Exogenous application of proline mitigates deteriorative effects of salinity stress in NFT closed-loop system: An ultrastructural and physio-biochemical investigation on hydroponically grown tomato (Solanum lycopersicon L.)

A nutrient film technique (NFT) experiment was carried out to assess the protective effect of foliarly-applied proline (10 mM) against deteriorative effects of salinity in closed-loop systems. For this purpose, Cooper's nutrient solution (NS) was salinized by either NaCl (68.4 mM) or CaCl2 (36.0 mM) to study the potential ultrastructural and physio-biochemical modulations on hydroponically-grown tomato. Endogenous proline concentration showed elevated levels in proline-treated plants (particularly those grown under NaCl stress). Tomato subjected to salinity stress (particularly NaCl-stressed plants) showed several ultrastructural deteriorations in leaf mesophyll cells. These deteriorations include irregular shape of chloroplast with visible of chloroplast protrusions (CPs) and Rubisco-containing bodies (RCBs), smaller volumes of nucleus and vanishing of nucleolus in some cells. Moreover, an obvious increase in the number of mitochondria, cell walls thickness, number of membrane vesicles and presence of Myelin-figures in the plasma membrane were observed in salinity-stressed plants. Negative effects of salinity on organelles were more pronounced in root tip cells due to the direct contact with hyperosmotic stress of salinized solutions. In this regard, abnormal increase in volume and number of vacuoles, which often filled with electron-dense globules (EDG), were clearly observed in root tip cells of stressed-plants. Scanning electron microscopy (SEM) micrographs showed significant alterations in stomatal characters of salinity stressed-plants. Besides, salinity-stressed plants (particularly under CaCl2) exhibited higher trichomes number, cuticular layers depositions on leaf epidermis and crystal formations in the base of trichomes. Salinity stress also caused reductions in photosynthetic pigments, water content and relative turgidity due to the disruption in membrane permeability. Findings of this study, therefore, provide insights regarding ultrastructural responses of proline-treated tomato grown hydroponically under different types of salinity stress.