Night-time water use of apple trees:: Results of sap flow rate measurements

This study was destined for investigating the contribution of night-time water use to the total daily water demand in apple trees. Sap flow velocity was measured in the trunks of five year old apple trees cv. 'Florina'/M.26 by a thermal dissipation (Granier) method from 20(th) of May to the end of September in 1998. The mean night-time sap flow velocity (night = light intensity < 2000 lux) varied markedly: it was between 6 to 83 % in the percentage of mean daytime sap flow velocity. The mean night time sap flow velocity was highly correlated with vapour pressure deficit of air and the length of the period while leaves were wetted by dew (r=0.52 and -0.59, respectively, n=82, p<0.001). On rainy nights these relationships were invalid, but the sap flow rate was relatively high, from 15 to 79 % compared to daytime sap flow rate. From these results is concluded that night-time sap movement reflects both the evaporative water loss and refilling depending on environmental conditions. On nights without dew or rain the water use of trees is considerable from point of view of water demand of tree and usefullness of some irrigation scheduling method, too.