ABSTRACIrEffects of varying light intensity and nitrogen nutrition on photosynthetic physiology and biochemistry were examined in the sun plant Phasolus vidgaris (common bean) and in the shade plant Alocasia macrorrhiza (Austalian rainforest floor species). In both Phaseolus and Alocauia, the differing growth regimes produced lrge changes in photosynthetic capacity and composition ofthe photosynthetic apparatus. COr saturated rates of photosynthesis were linearly related to leaf nitrogen (N) terized by significant alterations in the relative distribution of resources among the component parts of the photosynthetic apparatus (5). Light intensity also has a particularly dramatic effect on leaf N3 content, often a limiting resource for plant growth (14). A majority of this N in C3 plants is required for proteins involved in photosynthesis (10,25) and photosynthetic capacity is known to be generally proportional to leaf N content (6). A significant portion of any change in leaf N content which may result from differences in light intensity during growth is the result of a change in the concentration of RuBPCase3 (5), as RuBPCase represents approximately 20% of total N in leaves of well fertilized C3 sun plants (12,21). Changes in the activity of this enzyme are extremely well correlated with changes in photosynthetic capacity which occur with changes in the light intensity of growth (5), but it is unclear whether or not these changes represent an alteration in the relative proportion of total leaf N which has been allocated to this protein. Furthermore, it is important to understand how such changes affect the N-use efficiency of photosynthesis (8).We hypothesize that ifcertain components ofthe leafN budget predominate in the control of sun/shade acclimation, then redistribution of N among such key protein components of the photosynthetic apparatus may occur. Because large amounts of RuBPCase are required to support observed rates of photosynthesis in C3 plants and it is often rate-limiting for photosynthesis, comparative studies of environmental effects on N-use efficiency of photosynthesis in sun and shade plants should begin by assessing the relative importance of RuBPCase in the N budget of leaves grown under different environmental conditions. We have examined the effect of varying light intensity during growth, in combination with varying N availability, on photosynthetic performance, leaf N, RuBPCase, Chl, and metabolite pools of leaves. We have chosen to study two species, one of which is considered to be a 'sun plant' (Phaseolus vulgaris, common