Variation in Grain Mass, Grain Nitrogen, and Starch B ...

Download PDF
0 downloads 0 Views 81KB Size Report
Comment
The grain nitrogen content (GNmg) was then calculated from the product of GNC and grain mass. In total, 3,278 grains were analyzed. Starch B-Granule Content.
Variation in Grain Mass, Grain Nitrogen, and Starch B-Granule Content Within Wheat Heads F. L. Stoddard1 ABSTRACT

Cereal Chem. 76(1):139-144

Grain mass (mg) and grain nitrogen concentration (%) were determined in 3,278 individual grains from eight cultivars with reference to the position of the grain on the head (spikelet number and floret number). Selected grains were removed from certain heads at anthesis. Grain nitrogen content (mg) was determined as the product of grain nitrogen concentration and grain mass. Grain mass and starch B-granule content were determined in 3,030 grains from a further 12 cultivars with reference to grain position, and selected grains were removed from certain heads at anthesis. Grains from distal florets were always smaller and had lower B-granule contents, nitrogen contents, and nitrogen concentrations than those from the two proximal florets on each spikelet, which were not significantly different from each other. Grains in the basal two spikelets of the head were smaller with lower nitrogen contents and higher B-granule contents than those in most of the head. Their nitrogen concentration, however, did not differ from

that in the rest of the head. All four traits declined in the grains in the top four to five spikelets of the head. The differences in grain mass, nitrogen, and B-granule content between florets within spikelets and between spikelets within the head varied with cultivar. Grains on treated heads were larger, higher in nitrogen and slightly lower in B-granule content than those on untreated heads. The effects of floret and spikelet on grain mass and nitrogen were not significantly changed by the treatments. Grains from florets 1 and 2, excluding those from the bottom three and top five spikelets, therefore represented a particularly uniform population for grain mass, protein nitrogen and starch granule composition. Genetic analyses based on samples of these grains will be more robust and repeatable than those based on unselected samples. Implications for improving seedling vigor, plant yield, and grain protein concentration in breeding programs are discussed.

It has widely been observed in wheat, as in other crops, that large grains grow into more vigorous seedlings and higher yielding plants than small ones (Ries and Everson 1973, Evans and Bhatt 1977). Similarly, grains with a higher protein concentration (GPC) at a given size produce plants that outperform those with lower GPC (Millet and Zaccai 1991). It therefore appears that grain protein content (in milligrams per grain rather than percent) is an important contributor to the subsequent seedling vigor. Nevertheless, modern cultivars often have smaller grains (Fjell et al 1985) and lower GPC (Stoddard and Marshall 1990) than old varieties. This effect has been attributed to the combination of gains in yield and a common negative correlation between yield and GPC (Simmonds 1995). A major step in yield has been attributed to the Rht genes, which in turn have been associated with a direct reduction in both grain mass and GPC in near-isogenic lines (Allan 1986, 1989; Pinthus and Gale 1990). It has therefore been proposed that restoring grain mass would move the yield-protein relationship to a higher level (Fjell et al 1985; Stoddard and Marshall 1990). The mechanism behind the relationship between grain mass and grain nitrogen concentration is one subject of this article. New technologies offer opportunities for looking with greater detail at variation among individual grains. While grain mass could be determined with a micro-balance some years ago, the nitrogen content could not be determined until relatively recently. MicroKjeldahl methods required the pooling of several grains to get enough material for analysis. Questions about the partitioning of assimilate within the head, or about the contributions of maternal, embryo, and endosperm genotype to the composition of a grain, could be addressed only indirectly and with difficulty. Single-grain analysis for a number of components is becoming more popular (Delwiche 1995). If we are to apply such methods in a breeding program we need to have a map of the distribution of the quality factors within the head. We would then be able to focus on the grains that provide the best expression of the plant’s potential without the confounding influence of poorer grains.

MATERIALS AND METHODS

1 Plant

Breeding Institute, Woolley Bldg A20, The University of Sydney, NSW 2006, Australia; Quality Wheat CRC Ltd, Locked Bag No. 1345, North Ryde, NSW 2113, Australia. Phone: +61 2 9351 4594. Fax: +61 2 9351 4172. E-mail: [email protected]

Publication no. C-1999-0107-07R. © 1999 American Association of Cereal Chemists, Inc.

