Requirements of Ponkan on Environmental conditions

(i) Temperature

Ponkan is a tropical and subtropical fruit tree. Its planting is restricted by temperature, especially low temperature in winter. Therefore, temperature is the most important limiting factor in environmental conditions. Temperature not only affects tree growth, but also quality.

The optimum, minimum and maximum temperatures for growth

Ponkan fruit trees are more sensitive to temperature than to light. The optimum growth temperature is about 26℃, and the optimum growth temperature is between 23 ℃ and 34℃. Ponkan stopped growing at 12.8℃, the highest temperature was about 38℃, and it could endure extreme low temperature of-9℃.

2. Annual average temperature, annual activity accumulated temperature and cold month average temperature

Ponkan can withstand low temperature of-9~-8℃. The annual average temperature range of 15~22℃ can be planted. Annual active accumulated temperature is also called annual effective accumulated temperature, or annual active accumulated temperature ≥10℃ can be planted in the range of 5000~8000℃. The average temperature of cold months occurs mostly in January and a few in February. As one of the indexes of Ponkan planting, the average temperature in cold month is more important than the average temperature in year to the late-maturing Ponkan varieties, which is related to whether the fruits can survive through winter safely.

3. Effect of temperature on growth and development of Ponkan fruit trees

Too low temperature will stop the growth and development of Ponkan and freeze its organs; conversely, too high temperature will also affect the growth and development of Ponkan. Temperature has effects on shoot, root system, flower bud differentiation, fruit setting, fruit expansion, ripening and fruit quality.

(ii) Sunshine

Sunlight is an indispensable photothermal energy source for photosynthesis and production of organic matter in Ponkan fruit trees. The limiting effect of sunshine on Ponkan cultivation was inferior to that of temperature, moisture and soil.

1. Optimal illumination, saturation point and compensation point

Ponkan is a short-day fruit tree, likes diffuse light, more tolerant of shade. Too strong or too weak light is not conducive to growth and fruiting. It is generally believed that the annual sunshine is most suitable for 1200 to 1500 hours. The optimum illumination for Ponkan is 15000 lux. The photosynthetic intensity of Ponkan increased with the increase of illumination. However, when the light intensity increases to a certain extent, the photosynthetic intensity will no longer increase, and the light intensity at this time is called the light saturation point. The light saturation point of Ponkan is 35000~36000 lux. When the light is weakened to a certain extent, the photosynthetic intensity of Ponkan is equal to the respiratory intensity, so that all the substances produced by photosynthesis are consumed by respiration, and the light intensity at this time is called the light compensation point. The light compensation point of ponkan is usually 1000~2000 lux. Light is complementary, strong light can make up for the lack of heat, such as Yunnan high-altitude region Ponkan, temperature is slightly lower, but because of strong light, make up for the lack of temperature, still can make Ponkan quality, high yield.

2. The influence of light intensity on Ponkan

For example, in the stage of weak growth of young leaves, bud formation and new shoot maturity, even if the light dropped to 50%~60% of sunny day at about 12℃, it had no effect, but in the period of vigorous growth of new shoot and fruit, when the average temperature was 15~16℃, the light should not be lower than 70% of sunny day, especially in the late stage of fruit maturity, sufficient light could increase the soluble solids and sugar content of fruit. Insufficient or too strong light will bring disadvantage to Ponkan fruit trees.

(iii) Rainfall and humidity

1. Requirements for moisture and humidity of Ponkan fruit trees

Ponkan fruit trees not only require abundant heat, but also a humid environment. Appropriate precipitation and humidity are beneficial to the growth, development, high quality and high yield of Ponkan fruit trees. Generally, the optimum conditions for normal growth of Ponkan fruit trees are annual precipitation of 1000~1500 mm, air relative humidity of 75%~80% and soil relative moisture of 60%~80%.

2. Effect of precipitation and humidity on Ponkan

Too much rain, poor soil drainage and ponding, leading to rotten roots, causing leaves and flowers to fall off, and even dead trees. During the flowering period of Ponkan orange to the end of the second physiological fruit drop, continuous rainy weather will affect pollination and reduce fruit setting rate.

Ponkan water shortage, such as flowering and young fruit stage will aggravate flower drop fruit; fruit expansion stage affects fruit expansion; spring drought makes it difficult to shoot, short and thin; autumn and winter drought, although flower bud differentiation increased, but the proportion of leafless flowers increased, fruit setting rate decreased.

Relative humidity, which is closely related to moisture, also affects Ponkan: too much humidity (more than 85%) or too little humidity (less than 60%) at flowering and young fruit stage will affect fruit setting rate. Too low relative humidity during fruit expansion will hinder fruit expansion and reduce yield, and deteriorate quality, less juice, rough pericarp; too high relative humidity will increase the occurrence of diseases and insect pests, affecting yield and fruit quality.

4) Wind

The wind is both beneficial and harmful to Ponkan fruit trees. Breeze is beneficial to growth and development; strong wind and typhoon cause damage to Ponkan fruit trees, even broken branches and uprooted plants; cold wind makes trees fall leaves, serious frozen trees; dry hot wind causes leaves and fruit drop.

(v) Carbon dioxide

Carbon dioxide is the raw material for photosynthesis and production of organic substances in Ponkan fruit trees. Under suitable climatic conditions, ponkan trees that grow vigorously can absorb 130~150 kg of carbon dioxide per hectare per day. The photosynthetic intensity of Ponkan increased with the increase of CO2 concentration. Increasing CO2 concentration could increase the yield of Ponkan.

(vi) Soil

Ponkan suitable for planting soil requirements of deep soil layer, rich in organic matter, soil from sandy soil to clay soil can be loose, barrier-free layer, good drainage, groundwater level below 1 meter, fertilizer and water conservation. In many hilly areas of ponkan orchards in China, especially in red and yellow soil areas, soil organic matter content is low, nitrogen, phosphorus and potassium are deficient, so soil improvement and fertilization should be strengthened in the process of establishment and management.

The slope of ponkan growing in hilly and mountainous areas should not exceed 20℃ to facilitate soil and water conservation. The slope direction is south slope, southeast slope and southwest slope, followed by east slope and west slope. The south slope receives strong solar radiation, the lower soil accumulates more heat, and the night cooling is slower, so the temperature gradually increases from the north slope to the south slope. The lowest temperature on the south slope is about 1℃ higher than that on the north slope, and the ground temperature on the south slope is more than 2℃ higher than that on the north slope.