Influence of Paclobutrazol and Application Methods on High-temperature Stress Injury in Cucumber Seedlings

Paclobutrazol (PBZ) is a member of the triazole plant growth inhibitor group that is responsible for inducing tolerance to a number of biotic and abiotic stresses. An experiment was therefore conducted to examine whether the application of PBZ at various concentrations (0, 25, 50, and 75 mg l(-1)) by seed soaking or foliar spray would protect cucumber (Cucumis sativus) seedlings subjected to high-temperature stress. Thirty-five-day-old seedlings were exposed to heat stress at 40A degrees C for 4 h per day for 5 days. PBZ improved the majority of the physiological (for example, relative chlorophyll content and chlorophyll fluorescence ratio) and morphological parameters (for example, shoot and root fresh and dry weights) measured in cucumber seedlings subjected to high-temperature stress. PBZ ameliorated the injuries caused by heat stress by increasing leaf proline content and preventing an increase in leaf electrolyte leakage. PBZ was more effective at increasing the heat tolerance of cucumber seedlings when using the seed-soaking method rather than the foliar spray method. The best protection was obtained when seeds were soaked in 50 mg l(-1) PBZ.