Limitation to carbon assimilation of two perennial species in semi-arid south-east Spain

Abstract

Diurnal and seasonal changes of net photosynthetic rate (Pn) and the efficiency of photosystem 2 (Fv/Fm) were measured on two perennial species growing on a soil catena in semi-arid south-east Spain. Stipa tenacissima, a tussock grass, grows on shallow soil at the top of the catena and Retama sphaerocarpa, a leguminous shrub, grows in the valley bottom. A linear relationship was found between light saturated photosynthetic rates (Pmax) and diffusive leaf conductance (g1) in both Retama and Stipa indicating that the intercellular CO2 concentration (ci) was maintained constant in both species diurnally. Relatively high values of calculated ci in Retama cladodes suggested that was not the primary limitation to carbon assimilation. Fv/Fm for the two species when well watered was around 0.8. Although Retama cladodes maintained this value throughout the year, Fv/Fm decreased to a minimum of 0.43 in Stipa leaves, at the end of the dry season. Our data suggest that plants in the Rambla Honda can substantially reduce transpiration without reducing photosynthetic rates to the same extent by closing their stomata, because Pn is reduced primarily by high respiration, decreased mesophyll conductance and by photoinhibition or permanent damage of photosystem 2.