Study on kernel development and contribution of kernels from different sections of spikes to grain yield in winter wheat.

In recent years the grain yield of newly released wheat cultivars has been highly increased and have reached to a plateue level. Understanding of kernel development and potential kernel weight of kernels formed in various positions and how this is affected by assimilate supply and what possibilities exist for increasing grain yield are prerequisite for increasing grain yield in new cultivars. The objectives of this study were to determine: I) variation in national and foreign winter wheat genotypes for rate and duration of kernel filling, 2) if the position in the spikelet and position along the spike varied among genotypes and if they were important restrictions to cultivar grain yield potential and assimilate supply. The experimental design of all trials was a randomized complete block with ten genotypes at five locations (Karaj, Hamedan, Mashad, Zanjan and Ardabil) under both nomal irrigation and terminal drought stress conditions in 2003-04 and 2004-05. The highest yeild, particularly under terminal drought stress conditions obtained from genotypes with high kernel weight such as C-80-10 inbreed line. Overall rates and duration of kernel phases differed significantly between genotypes. Trends in kernel growth also differed in both environments and at different locations. Overall, the highest rate of kernel filling was observed in genotypes with large kernel size such as Gascogne and C-80-1 0 inbreed line.. All the genotypes had significant differences in the number and the weight of kernels in the A, B (basal), C and D (distal) positions in the spike let with fewer and lighter kernels in the C and D kernels of spiklets. However, the weight of kernels in the A and B position in the spiklets were heavier than kernels in basal positions. A significant genotype x kernel position interaction for number and weight of kernels indicated a considerable variation for this character. Significant genotype x location and genotype x environment interactions for kernel weight and numbers were also observed Cultivars like C-80-10, which produce heavier basal and dustal kernels are suitable for both emvironments. Cultivars that produce high kernel numbers and also heavier kernels from the basal kernels may be more desirable for water stress conditions. Removing spikelet numbers increased the kernel weight of the remaining kernels by increasing assimilate supply underterminal drought stress conditions. Kernel numbers and weights were also varied along the length of the spike with kernels from the lower and middle part of the spike significantly heavier than those from the upper part in bothenvironmental conditions. Considerable variation for a higher rate and duration of kernel filling, kernel weight and yield composition exist for increasing yield in the wheat gene pool. It is suggested that selection of genotypes with a relatively high rate of kernel filling and large kernel size for the regions prone to terminal drought stress and selection of genotypes with high rate and longer duration of kernel filling may increase wheat production in winter wheat areas of Iran. ­