Growth physiological biochemical and antioxidant responses of rice to exogenous proline and ascorbic acid under salt stress

Rice is the lifeblood of billions, a staple crop that feeds more than half the world's population. Yet, this vital grain is incredibly sensitive to its environment, particularly when the soil becomes salty. When salt accumulates in the ground, it creates a double threat for the plant. First, it acts like a sponge that pulls water away from the roots, making it difficult for the plant to drink even when the soil is wet. Second, the salt disrupts the delicate chemical balance inside the plant cells, allowing toxic sodium to build up while pushing out essential nutrients like potassium. This internal chaos damages the plant's ability to make food through sunlight, weakens its structure, and often stops it from producing seeds entirely. As coastal regions face rising seas and changing weather patterns, finding ways to help rice survive in salty conditions has become a matter of global food security.

Scientists have long known that plants have their own defense mechanisms against such stress. They can produce natural compounds that act as shields, helping cells hold onto water and neutralize the harmful byproducts of stress. Two of these protective compounds are proline, an amino acid that helps balance water inside cells, and ascorbic acid, a form of vitamin C that acts as a powerful antioxidant. While researchers have studied these substances individually, it remained unclear which one worked best, how often they should be applied, and whether mixing them together offered any extra benefit. A team of researchers at Patuakhali Science and Technology University in Bangladesh set out to answer these questions by testing these treatments on rice plants growing in salty soil.

The researchers grew rice in pots filled with soil that had been made salty to a specific level, simulating the harsh conditions found in coastal farmlands. They divided the plants into several groups to test different strategies. Some plants received no treatment, serving as a baseline to see how badly the salt would hurt them. Others were sprayed with a solution containing proline, while a different group received a spray of ascorbic acid. The team also tested whether applying these sprays once or twice made a difference, and whether mixing the two substances together worked better than using them alone. The goal was to see if these external applications could help the plants grow taller, produce more leaves, and, most importantly, generate a harvest of grain despite the salty soil.

The results were striking, revealing that a single spray of proline was by far the most effective treatment. The untreated plants in the salty soil struggled immensely; they were shorter, had fewer stems, and produced almost no grain. In fact, the salty soil reduced the amount of grain these plants produced by more than ninety percent compared to healthy plants growing in normal soil. However, the plants that received a single foliar application of proline showed a dramatic recovery. They grew nearly as tall as the healthy plants, produced a full complement of stems, and generated a harvest that was nearly eight times larger than the untreated salty plants. This single treatment restored the number of filled grains to levels close to those of the non-salty control group.

While the other treatments offered some help, they did not match the performance of the single proline spray. Plants treated with ascorbic acid grew better than the untreated salty ones, but they still fell significantly short of the proline-treated group. Similarly, applying the sprays twice did not improve the results; in fact, a single application of proline outperformed a double application. Mixing proline and ascorbic acid together also failed to provide an advantage over using proline alone. The data suggested that adding more of the treatment or combining different chemicals did not necessarily lead to better protection. Instead, the timing and the specific choice of the compound mattered most.

Beyond just looking at the size of the plants and the amount of grain, the researchers looked inside the leaves to understand why the proline worked so well. They found that the treated plants had higher levels of natural antioxidants, which are chemicals that protect cells from damage caused by stress. The plants sprayed with proline also maintained a better balance of minerals. Salt stress usually causes plants to accumulate toxic sodium and lose essential potassium, but the proline-treated plants managed to keep more potassium and less sodium in their tissues. This balance is crucial for the plant's internal machinery to function correctly. The treatment also helped the plants retain more of their green pigment, which is necessary for capturing sunlight, and increased their own production of vitamin C, further boosting their ability to fight off stress.

The study also highlighted that not all aspects of the plant responded in the same way. While the grain production and the number of stems improved dramatically with the proline treatment, the length of the leaves did not change significantly across the different groups. This suggests that the salt stress affected the plant's reproductive ability and overall structure more severely than it affected the length of individual leaves. The researchers noted that while the proline treatment clearly improved the plant's health, the exact molecular mechanisms—such as how the plant genes were turned on or off—were not measured in this study. Therefore, while the results are strong, they represent a physiological observation rather than a complete genetic explanation.

In the end, this research points to a simple and practical solution for a complex problem. For rice farmers facing salty soil, a single spray of proline applied at a specific stage of growth appears to be a powerful tool for saving the harvest. It helps the plant hold onto water, keep its internal chemistry balanced, and protect its cells from damage, allowing it to produce grain even when the environment is hostile. However, the researchers caution that this finding comes from a controlled experiment in a greenhouse setting. Before this method can be recommended for widespread use in fields, it needs to be tested across different varieties of rice and in real-world farming conditions. The path forward involves validating these promising results in the field, ensuring that this single spray can reliably turn a failing crop into a successful harvest for farmers around the world.