Exogenous melatonin improves salt stress adaptation of cotton seedlings by regulating active oxygen metabolism

Melatonin is a small-molecule indole hormone that plays an important role in participating in biotic and abiotic stress resistance. Melatonin has been confirmed to promote the normal development of plants under adversity stress by mediating physiological regulation mechanisms. However, the mechanisms by which exogenous melatonin mediates salt tolerance via regulation of antioxidant activity and osmosis in cotton seedlings remain largely unknown. In this study, the regulatory effects of melatonin on reactive oxygen species (ROS), the antioxidant system, and osmotic modulators of cotton seedlings were determined under 0-500 µM melatonin treatments with salt stress induced by 150 mM NaCl treatment. Cotton seedlings under salt stress exhibited an inhibition of growth, excessive hydrogen peroxide (H₂O₂), superoxide anion (O₂ ^-), and malondialdehyde (MDA) accumulations in leaves, increased activity levels of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX), and elevated ascorbic acid (AsA) and glutathione (GSH) content in leaves. However, the content of osmotic regulators (i.e., soluble sugars and proteins) in leaves was reduced under salt stress. This indicates high levels of ROS were produced, and the cell membrane was damaged. Additionally, osmotic regulatory substance content was reduced, resulting in osmotic stress, which seriously affected cotton seedling growth under salt stress. However, exogenous melatonin at different concentrations reduced the contents of H₂O₂, O₂ ^-, and MDA in cotton leaves, increased the activity of antioxidant enzymes and the content of reductive substances (i.e., AsA and GSH), and promoted the accumulation of osmotic regulatory substances in leaves under salt stress. These results suggest that melatonin can inhibit ROS production in cotton seedlings, improve the activity of the antioxidant enzyme system, raise the content of osmotic regulation substances, reduce the level of membrane lipid peroxidation, and protect the integrity of the lipid membrane under salt stress, which reduces damage caused by salt stress to seedlings and effectively enhances inhibition of salt stress on cotton seedling growth. These results indicate that 200 µM melatonin treatment has the best effect on the growth and salt tolerance of cotton seedlings.

Phytochrome-mediated accumulation of free amino acids in embryonic axes of New York lettuce seeds

Red light given to New York lettuce seeds incubated on a 0.3 m mannitol solution increased the amount of free amino acids in the embryonic axes, far-red light reversed this effect. Germination in response to red light given at various times during imbibition was closely related to the amount of free amino acids accumulated in the axes in response to the red light. The analyses of the free amino acids that accumulated in the axes of seeds incubated on 0.3 m mannitol indicated that the free amino acids that accumulated on red-light irradiation were produced by the degradation of storage protein. This is evidence that red-light-increased germination may be caused by the accumulation of free amino acids in the embryonic axes of New York lettuce seeds.

Effects of temperature, red light and hormones on the accumulation of free amino acids in osmotically growth-inhibited embryonic axes of New York lettuce seeds

Free amino acids accumulated in non-thermodormant embryonic axes of New York lettuce seeds from the 16th to the 24th hour of imbibition, even when germination was prevented with 0.3 m mannitol. The accumulation of free amino acids in the embryonic axes was affected by temperature, red light and hormones (gibberellic acid, kinetin and abscisic acid), and was correlated with the growth potential of the axes. These data are evidence that the accumulation of free amino acids in the embryonic axes is not the result of axes-growth, but one of its causes.

Accumulation of free amino acids in the tips of non-thermodormant embryonic axes accounts for the increase in the growth potential of New York lettuce seeds

Accumulation of free amino acids took place only in growing axes during the first 24 hr of imbibition at 18°C. A ninhydrin-positive section was shown to be located at the tips of the axes by a histochemical technique. The amount of free amino acids accumulated just at the breaking of the seed coats in the non-dormant axes was great enough to account for the increase in the growth potential, i.e., the force to rupture the testa. About 72% of the accumulated free amino acids in the non-thermodormant embryonic axes was comprised of glutamine and glutamate.