Section I: flower and fruit protection

1. Falling flowers and fruits

(1) Physiological fruit drop

Citrus flower and fruit fall can be divided into five stages: bud fall, flower fall, the first physiological fruit fall (10~20 days after flowering), the second physiological fruit fall (20~70 days after flowering without fruit stem falling from the honey tray), and fruit fall before harvest (fruit fall after fruit stabilization in June to before harvest). After the above process, the fruit setting rate is generally only 0.3%~5%. The main reasons for the low fruit-setting rate are the abnormal development of flower organs or poor fertilization, the insufficient nutrition of the tree body, the large number of summer shoots, the serious occurrence of diseases and pests, the bad weather or the hormone imbalance in the trees with stems, etc. The nutrients needed for the sprouting, spring shoot growth, flowering and early development of young fruit of the orange tree are mainly the storage nutrients of the previous year, while the amount of annual storage nutrients of each variety of orange tree is relatively fixed. Therefore, if the spring shoots of the orange tree grow too fast after sprouting and leafing, the storage nutrient consumption will be more, then the nutrient obtained from flowering and fruiting will be less, and the flowering and fruiting will cause a large number of flowers and fruits due to insufficient nutrients. The auxin and gibberellin are insufficient in the young fruit stage of citrus, especially in seedless varieties, which causes the fruit stalk to delaminate and cause flower and fruit drop. Navel orange is a parthenocarpic fruit, which mainly depends on the hormone produced by ovary to promote the expansion of young fruit. Abscisic acid inhibits growth and promotes fruit abscission. Gibberellin promotes cell elongation and fruit growth. The abscission of navel orange young fruit was positively correlated with the content of abscisic acid in the tree. The more abscisic acid content, the more fruit fell. There was a significant negative correlation between the abscission of navel orange young fruit and the content of gibberellin in the tree. The more gibberellin content, the less fruit abscission. The mechanism of gibberellin improving the fruit setting rate of navel orange is mainly that the high concentration of gibberellin improves the ability of fruit to transport nutrients, and the effect of gibberellin on ovary is to promote the metabolism of plant to the fruit (Tan Haiyu, 2003). Sweet oranges and satsuma mandarin have a large amount of young fruit loss in the first physiological fruit fall period, while red oranges have a large amount of young fruit loss in the second physiological fruit fall period (Wang Dajun et al., 1996). Cytokinetin (BA) has a significant effect on preventing the first physiological fruit drop, but it cannot prevent the second physiological fruit drop. The effect of gibberellin on reducing the first physiological fruit drop was weak; However, it can significantly inhibit the second physiological fruit drop. In citrus production, auxin and synthetic auxin 2,4-D are used to reduce pre-harvest fruit drop and delay fruit harvest. Applying 2,4-D to sweet orange trees will delay the shedding of flowers and young fruits, and eventually lead to the reduction of the number of mature fruits. However, it has also been reported that applying 2,4-D to navel orange trees will increase the number of fruits. Some other chemicals, such as chloropyrylphenylurea (CPPU) [1 - (2-chloro4-pyridyl) - 3-phenylurea], also inhibit the shedding of young citrus fruit (Wang Miao et al., 2009).

(2) Abnormal flower and fruit drop

In recent years, the citrus production areas in the Yangtze River basin often encounter abnormal high temperature weather with daily average temperature higher than 25 ℃ and daily high temperature higher than 30 ℃ during flowering and young fruit stage, which is often accompanied by abnormal flower and fruit drop, which greatly reduces the citrus production in the affected areas. According to the research, the potted Xingjin satsuma fruit trees grow for 2 to 5 days under the simulated conditions of abnormal high temperature with daily average temperature of 26.8 ℃ and daily high temperature of 35 ℃. The average length of spring shoots increases by 4.3% to 78.2% compared with the control growing under the natural conditions with daily average temperature of 27.3 ℃ and daily high temperature of 33 ℃. At the same time, it promotes the occurrence of dormant buds of some citrus trees and increases the number of spring shoots by 5% to 15.3%. As the number of new leaves increases, the proportion of new and old leaves increases, resulting in excessive nutrient growth of spring shoots, increased consumption of stored nutrients, and insufficient nutrients for flower and fruit development, resulting in a large number of flower and fruit drop. According to the survey, if the proportion of new and old leaves of potted Xingjin satsuma mandarin is greater than 1, the flowers and young fruits in the orange tree will fall off when the abnormal high temperature is harmful.

