Early water-status indicators under combined metal toxicity and drought in tomato leaves

Soil contamination by heavy metals, intensified by climate change, poses a growing threat to plant water relations and photosynthetic function. This study assessed the physiological responses of Solanum lycopersicum L. to two non-essential heavy metals (lead and arsenic) and one essential metal (nickel), under well-watered and drought–re-irrigation conditions. Each metal was tested at two concentrations. While plant height and leaf number were unaffected, lead and arsenic significantly reduced leaf area and shoot/root biomass, unlike nickel, which preserved these traits. Leaf mass area and dry matter content declined under lead and arsenic (up to −50% and −10%, respectively), but increased under nickel (up to +50% and +20%). Nickel also induced a more negative turgor loss point, indicating enhanced dehydration tolerance. Despite unchanged leaf hydraulic conductance under non-limiting water, metal exposure impaired membrane stability and increased dehydration sensitivity. Plants subjected to heavy metals showed earlier loss of leaf rehydration capacity and PSII efficiency at higher relative water content (RWC) compared to controls. Specifically, RWC thresholds for 10% loss in rehydration capacity (RWCPLRC10) and PSII function (RWCPLYP10) were increased by up to 20 and 10 percentile points, respectively. Membrane damage, assessed via electrolyte leakage, was exacerbated but not strongly predictive of functional decline. During drought recovery, high concentrations of all metals, especially lead and arsenic, hindered water status and gas exchange restoration. These findings elucidate the mechanisms by which heavy metals disrupt plant water relations and photosynthesis. RWC-based thresholds emerge as sensitive early markers of irreversible damage, supporting their application in physiological monitoring under combined metal toxicity and drought stress.