Grain Mass and Nitrogen Eight cultivars of wheat were sown in four replicates, one plant per 20 cm pot, in an unheated glasshouse. The potting mix was three parts sand to two parts peat by volume, with lime and a complete fertilizer mix added. Plants were fed weekly with half-strength commercial water-soluble fertilizer (N:P:K 23:4:18, Aquasol, Hortico Ltd., Laverton North, Vic., Australia). One head on each plant was left as a control. In another, the middle florets were removed from each spikelet within two days of anthesis, leaving only the first and second florets. In a third head, the first floret was removed from each spikelet, leaving all distal ones. The two treatments and control were applied in random order to the first three heads on each plant. Heads were harvested intact at maturity. Each grain was removed from the head, its spikelet number and floret number were recorded, and it was weighed to the nearest 0.1 mg. The grain’s nitrogen concentration (GNC%) was then determined by the Dumas total combustion method using an elemental analyzer (CHN-1000, Leco Inc., St. Joseph, MI). The grain nitrogen content (GNmg) was then calculated from the product of GNC and grain mass. In total, 3,278 grains were analyzed. Starch B-Granule Content Twelve cultivars of wheat were grown in a controlled-environment chamber, one plant per 15 cm pot, with three replicates. The chamber was set to a 14 hr day at 18°C and a 10 hr night at 13°C. The potting mix and fertilizer application were the same as in the previous experiment. In four heads on each of five cultivars, the bottom two and top four spikelets and the distal florets from the remaining spikelets were removed, the glumes were trimmed and a paper bag was placed over the head. Heads were harvested intact at maturity. Two untreated heads per plant were analyzed. Each grain was removed from the head, its spikelet number and floret number were recorded, and it was weighed to the nearest 0.1 mg. The starch was extracted and cleaned for particle size analysis. Each grain was crushed with a pair of smoothjawed pliers, put into a 2-mL Eppendorf microfuge tube with 0.5 mL of 0.5M NaCl, and soaked overnight at 4°C. The next morning it was ground with an Eppendorf pestle attached to a drill press until the gluten formed a tight ball. The starch slurry was decanted through a 0.2-mm mesh sieve into a fresh microfuge tube, and the residue Vol. 76, No. 1, 1999

139

was ground in a fresh 0.5 mL of NaCl solution. The grinding was repeated, the slurry added to the first, and a third cycle was conducted as well. The starch was precipitated by 2 min of centrifugation at 5,500 × g, then centrifuged through a sequence of 50%, w/w, CsCl, 2% SDS, water twice, and ethanol, then dried over silica gel and sealed until analysis. Starch particle size distribution was determined using a Malvern Mastersizer laser-diffraction analyzer with the stirred small-sample cell, 300-mm Reverse Fourier lens, 2.4-mm beam length, and polydisperse function optimized for the refractive index of starch in water. Particle sizes were calculated on Mie theory based on spheres of equivalent volume and were presented as diameters. Because A granules are lenticular to oblate, this diameter thus underestimated the long diameter and overestimated the short one. Each data point represented the volume percent of starch in the diameter size class. Data were transferred to a spreadsheet for manipulation and statistical

analysis. Particle size was bimodally distributed with the intermodal minimum in the 6.0-µm size class, so particles below this value were considered to be B granules and the remainder A granules. This boundary is smaller than the commonly reported 10-µm size (Morrison and Scott 1986). Starch B-granule content (SBGC, %) therefore refers to the volume percent of starch contained within the B granules. In total, 3,030 grains were analyzed. Seven cultivars were used for analysis of grain location, grain mass, and B-granule content, and the other five were used for analysis of floret removal treatments, grain mass, and B-granule content. Statistical Analysis Data were subjected to analysis of variance and generalized linear modeling. Means and standard errors were calculated for each grain position (florets 3 and 4 were pooled) and each treatment. Spikelet numbers were also reassigned counting from the top of the head instead of the bottom and the analyses were repeated. To describe the change in grain attributes with spikelet location, various curve forms were tested. RESULTS Effects of Floret and Spikelet In control heads, grain masses of floret 1 and floret 2 were not significantly different, but floret 3 produced considerably smaller grains (Fig. 1) with lower GNC (Fig. 2) and GNmg (Fig. 3). The bottom three spikelets produced smaller grains than the next seven (Fig. 1). The top four spikelets produced significantly smaller grains than the next six, and the trend was detectable as far down

