Effects of nitrogen nutrition on responses of rice seedlings to carbon dioxide

Global atmospheric CO2 concentration is increasing, likely increasing the productivity of crops as higher CO2 enhances plant photosynthesis. Responsiveness to nitrogen supply is an essential trait of modem rice cultivars, and may play a role in the response of rice cultivars to CO2. To determine the relationship between these two important production variables on young rice plants, seedlings of Oryza sativa L. 'IR72' and 'KDML 105' were exposed for 28 days after sowing to CO2 levels of 373, 545, 723 and 895 mu molmoI(-1), and 3 levels of nitrogen fertility. There were large increases in leaf CO2 assimilation and biomass production whereas leaf nitrogen concentration dropped sharply as CO2 increased from 373 to 545 mu mol mol(-l), with little additional effect from higher levels of CO2. Root and shoot biomass, and tiller number per plant increased with increasing nitrogen supply and with increasing atmospheric CO2 concentration. The biomass response to CO2 was slight at low N supply, but became dramatically greater as the N supply increased. Mean root/shoot ratio increased slightly as atmospheric CO2 concentration increased, but decreased sharply as nitrogen fertility rate increased. These results suggest that careful attention to nitrogen fertilization will be necessary for rice fanning to get the full benefit of any future increases in atmospheric CO2. (C) 1999 Published by Elsevier Science B.V. All rights reserved.