Abstract
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.
Highlights
Temperature is one of the most important environmental factors that influence plant physiology, being the photosynthetic activity especially sensitive to stresses caused by unfavorable temperatures (BERRY &BJÖRKMAN, 1980; BJÖRKMAN et al, 1980; GEORGIEVA, 1999)
This study aimed to evaluate the response of the photosynthetic O2 evolution to increasing temperature in sweet orange [Citrus sinensis (L.) Osbeck] leaf discs under non-photorespiratory conditions, considering respiratory changes and the interaction of temperature with light intensity
The photosynthetic O2 evolution showed an increasing trend up to 40oC, when decreases were probably caused by heat stress (Fig. 1A)
Summary
BJÖRKMAN, 1980; BJÖRKMAN et al, 1980; GEORGIEVA, 1999) It is well-known that photosynthesis determines plant growth and development by incorporating atmospheric carbon into carbohydrates. The effects of both instantaneous and growth temperatures on photosynthetic. The importance of studying the response of citrus photosynthesis to temperature is increased due to (i) the climatic diversity among citrus growing areas in Brazil, especially in the São Paulo State where citrus orchards are distributed in areas with distinct environmental characteristics ranging from climates Aw (in Western and Northern regions, characterized by dry winter and wet summer) to Cfa (in Central region, characterized by a dry season and mean air temperature higher than 22oC in the hottest month) according to the Köppen classification (PEREIRA et al, 2002; ROLIM et al, 2005); and (ii) the current biotic pressure caused by the attack of some pests and diseases and their relationship with environmental conditions (RIBEIRO et al, 2004), leading to differential productivity response of citrus orchards depending on geographic region and climate
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