2. Technical measures for flower and fruit conservation

(1) According to the test results of the Department of Horticulture of Zhejiang University, paclobutrazol was used to spray satsuma mandarin with 750 mg/L paclobutrazol or Ponkan with 1000 mg/L paclobutrazol in the evening of cloudy or sunny days at the flowering stage of satsuma mandarin and Ponkan. The young part of spring shoots should be sprayed when spraying. It can control the overgrowth of spring shoots, reduce the consumption of storage nutrients, and at the same time, the leaves turn green and thicken, enhance the ability to produce assimilation nutrients, ensure that flowers and fruits get sufficient nutrients, and significantly improve the fruit setting rate. However, the leaf area of orange trees decreased slightly after spraying paclobutrazol, which is a normal phenomenon and will not affect the growth and fruit of the next year. When using, first dissolve paclobutrazol with a small amount of water, and then add enough water to evenly spray the crown of the tree until the leaves drip. Due to the long residual effect period of paclobutrazol, in order to prevent the excessive inhibition of orange shoot growth caused by continuous use, it is recommended to use paclobutrazol after 1-2 years, and then stop using it for 1-2 years. At the same time, it is not suitable to use paclobutrazol during the flowering period and young fruit setting period of citrus, otherwise it will affect the growth and development of young fruit.

(2) The rate of fruit setting was significantly increased when gibberellin was sprayed once on the crown of citrus trees with a concentration of 30~50mg/L after 2/3 and 10 days after the flowering of satsuma mandarin, ponkan mandarin and other citrus trees, while the rate of fruit setting was significantly increased when gibberellin was sprayed once with a concentration of 100~200mg/L on the young fruit of citrus trees with a small amount of flowers after flowering, and the effect of fruit preservation was 10 points significant.

Due to the lack of endogenous hormones such as gibberellin, auxin and cytokinin produced by seed development, seedless sugar orange has a low level of endogenous hormones in fruit development, which can not meet the needs of growth and development, and a large number of fruit drops occur. Spraying 75% gibberellin (92O) powder 1g+water 35~50kg+0.4% urea+0.2% potassium dihydrogen phosphate+0.1% boric acid on the tree crown 20~25 days after flowering, and spraying it again every 15 days can significantly improve the fruit setting rate of seedless sugar orange. In addition, 100 mL of cytokine plus 200~300 kg of water or 500 mL of amino acid sugar phosphate plus 400 kg of water can also be added for spraying. If there are red spiders, aphids, Fenxun and other pests, they can also be mixed with pesticides for spraying (Ye Zizi et al., 2009).

(3) In the first physiological fruit fall period, the concentration of cytokinin in the crown of the tree is 200~400mg/L, which is sprayed once every 10 days, a total of 2-3 times, which has a significant effect on improving the fruit setting rate of citrus (Hu Yaosheng, 1996).

(4) At the flowering stage and the end of the first physiological fruit fall, the application of 0.1~0.5mg/1L of clopidouron (piromuron) on the canopy of satsuma mandarin can significantly promote fruit setting and increase fruit setting rate.

