Abstract
Production, remobilization and accumulation of assimilates in crops especially under water stress are essential factors for determination and studying the yield differences of species and cultivars. Field experiment was conducted using a split plot design based on a randomized complete block design with 3 replication s during 2007 growing season in agricultural research station (Khorasan Agricultural and Natural Resource Research Center, Mashhad-Iran). Main plots were consisted of 2 levels of water, water deficit after anthesis and normal condition (with out water stress) and factorial arrangement of photosynthesis status (non desiccation and chemical desiccation with potassium iodide) and 3 grain sorghum genotypes (Sepide, M5 and M2 promising lines) were assigned to sub plots. Results of variance analysis showed, that the effects of water stress on dry matter accumulation, efficiency of remobilization (REE), percent of remobilization (REP), biologic yield were significant in (p≤0.01 (and grain yield (economic yield) was significant in) p≤0.05), respectively. Water deficit caused an increase of 10.08%, 24.45 % and 12.43% in dry matter accumulation, percent of remobilization and efficiency of remobilization, respectively as compared to normal conditions. This in turn was led to decrease in seed yield, biological yield and harvest index by 36.38%, 5.43% and 31.60%, respectively. The effect of disturbance in current photosynthesis was significant in all of traits and caused the increase of 15.58%, 17.5% and 36.62% in dry matter accumulation, efficiency of remobilization and percent of remobilization, respectively. The role of remobilization was crucial in sorghum genotypes. Interaction between factors showed that highest dry matter accumulation, percentage of remobilization and efficiency of remobilization was in drought stress and disturbance in current photosynthesis and was 16.62%, 62.54 and 24.60%, respectively and was significantly different from other treatments. Genotypes showed significant influence on percent of remobilization and efficiency of remobilization and M5 genotypes had the highest percent for all traits. The results of correlation analysis showed positive and significant correlation between dry matter accumulation and percent of remobilization (r=0.722, p< 0.01), efficiency of remobilization (r=0.911, p< 0.01) and biological yield (r= 0.534, p< 0.05).

Keywords: Water deficit, Current photosynthesis, Efficiency of remobilization