In the case of dormancy in winter, cold resistance and frost resistance are commonly used to indicate the ability of plants to survive winter without being harmed, and they are represented by the lowest temperature that can be tolerated. However, the occurrence of frost damage is often not in the dormancy period with the lowest absolute temperature, but in late autumn and early spring.
When the temperature falls below 15.5°C in the fall, cool temperatures will slow or stop the growth of the plant roots. Daejeon plants can be insulated by soil, and container seedlings are prone to frost damage if they are not properly managed.
No matter how strong the cold resistance of the stem, the critical temperature of the root is the lowest temperature that plants can tolerate, and the cultivation matrix has a great influence on it. For example, the aerial parts of the genus Pyracantha could tolerate a low temperature of -25.5°C, while the old roots suffered freezing injury at -18.8°C, and the young roots suffered freezing injury at -6.1°C. Therefore, exposure to low temperature conditions, root death will directly lead to a large number of deaths above ground.
Growth regulators such as abscisic acid (ABA) affect the cold resistance of the roots. Studies have shown that the process of transporting abscisic acid from the top of the plant to the leaves and roots is a process in which the cold-resistance of old roots is increased, but it has no effect on the young roots. This is because of the effect of gibberellin (GA) produced on the top of the plant. An annular band formed by the mechanical damage of the plant stems can hinder the transport of abscisic acid and affect the cold resistance of the roots.
The old root is harder than the young root, and the cold-resistant temperature difference is generally 5°C to 10°C. Because the young root is at the edge of the container, it is most susceptible to freezing injury. In many cases, old roots can survive after the death of young roots. However, when the young root is subjected to freezing injury, the plant growth rate is slow, and the root system cannot absorb enough calcium to promote cell differentiation and growth, and is prone to disease. Therefore, before carrying out container nursery, it is necessary to understand the effect of low temperature on the cold resistance and nutrient absorption of roots.
Seeds or cuttings used for growing seedlings may also affect the cold resistance of the roots due to different genetics. In order to exercise the cold-resistance of the plant, it is best to carry out the nursery at a temperature slightly higher than the freezing injury, such as 0°C to 4.5°C. In prolonged warm climates, plants lose some of their ability to resist cold.
Low-temperature damage is due to the formation of ice crystals inside and outside cells. A sudden drop in the outside temperature, the formation of ice crystals in the cytoplasm, so that the protoplasm is severely dehydrated, causing cell death. If the formation of ice crystals is further extended, the entire plant will wither and die.
On the other hand, in the winter, crystals form outside the cells, and water flows out of the cells to resist cold temperatures. When the temperature rises, the moisture flows back into the interior of the cell. Most woody plants can tolerate this kind of injury, but freeze damage occurs at very low temperatures or when the cells are dehydrated for a long time. In the outdoors, the top of the container seedling is most vulnerable to damage, the surrounding temperature is high, the humidity is low, and the plant will evaporate a lot of water. If moisture cannot be added to the stem and leaves in time, further freezing damage will occur.
At the same time, the movement of the wind around the plants will also accelerate the loss of water, leading to its dehydration, characterized by browning of the evergreen plants, death of flower buds, longitudinal cracking of the bark, or death of the roots. In order to avoid damage to the plants, proper watering should be performed during the growing period and dormant period, pay attention to weather changes, and do not irrigate in cold, dry and windy weather. Irrigation can increase the cell's turgor pressure and help the cell's water absorption. Plants grown in clay are most vulnerable to drought and require multiple irrigations.
If the management measures are improper, even if the container seedlings can survive winter, there will be frost damage, such as frozen cracks, root deaths. In the days with the sun below 0°C, the southwestern direction of the plant can absorb the heat of the sun. However, in cold weather, due to the sun setting or dark clouds, it is easy to lose the absorbed heat. This change will cause longitudinal cracking of the container seedlings. In the course of one year, the growth of the root neck is delayed at the latest, and it enters the dormancy phase at the latest, and the activity begins and the dormancy is released earlier. In the case of a sudden drop in temperature, if it is not subjected to a good cold resistance exercise, it is vulnerable to early freezing injury.
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