Effect of drought stress on some physiological growth indices of sunflower cultivars

Introduction: Sunflower is one of the most important oil crops which plays an important economical and agronomical role in crop rotation because of high oil quality. This crop is very flexible in bad environments. Water deficiency lessen net assimilation rate, dry weight of leaves, stem, and root and causes total dry weight and slow growth rate (Turrner and Sobrado 1987; Ren et al., 2016). Leaf area index is the most important growth index of sunflower which shows the highest sensitivity to water deficiency. Then, the main sign of water stress in vegetative phase of sunflower is reduction of number and size of leaves. Net assimilation rate Sunflower generally 10 to 20 days after germination increased or remained constant. Goksoy and Demir (2004)announced that restricted irrigation reduced leaf area index due to yellowing and falling leaves. Sunflower can reach to its maximum leaf area under full irrigation in proportion to water deficiency, water stress in vegetative phase caused shorter plants and lower dry matter of sunflower (Cox and Jolliff. 1986). This study was conducted to examine the effect of drought stress on growth properties of their sunflower cultivars in Isfahan (Iran). Materials and methods: The study was conducted in 2013 as split plot in randomized blocks design with three replications. Main plots were irrigation in four levels (80, 150 and 180 mm evaporation from evaporation pan class A) and sub plots were four sunflower cultivars (Azargol, Euroflor, Ghasem and Farrokh). Soil preparation was done and seeds were sown at June 8th. Every plot had five lines with six meter length, 60cm inter row space and 14cm distance between plants to reach to twelve plants per square meter. Irrigation was done according to 80mm evaporation until plant establishment and after that irrigation treatments were enforced. Sampling was done every 15 days and at the time of maturity considering marginal effects. In order to calculate trends of total dry matter, leaf area index, net assimilation rate, and crop growth rate the best regression equations were used (soleymani et al, 2003). MSTAT-C software and mean comparison with Duncan’s test in 5% probability were used. Results and discussion: The effect of irrigation on maximum leaf area index was significant. The highest leaf area index was obtained by irrigation after 80 mm evaporation from class A .There was a significant difference with the other irrigation treatments. Lowest LAI by irrigation after 180 mm evaporation from class A pan evaporation so irrigation treatments had significant differences with other results show that with increasing severity of drought stress the leaf area index decreased. It seems that water stress during vegetative growth to remain small leaves and leaf area index during the period of reduced crop. The effect cultivars on the leaf area index was significant. Most leaf by leaf area index by as much as Euroflor significantly different from that of other varieties. Lowest leaf area index was obtained by the Qasem. Aargol and Farrukh was not significantly different. The results showed that by reducing crop water requirement LAI significantly reduced. Most of the dry weight of the plant by irrigation on 80 mm of Class A pan evaporation rate was obtained, which was significantly different to other treatments and the lowest plant dry weight by 180 mm evaporation from irrigation class A pan evaporation rate was 1080 g/m2, which is also a significant difference with other irrigation treatments. Effect variety on total dry weight of the plant is not significant. In all treatments, dry matter accumulation trend was maintained until maturity after the end of the dry season because of the falling leaves fell. With increasing drought stress reduced dry matter. The highest rates of net assimilation rate among irrigation treatments used for the treatment mm 180 and then to the moderate drought stress 150 mm evaporation from pan class (A) and then severe stress (80) mm evaporation from pan class A). Compare the net assimilation rate showed that NAR sunflower cultivars increased during the first growing season and then downward path and fell. So the Euroflora because more leaf area index and the shadow of the upper leaves on the lower leave NAR was less than the other varieties. Conclusions: Results showed that total dry matter, leaf area index, net assimilation rate and crop growth rate were decreased by drought increasing from 80 to 180mm. The least evaporation (150mm) had the highest amount of mentioned traits. Among cultivars, azargol had higher total dry weight because of having growth period and then higher LAI, total dry weight but it had lower NAR and CGR because of higher LAI and shading of upper leaves, then it is recommended to use Record cultivar.