Paclobutrazol-induced Drought Tolerance in Containerized English and Evergreen Oak

Abstract

The influence of paclobutrazol (PBZ) applied as a foliar spray and root drench on drought tolerance and recovery from drought, of containerized English and evergreen oak was investigated. PBZ treatment induced a suite of physiological adaptations that would allow both species to tolerate drought; more specifically, increased total leaf content of carotenoids (lutein:β-carotene:neoxanthin:α-carotene) and xanthophylls (zeaxanthin:antheraxanthin:violaxanthin), chlorophylls, proline, superoxide dismutase, and catalase. In addition, PBZ strengthened leaf membrane integrity and increased leaf photosynthetic efficiency and light-induced CO 2fixation before and at the cessation of the drought treatment. Irrespective of species, recovery rates of droughted trees treated with PBZ were 20% to 50% higher than non-PBZ-treated trees. In all cases, control trees (non-PBZ-treated) had the least capacity for recovery. Application of some of the PBZ treatments induced overregulation of newly emerged leaves. Results of this investigation indicate applications of the growth inhibitor PBZ either as a foliar spray or root drench induce a suite of physiological adaptations that confer a useful degree of drought tolerance and aid in the recovery from drought-induced damage. It is suggested that PBZ-induced protection of both English and evergreen oak from damage caused by drought stress is mediated by increased antioxidant enzyme and pigment activities.