Melatonin Counteracts Drought Induced Oxidative Damage and Stimulates Growth, Productivity and Fruit Quality Properties of Tomato Plants

Abstract

Melatonin "N-Acetyl-5-methoxytryptamine" (MT) has recently been considered as a new plant growth regulator with multiple physiological functions. Although many previous studies have confirmed that exogenous applied-MT can alleviate the deleterious effects of drought stress in many plant species, most of these studies were exclusive on seeds, seedlings, and young plants for a short period of their life cycles. Therefore, the knowledge of using MT as a potential promising agricultural foliar application to improve crop productivity and quality is still insufficient under adverse open field conditions. In this study, we investigated the effect of MT as a foliar application at 0, 20, and 40 ppm on tomato plants that were grown in the open field under the long term of optimal and deficit irrigation conditions. The results indicated that exogenous MT significantly enhanced plant growth, chlorophyll and activities of antioxidant enzymes, including ascorbate peroxidase (APX), catalase (CAT), and peroxidase (POX). This improvement was associated with a marked reduction in proline and soluble sugars. In addition, applied-MT worked as a protective agent against oxidative damage by reducing the cellular content of toxic substances such as H₂O₂ and malondialdehyde (MDA). Similarly, MT-treated plants showed greater total fruit yield with improving its quality attributes like total soluble solids (TSS), ascorbic acid, and lycopene. Generally, the highest significant fruit yield either under well-watered (13.7%) or water deficit (37.4%) conditions was achieved by the treatment of 20 ppm MT. These results indicate that exogenous MT played an essential role in enhancing tomato tolerance to deficit irrigation and could be recommended as a promising agricultural treatment under such conditions.

Keywords: Melatonin; Solanum lycopersicum L.; antioxidant enzymes; drought; quality attributes.

Figure 1

Effect of foliar application of melatonin (MT) at 0, 20 and 40 ppm on shoot fresh weight (A), shoot dry weight (B), and total chlorophyll (C) of tomato plants grown under well-watered and water deficit conditions in the seasons of 2018 and 2019. Means of three replicates were presented ±SD. Different letters are significant differences, according to Duncan’s multiple range tests (p < 0.05).

Figure 2

Effect of foliar application of melatonin (MT) at 0, 20 and 40 ppm on proline (A) and soluble sugars (B) of tomato plants grown under well-watered and water deficit conditions in the seasons of 2018 and 2019. Means of three replicates were presented ± SD. Different letters are significant differences, according to Duncan’s multiple range tests (p < 0.05).

Figure 3

Effect of foliar application of melatonin (MT) at 0, 20 and 40 ppm on (A) the concentration of H₂O₂, (B) lipid peroxidation as indicated by malondialdehyde (MDA) and (C) H₂O₂ distribution using DAB staining in the terminal leaflet of the 4th leaf of tomato plants grown under two different irrigation regimes (Brown DAB staining color indicates accumulation of H₂O₂. Bar = 1 cm). Means of three replicates were presented ±SD. Different letters are significant differences, according to Duncan’s multiple range tests (p < 0.05).

Figure 4

Effect of foliar application of melatonin (MT) at 0, 20 and 40 ppm on the activities of antioxidant enzymes; ascorbate peroxidase (POX; A), catalase (CAT; B) and peroxidase (POX, C) of tomato plants grown under well-watered and water deficit conditions in the seasons of 2018 and 2019. Means of three replicates were presented ±SD. Different letters are significant differences, according to Duncan’s multiple range tests (p < 0.05).

Figure 5

Effect of foliar application of d melatonin (MT) at 0, 20 and 40 ppm on (A) fruit yield, (B) total soluble solids (TSS), (C) ascorbic acid and (D) lycopene of tomato plants grown under well-watered and water deficit conditions in the seasons of 2018 and 2019. Means of three replicates were presented ±SD. Different letters are significant differences, according to Duncan’s multiple range tests (p < 0.05).