Ameliorative effect of melatonin improves drought tolerance by regulating growth, photosynthetic traits and leaf ultrastructure of maize seedlings

Abstract

Background: Melatonin is considered a potential plant growth regulator to enhance the growth of plants and increase tolerance to various abiotic stresses. Nevertheless, melatonin's role in mediating stress response in different plant species and growth cycles still needs to be explored. This study was conducted to understand the impact of different melatonin concentrations (0, 50, 100, and 150 μM) applied as a soil drench to maize seedling under drought stress conditions. A decreased irrigation approach based on watering was exposed to maize seedling after drought stress was applied at 40-45% of field capacity.

Results: The results showed that drought stress negatively affected the growth behavior of maize seedlings, such as reduced biomass accumulation, decreased photosynthetic pigments, and enhanced the malondialdehyde and reactive oxygen species (ROS). However, melatonin application enhanced plant growth; alleviated ROS-induced oxidative damages by increasing the photosynthetic pigments, antioxidant enzyme activities, relative water content, and osmo-protectants of maize seedlings.

Conclusions: Melatonin treatment also enhanced the stomatal traits, such as stomatal length, width, area, and the number of pores under drought stress conditions. Our data suggested that 100 μM melatonin application as soil drenching could provide a valuable foundation for improving plant tolerance to drought stress conditions.

Keywords: Antioxidant enzymes; Drought stress; Leaf ultrastructure; Maize; Melatonin.

Fig. 1

Effects of melatonin on net photosynthetic rate, transpiration rate, stomatal conductance, and intercellular CO₂ of maize seedlings under drought stress. CKW; indicate Melatonin 0 μM + Control well water, CKD; Melatonin 0 μM + Control drought stress, MT1; Melatonin 50 μM + drought stress, MT2; Melatonin 100 μM + drought stress; MT3; Melatonin 150 μM + drought stress. Vertical bars represent ± S.D. (n=3). Different small letters indicate significant differences as determined by the LSD test (p ≤ 0.05)

Fig. 2

Effects of melatonin on Chlorophyll a, chlorophyll b, carotenoid, and chlorophyll a & b (mg g^-1 FW) contents of maize seedlings under drought stress. The names of the treatments are the same as in (Fig. 1). Different lowercase letters indicate significant differences as determined by the LSD test (p ≤ 0.05)

Fig. 3

Effect of melatonin application on stomatal length (μM), stomatal width (μM), stomatal area (μm²), and the number of pores under drought stress condition. The treatments names are the same as those described in (Fig. 1). For stomatal number X300 magnification, scale bars =100 μm, while for stomatal length (μM), stomatal width (μM), stomatal area (μm²) X2000 magnification, scale bars =20 μm

Fig. 4

Effect of melatonin application on stomatal length (μM), stomatal width (μM), stomatal area (μm²), and the number of pores of maize seedling under drought stress condition. The names of the treatments are the same as in (Fig. 1). Different lowercase letters indicate significant differences as determined by the LSD test (p ≤ 0.05)

Fig. 5

Effect of melatonin application on soluble sugar and proline content of maize seedling under drought stress condition. The names of the treatments are the same as in (Fig. 1). Different lowercase letters indicate significant differences as determined by the LSD test (p ≤ 0.05)

Fig. 6

Effect of melatonin application on MDA, soluble protein, H₂O₂ and O₂ contents of maize seedling under drought stress conditions. The names of the treatments are the same as in (Fig. 1). Different lowercase letters indicate significant differences as determined by the LSD test (p ≤ 0.05)

Fig. 7

Effect of melatonin application on antioxidant enzymes activities (SOD, POD, CAT and APX) of maize seedling under drought stress conditions. The names of the treatments are the same as in (Fig. 1). Different lowercase letters indicate significant differences as determined by the LSD test (p ≤ 0.05)

Fig. 8

Diagrammatic sketch explaining the action of melatonin on various responses of plants under drought conditions