¼

/

Fig. 1. Grain mass (mg) for florets 1 ( ), 2 ( ) and 3 (∆) in the bottom 10 and top 10 spikelets of heads of wheat. Data show means of four replicates of eight cultivars. Bars show ± 1 standard error.

TABLE I Coefficients of Curves Fitting Changes in Grain Mass, Nitrogen Concentration, and Nitrogen Content to Spikelet Number in Florets 1 and 2 of Untreated Heads of Four Replicates of Eight Cultivars of Wheata From Bottom Cultivar

¼

/

Fig. 2. Grain nitrogen concentration (%) for florets 1 ( ), 2 ( ) and 3 (∆) in the bottom 10 and top 10 spikelets of heads of wheat. Data show means of four replicates of eight cultivars. Bars show ± 1 standard error.

¼

/

Fig. 3. Grain nitrogen content (mg) for florets 1 ( ), 2 ( ) and 3 (∆) in the bottom 10 and top 10 spikelets of heads of wheat. Data show means of four replicates of eight cultivars. Bars show ± 1 standard error. 140

CEREAL CHEMISTRY

Grain mass, mg Bluebird 4 Machete Najah PF70226 Schomburgk Spica Sunfield Vulcan Standard error Grain nitrogen concentration, % Bluebird 4 Machete Najah PF70226 Schomburgk Spica Sunfield Vulcan Standard error Grain nitrogen content, mg Bluebird 4 Machete Najah PF70226 Schomburgk Spica Sunfield Vulcan Standard error a

A 40.5 23.6 28.0 49.3 23.3 36.6 26.3 35.2 1.7 3.58 3.52 3.36 3.49 3.06 3.82 3.19 3.43 0.04 1.43 0.80 0.91 1.78 0.70 1.45 0.89 1.25 0.06

B −0.72 3.52 3.20 3.91 6.57 2.63 1.66 1.44 1.02 nsb ns ns ns ns ns ns ns ns −0.024 0.139 0.122 0.097 0.207 0.068 0.026 0.020 0.034

From Top A

B

30.4 18.9 18.5 42.9 29.2 25.5 19.7 26.5 1.6

2.72 4.25 6.73 6.26 3.44 7.00 4.64 5.50 1.06

3.36 3.18 3.13 3.15 2.86 3.44 2.95 3.22 0.06

0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.022

1.10 0.59 0.58 1.31 0.82 0.80 0.59 0.86 0.06

0.068 0.168 0.240 0.252 0.141 0.327 0.162 0.211 0.039

Curves were of the form y = A + B × ln (spikelet number), where spikelets were the first 10 counted either from the bottom of the head or the top of the head. b Not significant.

as the eighth spikelet from the top. Logarithmic curves provided good fits to the trends in grain mass at both ends of the head. GNC was nearly constant across all spikelets (Fig. 2). No curve was fitted to the bottom spikelets, and a curve with a small coefficient provided a good fit at the top of the head (Table I). GNmg showed similar trends to grain mass. The bottom three and top four spikelets had lower GNmg than the rest (Fig. 3) and logarithmic curves fit the data well. Starch B-granule content was generally lower in florets 3 and 4 than in the proximal two florets (Fig. 4). It was higher in the bottom two spikelets of the head and declined at the top of the head (Fig. 4). Effects of Cultivar The difference between the proximal florets and floret 3 in grain mass, GNC and GNmg varied among cultivars (Table II). The coefficients fitting the curves in grain mass and GNmg at the top and bottom of the head were also affected by cultivar. Cultivars differed for GNC but the effect of spikelet did not vary significantly among them.

