What kind of environmental conditions does ponkan (Citrus reticulata 'Ponkan') cultivation require?
Environmental requirements for ponkan
(1) Temperature
Ponkan is a tropical and subtropical fruit tree, and its cultivation is limited by temperature, especially low winter temperatures. Therefore, among environmental conditions, temperature is the most important limiting factor. Temperature not only affects tree growth but also has a great influence on fruit quality.
1. Optimum, minimum and maximum temperatures for growth
Ponkan trees are more sensitive to temperature than to light. The optimum growth temperature is around 26°C, and growth is suitable within the range of 23-34°C. The temperature at which ponkan stops growing is 12.8°C, the maximum temperature is around 38°C, and it can tolerate extreme low temperatures of around -9°C.
2. Annual average temperature, annual active accumulated temperature and average temperature of the coldest month
Ponkan can tolerate low temperatures of -9 to -8°C. It can be planted in areas with an annual average temperature of 15-22°C. The annual active accumulated temperature, also called the annual effective accumulated temperature, or the annual active accumulated temperature of ≥10°C, can range from 5000-8000°C for cultivation. The coldest month average temperature mostly occurs in January, and occasionally in February. The coldest month average temperature is used as one of the indicators for ponkan cultivation because the average temperature of the coldest month has a more important effect on late-maturing ponkan varieties than the annual average temperature, and it is related to whether the fruit can safely overwinter.
3. Effects of temperature on the growth and development of ponkan trees
Too low a temperature will cause ponkan to stop growing and developing, and organs will freeze; conversely, too high a temperature also affects the growth and development of ponkan. Temperature affects shoots, roots, flower bud differentiation, fruit set, fruit expansion, ripening and fruit quality.
(2) Sunshine
Sunlight (light) is an indispensable light and heat energy source for photosynthesis and the production of organic matter in ponkan trees. The limiting effect of sunshine on ponkan cultivation is secondary to environmental conditions such as temperature, moisture and soil.
1. Optimum light, light saturation point and light compensation point
Ponkan is a short-day fruit tree that prefers diffused light and is relatively shade-tolerant. Too strong or too weak light is unfavorable for growth and fruiting. It is generally considered that an annual sunshine duration of 1200-1500 hours is most suitable. The optimum light intensity for ponkan is 15000 lux. The photosynthetic rate of ponkan increases with increasing light intensity. However, when the light intensity rises to a certain level, the photosynthetic rate no longer increases; the light intensity at this point is called the light saturation point. The light saturation point of ponkan is 35000-36000 lux. When light decreases to a certain level, the photosynthetic rate of ponkan equals the respiration rate, so that the substances produced by photosynthesis are entirely consumed by respiration; the light intensity at this point is called the light compensation point. The light compensation point of ponkan is usually 1000-2000 lux. Light has a compensatory effect: strong light can make up for insufficient heat. For example, when ponkan is grown in high-altitude areas of Yunnan, the temperature is slightly lower, but because the light is strong, it compensates for the insufficient temperature, and ponkan can still be of high quality and high yield.
2. Effects of light intensity on ponkan
Ponkan trees have different light requirements at different growth stages. For example, at weak growth stages such as young leaves, flower bud formation and new shoot maturation, at around 12°C, even if light drops to 50%-60% of that on a sunny day, there is no effect; but during the period of vigorous growth of new shoots and fruit, when the average temperature is 15-16°C, light cannot be lower than 70% of that on a sunny day. Especially in the late fruit ripening stage, sufficient light can increase the soluble solids and sugar content of the fruit. Insufficient or excessive light will bring adverse effects to ponkan trees.
(3) Rainfall and humidity
1. Water and humidity requirements of ponkan trees
Ponkan trees not only require abundant heat but also a humid environment. Suitable precipitation and humidity are conducive to the growth and development of ponkan trees and to high quality and high yield. Generally, the normal growth of ponkan trees is best with an annual precipitation of 1000-1500 mm, an air relative humidity of 75%-80%, and a soil relative water content of 60%-80%.
2. Effects of precipitation and humidity on ponkan
Too much rain and poor soil drainage leading to waterlogging cause root rot, leaf and flower and fruit drop, and even tree death. From the flowering stage of ponkan to the end of the second physiological fruit drop, continuous rainy weather will affect pollination and reduce the fruit-setting rate.
When ponkan lacks water, for example during the flowering and young fruit stages, flower and fruit drop will be aggravated; during the fruit expansion stage, fruit expansion will be affected; spring drought makes spring shoots difficult to sprout and makes them short and thin; autumn and winter drought, although increasing the amount of flower bud differentiation, increases the proportion of leafless flowers and lowers the fruit-setting rate.
