Diurnal and nocturnl courses of needle xylem water potential and stomatal behaviour in young Norway spruce trees (Picea abies [L.] Karst.) at two soil moistures

Diurnal and nocturnal courses of needle xylem water potential and stomatal behaviour were measured on six young Picea abies trees together with environmental parameters on warm clear days. The plants were grown in containers (each container with two plants) at two soil moistures under field conditions and they were watered every second day. The water supply for the trees No. 1-2 and No. 3-6 corresponded to the precipitation amount of 624 mm and 150 mm per vegetation period, respectively. The daytime changes of needle water potential and stomatal opening show two culmination points with one pronounced midday depression. The higher the water stress the earlier the midday depression occurs and the greater the degree of stomatal closure during the depression (Fig. 1; 2). The daily maximum water potential reached 20 bar and more in both favorable and moderate watered trees. This indicates that lifting of planting stock should be avoided on warm clear days between 8 a.m. and 9 p.m. With predawn xylem water potentials up to 14 bar the plants were enabled to hold their stomata open. If the night time xylem water potential is less than 3 bar then the stomata remain open during the night (Fig. 3-5). © 1978 Verlag Paul Parey.