Shoot growth in willow (Salix viminalis) in relation to abscisic acid, plant water status and photoperiod

The decline in growth rate of field‐grown willow trees in Aberystwyth, U.K., began in mid‐summer and was followed by the senescence and abortion of shoot tips. These events were not triggered by a decline in the length of the natural photoperiod but were coincident with low leaf water potentials that developed in summer. Transient increases in the abscisic acid (ABA) content of shoot tips were observed during the period of declining water potential. These increases were roughly coincident with the onset of growth decline and preceded abortion and senescence of shoot tips. Under controlled conditions growth of both rooted cuttings and potted plants was arrested by short days (8 h) without any increase in tip ABA levels. Growth of rooted cuttings under long days (16 h) was inhibited by exogenous ABA; this inhibition could be relieved by addition of gibberellic acid (GA3) to the nutrient solution. Growth of aseptically cultured apices was also inhibited by ABA; this inhibition was relieved by joint application of GA9 and zeatin riboside.