Foliar fertilization of two dominant species in a semiarid ecosystem improves their ecophysiological status and the use efficiency of a water pulse

The role of soil fertility in modulating plant water utilization from rainfall pulses under dryland conditions is poorly understood. Moreover, the effectiveness of soil fertilization in drylands is difficult to ensure due to immobilization of nutrients such as phosphorus and micronutrients. The use of foliar fertilization could overcome these difficulties, but its effectiveness has not been evaluated in wild dryland plants so far. In this study, we assessed the responses to foliar P, Fe, Mn and Zn fertilization before a rainfall event in two Mediterranean species with contrasting strategies to cope with drought/light stress, namely, Stipa tenacissima (poikilochlorophyllous) and Rosmarinus officinalis (photoprotective). We hypothesized that foliar fertilization can mitigate soil nutritional constraints and trigger physiological improvements enabling a more efficient use of water pulses. In nonfertilized plants, the rainfall event triggered increases in chlorophyll content, epoxidation of xanthophyll cycle pigments and PSII efficiency, indicating stress alleviation and photosynthesis improvement, but also induced decreases in foliar concentrations of several elements. Foliar fertilization increased the concentrations of sprayed nutrients in the leaves of both species. In R. officinalis, P and Zn fertilization also increased carotenoid concentrations, PSII photochemical efficiency and photosynthesis rates, and changed delta O-18 isotope ratios. While in S. tenacissima, Fe and Mn fertilization increased photosynthetic pigment concentrations without affecting PSII efficiency. The results obtained show that foliar fertilization can mitigate soil nutritional constraints affecting dryland vegetation, thereby promoting species-specific physiological responses to cope with water and/or light stress and ultimately leading to a more efficient use of rainfall pulses.