Exogenously Applied Melatonin and Calcium Alleviate Salt Injuries to Growth, Physiological, and Biochemical Attributes of Borago officinalis

Abstract

Regulating roles of melatonin (Mel) and calcium (Ca) as signal molecules in mitigating salt stress-induced alterations in physiological and biochemical attributes have been confirmed for a number of crop species, but not for borage (Borago officinalis L.). Hence, borage genotypes were subjected to Mel (150 µM) and Ca (5 mM) alone or in combination under non-saline (Control) or saline (100 mM NaCl) conditions in a series of experiments. Salt-exposed plants indicated increases in the malondialdehyde (45%), H₂O₂ (33%), proline (121%), Na⁺ (163%), and total phenolic concentrations (85%), Na⁺/K⁺ (120%), and electrolyte leakage (40%), but decreases in the chlorophylls (36%) and carotenoids (40%) concentrations, maximum quantum efficiency of photosystem II (9%), leaf area (29%), relative water content (22%), K⁺ (32%) and Ca⁺² (45%) concentrations, and hence shoot (34%) and root dry mass (33%). Amending the salt-stressed plants with Ca, Mel, and particularly Mel + Ca decreased malondialdehyde, H₂O₂, and Na⁺ concentrations, Na⁺/K⁺, and electrolyte leakage, but increased chlorophylls, proline, phenolics, K⁺ and Ca⁺² concentrations, maximum quantum efficiency of photosystem II, leaf area, catalase and ascorbate peroxidase activities, relative water content, fresh and dry masses, and root volume. We indicated for the first time that while harmful effects of salt were not substantial, the amendment with Mel and Ca was effective in mitigating the adverse effects of salt on the photosynthetic components, ionic balance, and growth attributes of borage. These novel findings show that borage is a salt-tolerant medicinal species and the interactive effects of Mel and Ca increase its salt tolerance, and potentiate borage as a valuable alternative species for maintaining agricultural production under saline conditions.