Abstract:Effects of water stress on the diurnal changes of the net photosynthetic rate (Pn) in different seasons were investigated in 16-year-old trees of Litchi chinensis Sonn. cv. Nuomizi. In wet treatment the diurnal curves of Pn exhibited double peaks in autumn and summer, and a single peak in winter and spring, whereas in drought treatment they exhibited two peaks in winter and single peak in the other seasons. The maximum Pn or the first peak during daytime was generally the same in both treatments, which occurred at 9:00 in winter, 10:00 in spring and summer, 12:00 in autumn. The maximum Pn in wet- and drought-treated trees ranged from 6.25 μmol m-2s-1 to 8.14 μmol m-2s-1 and from 4.35 μmol m-2s-1 to 6.56 μmol m-2s-1, respectively. Moderate water stress could largely inhibit the photosynthesis of litchi tree, and the amount of daily Pn was decreased by 18.6%, 21.3%, 34.1% and 34.7% in winter, summer, spring and autumn, respectively. It was suggested that moderate water stress leads to Pn decrease by affecting stomatal conductance (Gs).