Effect of Foliar Application of Putrescine on the Morphological and Physiological Characteristics of Lime Seedlings (Citrus aurantifolia) under Drought Stress

Citrus fruits are one of the most commercial fruit products in the world, whose growth and production are affected by abiotic stresses. Drought stress is one of the most important abiotic stresses that affects all the vital processes of the plant. One of the ways to moderate the negative effects of drought stress is the use of polyamines. Polyamines are a group of biochemical compounds that are used as one of the most effective compounds to resist environmental stresses. Polyamines have a wide role in various plant growth processes, such that they play a significant role in modulating various types of biotic and abiotic stresses. Studies have shown that application of putrescine increases the fresh and dry weight of the shoot and root parts, leaf relative water content, photosynthetic pigments, leaf surface, and photosynthesis in plants under drought stress.

This study was conducted to investigate the effect of different levels of putrescine (0, 0.5, 1 and 2 mM) and different levels of irrigation (100, 75 and 50% of evotranspiration potential) on morpho-physical traits of lime seedlings as a factorial experiment based on randomized complete block design with 3 replications. Two-year-old lime seedlings were obtained from a commercial nursery located in Dezful city (approved by the Khuzestan Agricultural Jihad Organization). Then, they were located in 15-kilogram pots and kept for 2 months in the greenhouse to adapting to the environmental conditions. In order to apply the irrigation regime, 4 pots were considered as reference plants and the amount of irrigation water was determined by weighing these pots. First, the weight of reference pots was calculated in field capacity mode. Then, after 7 days, the pots were weighed again and the difference between the primary and secondary weights was considered as the amount of irrigation water of 100% plant evaporation and transpiration, and according to that, 75% irrigation and 50% evaporation and transpiration potential were applied. The first foliar spraying with putrescine was done at first of March in Field capacity (foliar spraying was done once every month for 4 months from March to June). At the end of the experiment, the fresh and dry weight of root and shoot, number of leaves, relative water content, leaf water potential, photosynthesis, transpiration, stomatal conductance, were measured. Statistical data analysis was done using MSTATC software and, Duncan's multi-range test was used to mean comparation at the 5% probability level.

Results showed that the rate of photosynthesis, stomatal conductance, relative water content of leaves, fresh and dry weight of aerial part and root decreased by reducing the amount of irrigation from 100 to 75 and 50%, of ETcrop. The reduction of growth parameters under drought stress can be due to the closing of the stomata and the reduction of carbon dioxide emission into the leaves, which can lead to lower levels of chlorophyll and photosynthesis, induction of oxidative stress, and finally less growth in plants. It has also been stated that the decrease in growth caused by drought stress in the initial stages of the stress can be due to the decrease in cell growth and development due to the decrease in turgor pressure and the decrease in the intensity of photosynthesis due to the closing of stomata. Also, the results showed that foliar spraying with 2 mM putrescine increased photosynthesis, stomatal conductance, relative water content of leaves, wet and dry weight of aerial parts and roots at all irrigation levels. The researchers believed that the increase in growth parameters, relative water content and photosynthetic pigments with putrescine foliar spraying can be related to the antioxidant properties of putrescine and its osmolality role in dry conditions. Other researches have shown that putrescine may modulate certain ion channels and increase the permeability of the membrane to calcium and cause a decrease in the entry of potassium into the membrane, which causes a decrease in the exit of water from the cell. Also, putrescine may increase leaf water potential and leaf content through osmotic regulation of the plant by increasing proline.

In general, the results showed that foliar spraying of putrescine, especially at 2 mM concentration has the greatest effect on increasing growth parameters, including fresh and dry weight of shoots and roots, leaf area, increasing the relative water content, leaf water potential and Gas exchanges and reducing the amount of ion leakage under drought stress conditions.