Evaluation of some physiological growth indices effective on growth and grain yield of three wheat cultivars under drought stress

 Growth analysis has been widely used to study crop and cultivar response to environmental conditions. Crop production is governed by the interception of radiant energy and the efficiency of converting this energy to dry matter. Numerous studies have shown a reduction in the leaf area index, crop growth rate, net assimilation rate and grain yield under drought stress conditions. Water deficit has a significant reduction on net assimilation rate, crop growth rate, total dry matter and grail yield. This has been attributed to promote leaf senescence and leaf abscission. Also water deficit stress has a significant effect on the reduction of net assimilation rate. This has been attributed to the phenomena of stomata closer and respiration increase. The aim of this study was to evaluate the effects of drought stress on growth indices and grain yield of wheat cultivar. This study was carried out in Tiran province, Isfahan, Iran during 2013-2014, as a split plot design in randomized complete block design with four replications. The main plots considered drought stress (irrigation after 70, 90 and 110 mm cumulative evaporation from class A evaporation pan), and sub-plots considered three wheat cultivars (Sepahan, Ghods and Pishtazs). Irrigation regimes applied from elongation phase. Plant samples were taken from 0.1 m-2 of all subplots with 20 days intervals and plants green leaf area were measured and oven dried at 75°C for 48 h plants weight mean were determined. The grain yield was measured at harvesting date. Statistical analysis of data was performed using the MINITAB statistical software and figures were drawn using by Microsoft excel software. The leaf area index in the irrigation after 110 mm, and in the irrigation after 70 and 90 mm cumulative pan evaporation treatments, in about in about 160 and 195 days after planting increased respectively, and was kept up for about 35 and 45 days at its higher surface in 110 mm evaporation and 70 and 90 mm evaporation respectively, then began to decrease. Delay in irrigation to 110 mm, plant had the lowest mean leaf area index and maximum leaf area index. Irrigation regime had a significant effect on total dry matter and delay in irrigation to 110 mm cumulative evaporation, significantly reduced total dry matter. The trend of changes in the total dry matter was more similar to the leaf area index. The leaf area index will be decreased by increasing irrigation intervals and that the total dry matter can also be decreased by reduction in photosynthetic capacity. The trend of changes in the net assimilation rate showed that in the irrigation after 110 mm, the net assimilation rate itself increased in about 165 days after planting and was kept up for about 30 days at its higher surface and in irrigation after 70 mm and irrigation after 90 mm the net assimilation rate itself increased in about 165 days after planting and was kept up for about 50 days at its higher surface, then began to decrease because of the extra leaves shading and of less absorption of the light by the leaves placed at the lower parts of the canopy. The trend of changes in the crop growth rate in the irrigation after 110 mm had been significantly lower than the irrigation after 70 mm and the irrigation after 90 mm. The trend of changes in the crop growth rate was more similar to the leaf area index but less resemble to the net assimilation rate. Irrigation regime had a significant effect on grain yield. Irrigation after 70 and 90 mm cumulative evaporation did not differ significantly for grain yield. Delay in irrigation to 110 mm cumulative pan evaporation, significantly reduced grain yield. Genotypes had a significant difference in gain yield. The Pishtaz cultivar had the highest grain yield. Result showed that irrigation after 70 and 90 mm cumulative evaporation did not differ significantly for leaf area index, net assimilation rate, crop growth rate and grain yield. Irrigation after 110 mm cumulative evaporation significantly reduced leaf area index, net assimilation rate, crop growth rate and grain yield. It was concluded that by irrigation wheat cultivar after 90mm cumulative pan evaporation, water could be saved by 22% with no significant loss in growth indices and grain yield under these conditions.