Response of Rice to p(CO2) Enrichment: The Relationship Between Photosynthesis and Nitrogen Metabolism

Rice (Oryza sativa L.) is grown in many different agroecological zones and it assimilates CO2 directly through C-3 photosynthesis. At current CO2 partial pressure [p(CO2)], rice is not photosynthetically saturated but immediately after exposure to 100 p(CO2), photosynthetic rates are increased by 48 percent on an average. However, this increase is not sustained during prolonged exposure and is associated with reduced amounts of ribulose-1-5-bisphospatc carboxylase/oxygenase (Rubisco) content and its maximum potential activity (V-cmax). Rubisco content and V-cmax were reduced by 33 and 22%, respectively, at elevated p(CO2) and this has an additive effect on leaf photosynthesis. Activation state of the Rubisco is also decreased by elevated p(CO2) but no deactivation of Rubisco occurs in rbcS antisense rice with 40% wild type Rubisco suggesting that deactivation is an optimize response. Large amounts of Rubisco (about 80-90%) is synthesized prior to full expansion of leaf when sugar accumulation is minimal. After the leaf has fully expanded, very little Rubisco (about 10-20%) is synthesized and a large amount of sugar accumulates during this period. Therefore, it is unlikely that downregulation of Rubisco at elevated p(CO2) is caused by decreasing the mRNA transcripts for Rubisco small subunit (rbcS) and Rubisco large subunit (rbcL) through soluble sugars. It is more likely that elevated p(CO2) mediated decline in Rubisco is due to rapid degradation of protein and lower nitrogen (N) allocation to Rubisco due to reduced N afflux. Leaf N content was reduced by 16% in plants grown at elevated p(CO2) though leaf N to Rubisco ratio remained consistently unchanged regardless of growth p(CO2) suggesting that N assimilation, distribution and remobilization at the whole plant level is tightly controlled at elevated p(CO2). Further, changes in N and C metabolisms at elevated p(CO2) are likely to reduce the critical N concentration by 30% at elevated p(CO2). (C) 2005 by The Haworth Press, Inc. All rights reserved.