Salt stress exerts numerous negative impacts on plants. It commonly slows down plant growth by influencing physiological processes such as photosynthesis. For instance, it reduces the content of photosynthetic pigments in leaves, the activity of hormones and enzymes, thereby inhibiting photosynthesis. This ultimately leads to a significant decrease in the grain yield of barley under salt stress conditions. The growth retardation is likely caused by factors such as osmotic pressure, ion toxicity, limitations in nutrient uptake, weakened photosynthesis, and the accumulation of Na⁺ in plant tissues.
Previously, numerous studies have shown that humic acid has a positive stimulating effect on crop growth, yield increase, and nutrient absorption. The use of humic acid or fulvic acid produces various effects, including direct or indirect effects. Indirect effects are mainly achieved through characteristics such as increasing soil nutrients, increasing cation exchange capacity (CEC), increasing microbial populations, and improving soil structure. While direct effects act directly on the cell membrane, cell wall, or cytoplasm, triggering a series of biochemical reactions, and the impact is not obvious, mainly hormonal in nature. In many studies, the hormone-like activity of humic acid has been well demonstrated, especially for auxins, gibberellins, and cytokinins. This is because organic amendments have a positive impact on yield through different and non – mutually exclusive mechanisms, including providing mineral nutrients, and the use of organic amendment treatments can significantly improve the growth parameters and yield of barley plants.
The Function of Organic Matter and Potassium Humate in Soil and Plant Growth
Organic matter plays a huge and important role in the soil ecosystem. The soil provides substrates for microbial decomposition (which in turn provides mineral nutrients for plants) and improves soil water – holding capacity and soil structure. The rapid availability of nutrients, especially the release and mineralization kinetic patterns of N, explains this. The positive role of potassium humate in promoting plant growth is due to its increase in the organic matter content in the growth medium, thereby improving water – holding capacity and water availability, preserving mineral nutrients and their availability, and enhancing the absorption of mineral nutrients by plant roots. In addition, potassium humate is more effective in promoting plant growth, which is attributed to the controlling effect of K on various enzymes in plants, and potassium humate also plays an important role as a biostimulant.
Under abiotic stress, plants relieve these stresses through multiple mechanisms. One of the mechanisms is to enhance the activity of antioxidant enzymes, such as catalase (CAT), peroxidase (POX), superoxide dismutase (SOD), and proline. These enzymes play a very important role in scavenging reactive oxygen species (ROS). Multiple studies have confirmed that under salt stress conditions, potassium humate treatment can increase the activity of antioxidant enzymes (CAT, SOD, and POX).
The Relationship between Salt Stress, Proline, and Plant Tolerance
The salinity of irrigation water or the soil in which barley grows can lead to a decrease in plant height, the number of ears, thousand – grain weight, and grain yield. At the same time, total chlorophyll, relative water content, leaf osmotic potential, proline, and K content are considered biochemical parameters of salt tolerance. Many early studies have confirmed that proline content and accumulation are not only one of the most important physiological indicators of barley salt tolerance but also related to osmotic tolerance and salt stress. In the studied salt – tolerant genotypes, the proline concentration increases with the increase of salt stress and changes progressively as the stress intensifies. Misra and Gupta also found a positive correlation between the accumulation of free proline and salt tolerance, which can be used as an index to determine the salt – tolerance potential of varieties, and the increase in free proline accumulation in salt – tolerant varieties is higher than that in sensitive varieties. Proline plays an important role in reducing the destructive effects of salt and accelerating the repair process after stress. This is because proline is the only molecule among many compatible solutes that can act as a free – radical scavenger and has antioxidant activity. Proline can stabilize proteins, DNA, and cell membrane structures. Research shows that under salt stress, a high concentration of proline in salt – tolerant plants may help maintain the structure and function of cellular macromolecules. Many reports also indicate that proline, as an osmoprotectant, is related to the tolerance mechanism of plants under salt stress. In addition to acting as an osmotic regulator, proline also has an enzyme – protecting effect and enhances membrane stability. Many studies have also found that under humic acid treatment and other treatments that improve plant salt tolerance, the accumulation of proline is related to the degree of salt tolerance and/or osmotic tolerance.
In summary, salt stress poses significant challenges to plant growth and productivity, especially for barley. However, the application of organic amendments such as HA and FA shows great potential in alleviating salt stress. They can improve soil conditions, promote plant growth, and enhance the plant’s tolerance to salt through various physiological and biochemical mechanisms. Further research in this area can help develop more effective strategies for sustainable agriculture in saline – alkali soil areas.