Early development and enlargement of wheat floret primordia suggest a role of partitioning within spike to grain set

Spike grain number, a major component in the yield of wheat, is suspected of being determined by the sudden death of most of the initiated floret primordia, which occurs at around booting. By counting twice weekly the number of floret primordia in the middle and top spikelets, the onset time for the death of floret primordia (T-d) was assessed in six genotypes of differing earliness and fertility subjected to three treatments that were applied during stem elongation. The variation in T-d was positively correlated with overall earliness, as well as with spike fertility. Ovary development and enlargement were quantified in eight specified positions within the spikes, but T-d could not be assigned to a developmental stage attained by any ovary. Development and enlargement were very strongly correlated with each other throughout the floret lifetime, yet with significant effects of genotype, treatment and position within spikes. In each position within a spike, the proportion of florets that set a grain correlated to the initial delay of development as compared to the most advanced floret (r(2) = 0.64), but this correlation was severely biased by genotype and the floret position effect. Better correlations were obtained with either the development or width of each ovary at T-d, thus highlighting the role of the preceding phase. The best prediction (r(2) = 0.93) was obtained from the ratio of ovary width to that of the most advanced floret at T-d. The importance of this width ratio emphasized the role of partitioning in grain set: only florets able to divert nutrients survived after T-d and eventually set a grain. This relationship was no longer biased by floret position, while some remaining variability due to genotype suggested potential for plant breeding: width ratios at T-d were generally related to the growth duration of various primordia, as well as to their relative growth rate. (C) 2008 Elsevier B.V. All rights reserved.