Acclimation of nitrogen uptake capacity of rice to elevated atmospheric CO2 concentration

Nitrogen (N) is a major factor affecting yield gain of crops under elevated atmospheric carbon dioxide concentrations [CO2]. It is well established that elevated [CO2] increases root mass, but there are inconsistent reports on the effects on N uptake capacity per root mass. In the present study, it was hypothesized that the responses of N uptake capacity would change with the duration of exposure to elevated [CO2]. The hypothesis was tested by measuring N uptake capacity in rice plants exposed to long-term and short-term [CO2] treatments at different growth stages in plants grown under non-limiting N conditions in hydroponic culture. Seasonal changes in photosynthesis rate and transpiration rate were also measured. In the long-term [CO2] study, leaf photosynthetic responses to intercellular CO2 concentration (Ci) were not affected by elevated [CO2] before the heading stage, but the initial slope in this response was decreased by elevated [CO2] at the grain-filling stage. Nitrate and ammonium uptake capacities per root dry weight were not affected by elevated [CO2] at panicle initiation, but thereafter they were reduced by elevated [CO2] by 31-41 % at the full heading and mid-ripening growth stages. In the short-term study (24 h exposures), elevated [CO2] enhanced nitrate and ammonium uptake capacities at the early vegetative growth stage, but elevated [CO2] decreased the uptake capacities at the mid-reproductive stage. This study showed that N uptake capacity was downregulated under long-term exposure to elevated [CO2] and its response to elevated [CO2] varied greatly with growth stage.