Review on Effects of Sunlight on the Internal Quality of Peach Fruit

Zhisheng MA Yuehui WANG Ruixia BAI Jianming LI Yunyun JIA

Abstract On the basis of referring plenty of literatures， we summed up the research advance in effects of sun light on the internal quality of peach fruit. This paper discussed the effect of light on the internal quality of peach fruit under the conditions of open cultivation， protected cultivation， bagging and surface covering with reflective film， the mechanism of occurrence and the technical measures to improve the utilization rate of light and light energy， and prospected the future research work.

Key words Peach tree; Sunlight; Fruit; Soluble solid content; Organic acid; Vitamin C; Protein

Peach originates in China and is the most important stone fruit tree in the world. China is the country that produces the most peaches in the world[1]. At present， due to the excessive pursuit of yield and fruit size， the cultivation and management measures are unreasonable， resulting in the deterioration of the internal quality of peach fruit. In recent years， the supply of peaches in China has changed from a shortage to a surplus， which requires quality improvement[2]. There are many factors that affect the internal quality of peach fruit， such as nitrogen， light， water， and mineral nutrition. Ma et al.[3]reviewed the effects of nitrogen on the internal quality of peach fruit. Strong light-loving is the main characteristic of peach trees. Light has an important impact on the growth and fruit of peach trees[4]. Photosynthesis is a decisive factor for the yield and quality of fruit trees. Light not only affects the photosynthesis of fruit trees， but also affects photosynthate distribution and utilization conversion[5]. On the basis of consulting a large number of literatures， this paper reviewed the research on the effect of light on the internal quality of peach fruit， hoping to provide a reference for improving the internal quality of peach fruit.

Factors Affecting Sunshine onto Peach Trees

Canopy structure

Tree shape

Compared with the trunk type peach trees， the light in the open heart-shaped trees increases by 15.2% to 27.9%， up to 50.0%[6]. Lu et al.[7]believed that the relative light distribution of the inclined trunk type peach trees is more than that of the Y-shaped trees， and the effective light ratio is higher. The photosynthetic rate of the open heart-shaped peach trees is higher than that of the trunk type[6]， and the net photosynthetic rate of the open heart-shaped peach trees is 18.1% higher than that of the trunk type[8]. Regardless of whether it is a heart-shaped or a spindle-shaped peach tree， the changes in the photosynthetic rate of different leaf curtain layers are shown as upper layer>middle layer>lower layer[9].

Canopy layers

From the lower canopy to the upper canopy， the relative light gradually increases. The height of the canopy is divided into three parts， of which one is from the ground to 1.0 m， the second is from 1.0 to 2.0 m， and the third is from 2.0 to 3.0 m. In June， there is no relative light proportion more than 80% at 0-1.0 m， and the relative light proportion more than 80% at 2.0-3.0 m can reach 27.7%， and the relative light distribution between 1.0-2.0 m is in the middle， at 3.33%. The relative light proportion of less than 30% from 0 to 1.0 m accounts for 5.83%; there is no area with relative light proportion of less than 30% from 2.0 to 3.0 m; and the relative light distribution between 1.0 to 2.0 m is in the middle， at 1.88%[10]. By September， with the growth of peach trees， the relative light proportion of different parts has changed greatly. There is no more than 80% of relative light at 0-1.0 m and 2.0-3.0 m， and the relative light proportion more than 80% at 2.0-3.0 m reduced to 3.13%. In the entire canopy， the proportion of relative light less than 30% is increasing. The relative light proportion less than 30% accounts for 19.79% at 0-1.0 m， 23.33% at 1.0-2.0 m， and increased from 0 in June to 4.79% at 2.0-3.0 m[10]. As the trees grow， the amounts of branches and leaves increase， and the light transmittance of the inner and lower parts of the crown decreases.

Facility cultivation

Due to the influence of the plastic film， the photosynthetic effective radiation in facilities reduced by 50% compared with the open field condition， and within a certain photosynthetic effective radiation range， the net photosynthetic rate of leaves in facilities increases with the increase of light intensity[11]. The leaf chlorophyll content in leaves of nectarine in facilities is higher than that in open field， and the light compensation point， light saturation point and utilization rate of strong light are lower than those in open field condition， but the light energy utilization rate of weak light is higher than that of peach trees in open field [11]. The average daily net photosynthetic rate of peach in facility is 16.7% higher than that outside facilitiy[12]. The plastic film mulching in facility decreases the net photosynthetic rate， and as the degree of shading increases， the net photosynthetic rate continues to decrease[13].

