Nutrient input by atmospheric particles is important or even essential for forest ecosystems. Many particles will first deposit to leaf surfaces. Competition is avoided by direct foliar uptake, compared to uptake via the soil. While there is meanwhile clear evidence for foliar uptake to happen via the stomata under natural conditions, this requires the establishment of a liquid water connection along the hydrophobic stomatal walls, called ‘hydraulic activation of stomata’ (HAS). Several possibilities for HAS to establish have been suggested including fungal hyphae, bacteria, and salts. Here we show for the first time the spreading of different salts on hydrophobic cuticles by repeated drying/wetting cycles, a plausible model for HAS.

NaCl, K2HPO4, and NaClO3 were placed on hydrophobic leaf surfaces or isolated cuticles. Gas exchange and water use efficiency were recorded for several days after the application of droplets on bean (Vicia faba) and on apple (Malus domesticus) leaves. Repeated drying/wetting cycles for the different salts were observed with an environmental scanning electron microscope (ESEM).

Apple leaves treated with NaClO3 on the astomatous adaxial surface were unaffected while treatment on stomatous abaxial surfaces caused photosynthesis and water use efficiency to decrease. Transpiration of bean leaves increased after K2HPO4 application, while water use efficiency decreased. Salts changed their appearance by drying wetting cycles, after some time spreading out in dentritic growth.

The study shows the crucial influence of stomata for the transport of ionic substances, the interaction of hygroscopic salts with plant water relations, and a possible mechanism for HAS establishment.