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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 μmol m −2  s −1  and was sufficient to achieve maximum net CO 2  assimilation rates ( A  CO 2  ). 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 °C above air temperature and 1–4 °C above the temperatures of shaded leaves. Although midday depressions of stomatal conductance ( g  s ) and  A  CO 2   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  CO 2   and leaf water-use efficiency than sunlit leaves. Estimated stomatal limitation to  A  CO 2   was generally less than 25% and did not differ between shaded and sunlit leaves. Leaf intercellular CO 2  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  CO 2   in sunlit leaves was greater than 40%. Reversible photoinhibition of photosystem II efficiency was more pronounced in sunlit than in shaded leaves. Thus, non-stomatal factors play a major role in regulating  A  CO 2   of citrus leaves during radiation and high temperature stress.

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