The curves at the bottom of the head were not significantly different from zero for grain mass in Bluebird 4 and for GNmg in Bluebird 4, Sunfield, and Vulcan (Table I). They were also not significantly different from zero for grain mass in Baldmin, Minister, Olympic, and Quadrat, and for SBGC in Bald Early and Currawa (Table III). They were particularly steep for grain mass and GNmg in Schomburgk and Currawa. At the top of the head, Najah, PF70226, and Spica had steeper declines in grain mass and GNmg than the other five accessions, and Bluebird 4 showed the least curvature (Table I, Fig. 5). The decline in SBGC was nonsignificant in Currawa, quite strong in Baldmin, and fairly uniform in the other five accessions (Table III, Fig. 6). In this set of cultivars, Bald Early had a particularly steep loss of grain mass at the top of the head but the other cultivars were consistent (Table III). TABLE III Coefficients of Curves Fitting Changes in Grain Mass and Starch B-Granule Content to Spikelet Number in Florets 1 and 2 of Untreated Heads of Six Replicates of Seven Cultivars of Wheata From Bottom Cultivar

¼

/

Fig. 4. Starch B-granule content (%) of florets 1 ( ), 2 ( ) and 3 (∆) in the bottom 10 and top 10 spikelets of heads of wheat. Data show means of four replicates of seven cultivars. Bars show ± 1 standard error. TABLE II Effects of Floret Position on Grain Mass (mg), Grain Nitrogen Concentration (%), and Grain Nitrogen Content (mg) in Spikelets 4–13 of Four Replicates of Eight Cultivars of Wheata Cultivar Grain mass, mg Bluebird 4 Machete Najah PF70226 Schomburgk Sunfield Spica Vulcan Grain nitrogen concentration, % Bluebird 4 Machete Najah PF70226 Schomburgk Sunfield Spica Vulcan Grain nitrogen content, mg Bluebird 4 Machete Najah PF70226 Schomburgk Sunfield Spica Vulcan a

Means ± standard errors.

Floret 1

Floret 2

Floret 3

40.3 ± 1.1 28.4 ± 1.0 33.3 ± 0.9 54.9 ± 1.8 34.5 ± 0.8 29.1 ± 0.5 40.5 ± 0.6 38.0 ± 0.4

36.5 ± 1.2 29.1 ± 0.8 32.7 ± 0.9 54.9 ± 1.8 35.8 ± 0.8 29.4 ± 0.6 40.2 ± 0.6 37.7 ± 0.5

29.8 ± 1.8 15.6 ± 1.1 19.5 ± 0.8 43.6 ± 1.9 26.4 ± 1.2 20.9 ± 1.0 29.0 ± 0.9 29.4 ± 0.8

3.56 ± 0.07 3.51 ± 0.04 3.35 ± 0.05 3.46 ± 0.03 3.12 ± 0.04 3.16 ± 0.06 3.79 ± 0.03 3.36 ± 0.06

3.44 ± 0.06 3.50 ± 0.04 3.37 ± 0.05 3.35 ± 0.12 3.11 ± 0.04 3.03 ± 0.07 3.76 ± 0.03 3.36 ± 0.07

2.98 ± 0.08 2.93 ± 0.05 3.16 ± 0.08 3.24 ± 0.04 2.73 ± 0.04 2.78 ± 0.09 3.37 ± 0.04 3.32 ± 0.07

1.42 ± 0.03 0.98 ± 0.03 1.11 ± 0.03 1.89 ± 0.05 1.07 ± 0.02 0.93 ± 0.03 1.53 ± 0.02 1.27 ± 0.02

1.24 ± 0.04 1.01 ± 0.03 1.10 ± 0.03 1.82 ± 0.08 1.11 ± 0.02 0.90 ± 0.03 1.51 ± 0.03 1.26 ± 0.02

0.88 ± 0.03 0.46 ± 0.03 0.61 ± 0.03 1.41 ± 0.06 0.72 ± 0.03 0.59 ± 0.04 0.98 ± 0.04 0.98 ± 0.03

Grain mass, mg Bald Early Baldmin Currawa Ghurka Minister Olympic Quadrat SE Starch B-granule content, % Bald Early Baldmin Currawa Ghurka Minister Olympic Quadrat SE a