Air relative humidity, which is closely related to water, also affects ponkan: too high humidity (above 85%) or too low humidity (below 60%) during the flowering and young fruit stages will affect the fruit-setting rate. If the relative humidity is too low during the fruit expansion stage, fruit expansion will be hindered and yield reduced, and quality will deteriorate, with less juice and rough peel; if the relative humidity is too high, the occurrence of pests and diseases will increase, affecting yield and fruit quality.
(4) Wind
Wind is both beneficial and harmful to ponkan trees. A gentle breeze is beneficial to growth and development; strong winds and typhoons cause damage to ponkan trees, and even break branches and uproot plants; cold wind causes trees to shed leaves, and in severe cases freezes them to death; dry hot wind causes leaf and fruit drop.
(5) Carbon dioxide
Carbon dioxide is the raw material for photosynthesis and the production of organic matter in ponkan trees. Under suitable climatic conditions, a vigorously growing ponkan tree can absorb 130-150 kg of carbon dioxide per hectare per day. The photosynthetic rate of ponkan increases with increasing carbon dioxide concentration, and increasing the carbon dioxide concentration can increase ponkan yield.
(6) Soil
Suitable soil for ponkan cultivation requires deep soil layers rich in organic matter; the soil texture can range from sandy loam to clay loam; the soil body should be loose, with no barrier layers, good drainage, a groundwater level below 1 m, and good fertilizer and water retention. Many ponkan orchards in China are located in hilly and mountainous areas, especially ponkan orchards in red and yellow soil areas, where soil organic matter content is low and nitrogen, phosphorus and potassium are deficient. During orchard establishment and management, soil improvement and fertilization should be strengthened.
For ponkan planting on hills and mountains, the slope should preferably not exceed 20°C, so as to help conserve soil and water. As for slope aspect, south-facing, southeast-facing and southwest-facing slopes are suitable, followed by east-facing and west-facing slopes. South-facing slopes receive strong solar radiation, store more heat in the lower soil layers, and cool more slowly at night. Therefore, air temperature gradually increases from north-facing slopes to south-facing slopes. The minimum air temperature on south-facing slopes is about 1°C higher than on north-facing slopes, and the ground temperature on south-facing slopes is more than 2°C higher than on north-facing slopes.
Ponkan is a tropical and subtropical fruit tree, and its cultivation is limited by temperature, especially low winter temperatures. Therefore, among environmental conditions, temperature is the most important limiting factor. Temperature not only affects tree growth but also has a great influence on fruit quality.
1. Optimum, minimum and maximum temperatures for growth
Ponkan trees are more sensitive to temperature than to light. The optimum growth temperature is around 26°C, and growth is suitable within the range of 23-34°C. The temperature at which ponkan stops growing is 12.8°C, the maximum temperature is around 38°C, and it can tolerate extreme low temperatures of around -9°C.
2. Annual average temperature, annual active accumulated temperature and average temperature of the coldest month
Ponkan can tolerate low temperatures of -9 to -8°C. It can be planted in areas with an annual average temperature of 15-22°C. The annual active accumulated temperature, also called the annual effective accumulated temperature, or the annual active accumulated temperature of ≥10°C, can range from 5000-8000°C for cultivation. The coldest month average temperature mostly occurs in January, and occasionally in February. The coldest month average temperature is used as one of the indicators for ponkan cultivation because the average temperature of the coldest month has a more important effect on late-maturing ponkan varieties than the annual average temperature, and it is related to whether the fruit can safely overwinter.
3. Effects of temperature on the growth and development of ponkan trees
Too low a temperature will cause ponkan to stop growing and developing, and organs will freeze; conversely, too high a temperature also affects the growth and development of ponkan. Temperature affects shoots, roots, flower bud differentiation, fruit set, fruit expansion, ripening and fruit quality.
(2) Sunshine
Sunlight (light) is an indispensable light and heat energy source for photosynthesis and the production of organic matter in ponkan trees. The limiting effect of sunshine on ponkan cultivation is secondary to environmental conditions such as temperature, moisture and soil.
