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Optimum leaf area index (LAI) is necessary to decrease in soil surface evaporation, to intercept maximum radiation and to increase in dry matter production and seed yield. In order to study the development of leaf area in canola (B. napus) and its relationship with seed yield, this experiment was conducted at agricultural research station of Gonbad during 2005-7. The experiment was a RCBD arranged in a split-plot in two conditions, i.e. supplemental irrigation and rain-fed. Five sowing dates (SD) (Nov. 6, Dec. 6, Jan. 4, Feb. 5 and Mar. 5) assigned as main plots and two cultivars (Hyola 401 and RGS003) as subplots. In all treatments, the number of leaves on main stem and LAI increased till the beginning of seed filling, and then decreased considerably due to sever shedding of leaves. LAI at the beginning of seed filling stage, varied from 6.1 in Nov. 6 to 3.1 at Mar. 5 SDs in supplemental irrigation conditions, and from 5.2 on Nov. 6 to 2.4 at Mar. 5 SDs in rain-fed conditions. The better early growth of Hyola401 compared to RGS003 cultivar caused that, in each year and conditions, LAI of Hyola401 was significantly more than RGS003 at the beginning of stem elongation and budding. The strong relationships between the number of leaves on main stem and LAI at different growth stages and seed yield, showed the positive effects of these traits on seed yield.

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Studyign of soybean cultivars under abiotic stress conditions to improve their yield is important. This experiment was carried out as a strip plot based on a randomized complete block design with three replications in the Safiabad Argicultural Research Center. Vertical factor consisted of foure levels of irrigation (Irrigation after 50, 90, 130 and 170 mm evaporation from class A pan) and horizontal factor was 6 soybean genotypes (504, Katoul, Safiabadi, BP×Safiabadi, Migmat and Salend). Results showed that the effect of drought stress, genotypes and their interactions were significant on grain yield. The mean comparison of drought stress × genotype interaction showed that the highest grain yield (4522 Kg ha-1) achieved in irrigation after 50 mm evaporation from class A pan in Migmat and the lowest grain yield (1017 Kg ha-1) belonged to Katoul genotype with irrigation after 170 mm evaporation from class A pan. Pod number per plant decreased by 16%, 28% and 51% with increasing irrigation intervals to 90, 130 and 170 mm compared to irrigation after 50 mm evaporation. The treatments did not have any significant effect on the number of grain per pod. It is concluded that, irrigation after 50 mm evaporation from class A pan would be suitable for Migmat and BP×Safiabadi genotypes. Irrigation after 90 mm evaporation from class A pan appropriate for 504, Salend, Katoul and Safiabadi genotypes.