From Top

A

B

A

B

45.5 61.8 56.9 48.1 62.4 52.2 48.4 2.0

3.43 −0.28 5.33 2.81 −0.05 −0.23 −0.04 1.53

41.1 12.7 51.2 36.4 43.7 25.6 34.4 2.7

9.13 17.09 6.27 8.08 8.73 10.05 8.22 2.52

22.5 27.2 21.4 22.3 19.3 23.5 19.7 0.4

−0.32 −1.41 −0.26 −0.96 −0.85 −1.22 −0.87 0.28

14.4 15.0 19.5 17.1 21.0 15.4 17.2 0.5

1.46 2.77 0.45 1.55 1.33 1.07 1.76 0.49

Curves were of the form y = A + B × ln (spikelet number), where spikelets were the first 10 counted either from the bottom of the head or the top of the head.

Fig. 5. Grain mass of florets 1 and 2 in the top 10 spikelets of untreated heads of Bluebird 4 ( ), Machete ( ), Najah (◆), PF70226 (l), Schomburgk ( ), Spica (∆), Sunfield (n) and Vulcan ([). Data show means of four replicates of two florets. Typical error bars, showing ± 1 standard error, are shown for Sunfield and PF70226 data only; others have been omitted to enhance clarity of the figure.

/

_

¼

Vol. 76, No. 1, 1999

141

Effects of Treatments Surgical removal of florets was associated with an increase in mass, nitrogen concentration and nitrogen content of the remaining grains (Table IV). Nevertheless, the difference between floret 1 and floret 2 remained small and nonsignificant, while that between floret 2 and floret 3 remained large and highly significant. The shapes of the declines (means across cultivars) in grain mass (Fig. 7), GNC, and GNmg at the top and bottom of the head were not significantly affected by the experimental treatments. Within cultivars, the effect of treatment on the shape of the curve was significant in only two cases, both involving GNmg at the top of the head. PF70226 with the first floret removed had a coefficient 0.2 greater than the other two treatments and Bluebird 4 control had a coefficient 0.2 less than the other two. Floret removal was associated with no significant change in SBGC in three cultivars, but increased it by 2% in Iran 74 and decreased it by the same amount in Kogat (Table V). This treatment was also associated with an increase in the mass of the remaining grains in two cultivars in this set, Croesus and Kewell, and no significant difference in the other three. For all the variates, standard errors could be decreased and discrimination between cultivars thereby increased when the bottom two and top four spikelets were eliminated from the calculations and only the proximal two florets per spikelet were included. For example, in Bald Early the mean SBGC of all 158 grains was 20.5% with a

standard error of 0.21. Restricting the data to the 116 grains from the recommended positions resulted in a small change in mean SBGC to 21.6% but a halving of the SE to 0.10. DISCUSSION These results have shown consistent effects of the grain’s position on its mass, nitrogen concentration, and starch composition. These effects have important implications for breeding and selection for improved relationships between yield and protein. Grain location affected grain mass in two ways. First, there was a major difference between proximal and distal florets. Second, there were gradients at the top and bottom of the head. These effects may be partly attributable to the shorter grain filling period of the smaller grains, due to their later anthesis but simultaneous maturity. This has been quantified for distal florets (Sofield et al 1977) and commonly observed for distal and proximal spikelets. Removal of floret 1 in the present experiment reduced competition for assimilates but did not affect the basic timing relationships among the florets. If the slope of the distribution was indeed due to timing, it would be expected to be parallel on treated and untreated heads. Two other critical factors determining final grain mass are the size of the floret cavity (Millet 1986) and competition from other grains (Fischer and HilleRisLambers 1978), but the genotype of the grain itself has not been shown to have an effect. Reducing the level of competition, by removing selected grains (as in the present experiment) or parts of the head (Fischer and HilleRisLambers 1978) has allowed the genetic potential maximum grain mass to be expressed, under conditions that allowed the maximum duration of grain filling. The present experiment has shown that even under these conditions, the distal grains remained smaller than the proximal ones, as would be expected from the smaller floret cavities and the effect of the later anthesis on the duration of grain filling. Similar arguments can be put forward for GNC. The differences between proximal and distal florets were smaller, 10% instead of 27% for grain mass, and the basal spikelets were hardly affected by position. In contrast, the effect of partial grain removal on GNmg was even greater than that on grain mass. Following removal of floret 3, the mass of florets 1 and 2 increased by 8% but the nitrogen content increased by 10%, giving a slight increase in nitrogen concentration. Similarly, following removal of floret 1, the mass of florets 2 and 3 increased by 23% and the nitrogen content by 35%. This implies that in these growing conditions and in spite of very high levels of both fertilizer input and average GNC, grain protein deposition was more limited by the nitrogen supply to the grains than by sink capacity. An average spikelet containing three grains in