(5) When gibberellin+benzyladenine was used in the young fruit stage of Washington navel orange, 250 mg/L gibberellin and 200 mg/L benzyladenine were used to coat the fruit, the fruit setting rate reached 31.78%, which was significantly higher than 0.85% of the control, and the yield was significantly increased. Citrus uses mixed solution of gibberellin and benzyladenine to keep flowers and fruits. The times and methods of application should be determined according to the type of citrus and the characteristics of falling flowers and fruits. Generally, for the citrus with less serious first physiological fruit drop, the first physiological fruit drop can be controlled by spray with 50 mg/L gibberellin, or the second physiological fruit drop can be coated with 100~500 mg/L gibberellin; For citrus with serious fruit drop in two times, the fruit can be coated with gibberellin 50-100mg/L plus benzyladenine 200-400mg/L before the first physiological fruit drop, and then sprayed with gibberellin 50-100mg/L before the second physiological fruit drop. Both the test and production practice have proved that the effect is more obvious when two or more substances are used in combination or in combination with topdressing outside the root, but the dissolution conditions of various substances must be paid attention to when compounding, and precipitation and turbidity are invalid. Secondly, we should also pay attention to the spraying on the leaf surface, as well as the topdressing outside the root, which should be carried out after 3pm every day. We should pay attention to the spraying on the back of the leaf, and the quantity should not be too much each time (Hu Ansheng, 1996).

3. Measures to prevent abnormal flower and fruit drop caused by abnormal high temperature of citrus

First of all, we must control the overgrowth of spring shoots to ensure that flowers and fruits get sufficient nutrients. At the citrus flower bud stage before the arrival of abnormal high temperature, spray the canopy with the concentration of 750 mg/L (satsuma mandarin) or 1000 mg/L (ponkan mandarin) of paclobutrazol solution, on the one hand, stop the growth of spring shoots ahead of time, mature the new leaves ahead of time, improve the photosynthetic capacity of the new leaves, increase the accumulation of assimilation nutrients in the citrus tree, and ensure that the flower and fruit development get sufficient nutrients; On the other hand, it can inhibit the number of dormant buds stimulated by abnormal high temperature, reduce the occurrence of new shoots, and reduce the consumption of new shoots on storage nutrients.

Secondly, after the occurrence of abnormal high temperature, it should be remedied by spraying in time to reduce the degree of harm. According to Tong Changhua and others, spraying 2,4-D of banned acetic acid+8 mg/L or 2,4-D of gibberellin+8 mg/L with a concentration of 100 mg/L or 50 mg/L on the tree crown after half a day of abnormal high temperature can significantly reduce the harm of abnormal flower and fruit drop caused by abnormal high temperature; Li Xuezhu and others coated the fruit with 125mg/L gibberellin+100mg/L furfuraminopurine (kinetin) after the occurrence of abnormal high temperature in the flowering period, which has a significant effect on improving the fruit setting rate of Xingjin satsuma and Jincheng.

4. Measures to prevent citrus fruit falling at low temperature in winter and before harvest

Fruits of many varieties, such as Fuling Summer Orange, need to be harvested from March to June next year after the winter solstice. Due to the relatively low temperature in winter, the water absorption capacity of root system is weak, and the content of auxin in the tree body is also reduced, which will cause a large number of fruit drop before and after the Spring Festival, causing heavy economic losses. Huang Dinggui applied 2,4-D with the concentration of 20~40mg/L to spray before the low temperature in November, which can significantly reduce the fruit drop caused by low temperature. If 0.1%~0.3% manganese sulfate is added to 2,4-D, the effect will be better. The reason is that adding manganese sulfate can reduce the activity of cellulase.

Spraying 2,4-D20~50mg/L before or at the beginning of fruit falling can not only prevent fruit falling, but also reduce defoliation. The concentration of medium-maturing varieties of citrus should be controlled at 20mg/L, and the concentration of late-maturing varieties should be appropriately increased. If the concentration is too high, it will cause curling leaves and falling leaves. Ponkan orange leaves easily in autumn (October to November). Spraying 2,4-D10mg/L can reduce leaf shedding.

The article is from the book "Application of plant growth regulators in fruit trees", and the content is for reference only.