High Concentration of CO2 Improve the Pb Resistance of Oryza sativa L. Seedlings by Enhancing Photosynthesis and Regulating Plant Endogenous Hormones

Rice seedlings were treated with different concentrations of Pb solutions or /and CO2 (Pb 0 μM + CO2 400 ± 20 μmol/mol, AC; Pb 0 μM + 800 ± 20 µmol/mol, EC; Pb 50 μM + CO2 400 ± 20 μmol/mol, L; Pb 150 μM + CO2 400 ± 20 μmol/mol, H; Pb 50 μM + CO2 400 ± 20 μmol/mol, EC + L; Pb 150 μM + CO2 800 ± 20 μmol/mol, EC + H;), so as to study the effects of Pb stress and/or elevated CO2 on growth, photosynthesis, and endogenous hormones. We measured the biomass, photosynthetic gas exchange parameters, fluorescence parameters and their curves (O-J-I-P, Variable fluorescence) and the contents and ratio of endogenous hormones in rice seedlings. The results show that: (1) Both L and H had negative effects on the growth of rice seedlings. However, H treatment had a significant negative effect on PSII of rice seedlings, but L treatment had no significant effect. In addition, JA (jasmonic acid) and ABA (abscisic acid) were significantly associated with the decrease of GS in Rice under Pb stress. (2) Under EC treatment, the secretion of growth hormones increased significantly. More importantly, the content of JA increased significantly while GS decreased, but ABA did not, which may mean that JA is more sensitive to stomatal regulation under EC. (3) Compared with L and H, EC + L and EC + H manifested in the enhancement of photosynthetic performance and adjustment of hormone ratio. Compound treatment significantly increased the ratio of “promoting-hormone to ABA” compare with L and H treatment. The biomass accumulation of rice seedlings increased significantly after being affected by changes in hormone ratios and enhanced photosynthesis. Therefore, the high concentration CO2 reduces the harm of Pb to a certain extent. However, this relief is also limited. Compared with H, EC + H has no alleviating effect on rice seedling most growth and photosynthetic parameters.