Photosynthesis, respiration and antioxidant enzymes in pepper leaves under drought and heat stresses

Leaf gas exchange, chlorophyll fluorescence and activities of antioxidant enzymes were studied in two pepper (Capsicum annuum L.) cultivars grown under drought (D) and heat (H), as well as under drought and heat in combination (HD). The drought-tolerant cv. Zhengjiao 13 exhibited greater net photosynthetic rate (P(N)) and cytochrome respiratory pathway activity (R(SHAM)), and lower contents of superoxide radical and hydrogen peroxide, as compared to the drought-sensitive cv. Longkouzaojiao. The P(N) and R(SHAM) decreased and ROS production increased under D and HD in both cultivars. As compared to the Longkouzaojiao, Zhengjiao 13 retained higher non-photochemical quenching (NPQ), photorespiration rate (R(L)), and alternative respiratory pathways (R(KCN)) under D and HD. Drought increased the superoxide dismutase (SOD) and ascorbate peroxidase (APX) activities in the cytosol, chloroplasts and mitochondria in the two cultivars. Although SOD and APX activities decreased in Longkouzaojiao under HD, SOD activity increased in Zhengjiao 13. There was no H-induced reactive oxygen species production due to increase of R(L), NPQ, R(SHAM), R(KCN) and activities of SOD and APX. However, H slightly decreased the P(N). The results indicated that HD was more detrimental than either stress alone.