The effect of GABA on some morpho-physiological parameters of watermelon seedlings under cold stress

Watermelon (Citrullus lanatus) belongs to the Cucurbitaceae family. All plants of this family are sensitive to chilling and their planting is done in the warm season. Plants that are always in a wide range of abiotic stresses that have adverse effects on the survival, growth, quality and quantity of agricultural products. In most plants, chilling stress occurs below 10 to 15 °C, and the cucurbitaceae family, including watermelon, shows chilling symptoms at temperatures below 10 °C. Watermelon is one of the most popular tropical products of this family, and the late spring chilling in temperate and cold regions always threatens its seedlings with chilling. γ-aminobutyric acid (GABA) is a non-protein amino acid that can reduce stress in various plants. Therefore, in present research, the effect of GABA treatments as seed priming and foliar spraying was studied on reducing the amount of chilling index and maintaining the quality of watermelon seedlings under cold stress.

The present study was designed and implemented in the research greenhouse of Ilam University in 2018 as a factorial experiment based on a completely randomized design with three repetitions and each repetition including five pots. The first factor included 0, 5 and 10 mM GABA and the second factor was the application method of GABA treatments including seed priming and foliar spraying. The seed priming treatment was applied before planting and the foliar spraying treatment was applied at the seedlings to the stage of four true leaves. At 48 h after foliar spraying, all the seedlings were subjected to cold stress at a temperature of 3 °C for 6 days by 6 h at night. They were placed inside the incubator at night (24:00 to 06:00). 48 h after the end of the cold stress, the seedlings were examined for different traits. The studied traits included chilling index, relative water content, fresh and dry weight of shoot and root, total chlorophyll and carotenoid, malondialdehyde, proline, ion leakage and activity of ascorbate peroxidase and catalase enzymes.

The results showed that the exogenous application of GABA effectively reduced the complications and chilling index, the amount of ion leakage, malondialdehyde and proline in watermelon seedlings, and the RWC, fresh and dry weight of the shoots and roots, total chlorophyll, carotenoids. Also, GABA increased the activity of ascorbate peroxidase and catalase enzymes in watermelon seedlings. This was despite the fact that foliar spraying treatments are more effective than the use of seed priming treatments. Also, in both application methods, the effect of GABA 10 mM in reducing chilling capital and biochemical changes of watermelon seedlings was more than 5 mM. Probably, the exogenous application of GABA has moderated the increase in cell osmolality with the intensity of cold stress by strengthening the antioxidant system and thus reduced the amount of chilling in watermelon seedlings. GABA has been reported as an osmolyte compatible with osmotic stress. As a result, the accumulation of this osmolyte compound in plant cells is used to maintain cell turgor and also to protect the cell membrane, nutrition and metabolism of the plant by preventing the loss of cell water.

The present study showed that keeping watermelon seedlings under cold stress causes symptoms of chilling and reducing the quality of the seedlings, but the exogenous application of GABA moderated the chilling and improved the quality of the seedlings compared to the control samples. In this study, the foliar application of GABA was more effective than seed priming and 10 mM concentration maintained the quality of seedlings better than 5 mM in both application methods. Therefore, based on the present results, the use of GABA 10 mM is recommended as a practical solution to prevent damage in "Orangeglo" watermelon seedlings under cold stress.