the tall rht isogenic lines. The increase in number of grains per spikelet and per ear in the modern varieties and dwarf Rht lines was due to both initiation of more florets per spikelet and higher floret survival (Table 8; Fig. 8). In the United Kingdom, increased grain mass appears to have been an important reason for increased grain-yield in the modern varieties when compared with old ones (Austin et al., 1980 ). In this case, instead of more florets, increased assimilate supply into the ear may have resulted in florets with larger carpels and consequently heavier grains. It was not possible to quantify instantaneous ear growth-rate of a floret because of the error of the mean of any sample, and the difficulty of finding a satisfactory growth function to describe d.m. growth in the period before anthesis (Kirby, 1988). More precise estimates of growth of the ear and its component may allow development of a more quantitative model to explain varietal differences.

Implications for variety improvement Some implications for variety improvement may be deduced from the relationships described in this paper. The correlation between length of life-cycle and ear:stem ratio indicate that any reduction in life-cycle (and number of leaves ) may tend to lead to a higher ear: stem ratio. However, in some modern varieties, the number of leaves is approaching the minimum (Kirby et al., 1989; Siddique et al., 1989) for varieties reported in the literature. Early maturity may also be undesirable, as critical phases may occur in suboptimal environments such as periods of frost risk (Loss, 1987) and the environment may not

77 be as fully exploited to produce the potential biomass as by later-maturing varieties. The parameters of the allometric relationship, which may determine ear: stem ratio, are interrelated, but like many correlations which affect yield, these relationships may be broken by breeding. An improvement in ear: stem ratio will depend both on obtaining and recognizing variation in the parameters. Modification in 'd' (the increment of stem d.m. from ear w e i g h t = l mg until anthesis), may come from selection for few ears per plant. Assuming that there is little variation in total biomass in the modern varieties (Austin et al., 1980; Perry and D'Antuono, 1989; Siddique et al., 1989), then plants with few, big ears may be expected to have bigger ear:stem ratios than plants with many small ears. Consideration of the ear: stem ratio in Miling showed that the improvement came chiefly from a higher slope value than would have been predicted from the relationship between slopes and intercepts (Equations ( 1 ) and (3); therefore, in this variety the correlation has to some extent been broken. There do not appear to be any morphological or physiological characters other than ear: stem ratio itself which might assist in the recognition of this character. The intercept showed greatest variation amongst varieties. This parameter is probably affected by the Rht gene and leads to high harvest indices in varieties containing those genes. The genetics and physiology of the Rht genes are well understood (Gale and Youssefian, 1985). The use of these genes, or other genes which affect the relationship between the size of ear and stem when the ear is small, may offer the best opportunity for progress. Further research may reveal the physiological and morphological basis of the difference in the parameters of the allometric function in terms of the size and activity of meristems established around the time of ear and internode initiation. ACKNOWLEDGEMENTS We gratefully acknowledge the assistance of Mrs. L. McLaughlin, Mr. C. Miller, Mr. D. Kaesehagen, and Mr. T. Parr with the experiments and collection of the data. The Commonwealth Wheat Research Council of Australia provided financial assistance for this work. We also wish to thank Dr. R.K. Belford for his comments on the manuscript.

REFERENCES Austin, R.B., Bingham,J., Blackwell,R.D., Evans, L.T., Ford, M.A., Morgan, C.L. and Taylor, M., 1980.Geneticimprovementin winterwheatyieldssince 1900and associatedphysiological changes.J. Agric.Sci., Camb., 94: 675-689. Brooking, I.R. and Kirby, E.J.M., 1981. Interrelationshipsbetweenstem and ear developmentin

78 winter wheat: the effects of a Norin 10 dwarfin gene, Gai/Rht2. J. Agric. Sci.,Camb., 97: 373-

381. Fischer, R.A. and Stockman, Y.M., 1986. Increased kernel number in Norin 10-derived dwarf wheat: evaluation of the cause. Aust. J. Plant Physiol., 13: 767-887. Gale, M.D. and Youssefian, S., 1985. Dwarfing genes in wheat. In: G.E. Russell {Editor), Progress in Plant Breeding, Vol. 1. Butterworths, London, pp. 1-35. Jain, H.K. and Kulshrestha, V.P., 1976. Dwarfing genes and breeding for yield in bread wheat. Z. Pflanzenzticht., 76: 102-112. Kirby, E.J.M., 1974. Ear development in spring wheat. J. Agric. Sci., Camb., 82: 437-447. Kirby, E.J.M., 1988. Analysis of leaf, stem and ear growth in wheat from terminal spikelet stage to anthesis. Field Crops Res., 18: 127-140. Kirby, E.J.M., Siddique, K.H.M., Perry, M.W., Kaesehagen, D. and Stern, W.R., 1989. Variation in spikelet initiation and ear development of old and modern Australian wheat varieties. Field Crops Res., 20: 115-130. Loss, S.P., 1987. Factors affecting frost damage to wheat in Western Australia. W.A. Dept. Agric. Div. Plant Res. Tech. Rep. No. 6, 30 pp. Perry, M.W. and D'Antuono, M.F., 1989. Yield improvement and associated characteristics of some Australian spring wheats introduced between 1860 and 1982. Aust. J. Agric. Res., 40 (3): (in press). Siddique, K.H.M., Belford, R.K., Tennant, D. and Perry, M.W., 1989. Growth, development and light interception of old and modern wheat cultivars in a Mediterranean type environment. Aust. J. Agric. Res., 40(3): (in press). Weir, A.H., Bragg, P.L., Porter, J.R. and Rayner, A.H., 1984. A winter wheat crop simulation model without water or nutrient limitations. J. Agric. Sci., Camb., 102: 271-382. Whingwiri, E.E. and Stern, W.R., 1982. Floret survival in wheat: significance of the time of floret initiation relative to terminal spikelet formation. J. Agric. Sci., Camb., 98: 257-268. Youssefian, S., 1987. The development of pleiotropic effects of GA-insensitive dwarfing genes in wheat. Ph.D. Thesis, University of Cambridge, 192 pp.