Deleterious Effects Of Long-term Salt Stress On Gerbera (Gerbera Jamesonii L.) And Salicylic Acid Induced Amelioration Of Its Photosynthetic Capacity And Antioxidative Defense System

Gerbera (Gerbera jamesonii L.; family Asteraceae) represents an important commercial cutflower crop, ranking the fifth most demanded cutflower in the international cutflower trade. The productivity and distribution of cut gerbera are threatened by ever shrinking cultivable land area and global constraints on fresh irrigation water supplies, due to salinity, especially under protected cultivation systems. However, the deleterious effects of long-term salt stress as well as effective alleviation of salt stress-damage in gerbera have rarely been studied. Hence, the present work briefed bellow, was carried out to investigate the effects of long-term NaCl salt stress on Gerbera jamesonii L. 'Amaretto' and the role of exogenously applied salicylic acid in improving its salinity-tolerance.1) Three-month old micropropagated plants were subjected to five salt treatments as 0, 10, 20, 30 and 40 mM NaCl for 10 weeks to investigate the effects of prolonged salt stress as characterized by growth, physiological, biochemical and ultrastructural responses. Increasing salt stress significantly declined the growth, plant survival, yield and flower quality. Salt stress significantly increased Na⁺ but decreased K⁺ concentration in leaves, stems and roots leading to a marked decline in K⁺/Na⁺ ratios. Salt stress significantly affected Mg²⁺ concentrations in stems and roots, too. Cationic nutrient concentrations, chlorophyll content, root and leaf proline content critically changed when plants switched from vegetative to reproductive growth. Plasmolysis and severe destruction of chloroplast and other organelle ultrastructure were prominent under higher salt stress. NaCl threshold level in irrigation water for the normal vegetative growth of gerbera is around 10 mM for this cultivar in particular. However, even this level of NaCl significantly reduces the flower yield and quality. Chlorophyll content, leaf fresh weight and K⁺/Na⁺ ratio are promising indicators of salt-sensitivity of gerbera.2) Protective roles of exogenously applied salicylic acid (SA) against salt-induced photosynthetic inhibition and oxidative damage in gerbera at reproductive stage were investigated. At the onset of flower initiation, plants pretreated with 0, 0.5 and 1.0 mM SA, were irrigated with 100 mM NaCl (three days after the first SA application) and continued for two weeks. Control plants did not receive either NaCl or SA. Salt stress negatively affected the gas exchange attributes, photosynthetic pigments content, membrane integrity and chloroplast ultrastructure. SA application significantly enhanced all gas exchange attributes tested. Further, the photosynthetic pigments content was also enhanced while the chloroplast ultrastructure was effectively prevented from damage by exogenous SA. Membrane permeability and lipid peroxidation were markedly lowered while superoxide dismutase and peroxidase activities were significantly enhanced by SA pretreatment. Results suggest that photosynthetic capacity and antioxidative defense system in salt stressed plants are enhanced by SA pretreatment. Generally, of the two concentrations tested, 0.5 mM SA seemed to have greater effect throughout the experiment showing no significant variation from control in some attributes. Responses of plants pretreated with SA, significant correlation among them as well as the low cost, plausibly predict its future potential in practical application to alleviate adverse effects of salt stress in gerbera.