Seasonal fluctuations in photoprotective (xanthophyll cycle) and photoselective (chlorophylls) capacity in eight Mediterranean plant species belonging to two different growth forms

Xanthophyll cycle components and their pool sizes were seasonally examined in eight Mediterranean plant species from two different growth forms (droug ht semi-deciduals and evergreen sclerophylls), in an attempt to correlate p ossible fluctuations in the above parameters with potentially photoinhibito ry conditions. The xanthophyll cycle was common in all species examined and was functional throughout the year. In semi-deciduals, the maximum midday values for conversion state, defined as the ratio (zeaxanthin + antheraxant hin)/(violaxanthin + zeaxanthin + antheraxanthin), and the maximum xanthoph yll pool sizes, expressed on a Chl basis, were recorded during the dry summ er, indicating that the period of water shortage may be the most demanding for a high photoprotective potential. However, the data suggests that scler ophylls may also need a high photoprotective potential during winter, since the xanthophyll pool sizes were equally high during both summer and winter . Corresponding spring and autumn values were low. Furthermore, winter pre- dawn conversion state values were the highest recorded amongst all species studied, indicating that the mildly low winter temperatures may be a consid erable stress factor for these plants. It is assumed that by preserving hig h concentrations of zeaxanthin and antheraxanthin overnight, sclerophylls r etain high energy dissipation activity early in the morning, when minimum t emperatures, coinciding with high photon fluence rates, are likely to drive the photosynthetic apparatus to potentially harmful overexcitation. Of sig nificance may be the fact that almost all species show a minimum in chlorop hyll content during the summer. In this way, overexcitation is alleviated a nd the photoprotective action of carotenoids per chlorophyll molecule is en hanced.