Elevated CO₂ and Nitrogen Interaction in Photosynthesis and Productivity of Modern and Local Rice (<i>Oryza sativa</i> L.) Cultivars

Abstract

Photosynthetic rate and productivity of modern and local rice (Oryza sativa L.) cultivars were compared at different levels of nitrogen and elevated CO₂ concentrations. BRRIdhan 39 and Shakkorkhora representing modern and local cultivars, respectively were tested under the conditions of three nitrogen levels and three CO₂ concentrations. Nitrogen levels were (i) control (0 kg N ha^-1), (ii) optimum (90 kg N ha^-1 for modern and 60 kg N ha^-1 for local cultivar) and (iii) supra optimum (135 kg N ha^-1  for modern and 90 kg N ha^-1 for local cultivar). Conversely, the three CO₂ concentrations were (i) open top chamber (OTC) with elevated CO₂ (570 ± 50 ppm), (ii) OTC with ambient CO₂ (~360 ppm) and (iii) open field. It was observed that the elevated CO₂ independently increased the photosynthetic rates in both cultivars. While in interaction, elevated CO₂ and supra optimal nitrogen increased the photosynthetic rate in modern rice cultivar only. Productivity of two rice cultivars corresponded to that of photosynthetic rates. The highest yield (50.82 g plant^-1) was found in modern cultivar at supra optimal nitrogen and elevated CO₂ treatments. In contrary, the lowest yield (15.09 g plant^-1) was observed in local cultivar at zero kg nitrogen treatment. However, productivity of local rice cultivar was substantially increased either by applied optimum nitrogen or by CO₂ enrichment.