Bagging

The light intensity inside white bags is 59.29% of the natural light intensity[14]. The light transmittance of white bags is better than that of yellow bags， and yellow bags are better than orange bags. The near-ultraviolet light， blue light， red light and far-infrared light transmitted by white bags are respectively 4.2， 4.2， 4.0 and 3.7 times of yellow bags， and nearly 9.4， 14.5， 3.6 and 3.7 times of orange bags [14]. Ma et al.[15]believed that the light transmittance of white single-layer bags， outer-yellow inner-white double-layer bags， and outer-yellow inner-black double-layer bags are 55.86%， 33.14% and 0， respectively. The study by Wang et al.[16]showed that the light transmittance of Kobayashi single-layer bags， Kobayashi double-layer bags and ordinary double-layer bags were 69.6%， 18.2% and 0， respectively. It can be seen that the light transmittance of white bags is better than dark bags， and single-layer bags are better than multi-layer bags.

Reflective film

Reflective film on the ground can improve the light in the middle and lower inner parts of peach tree crown[17]，and the degrees of light increase are different in different studies. Chen et al.[18]believed that laying reflective film between rows and between plants increases the light intensity by 0.5-1.0 time and 30%-50%， respectively. The research of Shen et al.[19]showed that the light intensity on the back of leaves in the middle of tree crowns with reflective film increased by 1.37 times than that without reflective film.

Reflective film on the ground significantly increased the net photosynthetic rate of the middle and lower leaves of tree canopies by 16.0%-27.8%， and the light energy utilization rate increased by 15.9%-22.9% compared with the control[20]. The results of Zhou et al.[21]showed that when the reflective film was laid on the ground for 20 d， the net photosynthetic rates of the upper， middle， and lower leaves of tree crowns increased by 35%， 93%， and 95% respectively compared with the control. It can be seen that the treatment with reflective film mainly affects the lower part of tree crown.

Effect of light on the internal quality of peach fruit

Effect of light intensity on the internal quality of peach fruit

Peach trees are sensitive to change of light intensity. Different tree shape and different parts of crowns receive different amounts of light. Measures such as facility cultivation， bagging， and reflective film will affect the intensity of light， the synthesis and transport of photosynthetic products， and thus affect the internal quality of peach fruits.

Open field

The soluble solid content in peach fruits on open heart-shaped trees with two main branches is 14.0% higher than that of the trunk-shaped trees[22]. The study by An et al.[23]showed that the soluble solid content of fruits in Y-shaped peach trees was 25.7% and 19.8% higher than those of spindle and natural open heart shaped trees， respectively. Studies have shown that the quality of fruits on Y-shaped and three-main-branch open heart-shaped trees is better than that of the trunk-shaped trees， and there is no obvious difference in the internal quality between the open heart-shaped and Y-shaped trees[24-25].

The soluble solid content of fruit in different parts of crowns is different. Zhang et al.[26]showed that the soluble solid content of fruit was the highest in the upper part of crown， followed by the middle part， and the lowest in the lower part. The lower part was 13.5% lower than the upper part. The directions with higher soluble solid content of fruit were the southeast and northeast of crown， the southwest was the lowest， and the northwest was in the middle. The soluble solid content of the upper， middle and lower fruits in the southwest varied greatly， and the upper fruit was 33.3% higher than the lower fruits.

The average soluble solid content in fruits in various crown layers of trunk-shaped trees is 11.8%. Specifically， the upper fruits had the highest soluble solid content， at 13.7%， which was significantly higher than the middle， which was the lowest， only 9.5%[8].

The study by He et al.[27]showed that the soluble solid contents of fruits in the upper， middle and lower parts of crown were significantly different， and the lower part was 17.2% less than the upper part. The soluble solid contents of fruits in the upper， middle and lower parts of crown were significantly different， and the lower part was 14.1% less than the upper part. The difference in the soluble solid content of fruits between the upper and lower crowns of the trunk-shaped trees is greater than that of the open heart-shaped trees[22].

Weak light environment

Shading： Shading at 40%， 60%， and 80%， the fruit soluble solid， soluble sugar， and vitamin C contents significantly reduced， and the titratable acid and soluble starch contents increased. When shading at 60%， the soluble solid and soluble sugar contents decreased respectively 24.4% and 31.0%; and when shading at 80%， the soluble solid and soluble sugar contents decreased by 40.0% and 22.68%， respectively， and the titratable acid and soluble starch contents increased by 71.4% and 114.0%， respectively[13]. After shading treatment， the titratable acid and soluble protein contents in peach fruits increased， and the titratable acid content increased by 26.7% compared with the control[28]. Shen et al.[19]covered peach trees with a layer of shading net with the shading rate of 75% at the upper 50 cm of crown about 20 d before peach fruits matured， and the soluble solid content of fruits decreased by 21.3%.