1. Optimum light, light saturation point and light compensation point
Ponkan is a short-day fruit tree that prefers diffused light and is relatively shade-tolerant. Too strong or too weak light is unfavorable for growth and fruiting. It is generally considered that an annual sunshine duration of 1200-1500 hours is most suitable. The optimum light intensity for ponkan is 15000 lux. The photosynthetic rate of ponkan increases with increasing light intensity. However, when the light intensity rises to a certain level, the photosynthetic rate no longer increases; the light intensity at this point is called the light saturation point. The light saturation point of ponkan is 35000-36000 lux. When light decreases to a certain level, the photosynthetic rate of ponkan equals the respiration rate, so that the substances produced by photosynthesis are entirely consumed by respiration; the light intensity at this point is called the light compensation point. The light compensation point of ponkan is usually 1000-2000 lux. Light has a compensatory effect: strong light can make up for insufficient heat. For example, when ponkan is grown in high-altitude areas of Yunnan, the temperature is slightly lower, but because the light is strong, it compensates for the insufficient temperature, and ponkan can still be of high quality and high yield.
2. Effects of light intensity on ponkan
Ponkan trees have different light requirements at different growth stages. For example, at weak growth stages such as young leaves, flower bud formation and new shoot maturation, at around 12°C, even if light drops to 50%-60% of that on a sunny day, there is no effect; but during the period of vigorous growth of new shoots and fruit, when the average temperature is 15-16°C, light cannot be lower than 70% of that on a sunny day. Especially in the late fruit ripening stage, sufficient light can increase the soluble solids and sugar content of the fruit. Insufficient or excessive light will bring adverse effects to ponkan trees.
(3) Rainfall and humidity
1. Water and humidity requirements of ponkan trees
Ponkan trees not only require abundant heat but also a humid environment. Suitable precipitation and humidity are conducive to the growth and development of ponkan trees and to high quality and high yield. Generally, the normal growth of ponkan trees is best with an annual precipitation of 1000-1500 mm, an air relative humidity of 75%-80%, and a soil relative water content of 60%-80%.
2. Effects of precipitation and humidity on ponkan
Too much rain and poor soil drainage leading to waterlogging cause root rot, leaf and flower and fruit drop, and even tree death. From the flowering stage of ponkan to the end of the second physiological fruit drop, continuous rainy weather will affect pollination and reduce the fruit-setting rate.
When ponkan lacks water, for example during the flowering and young fruit stages, flower and fruit drop will be aggravated; during the fruit expansion stage, fruit expansion will be affected; spring drought makes spring shoots difficult to sprout and makes them short and thin; autumn and winter drought, although increasing the amount of flower bud differentiation, increases the proportion of leafless flowers and lowers the fruit-setting rate.
Air relative humidity, which is closely related to water, also affects ponkan: too high humidity (above 85%) or too low humidity (below 60%) during the flowering and young fruit stages will affect the fruit-setting rate. If the relative humidity is too low during the fruit expansion stage, fruit expansion will be hindered and yield reduced, and quality will deteriorate, with less juice and rough peel; if the relative humidity is too high, the occurrence of pests and diseases will increase, affecting yield and fruit quality.
(4) Wind
Wind is both beneficial and harmful to ponkan trees. A gentle breeze is beneficial to growth and development; strong winds and typhoons cause damage to ponkan trees, and even break branches and uproot plants; cold wind causes trees to shed leaves, and in severe cases freezes them to death; dry hot wind causes leaf and fruit drop.
(5) Carbon dioxide
Carbon dioxide is the raw material for photosynthesis and the production of organic matter in ponkan trees. Under suitable climatic conditions, a vigorously growing ponkan tree can absorb 130-150 kg of carbon dioxide per hectare per day. The photosynthetic rate of ponkan increases with increasing carbon dioxide concentration, and increasing the carbon dioxide concentration can increase ponkan yield.
(6) Soil
Suitable soil for ponkan cultivation requires deep soil layers rich in organic matter; the soil texture can range from sandy loam to clay loam; the soil body should be loose, with no barrier layers, good drainage, a groundwater level below 1 m, and good fertilizer and water retention. Many ponkan orchards in China are located in hilly and mountainous areas, especially ponkan orchards in red and yellow soil areas, where soil organic matter content is low and nitrogen, phosphorus and potassium are deficient. During orchard establishment and management, soil improvement and fertilization should be strengthened.
For ponkan planting on hills and mountains, the slope should preferably not exceed 20°C, so as to help conserve soil and water. As for slope aspect, south-facing, southeast-facing and southwest-facing slopes are suitable, followed by east-facing and west-facing slopes. South-facing slopes receive strong solar radiation, store more heat in the lower soil layers, and cool more slowly at night. Therefore, air temperature gradually increases from north-facing slopes to south-facing slopes. The minimum air temperature on south-facing slopes is about 1°C higher than on north-facing slopes, and the ground temperature on south-facing slopes is more than 2°C higher than on north-facing slopes.