Seasonal Variation in the Hormone Content of Willow: II. Effect of Photoperiod on Growth and Abscisic Acid Content of Trees under Field Conditions.

Levels of abscisic acid were followed in the xylem sap, mature leaves, and apices of field-grown willow (Salix viminalis L.) during the summer months, under natural and artificially extended photoperiods. Although the long day treatment prevented the general onset of dormancy, the plants grown under natural daylengths showed lower concentration of abscisic acid than those kept under long days.