Effects of Foliar Application of Prohexadione- Calcium and Gibberellin on Photosynthetic Performance and Sucrose Metabolism in Rice under Salt Stress

Salt is a silent thief in the soil, stealing water from plant roots and choking the very machinery that turns sunlight into food. For rice, a crop that feeds more than half the world, this threat is particularly acute. When the ground becomes too salty, the plant's leaves struggle to breathe, closing their tiny pores to save water but inadvertently starving themselves of carbon dioxide. This stress damages the delicate solar panels inside the leaf cells, known as photosystems, and disrupts the complex chemical dance that converts sugar into the energy needed for growth. Without a way to counteract this, the plant withers, and the harvest fails. Scientists have long known that certain chemicals, called plant growth regulators, can help crops survive these harsh conditions, but the precise way they work—how they repair the solar panels and manage the plant's sugar reserves—remains a mystery. Understanding these mechanisms is not just an academic exercise; it is a matter of global food security, offering a potential key to unlocking productive farming on land that was once considered too toxic to cultivate.

In a recent study conducted at Guangdong Ocean University, researchers set out to compare two such regulators: a substance called prohexadione-calcium and gibberellin. They chose two varieties of rice, one known as Guanghong 3 and another called FL478, both of which possess a moderate ability to withstand salt. The scientists grew these plants in pots filled with soil treated with a high concentration of salt to simulate a harsh environment. To see how the regulators worked, they sprayed the leaves of the rice seedlings with either prohexadione-calcium or gibberellin at two different stages of growth: first when the seedlings had three leaves, and again when the plants began to break through the soil surface. Some plants received a second spray of the same regulator, while others received a different one, allowing the team to test the effects of single versus sequential applications. They then meticulously measured how well the plants breathed, how their internal energy systems functioned, and how they handled their sugar reserves.

The results revealed that both chemicals helped the rice plants breathe better under salt stress. The treated plants opened their leaf pores wider, allowing more carbon dioxide to enter and boosting their ability to photosynthesize. However, the two regulators took very different paths to achieve this, and their effects on the plant's internal machinery were almost opposite. The prohexadione-calcium treatment acted like a protective shield for the plant's energy centers. It kept the photosynthetic machinery running smoothly, ensuring that the energy captured from sunlight was efficiently passed along to power the plant's growth. In contrast, the gibberellin treatment, while also improving breathing, placed a heavy burden on these energy centers in the FL478 variety. In this specific variety, the gibberellin caused the energy systems to become less efficient, forcing the plant to waste more energy as heat rather than using it for growth, whereas in the Guanghong 3 variety, the energy efficiency remained relatively stable or slightly improved.

The difference became even more pronounced when the researchers looked at how the plants managed their sugar. Plants under salt stress naturally try to adjust their sugar balance to survive, but the regulators influenced this process in distinct ways. The prohexadione-calcium treatment helped the plants build up their sugar and starch reserves in the Guanghong 3 variety by slowing down the enzymes that break sugar down and speeding up the enzymes that create it. However, in the FL478 variety, this treatment actually led to a significant decrease in sugar content in the leaves, suggesting that the sugar was being rapidly transported to other parts of the plant, such as the roots, where it accumulated. Meanwhile, the gibberellin treatment created a chaotic situation in the Guanghong 3 variety, turning up the activity of both the sugar-making and sugar-breaking enzymes simultaneously. This resulted in a futile cycle where the plant burned through its resources without building up a meaningful reserve. In the FL478 variety, however, the gibberellin treatment inhibited the activity of the key enzyme responsible for sucrose synthesis, which directly limited the plant's ability to create new sugar, while also causing a significant drop in starch levels, leaving the plant with fewer energy stores to draw upon during the stressful period.

The study also highlighted that different rice varieties respond differently to these treatments. The Guanghong 3 variety showed a stronger overall reaction to both regulators, adapting its photosynthesis and sugar management more dramatically than the FL478. Yet, even within this responsive variety, the prohexadione-calcium consistently proved superior in protecting the plant's core functions. The researchers found that a specific measure of photosynthetic health, which tracks how well the plant captures and uses light energy, was closely linked to how well the plant managed its sugar. When this energy measure was high, the plant's sugar-making enzymes were working efficiently, and reserves were growing. When it was low, the plant was struggling to maintain its energy balance.

Ultimately, the research suggests that while gibberellin can help rice grow taller and breathe better in salty soil, it may do so at the cost of depleting the plant's energy reserves or disrupting sugar synthesis depending on the variety. Prohexadione-calcium, on the other hand, offers a more balanced approach, protecting the plant's energy-generating machinery while helping it stockpile the sugar and starch needed to survive. This distinction provides a clear guide for farmers and scientists working to grow rice on saline-alkali soils. By choosing the right regulator, it may be possible to not only help the plant survive the stress but to ensure it retains the strength to produce a viable harvest. The study confirms that the health of a plant's solar panels is deeply connected to its ability to manage its food supply, and finding the right chemical balance could be the key to feeding the world on land that was once thought barren.