Salicylic Acid in Combination with Protectants Enhances the Growth in Salt-Stressed Polianthes tuberosa L. Plants by Regulating Photosynthesis and Antioxidants

Salt stress (SS) poses a significant threat to cut flower geophytes. This study evaluated the impact of SS on vegetative growth and photosynthetic activities in tuberose (Polianthes tuberosa L.) and assessed the efficacy of 300 ppm salicylic acid (SA) alone or in combination with 100 ppm indole acetic acid (IAA), 300 ppm ascorbic acid (AA), and 300 ppm chitosan (CS), under saline water irrigations at 0, 80, and 120 mM NaCl concentrations. The SS negatively impacted the vegetative growth of Polianthes tuberosa L. At the highest salinity level (120 mM NaCl), leaf area decreased by 14% compared with the control group, while shoot fresh and dry weights dropped by 54% and 61%, respectively. Root fresh and dry weights were reduced by 48% and 62%, respectively, and the number of bulblets per plant decreased by 43%. Additionally, the average stalk length and the number of florets per plant declined by 15% and 2%, respectively. At 80 mM NaCl, the average stalk length and the number of florets per plant were reduced by 6%, compared to the control. The photosynthetic activity, including the parameters such as Phi2 (photosystem II), relative chlorophyll content and Fv/Fm was adversely affected. At the high salinity level, these parameters were reduced by 14%, 11%, and 21%, respectively, while moderate salt stress led to reductions of 7%, 67% and 14%, respectively. The response of antioxidative enzymes was varied at different salinity levels. At 80 mM NaCl, the catalase (CAT) and peroxidase (POD) increased by 22% and 46%, respectively, while at 120 mM NaCl, these activities decreased by 21% and 79%, respectively, compared with the control. SA alone and its combinations with IAA hindered the drastic effect of SS by enhancing superoxide dismutase (SOD) activity by 78% and 111% and DPPH scavenging property by 0.7% and 1% under SS. It also influenced the photosynthetic rates with higher nonphotochemical quenching (NPQt), Phi2 and photoprotection (PhiNPQ) in salt-stressed.