Effects of irradiation of corn seeds (SC740) with ultrasound for alleviation of drought stress corn

Introduction Ultrasound wave application is known as a novel technology. It is widely used for many purposes including industry, agriculture (e.g. plant breeding, raisin processing) and food technology. The experimental evidences regarding the irradiation of seeds with ultrasound have shown extraordinary results in affecting seedling growth. For instances, it has been reported that irradiation of seeds for 50 to 60 seconds increases the germination percent, radical and plumule length by 40, 32 and 5-8%, respectively (Fariabi et al., 2008). Irradiation of Myrtus communis has resulted in breaking seed dormancy, increasing germination rate and boosting seedling dry weight (Alvandian et al., 2013). Most of irradiation- related experiments have focused on seed germination and early seedling growth. Therefore there are no published reports regarding its possible effects on the plant at rest of growth and development stages. Considering the positive effect of irradiation on radical growth, it is hypothesized that such effect sustains even in adult stages of growth and development. Therefore, due to possible deeper rooting system, an increased drought resistance is expected. This filed experiment was aimed at investigation of possible alleviating effect of irradiation on decreasing impacts of drought stress on corn.   Materials and methods The variety of corn was SC740. The experiment was as factorial based on complete block design with 3 replications. Treatments were drought durations [control, long-term stress (no irrigation from flowering onward), and short-term stress (no irrigation from seed dough-stage onward)] and irradiation of seeds (control, 2, 4, 6, and 8 minutes) with ultrasound. The traits including leaf area index, relative water content (RWC), leaf carotenoid content, soluble sugar content of leaf and leaf K content were measured 10 days after seed dough stage. After maturity, seed protein content, shoot biomass and grain yield were measured.   Results and discussion The results indicated that, among source of variations, say experimental factors in ANOVA table, only the effects drought and irradiation were significant on RWC. The decrease in RWC was 13 and 19% for short and long duration drought stress, respectively. Six minutes irradiation was the best level which increased RWC by 18% compared to control. Surprisingly, leaf area index was not affected by irradiation. But it significantly responded to drought stress with probability level of 1%. The drought- resulted decrease in leaf area index was 25 and 27% for short term and long term cutting of irrigation. The difference of drought levels for leaf area index was statistically negligible. Accordingly, it seems that they may differ for lead thickness (say leaf dry weight) than leaf area. The highest content of K in leaf was obtained for long-term stressed plants (299.33 mg.g-1). Irradiated plants (say seeds) by 6 minutes had 14% higher leaf K content than control. Regarding leaf content of soluble sugars, the short drought stress appeared to be most effective. Its content was the lowest for non-stressed plants. Among irradiation levels, again 6-minute irradiation was the best as it increased leaf K content by 22%. The leaf carotenoid content was only showed response to drought stress. It tended to have decreasing trend with increasing drought stress duration. It is believed that in response to drought stress, carotenoid content gets increased, but subsequently shows decrease which is kind of plant adaptation to stress (Young, 1991). Seed protein content had significant response to both main and interactive effects of factors. This proves that the effect of drought stress on the mentioned trait is not the same for plants irradiated differently. For example, under 2 and 8 minutes irradiation conditions, seed protein content was lower for stressed plants when compared to non-stressed ones. But it was vice-versa for 6 minutes irradiation. The highest seed protein content was obtained for combination of long term drought stress and 6 minutes irradiation. The decrease in biomass was 24 and 31% for short and long drought stress, respectively. For grain yield, it appeared to be 18 and 34%, respectively. The best irradiation level, i.e. 6-minute one, positively affected the biomass and grain yield by 13 and 25%, respectively.   Conclusion The interactive effects of drought and irradiation were not significant on some traits including biomass and grain yield. This finding shows that the positive effect of irradiation on growth of corn did not differ statistically for stressed and non-stressed conditions. In another words, irradiation of seeds as long as 6 minutes could increase growth of both stressed and non-stressed corn. This is equivalent to alleviation of drought stress impacts on corn. This alleviation might be due to increased leaf K content (14%), and enhanced leaf concentration of soluble sugars (22%), and consequently promoted pant water status (18% increase in RWC).