Pretreatment with 5-aminolevulinic acid mitigates heat stress of cucumber leaves

Cucumber seedlings were pretreated with 3 mu M 5-aminolevulinic acid (ALA) followed by cultivation at normal (25/18 A degrees C) or high (42/38 A degrees C) day/night temperature to investigate the protective effects of ALA on heat stress in plants. Heat elevated the contents of malondiadehyde (MDA), superoxide radical (O-2 (.-)) and hydrogen peroxide (H2O2) in leaves of all plants but less in ALA-pretreated plants. Heat treatment resulted in higher antioxidant enzyme activities and proline and soluble sugar contents and weaker growth inhibition in ALA-pretreated plants than in those treated with heat alone. These results indicate that ALA pretreatment increased the tolerance of seedlings to heat stress.