Abstract
Tomato is an important vegetable that is highly sensitive to drought (DR) stress which impairs the development of tomato seedlings. Recently, melatonin (ME) has emerged as a nontoxic, regulatory biomolecule that regulates plant growth and enhances the DR tolerance mechanism in plants. The present study was conducted to examine the defensive role of ME in photosynthesis, root architecture, and the antioxidant enzymes’ activities of tomato seedlings subjected to DR stress. Our results indicated that DR stress strongly suppressed growth and biomass production, inhibited photosynthesis, negatively affected root morphology, and reduced photosynthetic pigments in tomato seedlings. Per contra, soluble sugars, proline, and ROS (reactive oxygen species) were suggested to be improved in seedlings under DR stress. Conversely, ME (100 µM) pretreatment improved the detrimental-effect of DR by restoring chlorophyll content, root architecture, gas exchange parameters and plant growth attributes compared with DR-group only. Moreover, ME supplementation also mitigated the antioxidant enzymes [APX (ascorbate peroxidase), CAT (catalase), DHAR (dehydroascorbate reductase), GST (glutathione S-transferase), GR (glutathione reductase), MDHAR (monodehydroascorbate reductase), POD (peroxidase), and SOD (superoxide dismutase)], non-enzymatic antioxidant [AsA (ascorbate), DHA (dehydroascorbic acid), GSH (glutathione), and GSSG, (oxidized glutathione)] activities, reduced oxidative damage [EL (electrolyte leakage), H2O2 (hydrogen peroxide), MDA (malondialdehyde), and O2•− (superoxide ion)] and osmoregulation (soluble sugars and proline) of tomato seedlings, by regulating gene expression for SOD, CAT, APX, GR, POD, GST, DHAR, and MDHAR. These findings determine that ME pretreatment could efficiently improve the seedlings growth, root characteristics, leaf photosynthesis and antioxidant machinery under DR stress and thereby increasing the seedlings’ adaptability to DR stress.
Highlights
In recent decades, water has been considered a vital environmental factor, and its deficiency is known as drought stress (DR) which leads to restricted plant growth, yield, and quality, mainly due to various morphological, physiological, anatomical, and biochemical responses
Stress, significantly reduced in plant height (57.52%), fresh shoot weight (FSW; 61.11%), dry shoot weight (DSW; 63.58%), fresh root weight (FRW; 63.96%), and dry root weight (DRW; 64.74%), compared with CK plants (Figure 2) This decline in the growth of tomato seedlings was alleviated by the exogenous application of melatonin (Figures 1 and 2)
After pretreatment with ME, growth limitations caused by DR stress were improved, and less reductions in plant height (26.43%), FSW (30.25%), DSW (37.06%), FRW (35.63%), and DRW (26.13%) was observed (Figure 2)
Summary
Water has been considered a vital environmental factor, and its deficiency is known as drought stress (DR) which leads to restricted plant growth, yield, and quality, mainly due to various morphological, physiological, anatomical, and biochemical responses. Water scarcity adversely influences plant growth by reducing relative water content (RWC), root length, seedling stem, leaf area, and leaf water potential [2]. Plants exhibit multiple and interconnected responses towards DR stress. Earlier studies revealed that DR stress can cause reduced pigment content, imbalanced ion homeostasis, decreased transpiration, stomatal closure, cell enlargement reduction, reduce canopy size, and lead towards plant death [4]. The impact of DR stress varies with the intensity and the growth stage of plants [5]
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