Vol. 82 No: 3

Title:
Reduced leaf photosynthesis at midday in citrus leaves growing under field or screenhouse conditions

Authors:
LI-MING HU, REN-XUE XIA, ZHI-YAN XIAO, REN-HUA HUANG, MEI-LIAN TAN, MING-YUAN WANG and QIANG-SHENG WU

pp: 387-392

Abstract:
Leaves on mature trees of Satsuma mandarin (Citrus unshiu Marc.) growing in full sun under field conditions (sun) or under shade in screenhouse conditions (shade) exhibited midday depression in their net photosynthetic rate (P_n) and stomatal conductance (gs). As the air temperature (T_a) increased from 32°C to 38°C, the relative humidity (RH) decreased from 71% to 42% and the vapour pressure deficit (VPD) increased from 235 Pa to 587 Pa. Decreased transpiration rates (E) and gs at midday were interpreted as a combined “feedforward” effect from the increased VPD. Carboxylation efficiency (CE) was highest at midday in both sunlit and shaded leaves. The apparent quantum yield (AQY) varied little during diurnal variations in shaded leaves, but the AQY in sunlit leaves showed lower values at midday than at other times. Photo-inhibition occurred from high photosynthetically active radiation (PAR) in full sun. The increased CO₂ compensation point (Γ) for both growth conditions also contributed to the reduction in P_n at noon. Leaf intercellular CO₂ concentrations (C_I) and stomatal limitation (L_s) were constant during the daytime in sunlit or shaded leaves, indicating that non-stomatal factors play a major role in regulating the P_n of citrus leaves during radiation and high temperature stress.

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