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

Abstract

Xanthophyll cycle components and their pool sizes were seasonally examined in eight Mediterranean plant species from two different growth forms (drought semi-deciduals and evergreen sclerophylls), in an attempt to correlate possible fluctuations in the above parameters with potentially photoinhibitory 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 + antheraxanthin)/(violaxanthin + zeaxanthin + antheraxanthin), and the maximum xanthophyll pool sizes, expressed on a Chl basis, were recorded during the dry summer, indicating that the period of water shortage may be the most demanding for a high photoprotective potential. However, the data suggests that sclerophylls 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 considerable stress factor for these plants. It is assumed that by preserving high concentrations of zeaxanthin and antheraxanthin overnight, sclerophylls retain high energy dissipation activity early in the morning, when minimum temperatures, coinciding with high photon fluence rates, are likely to drive the photosynthetic apparatus to potentially harmful overexcitation. Of significance may be the fact that almost all species show a minimum in chlorophyll content during the summer. In this way, overexcitation is alleviated and the photoprotective action of carotenoids per chlorophyll molecule is enhanced.