The isohydric strategy of <i>Platanus × hispanica</i> tree shapes its response to drought in an urban environment

Urban vegetation provides many ecosystem services like heat island mitigation. However, urban trees are subjected to the stresses that they are meant to alleviate, with drought being a main constraint. We investigated the drought response strategy of plane trees (Platanus × hispanica), focusing on stomatal regulation and metabolic remodelling. To address this question, a semi-controlled experiment was performed in an urban site with fourteen plane trees grown in containers. From May to June 2022, those trees were physiologically characterized in response to a controlled edaphic drought completed by a targeted metabolome analysis focused on amino acids, sugars, polyols and organic acids. Early P. × hispanica response to drought consisted in stomatal closure limiting carbon assimilation and osmotic adjustment, which was likely related to malate and trehalose accumulation. Both allowed the maintenance of stem water potential and Relative Water Content. As the drought became severe, when the extractable soil water content (eSWC) dropped below 30%, a non-stomatal limitation of photosynthesis was observed and was associated with photosynthetic apparatus damage (reduced chlorophyll content and decrease in F_v/F_m) and a further decline in carbon assimilation. When eSWC decreased below 25%, severe drought induced defoliation. Together, these results highlight the isohydric strategy of P. × hispanica, based notably on osmotic adjustment and explain its resistance to drought combined with other urban constraints. In the context of climatic change in cities, it would be interesting to analyse the impact of successive drought cycles in the long term, aiming for sustainable planning and management of urban trees.