Dimethoate modifies enhanced UV-B effects on growth, photosynthesis and oxidative stress in mung bean (Vigna radiata L.) seedlings: implication of salicylic acid.

The present study is aimed to investigate implication of salicylic acid (SA) in regulation of dimethoate (30 and 150 ppm designated as D1 and D2, respectively) and enhanced UV-B radiation (ambient + supplemental; ambient + 4.0 kJ m(-2) and ambient + 8.0 kJ m(-2), designated as UV-B1 and UV-B2, respectively) induced responses in mung bean seedlings. Seeds of Vigna radiata L. cv. Narendra 1 were surface sterilized, washed thoroughly and soaked for 24 h in sterilized distilled water. Soaked seeds were sown in acid washed sterilized sand filled in plastic trays, and incubated in dark at 26 ± 2 °C for 2 days. The seedlings were grown in growth chamber at 26 ± 2 °C with 12 h photoperiod (350 µmol photons m(-2 )s(-1), PAR) and watered regularly. Six day old seedlings of equal size were gently transferred in 0.2 strength Rorison nutrient medium (pH 6.8) for acclimatization. Thereafter, dimethoate (30 and 150 ppm designated as D1 and D2, respectively) and enhanced UV-B radiation treatments were given. On the 12th day, seedlings of each set were harvested and various parameters related to growth, pigments, photosynthesis, oxidative stress and antioxidant system were analyzed. The D2 dose of dimethoate and UV-B1 and UV-B2 alone and together significantly (P < 0.05) declined growth, photosynthetic pigments and photosynthesis (Fv/Fm and qP except NPQ) which were accompanied by significant decrease in SA level. Similarly, D2 and UV-B also enhanced (P < 0.05) accumulation of reactive oxygen species and concomitantly damaging effects on lipids, proteins and membrane stability were observed. In contrast, in SA-pretreated seedlings damaging impacts of D2, UV-B1 and UV-B2 alone and together were significantly (P < 0.05) alleviated. Besides this, interestingly D1 dose of dimethoate alone had stimulatory effect on growth and it also ameliorated damaging effects of both the doses of UV-B. The activity of superoxide dismutase was stimulated by all the combinations. However, catalase, glutathione reductase and dehydroascorbate reductase activities were significantly (P < 0.05) inhibited by D2, UV-B1 and UV-B2 while SA-pretreatment ameliorated D2 and UV-B-induced inhibitions in activities of these enzymes. Total ascorbate and glutathione pools also decreased by D2 and both doses of UV-B; however, in SA-pretreated seedlings their amounts were significantly (P < 0.05) higher than D2, UV-B1 and UV-B2 alone. Interestingly, D1 also alleviated damaging impact of UV-B1 and UV-B2 on total ascorbate and glutathione pools. Results revealed that D2, UV-B1 and UV-B2 might alter SA biosynthesis that results into declined SA level which might be related with their toxicity. However, SA-pretreatment might act as a signal that reduces oxidative stress by triggering up-regulation of antioxidants hence improved growth and photosynthesis noticed. Alleviation of UV-B toxicity by D1 suggests about hormesis that triggers SA biosynthesis and hence protection against both doses of UV-B was observed.