Impact of tryptophan on physiological and histological responses of pepper plants under salt stress

Salt stress is a significant abiotic stressor adversely affecting plant functions. Tryptophan, a precursor to IAA, plays a crucial role in plant growth under such stress. This study investigated the effects of different tryptophan doses (L-tr) on young pepper plants subjected to salt stress. Preceding salinity stress, three L-tr doses were administered to plant roots, excluding control and salt applications. Subsequently, all plants except controls received 150 mM salinity (NaCl) during irrigation. The experiment concluded after 35 days post-salinity treatment. Significant variations were noted in histological (cortex and cell diameters, xylem and midrib diameters, epidermis thickness) and physiological (leaf water content, membrane permeability, chlorophyll and carotenoid concentrations, total phenolic content) parameters among treatments. Evaluation of these parameters revealed L-tr’s positive impact on osmotic regulation. L-tr applications increased xylem, midrib, and cortical cell diameter, cortex, and epidermis thickness compared to salt stress, while cortical cell number also decreased. Particularly, NaCl + L-tr 100 µM treatment exhibited a 25% increase in xylem conduit diameter, a 30% increase in midrib diameter, and a significant 15% increase in epidermis thickness compared to controls. Overall, NaCl + L-tr 100 µM treatment displayed values closest to controls across various parameters. This study suggests the potential utility of L-tr in mitigating salinity stress in pepper plants.