INCREASED HEAT EMISSION AND ITS RELATIONSHIP TO THE XANTHOPHYLL CYCLE IN PEA LEAVES EXPOSED TO STRONG LIGHT STRESS

In vivo thermal energy dissipation was photoacoustically monitored in pea leaves before in pea leaves before and after strong light treatment. Concomitant with the conversion of the carotenoid violaxanthin into zeaxanthin, a marked increase in the heat emission signal was observed in the light-stressed leaves. However, when the xanthophyll cycle was blocked by dithiothreitol, the photothermal signal still increased, indicating that there was no causal relationship between these two phenomena. Increased heat emission was shown to result from pigment uncoupling, which caused the inhibition of the energy transfer from carotenoids to chlorophylls. It was also observed that the maintenance of a very low zeaxathin level by dithiothreitol led to an increase in both the oxygen evolution and the photothermal components of the photoacoustic signal in control leaves and to a strong increase in lipid degradation in light-stressed leaves. These results may suggest that a possible function of the xanthophyll cycle is to provide an accessory pigment (violaxanthin) in weak light and to furnish the lipid matrix of the thylakoid membranes with an efficient photoprotector (zeaxanthin) in strong light.