Effect of gamma-aminobutyric acid (GABA) on some morpho-physiological traits of rosemary (Rosmarinus officinalis L.) under salt stress
IntroductionSalinity stress is a major limiting factor for plant growth. Gamma-aminobutyric acid (GABA) is known as an important signaling molecule in response to environmental stresses. This study investigated the effect of exogenous GABA application on the morphophysiological and biochemical characteristics of rosemary plants under saline conditions. Materials and methodsA factorial experiment was conducted using a completely randomized design (CRD) with three replications. The study investigated the effects of NaCl salinity (0, 50, 100, and 150 mM) and foliar-applied GABA (0, 20, 40, and 60 mM) on the morphophysiological traits of rosemary (Rosmarinus officinalis L.) during the summer of 2014 at the research greenhouse of the Faculty of Agriculture, University of Zanjan. Fifteen-to-twenty cm tall rosemary seedlings, sourced from Zarrin Gyaa Urmia Company, were cultivated in size 3 pots containing equal proportions of perlite, cocopeat, and washed soil. Prior to treatment application, plants were acclimatized for two weeks with Hoagland nutrient solution. Greenhouse environmental conditions were set to a 16 h light/8 h dark cycle, with day/night temperatures of 25/20 °C. Salinity was applied gradually over several stages. Salinity stress was imposed by adding NaCl to the irrigation water, and the pots were irrigated every three days. Foliar spray solutions were prepared by dissolving the required amount of GABA in distilled water. The first foliar spray was applied immediately after the initiation of salinity stress, the second two weeks later, and a third application was performed thereafter, for a total of three applications. Twenty ml of solution was applied to each pot (Vojodi Mehrabani et al., 2016). The entire experiment lasted for 14 weeks. The dry weight of the aerial parts was measured after determining the fresh weight using a digital scale and the stem length with a ruler. Leaf proline content, leaf area, and leaf relative water content were measured. Data were analyzed using SAS software, and graphs were generated using Microsoft Excel. Duncan’s multiple range test was used to compare means at the 5% probability level. Results and discussionMean comparison results for shoot fresh weight (Table 2) indicated that the interaction between salinity and GABA was significant at the 1% level. The highest shoot fresh weight (25.130 g) was recorded under non‑saline conditions with the application of 40 mM GABA. The lowest shoot dry weight (4.941 g) was related to the independent effect of 150 mM salinity treatment, whereas the highest shoot dry weight (7.745 g) was observed in the salinity control treatment with 20 mM GABA. The interaction effect of salinity stress and GABA treatment on stem length was significant at the 1% level. The highest stem length (25.934 cm) was recorded under non‑saline (control) with 60 mM GABA. The interaction effect of salinity and GABA on rosemary leaf area was significant at the 1% level. The highest leaf area (4.5657 cm²) was observed under non‑saline (control) conditions with 60 mM GABA. In contrast, the lowest leaf area (2.1772 cm²) was recorded in the absence of GABA under 150 mM salinity. The interaction effect of salinity stress and GABA on electrolyte leakage percentage was statistically significant at the 1% level. The highest electrolyte leakage (98.031%) was observed at 150 mM salinity with 60 mM GABA, whereas the lowest value (67.231%) was recorded under non‑saline (control) conditions with 60 mM GABA. The main effect of salinity and GABA on leaf proline content was significant at the 1% level. Under salinity treatments, the highest proline content (0.0352 μmol g⁻¹ leaf fresh weight) was observed at 100 mM salinity, whereas the lowest value (0.0142 μmol g⁻¹ leaf fresh weight) was recorded in the control treatment. Similarly, among the GABA treatments, the highest proline content (0.0218 μmol g⁻¹ leaf fresh weight) was recorded at 60 mM GABA ConclusionThe present study demonstrated that salinity stress significantly affects the growth and physiological characteristics of the medicinal plant rosemary, resulting in reduced growth‑related traits and increased stress‑related indicators, such as electrolyte leakage and proline content. The application of GABA as a regulatory compound partially mitigated the detrimental effects of salinity. These results suggest that foliar application of GABA may represent an effective strategy to enhance rosemary tolerance to saline conditions and improve the performance of medicinal plants under environmental stresses.