Exceptional Photosynthetic Performance of Capparis spinosa L. Under Adverse Conditions of Mediterranean summer

Abstract

Diurnal and seasonal fluctuations in water potential (Ψ), stomatal conductance (g _s), transpiration rate (E), and net photosynthetic rate (P _N) were monitored in Capparis spinosa L., a Mediterranean plant growing during summer, i.e. at the period considered the most stressful for local plant life. In spite of the complete absence of rain, Ψ exhibited a modest drop at midday (−2.7 MPa), but was fully recovered overnight, indicating sufficient access to water sources. The stomata remained open throughout the day and season and the high E resulted in leaf temperatures up to 3.9 °C below air temperature. Additionally, P _N of the fully exposed leaves was higher than 25 μmol m⁻² s⁻¹ for more than 10 h per day throughout the summer growth period. No symptoms of photooxidative stress were shown, as judged by maximum photosystem 2 photochemical efficiency (F_v/F_m) and the function of xanthophyll cycle. Indeed, diurnal inter-conversions of the xanthophyll cycle components were modest during the summer and a more intensive function of the cycle was only evident during leaf senescence in autumn. In comparison with a semi-deciduous and an evergreen sclerophyll co-existing in the same ecosystem, C. spinosa assimilated up to 3.4 times more CO₂ per m² during its growth period (May to October) and up to 1.8 times more on an annual basis.