Effect of Humic Acid on Activity of Antioxidant Enzymes and ‎Yield of Castor Bean (Ricinus commonis) under Water Deficit ‎Condition

To evaluate the effect of humic acid on activity of antioxidant enzymes in castor bean a split plot experiment based on completely randomize block design with four replications was carried out at Islamic Azad University, Karaj Branch in 2014. Treatments were irrigation with three levels [irrigation at 80% of field capacity (S1) (normal), irrigation at 65% of field capacity (S2) (mid stress) and irrigation at 50% of field capacity (S3) (high stress)] assigned to main plots and humic acid with four levels (without foliar application, one time, twice and three times foliar application) to sub plots. In this study traits like seed yield, 100 seed weight and antioxidant enzymes were estimated. The results showed that the effect of humic acid was significant at 1% level on 100 seed weight and activity of antioxidant enzymes. Mean comparisons indicate that foliar application of humic acid could compensate relative effect of water deficit. It was also revealed that the rate of superoxide dismutase, glutathione peroxidase and catalase were increased with three times of humic acid foliar applications by 38.8, 34.75 and 37.88 percent under mid stress drought and 26.63, 41.15 and 43.14 percents under high stress drought as compared to control, respectively. Overall, it can be said that use of humic acid have different physiological effects. Humic acid not only increases seed yield of castor bean it also have a positive role in reducing water deficit effects and increasing activity of antioxidant enzymes.