GRAIN WEIGHT REDUCTIONS IN WHEAT ASSOCIATED WITH SEMIDWARFISM - AN ANALYSIS OF GRAIN WEIGHT AT DIFFERENT POSITIONS WITHIN THE SPIKE OF NEAR-ISOGENIC LINES

Semidwarf genes have made a major contribution to grain yield gains in wheat during this century. These genes have frequently been reported to increase grain number but decrease grain size. To analyse possible causes of this negative association between number of grains per head and mean grain weight, grains from different positions within the spik es were individually weighed in three isolines (standard-height, SH; s emidwarf, SD; and dwarf, DD) of the spring wheat cultivar Maringa grow n under field conditions. Dwarfing genes affected both grain number an d grain weight. Increasing doses of Rht genes resulted in a decreased average grain weight (ca. 22 %) bur in a greater increase in number of grains per m(2) (ca. 40 %). Since the isogenic lines did not differ i n either the number of spikes per m(2) or number of spikelets per spik e, the frequencies of appearance of grains in the distal positions (fl orets 3 and 4) of the central spikelets and in the top spikelets of th e spike was much higher for the SD and DD lines than for the SH line. Averaging across spikelets, basal grains (grains 1 and 2) of SH, SD an d DD lines represented ca. 82, 73, and 69 %, respectively, of the tota l number of grains. On the other hand, grain 4 was almost absent in th e SH line but represented around 10 % of the grains in the DD line. Av eraging across lines and spikelets, basal grains weighed ca. 30 mg, wh ich is approximately 20 % heavier than grain 3 (ca. 25 mg) and 85 % he avier than grain 4 (ca. 16 mg). Although there was a clear trend for a greater grain weight in the SH line (with no major differences betwee n the SD and DD line) when basal grains mere considered, grains 3 and 4 did not show any clear difference between lines. The fact that genot ypes with Rht alleles had grains in the distal positions with a much h igher frequency of appearance than in the SH line could be the main re ason for their lower average grain weight. Therefore, the negative rel ationship between grain number and average grain weight might be not r eflecting that competition for assimilates among grains of the DD and SD lines is stronger than that in the SH line.