Stomatal conductance (g s ) affects water use efficiency (WUE) through coordinating photosynthesis and transpiration and is regulated by chemical and/or hydraulic signals. However, the regulation mechanism of g s of tomato leaves has not been fully explored under combined water and salt stress. Here, we set up four salt treatments and two water treatments in a climate greenhouse and measured stomatal morphologies and conductance and other photosynthesis parameters. Water and salt stress reduced stomatal length (SL), width, perimeter, area (a max ), density (SD), and the maximum stomatal conductance (g smax ). Water and salt stress had a separate weakening effect on net photosynthetic rate (A) and transpiration rate but interactively reduced g s . The contents of abscisic acid (ABA) and Na + in tomato leaves increased with the NaCl concentration, while leaf water potential (Ψ l ) and chlorophyll content decreased. Under full irrigation, g smax was coordinated by SD and a max , and g s by ABA content under salt stress. Under water and salt combined stress, g smax was affected by a max , and g s was coordinated with ABA and Ψ l . The decrease of A was caused by both a reduction of chlorophyll content and g s under water and salt stress. Intrinsic WUE did not reduce under full irrigation or mild to moderate salt stress but decreased under a combination of water and severe salt stress, indicating that the leaves of the tested tomato cultivar performed better under moderate salt stress.Collectively, these results can provide useful insights for the efficient management of water and salt to adapt to drought and high salt environments.

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