Photoinhibition-induced reduction in photosynthesis is alleviated by abscisic acid, cytokinin and brassinosteroid in detached tomato leaves

Detached leaves of tomato (Lycopersicon esculentum Mill.) experienced photoinhibition associated with sharp reductions in net photosynthetic rate (Pn), quantum efficiency of PSII (I broken vertical bar(PSII)) and photochemical quenching (qP) even though they were exposed to mild light intensity (400 mu mol m(-2) s(-1) PPFD) at 28A degrees C. Photoinhibition and the reduction in Pn, I broken vertical bar(PSII) and qP, however, were significantly alleviated by 1 mg l(-1) ABA, 0.1 mg l(-1) N-(2-chloro-4-pyridyl)-N'-phenylurea (CPPU) and 0.01 mg l(-1) 24-epibrassinolide (EBR). Higher concentrations, however, reduced the effects or even exacerbated the occurrence of photoinhibition. Superoxide dismutase and ascorbate peroxidase activity in leaves increased with the increases in ABA concentration within 1-100 mg l(-1), CPPU concentration within 0.1-10 mg l(-1) and EBR concentration within 0.01-1.0 mg l(-1). Catalase and guaiacol peroxidase activity also increased with the increase in EBR concentration but CPPU and ABA treatments at higher concentrations caused a decrease. Malondialdehyde (MDA) content decreased with the increase in CPPU concentration. ABA and EBR, however, decreased MDA concentration only at 1 and 0.01 mg l(-1), respectively. In conclusion, detached leaves had increased sensitivity to PSII photoinhibition. Photoinhibition-induced decrease in photosynthesis, however, was significantly alleviated by EBR, CPPU and ABA at a proper concentration.