Photo of a salt crust formed on soil. Image from ImagGeo.

Plants vary greatly in their tolerance to sodium and saline conditions. Salt-sensitive plants have no ability to sequester Na, so it accumulates in their cytoplasm and quickly begins to disrupt photosynthesis. Salt-tolerant plants are able to concentrate excess Na in the vacuole to detoxify it, but as concentrations continue to increase their cellular functions are ultimately affected. Salt-stressed plants overproduce high levels of reactive oxygen species (ROS), which oxidize and damage cellular components.

The primary direct effect of salt stress is inhibited water uptake. Without water, photosynthesis is hindered, and plants are more susceptible to drought and heat stress. To compound this effect, salt stress can also inhibit root development, preventing plants from accessing available water.

Salt stress commonly causes nutrient imbalances in plants. As salt concentrations in the soil increase, the relative concentrations of beneficial nutrients such as K+ decrease, making it difficult for plants to maintain a proper ratio. Additionally, salt stress can destabilize the cell wall and alter membrane permeability, which can cause ion leakage and improper function of nutrient transporter proteins, further contributing to nutrient imbalances. These symptoms can significantly inhibit plant growth and reduce seed germination, yield, and fruit quality.

Salts most often accumulate in arid environments, where frequent rainfall does not flush them from the soil. Flushing the soil with plenty of irrigation water can significantly reduce salt concentrations. Flushing the soil with a calcium solution can be even more effective, as the calcium displaces sodium from soil exchange sites. This also replenishes soil calcium and builds soil structure. But in some cases, especially where water is scarce, flushing is unrealistic.

Promoting a healthy soil microbiome can help plants cope with saline conditions. Saline conditions reduce the colonization rates of arbuscular mycorrhizal fungi (AMF) and plant growth-promoting rhizobacteria (PGPR), but they can survive these conditions and have been shown to enhance salt tolerance in a wide range of host plants. AMF form a network of fine hair-like mycelia through the soil, accessing water and transporting it to the plant. The mycelia act as an initial filter for the plant’s roots, preventing the uptake of toxic ions such as Na+ and Cl-. PGPR form beneficial relationships with the roots of plants and help plants tolerate salt stress by producing beneficial organic acids and promoting nutrient uptake.

A wide range of organic compounds can be externally applied to make crops more tolerant of stressful conditions. Compounds such as proline, glycine betaine, and polyamines increase the osmolyte concentration inside plants, helping plants take up water without accumulating more salts. In saline soils, applications of these organic osmolytes in low concentrations may enhance salt tolerance and plant growth by improving the activity of the antioxidative enzyme system, photosynthetic activity, and nutrient and water uptake. Higher concentrations of these compounds may be deleterious to plant growth.

Similarly, foliar applications of organic acids such as ascorbic acid (vitamin C), jasmonic acid, and salicylic acid may improve yield and fruit quality in crops grown in saline conditions. These acids regulate different physiological processes within plants and can improve plant growth and health in a number of ways. In one experiment, the authors grew tomato plants in saline soil and treated them with jasmonic acid and nitric oxide. They found that the plants treated with 1 nM jasmonic acid had significantly greater dry weights than the control group, and this effect was compounded when nitric oxide was added to the solution.