Table 2.

A summary of exogenous melatonin application before abiotic stress treatment in enhancing abiotic stress tolerance.

Stress	Plant	Growth Stage	Melatonin Application		Activity under Stress (Compared to Plants without Exogenous Melatonin Stressed)	Reference
			Optimised Concentration	Application Method
Cold	Pepper (Capsicum annuum)	Seedling	5 µM	Soil drench with 25 mL melatonin solution one day before chilling stress	Decreased hydrogen peroxide, malondialdehyde contents and membrane permeability. Increased photosynthesis, water relation and antioxidant enzyme activity.	[64]
		Flowering	5 µM	Soil drench with 100 mL melatonin solution	Increased the yield when applied during flowering.	[64]
	Tea (Camellia sinensis)	Seedling (two-year-old)	100 µM	Foliage spray—three times with five-day intervals. (last treatment—24 h before the stress)	Increased antioxidant enzyme activity and stimulate photosynthesis.	[65]
	Barley (Hordeum vulgare)	Seeds	1 µM	Soaked the seeds for 12 h before germination.	Decreased malondialdehyde and soluble sugar content. Promoted seedling growth, increased chlorophyll content.	[66]
Drought	Rapeseed (Brassica napus)	Seedling	100 µM	Irrigation with 200 mL melatonin solution per pot each day for seven days.	Decreased malondialdehyde and hydrogen peroxide. Regulated leaf stomatal activity. Increased root growth and catalase activity.	[67]
	Tomato (Solanum lycopersicum)	Young plants (5 weeks old)	20 ppm	Foliar application	Decreased malondialdehyde and hydrogen peroxide. Increased yield and ascorbic acid content in fruits.	[68]
	Alfalfa (Medicago sativa)	Seedlings	100 µM	Sprayed at dark, two days before the stress and repeated every three days up to 20 days.	Increased chlorophyll and carotenoid contents, photosynthetic rate and stomatal conductance, soluble sugar and proline content. Decreased malondialdehyde, hydrogen peroxide, electrolyte leakage and superoxide anion.	[69]
	Coffee (Coffea arabica)	Seedlings	100 µM	Foliar spray of 20 mL and soil application of 30 mL three times per week.	Suppressed chlorophyll degradation and increased photosynthesis. Decreased malondialdehyde and electrolyte leakage. Increased enzymatic antioxidant activity.	[70]
	Soybean (Glycine max)	Seedlings	100 µM	Rhizosphere application	Increased chlorophyll content, photosynthetic activity, shoot and root growth, enzymatic antioxidation. Increased salicylic and jasmonic acid content. Decreased malondialdehyde, electrolyte leakage and hydrogen peroxide.	[71]
	Cotton (Gossypium hirsutum)	Seeds	100 µM	Soaked the seeds for 24 h prior to germination.	Increased soluble sugar and proline content. Increased stomatal regulation, germination rate, germination potential and fresh weight. Decreased the hydrogen peroxide, superoxide anion and malondialdehyde.	[72]
Water Logging	Soybean (Glycine max)	Seedling	10 µM	Root application at the same time of flood.	Increased root growth and development, increased root cell wall lignification. Increased alkaloid metabolism and ROS scavenging.	[73]
	Alfalfa (Medicago sativa)	Seedling	100 µM	Foliar spray one day before to the stress	Increased plant growth and photosynthesis. Increased endogenous melatonin levels. Increased polyamines and decreased ethylene. Decreased membrane damage and leaf senescence.	[60]
	Peach (Prunus persica)	seedlings	200 µM	Applied to soil every other day during the stress.	Increased root and shoot development. Positive development in photosynthetic and stomatal apparatus. Increased antioxidant activities. Increased anaerobic respiration through enhanced aerenchyma.	[74]
Salt	Tomato (Solanum lycopersicum)	Seedling	1 µM	Applied to the medium mixed with saline treatment	Increased photosynthesis and antioxidant enzyme activity. Decreased malondialdehyde and hydrogen peroxide content.	[75]
	Olive (Olea europaea)	Seedling	100 µM	Foliar spray	Increased shoot and root growth, photosynthetic pigments, proline and soluble sugars. Increased enzymatic antioxidation. Decreased hydrogen peroxide, malondialdehyde and electrolyte leakage.	[76]
