The Effect of Salicylic Acid and L-arginine on Morpho-physiological Properties and Leaf Nutrients of Catharanthus roseus under Drought Stress

Catharanthus roseus is a beautiful herbaceous plant, belonging to Apocynaceae family, which is widely cultivated due to its unique and beautiful appearance. It is native to the Caribbean Basin and has historically been used to treat a wide assortment of diseases. European herbalists used this plant for different conditions such as headache and a folk remedy for diabetes. The effects of drought range from morphological to molecular levels and are evident at all phonological stages of plant growth at whatever stage the water deficit takes place.The limitation of water resources in our country is one of the important issues in landscape. However, we can alleviate the adverse effects of water stress with the use of some biological and non-biological stimulators. Under severe stress conditions, the antioxidant capacity may not be sufficient to minimize the harmful effects of oxidative damage. Therefore, synthesis of signal molecules in plants is an important step in our better understanding of how plants respond to environmental stresses. Several such signal molecules have been identified in plants such as jasmonic acid, ethylene and salicylic acid (SA). SA is considered as a hormone-like substance, which plays an important role in regulating a number of plants’ physiological processes including photosynthesis. Besides, L-arginine is an amino acid, which can alleviate the adverse effect of drought. Among the 21 proteinogenic amino acids, arginine has the highest nitrogen to carbon ratio, which makes it especially suitable as a storage form of organic nitrogen. Synthesis in chloroplasts via ornithine is apparently the only operational pathway to provide arginine in plants. Therefore, the present study was conducted to evaluate the effect of salicylic acid (SA) and L-arginine on morpho-physiological properties of C. roseus.

This experiment was carried out as a factorial with two factors including the foliar application at five levels (control, L-arginine 1.5 and 3 mM, SA 100 and 200 mg /L) and water stress at three levels (100, 70 and 40% field capacity (FC)) in a completely randomized design with three replications. Water use efficiency (WUE), the root/shoot weight, flower number, flower diameter, leaf chlorophyll content, peroxidase and superoxide dismutase activity of leaf, and leaf macronutrients (nitrogen, phosphorus and potassium) were measured.

The results showed that WUE in plants under moderate stress (70% FC) was higher than the plants in control and severe drought condition (40% FC). Root/shoot weight at no foliar application (control) × 40% FC and L-arginine 1.5 mM × 40% FC was higher than other treatments. The number of flowers in the treatments of 100% FC and 70% FC was more than 40% FC. The highest flower diameter was obtained from the L-arginine 3 mM. Total chlorophyll of L-arginine 3 mM was higher relative to other treatments. Drought stress significantly increased the activity of antioxidant enzymes. The lowest amount of phosphorus and nitrogen was observed at no foliar application ×40% FC. Therefore, according to the optimum application of water and amendment substrates, at 70% FC stress, L-arginine 3 mM and SA 100 mg /l increased the nitrogen and phosphorus content. Overall, L-arginine 3 mM and SA 100 mg /l increased the flower number and flower diameter significantly. Accumulation of these organic solutes either actively or passively helps the plants to retain water within cells and protect cellular compartments from injury caused by dehydration or maintain turgor pressure during water stress. Turgor maintenance plays an important role in drought tolerance of plants which may be due to its involvement in stomatal regulation and hence photosynthesis. Foliar application with arginine resulted in elevated proline levels and radiotracer experiments demonstrated that both 3H and 14C from arginine can be recovered as proline. The physiological relevance and the biochemical pathway of the conversion of arginine to proline in plants remain unclear. The most prominent hypothesis is that ornithine, derived from arginine catabolism, is converted by δOAT to GSA/P5C, which then serves as substrate for proline synthesis by P5CR. This model has been doubted, since Arabidopsis δOAT was found to be exclusively localized in mitochondria, while P5CR is localized in the cytosol.

The findings of our study showed that water stress can morphologically and physiological change C. roseus. There was no significant difference between 70% and 100% FC for root/shoot, flower number and flower diameter traits. So, we can reduce the use of water to 70% FC increased the flower number and flower diameter and can be used to alleviate the adverse impact of water stress.