Rice (Oryza sativa L. cv. IR72) was grown in the tropics at ambient (345 μL L-1) or twice ambient (elevated, 700 μL L-1) CO2, concentration at three levels of supplemental nitrogen (N) (no additional N (N0), 90 kg ha-1 (N1) and 200 kg ha-1 (N2)) in open-top chambers under irrigated field conditions from seeding until flowering. The primary objective of the study was to determine if N supply alters the sensitivity of growth and photosynthesis of field-grown rice to enriched CO2. A second objective was to determine the influence of elevated CO2 on N uptake and tissue concentrations. Although photosynthesis was initially stimulated at the leaf and canopy level with elevated CO2 regardless of supplemental N supply, with time the photosynthetic response became highly dependent on the level of supplemental N, increasing proportionally as N availability increased. Similarly, a synergistic effect was noted between CO2 and N with respect to above-ground biomass with no effect of elevated CO2 observed for the No treatment. Most of the increase in above-ground biomass with increasing CO2 and N was associated with increased tiller and, to a lesser extent, root production. The concentration of above-ground N decreased at elevated CO2 regardless of N treatment; however, total above-ground N did not change for the N1 and N2 treatments because of the greater amount of biomass associated with elevated CO2. For rice, the photosynthetic and growth response to elevated CO2 may be highly dependent on the supply of N. If additional CO2 is given and N is not available, lack of sinks for excess carbon (e.g. tillers) may limit the photosynthetic and growth response.

The measurement of crop water stress under field conditions is fraught with technical and logistical problems. Although leaf water potential has become a standard measurement it has spatial and temporal sampling limitations. In the current study, a rice (Oryza sativa L.) crop was used to compare eight measurements indicating crop water status, namely leaf water potential ψleaf), stomatal resistance (ra, transpiration rate (T), net photosynthesis rate (PN), canopy temperature (Tc), canopy minus air temperature (Tc — Ta), crop water stress index (CWSI), and visual leaf rolling score (LRS). The eight measurements were compared across seven water stress levels created by a line source sprinkler irrigation system. The methods were compared for accuracy, sensitivity, sampling time, and the destructive‐disruptive nature of the sampling procedure. Accuracy was estimated by comparison with ψleaf and by the interaction between water stress level and time of day. All methods except PN were significantly correlated with ψleaf at the 1% level. However, Tc, Tc — Ta and CWSI showed less interaction between water stress level and time of day when total variance was partitioned into its relative components; water stress level, time, and the interaction between water stress level and time of day. All methods, with the exception of PN, were equally sensitive to the water stress gradient when “goodness of fit” response functions across the water stress gradient were compared. The visual LRS was the most rapid while the canopy temperature‐based measurements, Tc, Tc — Ta. and CWSI, were nearly three times faster than the gas exchange techniques and about two and a half times faster than ψleaf. Leaf water potential sampling was both destructive and disruptive to the crop plant community. The gas exchange methods were nondestructive but repetitive sampling was disruptive. Only the remotely sensed Tc Tc — Ta, CWSI and LRS were neither destructive nor disruptive to the crop. The interpretive value of various measurements is discussed. The CWSI was found to be highly correlated with mean daily PN and represents a significant advancement in crop level detection and measurement of water stress.