Modern rice (Oryza sativa L.) varieties produce very high yields under high input and favorable environments. Limited work has been done to develop plant types suitable for less than optimal environments at moderate N management levels. The objectives of this study were: 1) to examine the relationship between the uptake of N and different leaf characteristics, sink size, and grain yield of three morphologically distinct rice varieties; 2) to examine the CO2 exchange rates (CER) of rice varieties in relation to light, N management, water use efficiencies (WUE), and N use efficiency (NUE): and 3) to determine characteristics of rice varieties that are associated with productive potentials under low N management levels. A greenhouse experiment was conducted in 1989 at Virginia Polytechnic Institute and State University, Blacksburg, VA and a field experiment was conducted in Sri Lanka at two locations in 1990. In the greenhouse experiment three varieties (a traditional, an intermediate, and a modem variety) were grown under four N management (rate and time) treatments: viz 1) a 0 N control, 2) 200 mg N kg⁻¹ of soil split into two applications, 3) 200 mg N kg⁻¹ of soil split into three applications, and 4) 400 mg N kg⁻¹> of soil split into three applications. Competitive use of fertilizer N for the development of either large leaf area or high leaf N content per unit leaf area (LNLA) varied with the rate and timing of N fertilizer application and rice variety. When N supply was limited and early N applications were restricted, the intermediate variety increased LNLA in the flag leaf with little increase in total leaf area of the plant. Specific leaf weight did not differ with N management except for the low value of the N control treatment. Increased N applications reduced stomatal density. Total stomatal number leaf⁻¹ varied little within cultivars indicating that N fertilization enables the leaf to increase leaf area and thereby disperses the fixed number of stomates. Carbon dioxide exchange rates were higher in the flag leaf than lower leaves and were directly correlated to LNLA. / Ph. D.