Environmental effects on photosynthesis, nitrogen-use efficiency, and metabolite pools in leaves of sun and shade plants

Effects of varying light intensity and nitrogen nutrition on photosynthetic physiology and biochemistry were examined in the sun plant Phaseolus vulgaris (common bean) and in the shade plant Alocasia macrorrhiza (Australian rainforest floor species). In both Phaseolus and Alocasia, the differing growth regimes produced large changes in photosynthetic capacity and composition of the photosynthetic apparatus. CO2-saturated rates of photosynthesis were linearly related to leaf nitrogen (N) content in both species but photosynthesis per unit leaf N was markedly higher for Phaseolus than for Alocasia. Photosynthetic capacity was also higher in Phaseolus per unit ribulose 1,5-bisphosphate (RuBP) carboxylase (RuBPCase) protein. The leaf content of RuBPCase was linearly dependent on leaf N content in the two species. However, the proportion of leaf N which was RuBPCase was greater in Phaseolus than in Alocasia and was more sensitive to growth conditions, ranging from 6% of leaf N at low light to 20% at high light. In Alocasia, this range was much less, 6 to 11%. However, chlorophyll content was much more sensitive to light intensity in Alocasia. Thus, the RuBPCase/chlorophyll ratio was quite responsive to N availability and light intensity in both species (but for different reasons), ranging from 6 grams per gram for Phaseolus and 2 grams per gram for Alocasia at high leaf N and 1.5 gram per gram for Phaseolus and 0.5 gram per gram for Alocasia at low leaf N. These large changes in the proportions of components of the photosynthetic apparatus had marked effects on the sensitivity of these species to photoinhibition. These environmental effects also caused changes in the absolute levels of metabolites of the photosynthetic carbon reduction cycle. Concentrations of RuBP and P-glycerate were approximately 2-fold higher in high light-grown than low light-grown Phaseolus and Alocasia when expressed on a leaf area basis. However, if metabolite pool sizes are expressed on the basis of the RuBPCase catalytic site concentration, then they were little affected by the marked changes in leaf makeup. There appears to be fundamental differences between these species in the mechanism of sun-shade adaptation and N partitioning in the photosynthetic apparatus that result in significant differences in the N-use efficiency of photosynthesis between Phaseolus and Alocasia but similar RuBPCase:substrate:product ratios despite these differences.