Temperature response of photosynthesis and its interaction with light intensity in sweet orange leaf discs under non-photorespiratory condition

摘要

This study aimed to evaluate the response of photosynthesis (A), given by photosynthetic O2 evolution, to increasing temperature from 25 to 50oC 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 (Rd) rates were also measured, i.e. the O2 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 μmol m-2 s-1 at 25 and 40oC. The highest A values were observed at 35 and 40oC, whereas the highest Rd values were noticed at 50oC. A higher relationship A/Rd was found at 30 and 35oC, suggesting an optimum temperature of 35oC 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/Rd. In light response curves, higher A values were also found at 40oC under PPFD higher than 300 μmol m-2 s-1. Light saturation point of photosynthesis was increased at 40oC, without significant change of quantum efficiency under low PPFD. Respiration was also enhanced at 40oC, and as a consequence, the light compensation point increased. The better photosynthetic performance at 35-40oC 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.