MORPHOLOGICAL AND PHYSIOLOGICAL RESPONSES OF YOUNG APPLE TREES TO FOLIAR APPLIED UNICONAZOLE

Abstract

Uniconazole (UCZ) is a gibberellin biosynthesis inhibitor reported to be more effective than paclobutrazol at similar concentrations. It was labeled in Australia to improve avocado yield, received a supplemental label in the USA for vegetable trans-plants, and was reported to improve Arabidopsis drought tolerance. In this experiment one-year-old 'Golden Delicious'/MM. 106 (Malus ×domestica) trees were pruned to 15 cm above the graft union, one shoot selected and trained as a leader. Trees were sprayed with 0, 100, 200, 300, 400, or 800 mg/L UCZ on 15 June (≈ 12 new leaves formed). The topmost fully expanded leaf was tagged at treatment and referred to as mature leaf. Net photosynthesis (Pn), stomatal conductance (gs), and transpiration (E) of mature leaves and recently fully expanded leaves were measured at 0, 21, 52, and 102 days after treatment (DAT). The Pn of young leaves on UCZ treated trees were higher at 21 and 102 (P≤0.05) DAT than control leaves. However, the Pn of old leaves were numerically lower than controls at 21 DAT but higher at 52 and 102 DAT. New and old leaves of 300 mg/L treated trees had higher numerically maximum daily Pn (in the diurnal cycle) than control leaves and remained higher throughout the afternoon. In a separate experiment, trees were saturated for three days, and then water withheld from 10 July until 3 Aug. New leaves on trees treated with 300 mg/L had higher Pn than controls at four, eight, and 14 days after imposed water stress.