Seeds pre-treatment by abscisic acid for inducing tolerance to heat stress in corn seedling

Introduction Abscisic acid (ABA) is a phytohormone which has a main role in plant life cycle and regulates many physiological processes including plant development and adoptability to environmental stresses. ABA controls water status of plants under stress and noticeably decreases undesirable heat stress damages. Since high temperature stress is a limiting factor for many crops growth and according to literature review there are evidences that ABA can induce thermo tolerance in plants, the present study was designed to evaluate the response of corn to heat stress when seeds are pretreated by ABA. Materials and methods The experiment was factorial in a completely randomized block design with three replications and was done in agricultural college of Vali-e-Asr University of Rafsanjan in 2013. First factor was seed pretreatment by ABA solutions (zero, 50, 100 and 200 µM L-1) and second factor was different durations (zero, 8, 16 and 24 h) of heat stress in 40º C exerted on hybrid S.C.704 corn seedlings. In darkness seeds were soaked in ABA solutions for 24 h at the temperature of 25 ± 1º C. Following pretreatments seeds were sterilized by distilled water and finally were planted in pots in the depth of 1.5 cm. Seedlings were transported to growth chamber with the temperature of 40º C in four foliate stage to be treated by different heat durations. After thermo treatments seedling were transported to green house and plant responses were measured two weeks later. Results and discussion Results indicated that the highest dry weight of shoots was obtained in ABA concentration of 50 µM L-1 but there were no significant differences at probability level of one percent among other concentrations. In addition corn seedlings were able to tolerate 8 h heat duration but negative effects of heat stress was significantly increased in 16 and 24 durations which led to the reduction of shoot dry matter production. Increase in heat duration and ABA concentration caused a significant raise in shoot to root weight ratio so that the highest ratio (31.66 percent) was observed in control treatment and the lowest (3.83 percent) in heat duration of 24 h accompanied with 200 µM L-1 ABA. Leaf relative water content in ABA concentrations of 50 and 100 µM L-1 showed a significant increase at probability level of five percent in respect to control and ABA concentration of 200 µM L-1 which can probably be due to stomata resistance in these treatments and may be used as suitable approach to increase plant thermo tolerance. Negative effect of ABA concentration of 200 µM L-1 on leaf relative water content indicated that ABA concentration must be chosen precisely. Increase in heat duration caused a significant raise in leaf temperature. In heat durations of 16 and 24 h the highest leaf chlorophyll index (27.2 and 24.5 respectively) was obtained in 50 µM L-1 ABA and the lowest ones (21.9 and 18.7 respectively) in 200 µM L-1. Increase in ABA concentration and heat stress led to a significant reduction of chlorophyll a content in corn leaf so that in heat duration of 24 h and ABA concentration of 200 µM L-1 a reduction of 36.9 percent was observed compared to control. In addition a reduction of 17 percent in carotenoids was associated with 24 h heat stress in respect to control. In all three durations of heat stress, rise in ABA concentration up to 100 µM L-1 made a decline in ion leakage while in the highest concentration (200 µM L-1) an increment in ion leakage was observed. Conclusions In conclusion the results of the present study showed that high levels of heat stress in combination with undesirable concentrations of ABA can create negative effect on plant ability to preserve leaf relative water content and can lead to the destruction of chlorophyll and carotenoids structure and increase the ion leakage. On the other side ABA concentration of 50 and 100 µM L-1 decreased ion leakage while increased the leaf relative water content and chlorophyll index. By these simultaneous effects the corn shoot dry weight was improved under heat stress condition. It seems that external use of ABA can have positive and negative effects on plant growth depending on its concentration. Concentrations higher than threshold tolerance to ABA can cause disturbance in plant physiological processes and restrict plant growth and development.