The Role of Humic Acid Under Various Stress Conditions

A plant is an open system that lives in a natural environment and is therefore inevitably subjected to adverse environmental conditions. Such conditions that harm plants are commonly referred to as abiotic stresses or environmental stresses.

When these adverse conditions arise, plants undergo a series of physiological and morphological changes: drought causes wilting of leaves and young stems; waterlogging leads to yellowing and desiccation of leaves, browning of roots, and even root rot; under high temperatures, leaves turn brown, develop necrotic spots, and the bark cracks; and pathogen infection results in the formation of disease lesions on the leaves. At mild levels, these stresses impair plant growth, yield, and quality; at severe levels, they can even cause outright mortality.

Unlike humans and animals, plants cannot escape from any danger they face. Since they cannot flee, they have adapted by developing a remarkable ability to withstand adversity—this is the origin of plant stress tolerance.

For example, under drought conditions, plants maintain water balance by regulating the opening and closing of stomata on their leaves; when exposed to high-salinity stress, they enhance cell membrane permeability to prevent excessive influx of saline solutions into the plant; and even in the face of pest or pathogen attacks, some plants can synthesize chemical compounds to defend themselves or attract natural enemies that prey on these pests and pathogens.

In general, plants exhibit relatively low stress tolerance during their peak growth phase, but their stress resistance increases once they enter dormancy; they display strong stress tolerance during the vegetative stage, while their resilience is weaker during the flowering period. However, if environmental stresses exceed their capacity to withstand them, even plants cannot escape adverse outcomes.

Humic acid is a natural mixture of organic macromolecular compounds, widely found in nature. Owing to its unique physiological functions, it has been extensively applied in agricultural production in recent years. In addition to improving soil quality, enhancing fertilizer efficiency, and promoting crop growth, its role in plant stress resistance has also attracted increasing attention.

1. Plant Drought Stress

Under drought conditions, the application of humic acid–containing fertilizers can enhance the accumulation of proline in plant leaves, thereby improving cellular water‑holding capacity. At the same time, it increases the activities of cellular protective enzymes—such as nitrate reductase, superoxide dismutase, and peroxidase—and reduces plasma membrane permeability, collectively enhancing crop drought tolerance.

2. Salt-Alkali Stress in Plants

Humic substances can indirectly enhance plant stress tolerance and resilience by improving the soil’s physicochemical properties. In addition, humic acids can modulate fructose concentrations, thereby alleviating plasma membrane damage under salt stress and bolstering salt tolerance.

3. Plant Temperature Stress

Under low-temperature stress, humic acid can enhance crop cold tolerance by increasing proline and abscisic acid levels, boosting polyphenol oxidase activity, and mitigating the extent of reductions in transpiration rate, stomatal conductance, and net photosynthetic rate following cold stress.

Under high-temperature stress, humic acid also exhibits favorable effects. The activities of superoxide dismutase and peroxidase in crops are enhanced, while the rate of superoxide anion production and the level of hydrogen peroxide are reduced, thereby improving the plants’ heat tolerance.

4. Heavy Metal Stress in Plants

Humic acid contains a variety of functional groups that, through complexation, chelation, and reduction, can immobilize heavy metals on the surfaces of soil particles, thereby influencing their mobility and reducing the water‑soluble forms of these metals, thus lowering their toxicity. Meanwhile, humic acid can also alleviate the phytotoxic effects of cadmium on seedlings, primarily by inhibiting cadmium uptake while promoting the absorption and accumulation of essential nutrients such as copper, zinc, iron, and manganese.

5. Plant Disease Resistance

Humic acids can promote plant growth by increasing leaf sugar and chlorophyll levels, thereby indirectly enhancing crop resistance to diseases. Furthermore, studies have shown that sodium humate can inhibit the germination of downy mildew sporangia, and when combined with other pesticides, it can significantly reduce the incidence of downy mildew. In addition, humic substances can enhance control of root-knot nematodes. Due to their unique structure and multifunctional properties, humic acids are widely used as adjuvants in agrochemical formulations, underscoring their beneficial effects on plant disease resistance.

Humic acid is an important constituent of organic matter; it can reduce fertilizer application, enhance nutrient-use efficiency, partially substitute for bio‑synthesized plant growth regulators, improve the quality of fruits and vegetables, strengthen plants’ stress tolerance, decrease the incidence of diseases and pests, and promote early vegetative growth and flowering. With such powerful functions, it deserves widespread adoption.