Effect of elevated carbon dioxide on growth photosynthesis, pigments and Abscissic acid content in phosphoribulokinase deficient transgenic tobacco plants grown under drought condition

Abstract

Study investigate effect of ambient (350 mu mol mol-1) and elevated (700 mu mol mol-1) CO2 under water stress conditions on phosphoribulokinase deficient transgenic tobacco plants (C8) generated through antisense technique and found to have only 5 percent phosphoribulokinase activity (referred to as transformants) as compared to wild type (Bin-19) on growth, net CO2 assimilation, chlorophyll fluorescence, photosynthetic pigments, ABA content and activity of peroxidase. CO2 enrichment resulted in significant increase in biomass in transformant and wild type plants and almost completely mitigated the effect of water stress on biomass. Rate of net photosynthesis (Pn) was also higher in plants grown at elevated CO2 irrespective of wild or transformant and showed even greater Pn even under water stress conditions. Stomatal conductance was also higher in plants grown at 700 mu mol mol-1 CO2 level and decreased relatively to lesser extent even when water stressed than seen at ambient CO2 level. There was no change in chlorophyll fluorescence parameters (Fv/Fm, qP and qN) in wild or transformant plants nor water stress influenced the fluorescence. However, CO2 enrichment resulted in slight increase in qP and qN. Peroxidase activity was greater in wild type plants. Elevated level of CO2 further increased the level of Peroxidase activity in both wild as well as in transgenic plants. Plants experiencing water stress showed even higher level of peroxidase activity. Wild type plants showed higher level of pigments including xanthophylls compared to transformants. CO2 enrichment did not affect the pigment content appreciably. De-epoxidation of V to Z was greater in wild type plants than seen in transformants. Ambient level of CO2 resulted in better de-epoxidation of V to Z than CO2 enrichment in both type of plants. ABA content showed very little difference in wild type or transformant plants grown either at elevated or ambient level of CO2. However, plants subjected to water stress increased the level of ABA many fold in both transformants as well as in wild type plants irrespective of level of CO2 they were grown. The data presented here show that plants grown at elevated level of CO2 resulted in better growth and better water stress tolerance. C8 plants showed relatively more sensitivity to stress which could be result of substrate (RUBP) limitation, as a result of 95 percent less of PRK enzyme resulting in low carbon metabolism as well as over energization (reduction) of the photosynthetic electron transport chain.