Investigating the Effect of Foliar Application of Glycine Betaine on Some Quantitative and Qualitative Characteristics of Mint (Mentha spicata L(. under Salinity Stress

Spearmint is a perennial plant with creeping and underground stems, which belongs to the Labiatae family. After drought, salinity is the second most common environmental agent that restricts agricultural production. Glycine betaine is the most widely used organic solution known for its quadrature ammonium compounds and the largest and most abundant compound in response to stress. The purpose of this experiment was to investigate the effect of glycine betaine on some quantitative and qualitative properties of mint under salinity stress.

A factorial experiment was conducted based on randomized complete block design with three replications in 2017-18. Experimental factors included salinity stress (4 levels, control, 30, 60 and 90 mM NaCl) and glycine betaine (at 3 levels of 0, 100 and 200 mM). In the present study, some growth traits, height of plant, fresh weight and dry weight of shoot, root volume, fresh and dry weight of root, photosynthetic pigments (chlorophyll a and b and total), protein, proline and soluble sugars were measured.

The results showed that salinity stress was significant for all characteristics, so that by increasing levels of salinity all traits were reduced except chlorophyll b, total chlorophyll, proline and soluble sugars. Salinity reduced plant height (29.47%), fresh weight of shoot (49.56%), dry weight of shoot (56.87%), root volume (47.89%), fresh weight of root (52.15%), dry weight of root (43.92%), photosynthetic pigments (a) (18%) and (b) (29.41%), protein content (17.79%), proline (30.98%) and soluble sugars (3.58%). Application of glycinebetaine reduced the negative effects of salinity stress, but it caused reduced height (9.16%) and chlorophyll b (26.31%) compared to control. Also, interactions between salinity stress and foliar application of glycinebetaine were significant for shoot fresh weight, shoot dry weight, root fresh weigh, root dry weight, proline, protein (p < 0.01), root volume and soluble sugars (P<0.05). The highest values of the studied traits were obtained for the interaction between 7 mM NaCl and 200 mM glycinebetaine.

Salinity stress reduces the growth of plants through osmotic stress, ion toxicity and nutritional imbalance. However, glycine betaine, as an important osmolite in plants, increases the osmotic potential and improves the water relations and increases the activity of photosynthesis and the production of hydrocarbon materials. In this way, glycine betaine can protect against the stress of salinity by increasing the mechanisms of tolerance to salinity and will provide better conditions for plant growth in a saline environment.