Fig. 6. Starch B-granule content (%) of florets 1 and 2 in the top 10 spikelets of untreated heads of Olympic (n), Baldmin (l), Quadrat ( ), Bald Early (◆), Minister ( ), Currawa (∆) and Ghurka ( ). Data show means of six replicates of two florets. Typical error bars of ± 1 standard error are shown for the Baldmin data.

¼

_

/

TABLE IV Effects of Removal of Middle or First Floret on Grain Mass, Grain Nitrogen Concentration, and Grain Nitrogen Contenta Treatment Grain mass, mg Control Middle First Grain nitrogen concentration, % Control Middle First Grain nitrogen content, mg Control Middle First a

Floret 1

Floret 2

Floret 3

37.0 ± 0.6 40.1 ± 0.5 Removed

36.6 ± 0.6 39.1 ± 0.5 45.0 ± 0.7

26.7 ± 0.8 Removed 32.8 ± 0.7

3.41 ± 0.02 3.48 ± 0.02 Removed

3.36 ± 0.03 3.42 ± 0.03 3.61 ± 0.02

3.06 ± 0.03 Removed 3.39 ± 0.03

1.26 ± 0.02 1.40 ± 0.02 Removed

1.23 ± 0.02 1.35 ± 0.02 1.65 ± 0.03

0.83 ± 0.03 Removed 1.14 ± 0.03

Means ± standard errors of spikelets 4–13 in four replicates of eight cultivars.

142

CEREAL CHEMISTRY

¼

Fig. 7. Grain mass for florets 1 and 2 of untreated heads ( ), heads without floret 1 ( ) and heads without floret 3 (∇) in the top 10 spikelets of heads of wheat. Data show means of four replicates of eight cultivars. Bars show ± 1 standard error.

£

the present experiment would yield 100 mg of grain with an average of 3.32% nitrogen. Every additional distal floret would reduce the average nitrogen concentration, whereas the data suggest that additional spikelets would not affect it, but methods of testing this by surgically increasing the number of spikelets are not appealing. Distal florets are clearly detrimental to average grain mass and protein levels. The Rht genes have been associated with an increase in distal floret fertility and a decrease in GNC (Allan 1986, 1989). The present results show how this association may have come about, with an increase in floret fertility leading in turn to an increase in the numbers of small, low-protein grains. This effect is not one of number of grains per spikelet, as shown by the effects of removal of the first grain, where the third grain remained smaller and lower in nitrogen than the second. The effect of the spikelets at the top and bottom of the head on grain mass, if not on GNC, also was important in this context. It showed that the link between grain mass and GNC was not unbreakable. Grains that were small because of being in particular spikelets were not as low in protein as those that were small because of being in distal florets. In some hard wheats (Marshall et al 1986) but not in some soft wheats (Gaines et al 1997), small grains were also associated with a lower milling yield because of higher surface to volume ratio. For both these reasons it may be desirable to minimize the relative numbers of small grains. Chojecki et al (1986) also demonstrated that the proximal two and distal two spikelets contained smaller grains than the rest of the head, but their analysis of the rest of the spikelets was not as detailed as the current one. Their graphs showed that the top third of the head in both Chinese Spring and Spica contained slightly smaller grains than the bottom twothirds of the head. This has been corroborated by the present detailed demonstration of small grains in several distal spikelets. The increase in starch B-granule content in the bottom two spikelets of the head contrasted with all other traits. The differential sensitivity of cultivars to floret removal was also unexpected. These results indicate that when the SBGC of F1 grains is evaluated, the appropriate control is a similarly treated head rather than an untreated one. Thus for genetic or breeding studies on grain mass, protein concentration, or B-granule content, grains should be selected from the uniform portion of the head rather than from a bulked sample. This will minimize statistical “noise” from nongenetic variance. Where the trait in question is determined by the maternal plant, the uniformity of these selected grains will allow the greatest discrimination between segregating plants. Where the trait is determined by the embryo or the endosperm of the individual grain, these genotypes will be much more easily detected because of the otherwise uniform nature of the selected grains. There are two further implications of these results for breeding for improved wheat quality. First, distal florets are detrimental to average grain mass and GNC, so a readjustment of yield components against grains per spikelet may be of value. Where a reduction of SBGC is desirable, however, we may need more of the distal grains. Second, because distal spikelets also carry smaller, lower protein grains, this redistribution should be in favor of spikelets per head rather than

