Effect of humic acid on physiological and biochemical indices and yield of tomato under deficit irrigation

In order to study the effect of water deficit stress and foliar spray of humic acid (HA) on physiological and yield traits of tomato, an experiment was carried out in a split plot based on randomized complete block design with three replications at the research filed of University of Zanjan, in 2016. Treatments consisted arrangement of three irrigation levels (starting irrigation at 100, 80 and 60 percent ETc) and 4 levels of HA (0, 100, 200 and 300 mg.l-1). The results showed that deficit irrigation significantly decreased stomatal conductance, membrane stability index, leaf relative water chlorophyll a and b content and plant yield, and increased the proline content and proxidase activity. Treatment of HA enhanced proxidase activity, leaf relative water content, proline, chlorophyll, membrane stability index and plant yield under normal and deficit irrigation, and decreased stomatal conductance. The highest peroxidase activity (1.207 uints.g-1FW.Min-1) and proline content (11.5 mg.gFW-1) was obtained with treatment of HA 200 mg L-1 under irrigation 60 percent ETc. Also, maximum leaf relative water content (78.6 percent) and membrane stability index (70.01 percent) was achieved in HA 300 and 200 mg L-1, respectively under irrigation 100 percent ETc. The maximum plant yield was obtained with application of HA 200 mg L-1 under irrigation 100 %ETc. According to the results, application of HA 200 mg L-1 can be proposed to improve physiological traits and increase fruit yield.