The effect of soil potassium and carbohydrates on xylem conductivity and embolism in an evergreen angiosperm tree and a gymnosperm tree before and after drought

Xylem embolism is a major threat to tree function and survival under drought, in natural and agricultural settings alike, with its impact increasing in light of global climate change. Conversely, potassium (K+) has been shown to increase xylem conductivity (Ks) in trees, and carbohydrates were reported to impact leaf gas exchange. In this study we examined the effects of K+ and carbohydrates on Ks in two divergent evergreen tree species that are regularly exposed to drought: pine (Pinus brutia) and lemon (Citrus x limon). Five-year-old trees were pretreated with zero, moderate, and high K+, and with ambient or elevated CO2, to experimentally increase their xylem K+ or carbohydrates levels, respectively. Trees were then monitored for Ks and embolism (using a microCT), along with leaf gas exchange and water potential, before and after a 1.5-2.5 month drought period. Potassium fertigation had a positive effect on Ks, in both species when irrigated, which was eliminated following drought. Drought decreased Ks about 10-fold in lemon, with little effect in pine. CO2-treated trees had the same Ks as control trees before and after drought. Our results indicate a positive effect of K+ on tree hydraulics, which was more pronounced in lemon than in pine, supporting the hypothesis of interaction with the angiosperm pit membrane, and not with the gymnosperm bordered pit. Yet, the elimination of this benefit following drought, and the lack of benefit from elevated carbohydrates following a short-term CO2 treatment, question the relevance of these components to tree drought resistance mechanisms. Key massagePotassium fertigation increases hydraulic conductivity and reduces xylem embolism in the gymnosperm pine, and more so in the angiosperm lemon tree, benefits which were eliminated following drought.