Although tea trees have a certain ability to withstand adverse weather conditions during their growth and development, they can be harmed if the conditions exceed a certain limit. This can range from reduced yield and quality of the leaves to the death of the Tree. So what are the effects of low temperatures on tea trees?
I. Common types of Frost damage in tea trees
The common types of frost damage in tea trees include Snow frost, frost, and dry cold wind frost. In tea-producing areas south of the Yangtze River, snow frost and frost are predominant, while all three types of frost occur in areas north of the river. After being affected by frost, tea trees typically exhibit red withering or green withering. In areas south of the Yangtze River, red withering is more common, while both forms occur in areas to the north.
II. The sequence of frost damage development in tea trees
Tea trees have a certain ability to resist low temperatures, but different organs have varying degrees of cold tolerance. Among leaves, stems, and roots, cold tolerance decreases in that order. However, under field cultivation conditions, leaves protect the stems, and the stems and leaves together protect the roots. Therefore, during the winter, the temperature at the top, middle, and base of the tea tree Canopy increases from top to bottom. As a result, in practice, frost damage often starts at the top branches and leaves, then affects the stems, and only under extremely cold conditions do the roots suffer, leading to the death of the entire plant.
III. The mechanism of frost damage caused by low temperatures in tea trees
Frost damage in tea trees due to low temperatures occurs through ice formation within the cells of leaf tissues. When the temperature drops to -1.07°C, the free water in the cell gaps begins to freeze, forming ice nuclei. As the temperature continues to fall, the ice expands, drawing moisture from the protoplasm, causing it to slowly dehydrate and denature. At the same time, the ice crystals grow in size, applying mechanical pressure on the cells. Due to differences in the speed of response to water between the protoplasm and the cell wall, when the temperature suddenly rises or falls, the cell contents are torn away from the cell walls, damaging the cells. Rapid warming can also cause water stress as the intercellular water evaporates quickly and the protoplasm cannot absorb water fast enough, leading to dehydration and denaturation. When the temperature drops sharply and becomes exceptionally cold, ice formation can damage the cell membranes, resulting in cell death.
Research in the 1980s showed that the occurrence of frost damage in tea trees is closely related to the types and numbers of ice nucleating bacteria on the leaf surfaces. When ice nucleating bacteria are present on the surface of tea leaves, ice usually forms around these bacteria as nuclei. Typically, at temperatures between -2°C and -5°C, ice nucleating bacteria trigger the freezing of water in plant cells, causing frost damage. Tea leaves without ice nucleating bacteria can withstand temperatures down to -7°C to -8°C without frost damage. Major ice nucleating bacterial species found on tea trees include Pantoea ananatis, Pantoea agglomerans, Xanthomonas campestris, and Erwinia ananas. Therefore, controlling ice nucleating bacteria on the surfaces of tea leaves can help reduce the occurrence of frost damage.
IV. Factors affecting frost damage in tea trees
1. Relationship between meteorological conditions and frost damage in tea trees
The severity of frost damage is closely related to the monthly average temperature, extreme minimum temperature, negative degree days, and consecutive days of low temperatures. The greater the absolute values, the more severe the frost damage; conversely, milder or no frost occurs when these values are lower.
2. Relationship between soil freezing depth and frost damage in tea trees
The severity of frost damage in tea trees is positively correlated with the lowest ground temperature, total negative degree days, maximum soil freezing depth, and consecutive days of frozen soil. If tea trees are in frozen soil for long periods, it can cause difficulties in root absorption and transport of water, leading to dehydration and death of above-ground parts, as well as root shrinkage and decay.
3. Relationship between atmospheric relative Humidity and frost damage in tea trees
During winters with less rainfall, low atmospheric relative humidity, and insufficient soil moisture, frost damage in tea trees tends to be more severe. If strong winds occur, tea trees may rapidly exhibit green withering type of frost damage. Such frost damage is uncommon in southern China's tea regions but frequently occurs in northern regions.
4. Relationship between frost damage in tea trees and variety and age
Different tea varieties have varying degrees of cold tolerance. For example, Yunnan large-leaf tea, which grows in environments without severe winters or hot summers, has weaker cold resistance compared to other varieties and may experience frost damage at temperatures around 0.5°C. On the other hand, small and medium-leaf tea varieties generally have stronger cold tolerance and can withstand temperatures as low as -10°C if the duration of the cold is not prolonged.
The degree of frost damage in tea trees is also related to their age, with older trees generally having stronger cold resistance.
5. Relationship between frost damage in tea trees and geographical conditions
As altitude and latitude increase, the absolute low temperature during the dormant period, total negative degree days, and number of consecutive cold days also increase. Therefore, in high-latitude and high-altitude environments, tea trees are more susceptible to frost damage.
The saying “snow hits the mountains, frost hits the valleys” is a meteorological proverb. Mountains receive more snow than plains and lowlands, and since cold air sinks, heavy frost often occurs in winter. Planting tea in such locations makes the trees highly prone to snow and frost damage. Additionally, areas exposed to strong winds and cold air currents are also prone to frost damage, with affected trees often showing a band-like distribution.
6. Relationship between orchard management and frost damage in tea trees
Proper orchard management techniques can strengthen the growth of tea trees, enhance their cold resistance, and ensure safe overwintering, thereby reducing the severity of frost damage. Conversely, improper management can exacerbate the occurrence and development of frost damage.
Preventive and remedial measures for low-temperature frost damage in tea trees will be covered in subsequent articles, so stay tuned~
This article is excerpted from “The Chinese Tea Classic,” edited by Zongmao Chen and Ya Jun Yang. Some images are sourced from the internet.