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Background and aims: The beneficial effects of Si have mainly been observed in herbaceous plants, while little is known about its role in deciduous trees. The aim of this work was to evaluate the effect of foliar application of Si on chestnut leaf growth, photosynthesis and water relations in the presence of short, but intense water deficit. Methods: Sili-K® solution (containing 0. 12 % Si and 0. 15 % K) was repeatedly (× 3) sprayed onto leaves of potted chestnut plantlets and irrigation was suspended 7 weeks later, for 8 days. Leaf growth, anatomy, as well as physiological and biochemical traits of the plantlets were studied. Results: Si application enhanced chestnut growth, due to increased photosynthetic traits, including higher chlorophyll content and chlorophyll a to b ratio, photochemical efficiency of PSII, gas exchange (stomatal conductance, transpiration rate, net CO₂ assimilation) and oxygen evolution rate. Meanwhile, Si yielded larger and thinner leaves, higher xylem, specific leaf area and transpiration rate, thus being beneficial to the tree in absorbing sunlight energy for photosynthesis and in alleviating heat stress. However, Si also lowered leaf sap osmotic pressure, causing the plant to lose water more quickly, thus being more susceptible to water stress. Conclusions: Si improved chestnut photosynthesis, growth, and heat stress tolerance, but it also increased the susceptibility to drought. © 2012 Springer Science+Business Media B.V.