| The “leaf-to-fruit ratio”(LFR)affects the “supply and demand relationship” of nutrients and carbohydrates between roots and leaves and fruits,which is closely related to the economic yield of fruit trees.However,the reason why LFR regulates carbohydrates and affects fruit growth during fruit growth is not yet fully understood.In view of this,this paper used fruitbearing branches with different ratios of leaf(L)and fruit(F)in J.regia ‘Xinxin 2’ as the research material,and focused on the effect of LFR controlling carbohydrates on fruit growth.Starting from the demand for carbohydrates and nutrients for fruit growth,ring-cutting experiments were conducted on the fruit-bearing branches.Field investigation,microscopic observation,and physical and chemical determination were used as data acquisition methods.Cluster analysis,correlation analysis,network analysis,canonical analysis,structural equation,and weight analysis was used.By studying the interaction between leaves,branches,and fruits in the “source-sink” system of fruit-bearing branches,and the starvation effect of fruit-bearing branches and fruit growth in response to LFR,to explore the dynamic causes of the regulation of carbohydrate assimilation and transport by LFR on fruit growth by changing the characteristics of fruit-bearing branches,to find out the key periods and ways of obtaining carbohydrate and nutrients in fruit growth,and to identify the important target traits of fruitbearing branches affecting fruit growth.The main findings are as follows:The number of pinnately compound leaves significantly affected the bearing branch characteristics(P<0.05),but the number of fruits did not(P>0.05).The fruit-bearing branches(length and diameter),spring shoots(length and diameter),and leaf area of 5L were significantly higher than those of 3L and 1L(P<0.05).Increasing the number of pinnate compound leaves on fruit branches can promote the growth of carbohydrate assimilation tissue and transport tissue.The development of 1L leaf tissue(main vein length,main vein width,main vein bundle,palisade tissue,and spongy tissue)and stalk tissue(pulp length,pulp width,xylem,phloem,cambium,and cortex)were lower than that of 3L and 5L.The ring-cutting treatment of the fruit-bearing branches reduced the development of leaf tissue,and the treatments with a higher load on the fruit-bearing branches(1L∶2F and 1L∶3F),the ringcutting treatment of the fruit-bearing branches increased the number and area of voids in mesophyll sponge tissue,which easily inhibited leaf growth and caused premature leaf senescence.During the entire fruit growth process,stomatal limiting factors,leaf area of pinnate compound leaves,and fruit storage capacity were important factors for LFR to regulate carbohydrate assimilation ability.Fruit storage capacity was the main factor for LFR to regulate carbohydrate transport ability.They are related to the ratio of the number of pinnate compound leaves and the number of fruits on the fruit-bearing branches,which affected the nutritional characteristics of the fruit-bearing branches.The leaf area,fruit-bearing branch length,and fruit-bearing branch diameter were significantly positively correlated with net photosynthetic rate,stomatal limiting factors,total dry weight of fruit,extracellular material transport rate,leaf sucrose phosphate synthase activity,leaf sucrose synthase decomposition direction enzyme activity,fruit peel sucrose synthase synthesis,and decomposition direction enzyme activity,and seed sucrose synthase synthesis and decomposition direction enzyme activity(P<0.05),but they were significantly negatively correlated with the carbohydrate content of the fruit stem(P<0.05).Fruit growth was not only related to the influence of LFR on carbohydrate assimilation and transport capacity but also closely related to fruit-bearing branch factors.Among the fruitbearing branch factors,the contribution rate of the area of pinnate compound leaves on fruit growth was relatively high(50.1%),which was 23.4%,31.7%,and 45.3% higher than the length,diameter,and load of fruit-bearing branches,respectively.Compared with the level with 1pinnate compound leaves,the 3 and 5 levels had better physical and chemical quality,carbohydrate assimilation,and transport capacity,and had more complex interaction network relationships to regulate fruit growth.Photosynthesis,chlorophyll fluorescence,activity of sucrose synthesis and decomposition related enzymes in leaves and fruits,and dry matter accumulation rate in fruits with 3 and 5 levels of pinnate compound leaves were significantly higher than level 1(P < 0.05).Moreover,the number of interaction network links and extracellular transport rate regulating fruit growth at levels 3 and 5 of pinnate compound leaves were higher than level 1,but the content of non-structural carbohydrates in the fruit stem was significantly lower than level 1(P<0.05).The nutritional level of a tree can affect the competition for carbon and nutrients between leaves and fruits,and early good tree nutritional status was one of the key factors affecting the improvement of fruit quality.During the rapid growth stage of fruits,excessive load on fruitbearing branches was one of the reasons for premature fruit senescence and insufficient kernel filling.During the entire fruit growth period,nut kernels with high fruit-bearing branches loading(1L∶2F and 1L∶3F)have a low plumpness rate of 92.7%,which was related to the high fruit quantity and low leaf quantity,which can easily inhibit the fruit from obtaining carbohydrates,hinder kernel plumpness,and disrupt kernel “carbon and nitrogen metabolism”.The ring-cutting experiment further showed that,during the rapid growth stage of fruits,the starvation effect of carbohydrates and nutrients would intensify the competition for carbon between fruits and leaves,inhibit chlorophyll synthesis,and reduce the accumulation of nonstructural carbohydrates such as starch and soluble sugars in leaves.There were differences in the availability of carbohydrates and nutrients at different locations on the stalk.The single fruit weight at the top and middle of the stalk was 1.1 g and 2.9 g higher than that at the bottom,respectively.Given this,fruit growth was not only related to the impact of LFR regulation on carbohydrate assimilation and transport on fruit vitality and carbohydrate acquisition ability but also influenced by tree nutrient levels and material transport modes at the time scale.Therefore,before the rapid growth period of the fruit,paying attention to the reasonable fruit load of the fruit-bearing branches can help strengthen the concentrated supply of nutrients and carbohydrates to the fruit,promote fruit growth and improve kernel plumpness,increase fruit yield and improve fruit quality.It is advisable to manually retain one fruit at the top of the fruit stalk when the number of pinnately compound leaves on the fruit-bearing branches is less than3. |
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