© 2003 Heron Publishing—Victoria, Canada
Moderate shade can increase net gas exchange and reduce photoinhibition in citrus leaves
John L. Jifon (1, 2) and James P. Syvertsen (1, 3)
1. Citrus Research and Education Center, University of Florida, IFAS, 700 Experiment Station Road, Lake Alfred, FL 33850, USA / 2. Texas A&M University, 2415 E. Highway 83, Weslaco, TX 78596, USA / 3. Author to whom correspondence should be addressed (jmsn@lal.ufl.edu) / Received May 16, 2002; accepted July 15, 2002; published online January 2, 2003
Summary
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 CO2 assimilation rates (ACO2). 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 (gs) and ACO2 were observed in both sunlit and shaded leaves, shaded leaves had lower leaf-to-air vapor pressure differences (D) along with higher gs, ACO2 and leaf water-use efficiency than sunlit leaves. Estimated stomatal limitation to ACO2 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 ACO2 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 ACO2 of citrus leaves during radiation and high temperature stress.
Keywords:
chlorophyll fluorescence, grapefruit, leaf temperature, orange, photosynthesis, stomatal conductance, vapor pressure deficit.
