Capacity of photoassimilate storage in the stems and the efficiency of its remobilization to grain can be considered as two important traits stabilizing grain yield under water stress in grain filling period. These traits were evaluated in four climatically different wheat cultivars (Chamran, Tajan, Alvand and M-75-7) under water stress and non — stress conditions. The results showed that the contribution of stem reserves to grain yield increased under water stress condition (19.3 and 24.4 percent under non — stress and stress conditions, respectively). This increase was coincided with a decline in current photosynthesis contribution to grain yield. In Chamran and M-75-7 cultivars a greater portion of stem reserves were remobilized to grain as compared to Alvand and Tajan. In addition, the maximum stem weight, at 17 days after anthesis, was also greater in the two former cultivars and was less affected by water stress. The efficiency of stem reserve remobilization was increased under stress conditions. However, this increase was observed only in Chamran and M-75-7, but a marked reduction was observed in the other three cultivars. The concentration of stem soluble sugar decreased from 21.3% at flowering to 2.7% during grain filling. These compounds accounted for 13% of grain yield. The contribution of stem soluble sugars to yield did not vary in cultivars under either moisture regimes. However, at maturity, differences between cultivars were evident. A significant and positive correlation was detected between stem soluble sugar concentration at 17 days after anthesis and its contribution to grain yield. On the other hand, for yield contriution, a negative and significant correlation was observed between stem sugars and ongoing photosynthesis. Water stress did not affect grain filling rate until 24 days after anthesis. The highest and lowest grain filling rates were observed for Alvand and M-75-7, respectively at this stage. The stem weight losses were evident in all cultivars at the aforementiond period of grain filling. It appeared that water stress reduced final grain weight through reduction of grain filling duration, rather than the rate of grain filling. Reductions in current photosynthesis and grain yield under water stress condition were 20% and 15%, respectively. Buffering effect in stem remobilization could compensate for part of the reduction in current photosynthesis.
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