Investigating the response of tropical maize (Zea mays L.) cultivars against elevated levels of O3 at two developmental stages

Tropospheric ozone (O-3) concentrations are rising in Indo-Gangetic plains of India, causing potential threat to agricultural productivity. Maize (Zea mays L.) is the third most important staple crop at global level after rice and wheat. Two high yielding cultivars of Indian maize (HQPM1-quality protein maize and DHM117-normal/non quality protein maize) were exposed to two levels of elevated O-3 above the ambient level (NFC) viz. NFC + 15 ppb O-3 (NFC + 15) and NFC + 30 ppb O-3 (NFC + 30) using open top chambers under field conditions. The study was conducted to evaluate the biochemical responses of two cultivars at different developmental stages leading to change in yield responses. Initially at lower O-3 dose, photosynthetic pigments showed an increase but reduction at later stage, while higher dose caused a decline at both the stages of sampling. Levels of superoxide radical (O-2 (-)) and hydrogen peroxide (H2O2) significantly increased and contributed to lipid peroxidation at elevated O-3. Histochemical localization assay of O-2 (-) and H2O2 showed that guard cells of stomata and cells around trichomes took deeper stain at elevated O-3 reflecting more formation of reactive oxygen species. Secondary metabolites like total phenol, flavonoids and anthocyanin pigments also increased in plants under O-3 stress. Enzymatic antioxidants were triggered in both the cultivars due to elevated O-3, while induction of non-enzymatic antioxidants was more in HQPM1. Native PAGE analysis also showed that SOD, POX, CAT, APX and GPX were stimulated at elevated O-3 concentrations compared to NFC. SDS-PAGE showed reductions of major photosynthetic proteins with higher decrease in DHM117. Principal Component Analysis showed that both the cultivars showed differential response against O-3 at two developmental stages. HQPM1 maintained the analogous defense strategy at both the sampling stages while DHM117 showed variable response. Overall metabolic induction of antioxidants related to defense was more in DHM117 than HQPM1. This suggests that DHM117 utilized more assimilates in maintaining the homeostasis against imposed oxidative stress, causing less translocation of assimilates to reproductive parts and thus affecting the final yield. In terms of yield it is suggested that performance of HQPM1 (quality protein maize) was better than the DHM117 (non quality protein maize).