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The pattern of changes in dry matter(DM) of plants and crop growth rate (CGR) of two soybean Glycine max (L.) Merr).Cultivars, williams and Clark 63, were investigated under three irrigation treatments (irrigation after 30, 70 and 110 mm evaporation of water from class A pan, named Ii, I2 and I3? respectively) at Agricultural Research Stalion, Colledge of Agriculture, Esfahan University of Technology, during 1986 to explain the differences between grain yiely levels.
A higher amount of DM was produced by cultivars under I2 but equal amounts of DM were produced under I1 and 13. The pattern of DM accumlaion and total DM produced by both cultivars were similar. The rate of DM accumulation by cultivars under irrigation treatments increased at full flowering stage (R2) and proceeded through the early maturity stage (R7) linearly. Total DM decreased as leaf shedding started.
A higher CGR in 12 produced more grain yield than the I1 and I3 treatments, however in I3 treatment GCR was lower than I upto the completion of seed filling stage, but later on, it declined rapidly in I1. The CGR was similar for both cultivars, the maximum of which occured at early seed filling (R5 ), and decreased after this stage. Leaf shedding at the start of maturity resu1te in negetive CGR. However, in I and I3treatments the maximum CGR occured before and after R5 stage, respectively. In these treatments 1i and 13) negative CGR occured before and after R7, respectively. Higher and lower vegetative growth at I and 13 treatments, respectively, had caused the above conditions.
Evaluation of yields obtained by cultivars under irrigation treatments showed that the total DM produced, the pattern of DM accumultion and CGR can not exclusively explain the differences between the yields. Therefore, evaluating the other growth indices such as net assimilation rate (NAR) and the amount of grain yield on branches seem to be important too (1,5).