THE DIURNAL CHANGE OF LEAF WATER USE EFFICIENCY IN SIX FRUIT TREES

The diurnal changes of leaf water use efficiency (WUE) were studied by LCA-4 photosynthesis and transpiration measurements in six fruit tree species: sweet cherry (Prunus. Avium), Chinese cherry (P. pseudocerasus), apricot (P. armeniaca), peach (P. persica), plum (P. salisina) and apple (Malus pumila Mill). The diurnal changes of WUE in summer and autumn showed the same tendencies. WUE was the highest at 7:00-8:00 am, the lowest at noon, and recovered a little by 16:00-17:00 pm. WUE was higher in autumn than that in summer at the same time of the day. The pattern diurnal changes of WUE in apple showed a decrease during the morning and an increase during the afternoon in the two seasons studied. The WUE of peach changed gradually in autumn. Apricot WUE in summer followed a wavy decline but declined smoothly in autumn. The diurnal change of Chinese cherry WUE had two peaks in summer at 7:00 and 15:00 respectively, but in autumn it has only one peak at 7:00, declined to a low level at 9:00 and remained at the same level later in the day. WUE of sweet cherry in the daytime followed the same pattern as for apple in the two seasons. Plum WUE was highest at 7:00, lowest at 11:00 and recovered a little at 17:00 in summer, and was highest at 7:00 and 17:00, and lowest at noon in autumn. The leaf WUE in two shoots with or without fruit declined from their maxima at 7:00-8:00 and recovered at 14:00, but the WUE of the shoot without fruit was always higher than that of the shoot with fruit at the same time of day. Path analysis showed that leaf temperature, among external factors, and net photosynthetic rate, among internal factors, were the most important variables directly affecting WUE. Vapor pressure deficit (VPD) indirectly affected WUE through leaf temperature, air humidity and net photosynthetic rate.