Abstract
Melatonin is an indolamine bioactive molecule that regulates a wide range of physiological processes during plant growth and enhances abiotic stress tolerance. Here we examined the putative role of exogenous melatonin application (foliar or root zone) in improving drought stress tolerance in soybean seedlings. Pre-treatment of soybean seedlings with melatonin (50 and 100 µM) was found to significantly mitigate the negative effects of drought stress on plant growth-related parameters and chlorophyll content. The beneficial impacts against drought were more pronounced by melatonin application in the rhizosphere than in foliar treatments. The melatonin-induced enhanced tolerance could be attributed to improved photosynthetic activity, reduction of abscisic acid and drought-induced oxidative damage by lowering the accumulation of reactive oxygen species and malondialdehyde. Interestingly, the contents of jasmonic acid and salicylic acid were significantly higher following melatonin treatment in the root zone than in foliar treatment compared with the control. The activity of major antioxidant enzymes such as superoxide dismutase, catalase, polyphenol oxidase, peroxidase and ascorbate peroxidase was stimulated by melatonin application. In addition, melatonin counteracted the drought-induced increase in proline and sugar content. These findings revealed that modifying the endogenous plant hormone content and antioxidant enzymes by melatonin application improved drought tolerance in soybean seedlings. Our findings provide evidence for the stronger physiological role of melatonin in the root zone than in leaves, which may be useful in the large-scale field level application during drought.
Highlights
In the natural environment, plants are exposed to a wide range of abiotic and biotic stresses such as flooding, drought, heat, salinity, heavy metals and pathogenic microbes, which affect the growth, development and yield of agricultural crops and impact global food security
The results of this study showed that drought stress led to a significant decrease in root/shoot lengths and fresh/dry weights
The application of melatonin via root irrigation (RM50 and RM100) promoted plant growth and development by significantly enhancing the shoot length (44.2 and 55.7 %, respectively), root length (41.4 and 81.1 %, respectively), fresh weight (32.2 and 96.7 %, respectively), dry weight (35.2 and 94.1 %, respectively) and chlorophyll content (20.1 and 36.9 %), respectively, compared with those in plants under drought conditions and treated via foliar application in a concentration-dependent manner (Table 1; Fig. 1). These findings suggest that the application of melatonin via root irrigation has more positive effects on plant growth and development under stress conditions than foliar application, and 100 μM melatonin application can provide more effective protection against drought than 50 μM melatonin
Summary
Plants are exposed to a wide range of abiotic and biotic stresses such as flooding, drought, heat, salinity, heavy metals and pathogenic microbes, which affect the growth, development and yield of agricultural crops and impact global food security. In this context, drought stress is a crucial problem in arid and semiarid environments. Received: 3 December 2020; Editorial decision: 16 April 2021; Accepted: 8 June 2021 To overcome drought-induced damage, plants activate their defence system by enhancing the accumulation of antioxidant enzymes (ascorbate peroxidase, superoxide dismutase, catalase and guaiacol peroxidase) and nonenzymes such as glutathione, carotenoids, α-tocopherol and ascorbic acid, and activating defence mechanisms involving osmotic adjustment and stomata regulation (Shi et al 2016)
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