heads per plant. This appears to be a complex set of recommendations, unlikely to appeal to practical breeders. Nevertheless, it reduces to a single trait, grain mass, that has many positive effects on seedling vigor (Evans and Bhatt 1977) and, all other things being equal, on yield. Cultivar PF70226 has shown some promise as a parent in preliminary breeding experiments by the author, combining large grains with high GNC. The positive correlation between GPC and grain mass has already been noted (Jain et al 1975, Stoddard and Marshall 1990). The present results show that where there are larger numbers of fertile distal florets per spikelet, both mean grain mass and mean GNC would be reduced. Fatih (1986) also found a negative correlation between the number of grains per spikelet and GPC. Such a correlation need not be universal, as some crosses, possibly including those analyzed by Jain et al (1975), would segregate for the production of distal fertile florets while others clearly would not. Attempts to use mass selection based on grain density to increase GPC have been impeded by considerable nongenetic contribution to variation in protein content (Peterson et al 1986). The present results illustrate that grain position on the spike is an important contributor to this nongenetic variation. A genetically low-protein grain on a segregating head may be favored by its location as a proximal floret, whereas a genetically higher protein grain may be in a distal position. The protein concentration of a grain is primarily determined by the maternal plant and not detectably by the genotype of the grain or by the maternal cytoplasm (Millet et al 1984). This generality, however, would not apply to any genetic sources of high-protein content that affected protein partitioning at the sink instead of the source, such as null alleles for high molecular weight glutenins. The uniformity of many grain parameters across florets 1 and 2 in spikelets 4 to 13 will be useful in a number of genetic and physiological studies. In particular, the elimination of floret 3 and the proximal and distal spikelets will allow the detection of genetic effects in the remaining grains that would otherwise be obscured by positional effects. CONCLUSIONS A uniform subset of grain positions has been identified in wheat. Grains from florets 1 and 2, excluding the bottom 2–3 and top 3–5 spikelets of the head, were uniform for grain mass, grain nitrogen, and starch B-granule content. Focusing on these positions will allow greater resolution of differences in genetic and breeding studies. ACKNOWLEDGMENTS I thank Neroli O’Toole, Toni Swain, Shiranee Gunasekera, and Ranjana Sarker for technical assistance and Hak-Kim Chan, Dept. of Pharmacy, The University of Sydney, for use of his particle size analyser. Part of this work was funded by the Grains Research and Development Corporation under grant US64: Genetically enhanced grain protein in wheat, and the remainder by the Quality Wheat CRC Ltd. LITERATURE CITED

TABLE V Effects of Head Preparationa on Grain Mass and Starch B-Granule Content in Four Heads of Five Cultivars of Wheat Grain Mass (mg) Cultivar Croesus Kewell Kogat Iran 74 Turkey 187 Standard error a

Starch B-Granule Content (%)

Control

Treated

Control

Treated

37.7 48.6 35.2 57.2 45.9

44.4 58.6 33.2 54.4 43.5

25.1 20.9 20.1 17.3 15.0

24.5 20.6 17.7 19.2 15.8

0.7

0.2

Removal of proximal two and distal four spikelets and all but the proximal two florets from the remaining spikelets; covered with a paper crossing bag.

