In vitro germination and storage optimization of Fraxinus chinensis pollen

In the quiet work of plant breeding, success often depends on a single, fleeting moment: the meeting of pollen and pistil. For many trees, this encounter happens by chance, carried by the wind to the right branch at the right time. But for scientists trying to create new, hardier, or more beautiful varieties of trees, chance is not enough. They need to control the process, moving pollen by hand from one tree to another to combine the best traits of both parents. This is where the science of pollen viability comes in. Pollen is not just a static powder; it is living tissue that must be kept alive, fed, and coaxed into growing a tiny tube to reach the egg. If the conditions are wrong—if the food is too rich or too poor, if the temperature is too hot or too cold—the pollen dies before it can do its job. Understanding exactly what a specific tree needs to stay alive and ready to fertilize is the key to unlocking its potential for creating new forests and landscapes.

This is the challenge researchers faced with the Chinese ash tree, a species valued for its strong timber, its use in traditional medicine, and its beauty in city parks. While these trees are common across China and parts of Asia, creating new, improved versions of them has been difficult because scientists lacked a reliable way to test if the pollen was still alive or to store it for later use. A team of researchers at Shenyang Agricultural University set out to solve this problem by treating the pollen like a delicate guest that needs the perfect environment to thrive. They wanted to find the exact recipe of nutrients, the ideal temperature, and the best way to wake the pollen up after it had been frozen, all to ensure that when breeders tried to cross-pollinate these trees, the effort would succeed.

The first step was to figure out what the pollen needed to eat and drink to grow. In a test tube setting, the researchers tried mixing different amounts of sugar, boric acid, and calcium nitrate, while also adjusting how acidic or basic the solution was. They found that the pollen was very particular about its diet. It performed best in a solution containing a high amount of sugar, specifically 150 grams per liter, which provided the energy needed for growth. It also required a specific amount of boric acid, 0.1 grams per liter, and a small dose of calcium nitrate, 0.05 grams per liter. Perhaps most importantly, the acidity of the mixture had to be just right; a neutral level of 6.0 worked far better than more acidic or basic conditions. When they combined these specific ingredients, the pollen responded with vigor, growing long tubes that signaled it was healthy and ready for fertilization.

Once they had the perfect food mixture, the team turned their attention to the temperature. Just as a person might feel sluggish in the cold or overheated in the heat, pollen has a narrow window where it functions best. The researchers tested temperatures ranging from chilly 5 degrees Celsius to a warm 30 degrees. They discovered that the pollen was slow to start in the cold but grew rapidly as the temperature rose, peaking at exactly 25 degrees Celsius. At this temperature, after 48 hours, the pollen reached its highest success rate. If the temperature went any higher, the growth slowed down again. This finding gave breeders a clear target: keep the pollen at a comfortable room temperature of 25 degrees to get the best results.

The study also tackled the difficult problem of storage. In nature, pollen is often only available for a short time, but breeders need to save it to use when the trees are not flowering or to transport it to different locations. The researchers tested keeping the pollen at various temperatures, from room temperature down to deep freezing. They found that at room temperature, the pollen died within two weeks. Even in a standard refrigerator at 4 degrees, it only lasted about two months before losing its ability to grow. However, when the pollen was stored in a deep freezer at minus 20 degrees or minus 80 degrees, it remained alive and healthy for a full year. This means that breeders can collect pollen in the spring, freeze it, and use it to create new trees the following year, effectively extending the breeding season indefinitely.

There was one final hurdle to clear: how to wake the pollen up after it had been frozen. Taking a frozen sample and putting it directly into the warm growing medium often shocks the delicate cells, causing them to fail. The researchers developed a gentle thawing process to avoid this. They found that the best method was to move the frozen pollen to a refrigerator at 4 degrees and let it sit there for 24 hours. After this slow thaw, they placed the pollen in a humid environment at 25 degrees for another four hours to rehydrate it. This two-step process acted like a gentle wake-up call, allowing the pollen to recover its strength. When treated this way, the pollen achieved a germination rate of over 91 percent, proving that the combination of the right food, the right temperature, and a careful thawing routine could keep these tree seeds of life ready for years.

The work of these researchers provides a complete toolkit for anyone looking to improve the Chinese ash tree. By defining the exact chemical recipe for growth, identifying the perfect temperature for development, and establishing a reliable method for long-term storage and revival, they have removed the guesswork from the process. This knowledge allows breeders to confidently mix and match traits, creating new varieties that can grow faster, resist disease better, or look more beautiful. The study confirms that with the right care, even a tiny grain of pollen can be preserved and revived, ensuring that the legacy of this valuable tree continues to grow for future generations.