Exogenous D-ornithine Enhances Drought Tolerance in Sage (Salvia officinalis L.) Through Modulating Water Status, Photosynthetic Performance, Osmoprotectants, and Defense Mechanisms

Ornithine, a non-proteinogenic amino acid, plays a crucial role in plant metabolism, especially in polyamines (PAs) biosynthesis and stress response mechanisms. This study demonstrates, for the first time, that both L- and D-ornithine enantiomers improve growth parameters, antioxidant activities, and photosynthetic efficiency in drought-stressed Salvia officinalis (sage). Plants were subjected to irrigation regimes of 20% and 80% available soil water depletion for control and drought stress conditions, respectively, followed by foliar application of L- and D-ornithine at 0- and 1 mM concentrations. Drought stress significantly decreased photosynthetic parameters such as quantum yield of PSII photochemistry (Phi PSII), photochemical quenching (qP), maximum quantum yield of PSII (Fv/Fm), and photosynthetic pigments (chlorophyll (Chl), Chl a, Chl b, total chlorophyll, and carotenoids), as well as growth indicators such as leaf area index (LAI) and relative water content (RWC). Conversely, drought stress increased oxidative stress markers, osmolytes (proline, glycine betaine, soluble sugars), PAs, and antioxidant enzyme activities (superoxide dismutase, catalase, ascorbate peroxidase, and peroxidase). Foliar application of L-ornithine increased LAI (102%), RWC (20%) and total chlorophyll (30%) compared to drought stress without L-ornithine treatment. In addition, foliar application of D-ornithine significantly increased the levels of total polyamines (107%), bound putrescine (111%), bound spermidine (124%), and bound spermine (140%) levels, as well as osmolytes including proline (152%) and total soluble sugars (78%) compared to drought stress without D-ornithine. In conclusion, our findings suggest that foliar application of D-ornithine improves drought tolerance in sage under stressful conditions.