Increased atmospheric [CO2] is likely to affect photosynthesis, plant growth, and yield potential of plants. Mustard (Brassica juncea L.) is an important oil seed crop that is widely grown in India. Therefore, the impact of elevated [CO2] (585 μmol mol(-1)) on pigment and protein content, chlorophyll a fluorescence, photosynthetic electron transport reactions, CO2 assimilation, biomass production, and seed yield potential was measured in B. juncea cv Pusa Bold, grown inside free air carbon dioxide enrichment (FACE) rings installed on the campus of Jawaharlal Nehru University, New Delhi, India. Plants were grown for three consecutive winter seasons (2010-2013), in ambient (385 μmol mol(-1)) or elevated [CO2], in field conditions. Elevated [CO2] had no significant effect on the minimal chlorophyll fluorescence (F 0), while the quantum efficiency of Photosystem II, measured as variable fluorescence (F v = F m-F 0) to maximum fluoresence (F m), increased by 3 %. Electron transport rate, photosystem I, photosystem II, and whole chain electron transport rates increased by 8 % in elevated [CO2]. However, the net photosynthesis rate increased by ≈50 % in three growing seasons under elevated [CO2] condition. The stomatal conductance and transpiration rate decreased resulting in higher photosynthetic water use efficiency. The photosynthesizing surface, i.e., leaf area index substantially increased leading to higher biomass and seed yield under elevated [CO2] condition. Acclimatory downregulation of photosynthesis and plant productivity was not observed in three consecutive growing years suggesting that in the absence of nutrient limitation, B. juncea is highly responsive to elevated CO2 whose yield potential shall increase in changing climatic conditions.

Summary An important method to improve photosynthesis in C3 crops, such as rice and wheat, is to transfer efficient C4 characters to them. Here, cytosolic carbonic anhydrase (CA: βCA3) of the C4 Flaveria bidentis (Fb) was overexpressed under the control of 35S promoter in Arabidopsis thaliana, a C3 plant, to enhance its photosynthetic efficiency. Overexpression of CA resulted in a better supply of the substrate HCO3‐ for the endogenous phosphoenolpyruvate carboxylase in the cytosol of the overexpressers, and increased its activity for generating malate that feeds into the tricarboxylic acid cycle. This provided additional carbon skeleton for increased synthesis of amino acids aspartate, asparagine, glutamate, and glutamine. Increased amino acids contributed to higher protein content in the transgenics. Furthermore, expression of FbβCA3 in Arabidopsis led to a better growth due to expression of several genes leading to higher chlorophyll content, electron transport, and photosynthetic carbon assimilation in the transformants. Enhanced CO2 assimilation resulted in increased sugar and starch content, and plant dry weight. In addition, transgenic plants had lower stomatal conductance, reduced transpiration rate, and higher water‐use efficiency. These results, taken together, show that expression of C4 CA in the cytosol of a C3 plant can indeed improve its photosynthetic capacity with enhanced water‐use efficiency.

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One of the important ways to improve photosynthetic capacity in C3 crops, such as rice and wheat, is to transfer efficient C4 characters to them. Here, cytosolic carbonic anhydrase (βCA3) of the C4 Flaveria bidentis (Fb), having low Km for CO2, was overexpressed under the control of 35S promoter in Arabidopsis thaliana, a C3 plant, to enhance its photosynthetic efficiency. Overexpression of CA resulted in higher [HCO3-] in the cytosol of the overexpressors, and increased endogenous phosphoenolpyruvate carboxylase activity generating oxaloacetate that feeds into the tricarboxylic acid cycle. This provided more carbon skeleton for increased synthesis of amino acids and proteins. Further, transgenic expression of FbβCA3 in Arabidopsis led to pleiotropic expression of several genes/proteins involved in chlorophyll biosynthesis and photosynthesis leading to higher chlorophyll content and photosynthetic capacity in the transformants. Due to the presence of higher CO2 in the chloroplast, pleiotropic effect overexpressors had enhanced CO2 assimilation, starch content, and plant dry weight. In addition, transgenic plants had lower stomatal conductance, reduced transpiration rate and higher water use efficiency. These results, taken together, show that expression of C4 CA in the cytosol of a C3 plant can indeed improve its photosynthetic capacity with enhanced water use efficiency.