The photosynthetic capacity and leaf nitrogen concentration as related to light regime in shade leaves of a montane tropical forest tree, Tetrorchidium rubrivenium

1. In order to evaluate quantitatively the photosynthetic performance and related nitrogen allocation of understorey plants in relation to the light environment in which they grow, two approaches may be used: the optimization theory and the co-ordination theory. 2. The optimization theory assumes that there is an optimal leaf nitrogen concentration (N-op) at which photosynthesis per unit nitrogen is maximized. The co-ordination theory, on the other hand, hypothesizes that plants maintain the nitrogen concentration in the leaves such as to keep a balance between the processes which potentially limit photosynthesis, namely the Rubisco- and the electron-transport-limited rates of carboxylation. 3. These approaches were applied to analyse the photosynthesis-N relationship of trees (1.4-1.8 m in height) of the species Tetrorchidium rubrivenium (Euphorbiaceae) growing in the understorey of a tropical montane rain forest in northern Venezuela. Light-distribution patterns in the understorey and photosynthetic variables in relation to leaf N concentrations (N-L) and light were measured, The optimal N-L values (N-op) and the N-L at which the processes which limit photosynthesis are in balance (N-orday) were calculated. 4. Measured N-L values were considerable higher than the calculated values of N-op and N-crday. However, the N-L values were close to the values at which carbon gain per unit leaf area is maximal. 5. It was suggested that growth and therefore probably leaf-area production of Tetrorchidium in the understorey are limited by light and not by nitrogen availability. Under these conditions, maximization of photosynthesis per unit of leaf area might be more important than maximization of photosynthesis per unit of N-L.