Exceptional photosynthetic performance of Capparis spinosa L. under adverse conditions of Mediterranean summer

Diurnal and seasonal fluctuations in water potential (T), 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, T 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 degreesC below air temperature. Additionally, P-N of the fully exposed leaves was higher than 25 mol 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 in 2 during its growth period (May to October) and up to 1.8 times more on an annual basis.