Characteristics of leaf photosynthesis and simulated individual carbon budget in Primula nutans under contrasting light and temperature conditions.

Primula nutans Georgi is widely distributed in hummock-and-hollow wetlands on the Qinghai-Tibetan Plateau. To assess the ecophysiology of this species in responding to microenvironments, we examined the photosynthetic characteristics and individual carbon gain of plants growing in different microsites from a hummock-and-hollow wetland on the Qinghai-Tibetan Plateau and under laboratory conditions. Plants from wetland hummock microsites showed significantly higher light-saturated photosynthetic CO(2) uptake (A (max)) than those from microsites in hollows at a controlled temperature of 15 degrees C in leaf chamber. Leaf dark respiration rate (R) was only significantly higher in plants from hummocks than hollows at the measuring temperature of 35 degrees C. Optimum temperature for A (max) was 15 degrees C for all plants in the field despite different microsites. In plants growing under laboratory conditions differing in light and temperature, both A (max) and R were significantly higher under higher growth light (photosynthetic photon flux density, PPFD: 800 or 400 micromol m(-2) s(-1)) than low light of 90 micromol m(-2) s(-1). No statistically significant differences in A (max) and R existed in plants differing in growing temperatures. Estimates derived from the photosynthetic parameters of field plants, and microsite environmental measures including PPFD, air temperature and soil temperature showed that the optimum mean daily temperature for net daily carbon gain was around 10 degrees C and the net daily carbon gain was largely limited under lower daily total PPFD. These results suggest that the differences in A (max) and R in P. nutans are strongly affected by growing light regimes but not by temperature regimes.