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Seedling growth of the invasive weed Parkinsonia aculeata L. may occur during water shortage under dense canopy at low light intensity. Experiments performed under controlled conditions during 22 days showed that water stress inhibited growth and decreased water, osmotic and turgor potentials. Drought conditions reduced stomatal conductance and net photosynthesis but had no obvious impact on chlorophyll fluorescence or chlorophyll content. Maximum photochemical efficiency of PSII (Fv/Fm) was affected only after 22 days of stress. Despite the low light intensity, a precocious activation of the xanthophyll cycle allowed an efficient protection of PSII through non-radiative energy dissipation and a significant decrease of epoxidation state was observed already after 11 days of treatment. Water stress reduced photosynthesis mainly through stomatal closure but increased water use efficiency. Active osmotic adjustment was a late process observed after 15 days of treatment and proline over synthesis may account for it while sugar accumulation appeared as a consequence of growth inhibition. Malondialdehyde did not accumulate in stressed plants after day 11, suggesting a limited oxidative stress. However, a putative involvement of phenolic compounds in the management of stress-induced oxygen reactive species is hypothesized. Young seedlings of P. aculeata exposed to water stress in semi-arid areas under low light intensity may, to some extent, adopt physiological strategies allowing them to survive and contribute to the invader potential of this species.