The relationship between maximum in situ rates of CO2 gas exchange and leaf carbon budgets in tropical gap plants of the genus Macaranga

Diurnal courses of the net CO2 gas exchange of leaves of five species of Macaranga Thouars (Euphorbiaceae) were determined under natural tropical conditions in Peninsular Malaysia to investigate the relationship between the maximum rates of net photosynthesis (NPmax) and diurnal leaf carbon budgets (NPday) in these gap plants. The observed rates of NPmax ranged from 0.8-21.2 mu mol CO2 m(-2) s(-1), while NPday was between -22 mmol m(-2) 12 h(-1) and 370 mmol m(-2) 12 h(-1). Similar to earlier results with leaves from the outer forest canopy, a rather tight correlation between NPmax and NPday was found, but the slope of the regression was significantly less steep. The relationship between regression slope and both the duration of direct radiation (PFD) and the intensity of background PFD are discussed. I conclude, that in leaves of gap plants - similar to those of canopy plants reasonable estimates of leaf carbon budgets can be obtained from spot measurements of the maximum rate of net CO2 exchange.