TEMPERATURE RESPONSE OF PHOTOSYNTHESIS AND ITS INTERACTION WITH LIGHT INTENSITY IN SWEET ORANGE LEAF DISCS UNDER NON-PHOTORESPIRATORY CONDITION

This study aimed to evaluate the response of photosynthesis (A), given by photosynthetic O-2 evolution, to increasing temperature from 25 to 50 degrees C in sweet orange (Citrus sinensis (L.) Osbeck) leaf discs under non-photorespiring conditions. In order to evaluate the response of gross photosynthesis to temperature and the balance between photosynthetic and respiratory activities, respiration (R-d) rates were also measured, i.e. the O-2 uptake in each temperature. In addition, light response curves of photosynthesis were performed by varying the photosynthetic photon flux density (PPFD) from 0 to 1160 mu mol m(-2) s(-1) at 25 and 40 degrees C. The highest A values were observed at 35 and 40 degrees C, whereas the highest R-d values were noticed at 50 degrees C. A higher relationship A/R-d was found at 30 and 35 degrees C, suggesting an optimum temperature of 35 C when considering the balance between photosynthesis and respiration under non-photorespiring condition. Overall, heat effects on plant metabolism were more evident when evaluating the relationship A/R-d. In light response curves, higher A values were also found at 40 degrees C under PPFD higher than 300 mol mu m-2 s(-1). Light saturation point of photosynthesis was increased at 40 degrees C, without significant change of quantum efficiency under low PPFD. Respiration was also enhanced at 40 degrees C, and as a consequence, the light compensation point increased. The better photosynthetic performance at 35-40 degrees C was supported by higher photochemical efficiency in both light and temperature response curves. The temperature-dependence of photosynthesis was affected by growth temperature, i.e. a high air temperature during plant growth is a probable factor leading to a higher photosynthetic tolerance to heat stress.