Moderate shade can increase net gas exchange and reduce photoinhibition in citrus leaves

Daily variations in net gas exchange, chlorophyll a fluorescence and water relations of mature, sun-acclimated grapefruit (Citrus paradisi Macfady.) and orange (Citrus sinensis L. Osbeck) leaves were determined in tree canopies either shaded with 50% shade screens or left unshaded (sunlit). Mean daily maximum photosynthetic photon flux density (PPFD) under shade varied from 500 to 700 mumol m(-2) s(-1) and was sufficient to achieve maximum net CO2 assimilation rates (A(CO2)). Responses of grapefruit and orange leaves to shading were remarkably similar. At midday, on bright clear days, the temperatures of sunlit leaves were 2-6 degreesC above air temperature and 1-4 degreesC above the temperatures of shaded leaves. Although midday depressions of stomatal conductance (g(s)) and A(CO2) were observed in both sunlit and shaded leaves, shaded leaves had lower leaf-to-air vapor pressure differences (D) along with higher g(s), A(CO2) and leaf water-use efficiency than sunlit leaves. Estimated stomatal limitation to A(CO2) was generally less than 25% and did not differ between shaded and sunlit leaves. Leaf intercellular CO2 partial pressure was not altered by shade treatment and did not change substantially with increasing D. Radiation and high temperature stress-induced non-stomatal limitation to A (CO2) in sunlit leaves was greater than 40%. Reversible photoinhibition of photosystern II efficiency was more pronounced in sunlit than in shaded leaves. Thus, non-stomatal factors play a major role in regulating A(CO2) of citrus leaves during radiation and high temperature stress.