Ecophysiological significance of chlorophyll loss and reduced photochemical efficiency under extreme aridity in Stipa tenacissima L.

Stipa tenacissima L., a perennial tussock grass widely found in semi-arid environments of the Iberian Peninsula and North Africa, is subjected to multiple stresses during the extreme summer conditions of south-east Spain. We characterised the photoprotective mechanisms of S. tenacissima during the transition from spring to summer and autumn. S. tenacissima experienced a marked water deficit (Psi (pd)<-8.4 MPa) and the complete suppression of CO2 assimilation in August, associated with a 72% reduction of maximal photochemical efficiency of PSII (F (v)/F (m)). These reduced F (v)/F (m) values were related to the pre-dawn maintenance of high levels of epoxidized forms of xanthophyll-cycle pigments (DPS (pd), ca. 42% higher than spring values), and with a 60% reduction in the concentration of total chlorophyll (Chl a+b). These changes were associated with a low capacity of dissipation of the excitation energy non-radiatively (measured as NPQ). Leaves showed a complete recovery of F (v)/F (m) and xanthophyll and chlorophyll concentrations after the autumn rainfall, which reached levels similar to that of spring. This poikilohydric-type response of S. tenacissima to stress allows for a greater tolerance of water shortage, high temperature and high light intensity, which are typical in these semi-arid environments and accounts for its distinctive opportunistic growth.