response of modern and old wheat cultivars improved to fit moderate, cold and warm climates of iran to source strength reduction

source limitation during grain filling is important for both management and breeding strategies of grain crops. there is little information on the sensitivity of grain yield to various source-sink ratios in various wheat (triticum aestivum) cultivars. the objective of the current study was to evaluate the response of grain yield as well as yield components to different source-sink ratios during grain filling in wheat cultivars improved for contrasting environments. pot experiment was carried out outside, under natural conditions in greenhouse areas of college of agriculture, university of tehran, karaj, in 2007-08 cropping season. treatments consisted of 7 cultivars of bread wheat (new and old cultivars from moderate, cold and warm climates of iran) and 7 defoliation treatments were carried out at the late joining stage and at 20 days after anthesis. treatments were arranged in a 3-replicated factorial combination based on rcbd design. in general, defoliation treatments improved soil water content. the results indicated that defoliation increased photosynthesis and transpiration rates, stomatal conductivity and chlorophyll stability in flag leaf as compared to control. there was no significant interaction between cultivars and defoliation treatments regarding yield and yield components. although severe defoliation treatments reduced grain yield, other defoliation treatments (2l and 3l) did not much affect grain yield. the number of grains in spike was affected by defoliation treatments, while it didn’t significantly affect the 1000-seed weight.