	Alfalfa (Medicago sativa)	Seeds	10–100 µM	Seeds immersed and air-dried prior to germination	Increased seed germination, root length, seedling growth and enzymatic anti-oxidation.	[77]
		One-month-old plants	50 µM	Foliar spray every night	Decreased hydrogen peroxide, malondialdehyde and electrolyte leakage
	Rice (Oryza sativa)	Seedlings	20 µM	Applied for 24 h prior to salt stress	Increased root and shoot growth. Increased the expression of stress-responsive genes.	[78]
	Rapeseed (Brassica napus)	Seeds	50 µM	Seed primed for 8 h prior to germination	Decreased hydrogen peroxide and superoxide anions. Increase the regulation of antioxidant enzymes, chlorophyll content, photosynthetic rate and proline content. Improved the oil quality.	[79]
Cadmium	Wheat (Triticum aestivum)	Seedling	0.5–100 µM	Applied directly to the roots of seedlings growing in Petri dishes	Increased root and shoot growth, Increased enzymatic and non-enzymatic anti-oxidants. Decreased hydrogen peroxide content.	[80]
Aluminium	Wheat (Triticum aestivum)	Seedling	10 µM	Treated for 12 h prior to the stress	Increased enzymatic and non-enzymatic antioxidant activity	[81]
Nickel	Tomato (Solanum lycopersicum	Seedling	100 µM	Foliar sprayed with 80 mL solution with 3 days interval during the stress	Improved photosynthesis and gas exchange. Increased enzymatic anti-oxidation. Upregulation of stress-responsive genes. Decreased hydrogen peroxide, malondialdehyde and electrolyte leakage	[59]
Heat	Tomato (Solanum lycopersicum)	Seedling	100 µM	Foliar sprayed every two days for seven days one week before the stress	Increased photosynthesis and stomatal activity. Decreased hydrogen peroxide, malondialdehyde and electrolyte leakage. Downregulation of genes encoding ROS accumulation.	[82]
	Wheat (Triticum aestivum)	Seedling	100 µM	Sprayed 80 mL of melatonin solution on leaves each day for seven days one week before the stress.	Increased chlorophyll content, enzymatic and non-enzymatic antioxidant activity and proline content. Decreased hydrogen peroxide and malondialdehyde.	[58]
	Cherry radish (Raphanus sativus)	Seedling	29.0 mg/L	Applied on roots mixed with Hogland’s nutrient solution.	Increased chlorophyll, carotenoid content and enzymatic antioxidation. Decreased malondialdehyde.	[83]
	Strawberry (Fragaria × ananassa)	Young plants (3 weeks old)	100 µM	Foliar spray three times at one-day intervals (last treatment—10 h prior to the stress)	Increased enzymatic and non-enzymatic antioxidant activity. Upregulation of stress-responsive genes. Decreased hydrogen peroxide, and malondialdehyde.	[57]
	Rice (Oryza sativa)	Flowering	200 µM	Sprayed one day before the stress treatment.	Increased chlorophyll content and stomatal conductance.	[84]
	Tomato (Solanum lycopersicum	Flowering	20 µM	Applied on roots on each day for 7 days followed by the heat stress.	Alleviated pollen abortion. Increased stability of tapetum cells and avoid pollen deformity by inducing stress-responsive genes. Increased ROS scavenging and enzymatic antioxidant activity.	[85]
	Soybean (Glycine max)	Seedling	100 µM	Applied on root zone (30 mL) twice daily for 6 days prior to stress.	Increased chlorophyll content and non-enzymatic anti-oxidation. Decreased hydrogen peroxide, superoxide, malondialdehyde and electronic leakage.	[86]
	Kiwifruit (Actinidia deliciosa)	Seedling	200 µM	Treated five times every two days prior to stress.	Increased proline, enzymatic and non-enzymatic antioxidant activity. Reduced hydrogen peroxide.	[87]
	Tall fescue (Festuca arundinacea Schreb.)	Seedling	20 µM	Seedlings were transferred to MS medium containing melatonin two days prior to the stress.	Increased antioxidant enzyme activity and chlorophyll content. Changed stress-responsive gene regulation. Reduced hydrogen peroxide, superoxide anion, malondialdehyde and electronic leakage.	[88]