High temperature barriers When the temperature in the greenhouse is higher than the maximum temperature in the optimum temperature range and higher than the maximum temperature that can be tolerated by vegetable crops, high temperature hazards will occur. The main damage mechanisms are: high temperature changes the physicochemical properties of the protoplasm, reduces the dispersion of biological colloids, extravasation of electrolytes and non-electrolytes, ester compounds into layers, and sometimes leads to the destruction of cell structure, so that the nuclear enlargement Loose, cracked. The high temperature also changes its respiratory intensity, impedes the synthesis of the plant body, and the decomposition of the protoplasm is greater than the synthesis. In particular, under the condition of insufficient illumination and high temperature, the damage is particularly severe. In addition, the temperature may easily affect the temperature of the leaf. If the leaf temperature is higher than the ambient temperature, photosynthesis is inhibited, dead spots appear on the leaves, the chlorophyll is destroyed, the leaf color becomes brown, premature aging, and affects the formation of normal pigments and flower buds. Differentiation leads to sunburn, fruit drop, fruit malformation, degenerative degradation, seed abnormalities, and male infertility. The coldest plants in different growth and development periods, the lowest temperature that can begin to grow and grow is called biological zero. When the temperature falls below biological zero, plant growth ceases. The lowest biological temperature varies according to the crop variety and growth period. When the vegetables in the shed are above 0°C and below the biological zero point, the damage caused by the cold is called cold damage. Thermophilic crops are prone to chilling at low temperatures. The cause of the chilling damage is the decline of the absorption function of the plant body, which hinders the absorption of water and nutrients. Such as tomato, cucumber, bitter gourd and other thermophilic vegetable crops are sensitive to temperature, their root hair protoplasm stop flowing at 10 °C -12 °C, the absorption of nutrients is inhibited, and some nutrient elements deficiency will follow. Cold damage also inhibited the formation of chlorophyll and reduced photosynthesis. Young leaves lost green or white, and the starch stored in green tissue was hydrolyzed into soluble sugars and converted into anthocyanins, which changed from green to purple. Anthocyanins can increase leaf temperature by about 1.8°C, which is beneficial from the point of view of enhancing cold resistance. However, poor growth of plants or growth stops, resulting in short plants and reduced yield. Freezing damage When the temperature is below 0°C, frost damage caused by crops is called freezing injury. The extent of frost damage depends on the extent of cooling, the length of duration, the coming of frost and the speed of thawing, and the cold resistance of crops. Generally, the larger the temperature reduction, the longer the frost duration, and the colder the crop, the worse The heavier frost damage. The cause of death of vegetables caused by freezing injury is intracellular ice formation, which directly destroys the structure of the protoplasm and causes cell death or mechanical extrusion of the dehydration of the protoplasm and degeneration and coagulation of the protoplasmic colloid (The difference between cold damage and cold injury is that the damage to the protoplasm cannot be caused when the cold damage is severe, but only Cell dehydration). Freezing damage sometimes does not occur when the cold weather arrives, but rather when the thawing occurs. If the temperature rises slowly when thawing, the cells in the dehydrated tissue will be able to reabsorb the lost water. However, if the temperature suddenly rises during thawing, the external water of the cell will not be able to be quickly recovered after being recovered by the protoplasm, and the tissue of the crop after the protoplast dehydration will dry out and cannot be restored. Therefore, when the vegetables are frost-damaged, during the thawing process, if the temperature rises too quickly, shading coverage is needed to gradually warm the vegetables so as to reduce the degree of frost damage.
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