Exogenous Carnitine Mitigates the Deleterious Effects of Mild-Water Stress on Arugula by Modulating Morphophysiological Responses

Water deficiency is a threat to food security and limits the yield of vegetables, including arugula (Eruca sativa Mill.). Despite the increasing use of amino acid-based bioregulators to mitigate water stress, the effects of carnitine as a water deficit mitigation agent have not yet been elucidated. Here, the effects of different carnitine concentrations on the morphophysiology of arugula plants under different water deficit conditions were evaluated. Plants were irrigated to 100%, 80%, 60%, or 20% field capacity (FC) with water alone (control) or water with carnitine at concentrations of 100 mu M or 100 mM. Forty days after sowing, the photosynthetic pigment content, gas exchange, chlorophyll a fluorescence, electrolyte leakage, relative water content, and growth parameters were measured. The application of 100 mu M carnitine to well-watered plants (80%-FC) and plants under moderate water stress (60%-FC) resulted in greater photosynthetic plasticity, as demonstrated by the unaltered maximum quantum yield of photosystem II. However, under severe water deficit (20%-FC), plants treated with carnitine exhibited a reduced maximum quantum yield of photosystem II, indicating damage to photosystem II. Application of 100 mu M carnitine increased the shoot biomass of arugula plants exposed to 80%-FC conditions and reduced electrolyte leakage in plants receiving 60%-FC irrigation. In contrast, 100 mM carnitine was toxic to arugula, hampering overall plant growth. The results suggest that low carnitine concentrations have the potential to mitigate the effects of moderate water stress on arugula, maintain membrane integrity and photosynthetic plasticity, and enhance shoot growth, which indicates this amino acid as a promising biostimulant in plants.