Allan, R. E. 1986. Agronomic comparisons among wheat lines nearly isogenic for three reduced-height genes. Crop Sci. 26:707-710. Allan, R. E. 1989. Agronomic comparisons between Rht1 and Rht2 semidwarf genes in winter wheat. Crop Sci. 29:1103-1108. Chojecki, A. J. S., Bayliss, M. W., and Gale, M. D. 1986. Genetic analysis of grain weight in wheat. Heredity 57:93-99. Delwiche, S. R. 1995. Single wheat kernel analysis by near-infrared transmittance: Protein content. Cereal Chem. 72:11-16. Evans, L. E.. and Bhatt, G. M. 1977. Influence of seed size, protein content and cultivar on early seedling vigour in wheat. Can. J. Plant Sci. 57:929935. Fatih, A. M. B. 1986. Genotypic and phenotypic associations of grain yield, grain protein and yield related characteristics in wheat-Agropyron derivatives. Hereditas 105:141-153. Vol. 76, No. 1, 1999

143

Fischer, R. A., and HilleRisLambers, D. 1978. Effect of environment and cultivar on source limitation to grain weight in wheat. Aust. J. Agric. Res. 29:443-458. Fjell, D. J., Paulsen, G. M., and Walter, T. L. 1985. Relationship among planted and harvested kernel weights and grain yield and protein percentage in winter wheat. Euphytica 34:751-757. Gaines, C. S., Finney, P. L., and Andrews, L. C. 1997. Influence of kernel size and shriveling on soft wheat milling and baking quality. Cereal Chem. 74:700-704. Jain, H. K., Singhal, N. C., Singh, M. P., and Austin, A. 1975. An approach to breeding for higher protein content in bread wheat. Pages 39-46 in: Breeding for Seed Protein Improvement Using Nuclear Techniques. IAEA: Vienna. Marshall, D. R., Mares, D. J., Moss, H. J., and Ellison, F. W. 1986. Effects of grain shape and size on milling yields in wheat. II. Experimental studies. Aust. J. Agric. Res. 37:331-342. Millet, E. 1986. Relationships between grain weight and the size of floret cavity in the wheat spike. Ann. Bot. 58:417-423. Millet, E., and Zaccai, M. 1991. Effects of genotypically- and environmentally-induced differences in seed protein content on seedling vigor in wheat. J. Gen. Breed. 45:45-49.

Millet, E., Levy, A. A., Avivi, L., Zamir, R., and Feldman, M. 1984. Evidence for maternal effect in the inheritance of grain protein in crosses between cultivated and wild tetraploid wheats. Theor. Appl. Genet. 67:521-524. Morrison, W. R., and Scott, D. C. 1986. Measurement of the dimensions of wheat starch granule populations using a Coulter Counter with 100channel analyzer. J. Cereal Sci. 4:13-21. Peterson, C. J., Liu, G. T., Mattern, P. J., Johnson, V. A., and Kuhr, S. L. 1986. Mass selection for increased seed protein concentration of wheat based on seed density. Crop Sci. 26:523-527. Pinthus, M. J., and Gale, M. D. 1990. The effects of ‘gibberellin-insensitive’ dwarfing alleles in wheat on grain weight and protein content. Theor. Appl. Genet. 79:108-112. Ries, S. K., and Everson, E. H. 1973. Protein content and seed size relationship with seedling vigour and wheat cultivars. Agron. J. 65:884-886. Simmonds, N. W. 1995. The relation between yield and protein in cereal grain. J. Sci. Food Agric. 67:309-315. Sofield, I., Wardlaw, I. F., Evans, L. T., and Zee, S. Y. 1977. Nitrogen, phosphorus and water contents during grain development and maturation in wheat. Aust. J. Plant Physiol. 4:799-810. Stoddard, F. L., and Marshall, D. R. 1990. Variability in grain protein in Australian hexaploid wheats. Aust. J. Agric. Res. 41:277-288.

[Received July 8, 1998. Accepted October 21, 1998.]

144

CEREAL CHEMISTRY