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To study the effect of heat stress on physiological and structural characteristics of rice cultivars, an experiment was carried out in a split-plot arrangement using randomized complete blocks design with three replications in 2006 and 2007 cropping seasons at Shavoor Agricultural Research Station, Ahwaz, Iran. For exposing rice cultivars to different higher temperature regimes, main plots were assigned to three sowing dates from 5 May with 20 days intervals and cultivars Hoveizeh, Hamar (heat tolerance) Red Anbori, Champa (heat sensitive) and Danial (relatively heat tolerant) were randomized in sub-plots. The results of combined analysis of variance showed that sowing date had significant effect on grain yield. The highest grain yield was harvested from the second sowing date (26 May). cv. Hoveizeh with grain yield of 4977.6 Kg.ha-1 was the superior cultivar. Chlorophyll content at 50% heading and harvesting time were also affected by sowing date and cultivars. The first sowing date (5 May) had the highest chlorophyll content, however, the lowest chlorophyll content was measured for cv. Hoveizeh. The simple analysis of variance also indicated that the third (15 June) and first (5 May) sowing dates had the highest and lowest cell membrane stability, respectively. Cv. Hoveizeh had greater cell membrane stability when compared with the other cultivars, in all three planting dates. cv. Hoveizeh had also the lower environmental sensivity, hence, higher grain yield stability.

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To study effects of different levels of nitrogen fertilizer and cytokinin on some physiological traits of wheat cv. Chamran under normal and post anthesis heat stress conditions, this experiment was conducted at Ramin Agriculture and Natural Resources University, Ahvaz, Iran, in 2012-2013 growing season. The experiment was carried out in two separate factorial experiments using randomized complete block design with four replications.The first experiment was planted in normal date of sowing (05 December 2012) and the second experiment was planted late (20 January 2013) to expose the experimental treatments to terminal heat stress. In both experiments, nitrogen (N) was applied at four levels (0, 75, 150, and 225 kg.ha^-1) and cytokinin (Ck) at three levels (0, 50 and 100 µmol). Results showed that intraction effect of sowing date× nitrogen× cytokinin on prolin content and SPAD value, intraction effects of nitrogen × cytokinin and sowing date × nitrogen on FLAD (Flag Leaf Area Duration), stomatal conductance and grain yield was significant. The highest grain yield was obtained from application of 150 kg.ha^-1 nitrogen and 100 µmol cytokinin, however, it was not significantly different from 225 kg.ha^-1 nitrogen and 100 µmol cytokinin and 150 kg.ha^-1 nitrogen and 50 µmol cytokinin. It was concluded that with increasing application levels of nitrogen and cytokinin the negative effects of terminal heat stress could be mitigated.