SEASONAL-CHANGES IN XANTHOPHYLL COMPOSITION AND PHOTOSYNTHESIS OF CORK OAK (QUERCUS-SUBER L.) LEAVES UNDER MEDITERRANEAN CLIMATE

Seasonal changes in pigment composition of sun and shade leaves of cor k oak (Quercus suber) were studied under field conditions in Portugal, Expanding leaves showed a high concentration of xanthophyll cycle com ponents, violaxanthin, antheraxanthin and zeaxanthin. The pool of viol axanthin plus antheraxanthin plus zeaxanthin (V+A+Z) varied greatly be tween the seasons, being higher at the end of summer and in winter whe n photosynthesis was limited by water stress and cold, respectively, T he size of V+A+Z pool was associated to synthesis of zeaxanthin in res ponse to an excess of light, In sun leaves, midday A+Z relative conten t was positively correlated with the V+A+Z pool, whereas in shade leav es A+Z decreased with leaf ageing, In both leaf types A+Z was positive ly correlated with the non-photochemical quenching (NPQ) of chlorophyl l a fluorescence, However, in winter NPQ did not change significantly throughout the day, whereas the (A+Z)/(V+A+Z) increased following the typical daily trend observed in other seasons.