Facility cultivation： After shading treatment， the contents of total soluble sugar and vitamin C in peach fruits decreased， and the total soluble sugar content was 14.83% lower than the control[28]. The contents of titratable acid， starch， dry matter and soluble protein in peach fruits after shading treatment increased， among which the titratable acid increased by 26.7% compared with the control[28]. Shading reduced the glucose content of peach fruits cultivated in facilities by 31.0%[29]. Yin et al.[30]showed that the citric acid content of ‘Chunmei peach fruit in facilities increased by 81.5% in the later stage of its development compared to that in open field.

Bagging： Jin et al.[31]studied the ‘21-century peach and showed that the soluble solid contents of fruits covered by single-layer， double-layer and three-layer bags decreased by 24.7% on average compared with the control. The largest decrease was in the three-layer bags， and the highest decrease was 39.5%. Bagging reduced the soluble sugar content of fruit， and the decrease of the single-layer bag was less than that of the double-layer bag[32-33]. The outer-yellow and inner-black double-layer bag reduced the soluble solids and soluble sugar contents of fruits by 8.1% and 12.0%， respectively[15]， and the soluble solid content of fruits in white single-layer bags was closer to the control fruits[34-36].

Studies also found that different varieties have different sensitivity to fruit bagging， and they differ greatly. Jin et al.[31，37]carried out bagging studies on‘21-centuryand ‘Meishuai peach and showed that the soluble solid content of the former fruits reduced by an average of 24.7%， while ‘Meishuai had almost no change， and even some treatments were 14.8% higher than the control. There were also studies reporting that some bagging treatments increased the soluble solids and soluble sugar contents of fruits[38-40].

Zhisheng MA et al. Review on Effects of Sun Light on the Internal Quality of Peach Fruit

Bagging reduces the contents of aroma substances in fruits， but the content of ester aroma substance （γ-hexa， octa， decanolactone） in fruits in white bags is higher than those in other color bags， and orange bags are second only to white bags[14]. Guo et al.[41]believed that， compared with the control， bagging increased the content of alcohols and acids in fruit， but reduced the content of aldehydes and esters. Compared with other fruit bags， the lactones in fruits covered with single-layer outer-yellow inner-black paper bags had the least decrease， while the fruits in double-layer outer-red inner-black paper bags had the most decrease. The results of Luo et al.[42]showed that bagging reduced 6 kinds of aroma substances compared with the control， and the relative contents of some components reduced. For example， the content of ethyl 4-octenate reduced by 85%， and that of γ-decanolide reduced by 41.6%. Some components increased， such as hexanal.

Bagging increased the titratable acid content of peach fruit by 11.3%-46.7%[14-15，33，43]. Luo et al.[42]showed that bagging increased the organic acids contents by more than 150%. The content of malic acid in fruits in yellow bags was significantly higher than those of the control and the fruits in newspaper bags， but there were no significant differences in orange bags and white bags[14]. Wang et al.[16]showed that bagging significantly reduced the titratable acid content. Studies have shown that bagging increased the soluble protein content by 21.4% compared with the control[43].

Reflective film

Laying reflective film can significantly increase the soluble solid content and sugar-acid ratio of peach fruit[18，44，45]. Laying reflective film significantly increases the soluble solids and soluble sugar contents of fruits in the middle and lower inner bore of crowns. Fruits in the lower inner bore increased the most， up to 65.2%， but the soluble solid contents of the upper periphery， upper inner bore and middle periphery fruits did not increased[46]. The soluble solid content of peach fruits increased during the reflective film treatment， mainly due to the increase of sucrose content[47]. Numerous studies have shown that laying reflective film can reduce the acid content of fruits[44，46-47]. The main reason was that it reduced the contents of quinic acid and isocitric acid[47].

Effect of light quality on the internal quality of peach fruit

There are few reports on the effect of light quality on the internal quality of peach fruit. The results of blue light treatment on ‘Shuguangnectarine in facility showed that with the second expansion of fruits， the total acid content continued to decrease， and when it reached the maturity stage， it dropped to the lowest level， which was significantly lower than the control. However， the fructose and glucose contents in fruits under the supplemental blue light treatment were higher than the control， resulting in a higher total sugar content in fruits than the control[48]. The study on bagging with different colors of fruit bags showed that the citric acid content of fruitsin white bags， red bags and blue bags were lower than that of the control. In addition， the contents of sucrose， quinic acid and malic acid of the red bag treatment were higher than that of the control. The content of citric acid in green bag treatment was higher than the control， but the contents of sucrose， quinic acid， and malic acid were lower[49]. Ma et al.[50]covered peach trees with four different colors of films， and the results showed that the soluble sugar content was the highest under the green film treatment; the red film treatment exhibited the lowest soluble protein and the highest titratable acid contents; the blue film treatment had the lowest soluble sugar content and the highest soluble protein content; and the while film treatment had the highest titratable acid content， sugar-acid ratio and vitamin C content. It can be seen that to improve the internal quality， green film or white film should be used.

