The Function of Humic Acid in Different Adversities
Drought Stress
Under drought conditions, application of humic acid fertilizer can promote the increase of proline content in plant leaves, the water holding capacity of cells and the activity of plant cell protective enzymes (nitrate reductase superoxide dismutase peroxidase, etc.). It can also reduce the permeability of cytoplasmic membrane to comprehensively improve the drought resistance of crops.
Saline-Alkali Stress
Humic acid substances can indirectly improve the stress resistance of plants by improving the physical and chemical properties of soil. In addition, humic acid can also regulate the change of fructose concentration to alleviate the damage of membrane under salt stress, so as to enhance its salt resistance.
Temperature Stress
Under low temperature stress, humic acid can enhance the activity of polyphenol oxidase by increasing the content of proline and abscisic acid of crops. It also can reduce the reduction of transpiration rate, stomatal conductance and net photosynthetic rate of plants to improve their cold resistance.
Under high temperature stress, humic acid also had a good performance. The activities of superoxide dismutase peroxidase and other activities of crops were improved, which reduced the production rate of superoxide anion and the content of hydrogen peroxide, and enhanced the thermal resistance of crops.
Heavy Metal Stress
Humic acid contains a variety of functional groups, which can fix heavy metals on the surface of soil particles through complexation, chelation and reduction, thereby affecting their migration, reducing the water-soluble state of heavy metals and reducing their toxicity. Meanwhile, humic acid can also alleviate the toxic effect of heavy metal cadmium on seedlings, the main mechanism is to inhibit the absorption of cadmium, and promote the absorption and accumulation of copper, zinc, iron, manganese and other nutrients.
Plant Disease Resistance
Humic acid can promote growth by increasing the content of sugar and chlorophyll in leaves and indirectly improve the disease resistance of crops. And research shows that sodium humate can inhibit the sporangium germination of downy mildew, and the combination with other pesticides can significantly reduce the occurrence of downy mildew. And humic acids can also enhance the control effect against root knot nematodes. Humic acid is widely used as a pesticide assistant because of its special structure and function.