Effect of irrigation withdrawal at different growth stages on gas exchange, photosynthesis and grain yield of four sunflower cultivars

To study the effect of irrigation deficit stress on photosynthesis, gas exchange and yield of four sunflower (Helianthus annuus L.) cultivars, a split-plot study experiment conducted at Agriculture and Natural Resources Research Station of Zanjan, Iran in 2009 on the basis of a Randomized Complete Block Design with three replications. The irrigation levels includdingfull irrigation (control) and water deficit stress at heading, flowering and grain filling stages were regarded as main plot and four hybrids including Master Lakomka, Azargol and Euroflour as the sub-plot. Analysis of variance showed that water deficit significantly influenced grain yield and decreased the yield of all four cultivars at all three stress stages compared to control. The highest grain yield was obtained from Azargol under optimum irrigation conditions. Stress at heading stage had the highest effect on yield and severely decreased it, so that it decreased the grain yield of Master, Lakomka, Azargol and Euroflour by 62, 52, 65 and 63%, respectively. In addition, stress level significantly affected physiological traits, so that irrigation deficit stress at heading decreased photosynthesis rate of Master, Lakomka, Azargol and Euroflour by 26, 27, 20 and 30%, stress at flowering decreased them by 42, 24, 22 and 39% and stress at grain filling stage decreased them by 34, 22, 16 and 25%, respectively. Stress at all three stages resulted in the loss of stomatal conductance of the studied cultivars and increased substomatal CO 2 concentration. In total, Azargol was superior to the other cultivars in terms of yield and photosynthesis attributes.