Relationship between internal fruit quality and light or photosynthetically active radiation

Under open field cultivation condition， the soluble solid content of peach has a significant correlation with the relative light intensity[27]. Gao et al.[10]showed that the contents of soluble solids and total sugars were significantly positively correlated with the photosynthetically active radiation intensity. When the light hours during the second expansion period of peach fruit increased by 12.6%， the soluble solid content increased by 15.4%[51]. Teng et al.[52]showed that the number of sunshine hours in August had an important effect on the soluble solid content of yellow peaches. With the increase of photosynthetically effective radiation intensity， the total acid content and vitamin C content of peach fruit had a slight decrease trend[10].

Taking the relative light intensity or photosynthetically active radiation intensity as the independent variable， and the fruit quality factors （soluble solid content and soluble sugar content） as the objective function， the regression equations were established （Table 1）. It can be seen from Table 1 that the soluble solid content of peach fruit requires a higher relative light intensity in September than in June. The optimal light intensity required in June and September was calculated from the equations to be 76% and 119%， respectively. The optimal light intensity value exceeded the relative light intensity， indicating that the soluble solid content of fruits requires high relative light intensity[27]. Lu et al.[7]showed that the lowest relative light intensity for producing high-quality fruit of ‘Ruipan No. 5peach was 34%， and the relative light intensity for obtaining the best fruit quality index was 71.66%.

Requirements for light at different stages of fruit development

In the various processes of peach fruit development， the middle stage is the least sensitive to low light， followed by the young fruit stage， and the later stage of fruit development has the greatest impact on the internal quality of peach fruit[28]. Hao et al.[53]showed that light had the greatest impact at the later stage of fruit development， and the least impact at the hard-core stage， and the young fruit stage was in the middle. Facility conditions are the same as open field cultivation. In different growth and development stages of peach fruit， shading treatment has different effects on fruit quality， which is characterized by a large impact in the later stage and a small impact in the early stage[28]. It can be seen that light has different effects on different stages of development. Fruits are more sensitive to light at later stage of fruit development， when light has a greater impact on the internal quality of fruits.

Mechanism of Effect of Light on the Internal Quality of Peach Fruit

Open field

The acid invertase activity of peach fruits under shading treatment was lower than that of the control[54]. In the fruit maturity period （70 and 80 d after blooming）， the sucrose synthase activity of fruits under low light environment was significantly lower than that of fruits under normal light[29].

Facility cultivation

There was a significant positive correlation between the fructose content and acid invertase activity of shaded fruits. Extending the light time， the glucose and fructose contents of peach fruits were significantly positively correlated with the neutral invertase activity， and the sucrose content was significantly positively correlated with the sucrose synthetase activity[54]. Yin et al.[30]showed that the activity levels of phosphoenolpyruvate carboxylase and citrate synthase at the late stage of fruit development were significantly higher than those in the open field， which promoted the synthesis of citric acid. In addition to affecting photosynthetic efficiency， light also affects the distribution of photosynthetic products. After the nectarine was lightly shaded in greenhouse， the photosynthetic product mainly supplied the nearby fruits， the transportation to the distant fruits reduced， and the distribution ratio of fruit assimilate decreased[55].

Bagging

The main enzyme that affects the metabolism of sucrose in peach fruits is invertase. Experiments have shown that the acid invertase and neutral invertase activities of bagged and control fruits were negatively correlated with the sucrose content， and positively correlated with the fructose and glucose contents， and some had significant or extremely significant correlation[56].

Reflective film

There are very few reports on the relationship between the reflective film and enzyme activity in peach trees. The treatment with reflective film increased the activity of enzymes related to sucrose metabolism in peach fruit， and the fructose content was significantly positively correlated with the activity of SS-c[54]. Studies showed that the activity of sucrose phosphate synthase in pear fruits with the silver reflective film was higher than that of the control， while the activity of neutral invertase was significantly lower than that of the control[57]. In addition， laying reflective film increases the light， improves the photosynthetic efficiency of the leaves， increases the source of storage， and helps improve the internal quality.

