Abscisic acid induced protection against photoinhibition of PSII correlates with enhanced activity of the xanthophyll cycle.

The exogenous application of abscisic acid (ABA) to barely seedlings resulted in partial protection of the PSII photochemistry against photoinhibition at low temperature, the effect being most pronounced at 10(-5) M ABA. This was accompanied by higher photochemical quenching (qP) in ABA-treated leaves. A considerable increase (122%) in the amount of total carotenoids and xanthophylls (antheraxanthin, violaxanthin and zeaxanthin) was also found in the seedlings subjected to ABA. The activity of the xanthophyll cycle measured by the epoxidation state of xanthophylls under high-light treatment was higher in ABA-treated plants compared with the control. This corresponds to a higher value (0.411) of non-photochemical quenching (qNP) observed in ABA-treated than in control (0.306) leaves.