Measures to Improve Light and Light Efficiency

Open field cultivation

Under open field cultivation， the open heart shape is mainly used in peach production. It is characterized by an open crown， simple tree structure， three-dimensional fruiting， high light utilization efficiency， and improves light conditions inside the crown especially[27]. In order to obtain high-quality peach fruits， it is necessary to ensure that the relative light intensity of each part of the open heart-shaped tree crowns during the growing season is higher than 36.1%[27]. It is necessary to strengthen summer pruning， adjust the density of branches and leaves in crowns， strengthen the pruning before fruit harvest to ripening period， improve the light transmission， and increase the proportion of light distribution with a relative light intensity above 80% in crowns[27，58].

Facility cultivation

Under the facility cultivation， we must first choose the suitable varieties. The ‘Beijixing nectarine has strong low light tolerance， high light utilization efficiency， high quality and high yield， and it is recommended to choose ‘North Star as a good variety for facility cultivation[59]. The second is to choose covering materials with good light transmission properties， which can make full use of reflected light to increase the light intensity in facilities[12]. Meanwhile， the debris and dust on the covering material should be cleaned in time[60]. The third is to adjust the balance of vegetative growth and reproductive growth through pruning， and reduce the adverse effects of low-light environment on fruits[28]. Like open field peach， light has the greatest impact on facility peach at the later stage. Measures such as pruning， laying reflective screens， and artificial light supplementation should be used to improve the light conditions at the later stage of peach fruit growth and improve the internal quality of fruits[55， 60]. The hard-core period is critical for blue light treatment to increase the sugar-acid ratio of fruits， and supplementing blue light at this time can improve the photosynthetic performance and fruit quality of nectarine in facilities. This technical measure is expected to be applied to the cultivation of facility fruit trees[48].

Bagging

For fruit bagging， it is necessary to choose fruit bags suitable for improving the internal quality. Mi et al.[35]believed that white bags are better than other fruit bags， and white fruit bags should be selected for nectarine production in the middle and lower reaches of the Yangtze River to improve the overall quality of fruits[49]. Single-layer bags with high light transmittance are better than double-layer bags[16]. Single-layer orange fruit bags can produce high-quality peach fruits with rich aroma[61]. Wang et al.[16]found that the Kobayashi single-layer bags are more suitable for nectarine in greenhouse.

Reflective film

Hu et al.[47]suggested that peach fruits should be covered with a moisture-permeable reflective film during the development period to improve the internal quality. Zhou et al.[17]believed that in order to cooperate with winter pruning and summer pruning， the application of reflective film was an extremely effective measure to improve internal quality on the basis of ventilation and light transmission in peach orchard.

Prospects

The open-field cultivation area of peach trees in China accounts for more than 90% of the total area of peach trees in China. It is an important task to improve the internal quality of fruits in open field by increasing light and improving the light utilization efficiency. However， under the conditions of open field cultivation， there are relatively few literatures on the effect of light on internal quality. In-depth and systematic research on related content should be strengthened to explore the related mechanisms and measures.

The internal quality of peach fruit under current facility conditions is poor. Although there are many influencing factors， light is also one of the more important factors. Starting from many aspects such as tree shape selection， pruning and supplementing light， studies can be conducted to improve the relative light rate and photosynthetic efficiency， make the photosynthetic products reasonable distribute， and discuss related technical measures and mechanisms. The study of light quality on the internal quality of peach fruits should be strengthened.

There are many research literatures on the effect of bagging on fruit internal quality， but the results are quite different. Some had greatly improved the internal quality， and some were on the contrary， that is， the internal quality decreased a lot， which was related to the type of fruit bags and fruit varieties. However， in-depth and systematic research should be carried out on the basis of adequate collection and analysis of relevant literatures， to screen out the types of fruit bags that can improve the internal quality of peach fruits， suitable varieties and supporting technologies.

Reflective film has a great effect on improving the internal quality of peach fruit. It is also necessary to study and screen the best reflective film and corresponding supporting technical measures on the basis of fully understanding the research progress to maximize the internal quality of peach fruits at the lower part of crowns， so as to minimize the difference in internal quality of fruit at different parts of crowns.

The internal quality of peach fruit includes more content， some of which are beneficial， such as soluble solids， soluble sugar， aroma substances， etc. There are also some substances that reduce the internal quality of peach fruit， such as protein， amino acids， and soluble acids， as well as some nutrients. In addition， there are some nutritional ingredients， such as flavonoids. In-depth study should be carried out on different types of sugars， different types of organic acids and amino acids. The research on nutrients and related hormones should be strengthened.

There are few reports on the effect of light quality on the internal quality of peach fruit， but more studies on strawberry[62-63]. It is necessary to strengthen the research on the internal quality of peach trees.

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