Abstract Light is crucial to the uniform production of high-quality fruit since it is the driving force for leaf photosynthesis and hence overall plant nutrition. The objective of this study was to evaluate the relationship between the distribution of relative light intensity in the peach tree (Prunus persica) canopy and the yield and texture of peach fruits. The canopy of 7-year-old ‘Qiuyan’ was divided into cubical volumes and the relative light intensity distribution was measured for each cube, along with yield distribution and fruit textural properties at different growing times. The relative light intensity decreased gradually from outside to inside the canopy and from top to bottom. The yield distribution and the relative light intensity were clearly correlated. The percentage of the canopy receiving <30% relative light intensity was 49.07% in May, 56.02% in June, and 58.95% in July, whereas the percentage receiving >80% relative light intensity was 32.72%, 17.28%, and 10.96%, respectively. Consistent with this, peaches were found in the upper and middle portions of the canopy, within 1.5-3.0 m of the top. The regression equation showed that fruit texture index correlated significantly with relative light intensity. Relative light intensity is the main factor affecting peach yield and texture and must be above 41.83% for good peach quality. Orchardists should carefully plan summer pruning strategies to adjust the number and spatial distribution of branches accordingly.
　　Key words Canopy; Central leader form; Relative light intensity; Yield; Texture property
　　Received: March 3, 2021  Accepted: May 4, 2021
　　Supported by China Agriculture Research System (Nos.2019-3-5-1-02, 2019-3-4-4, F18R06001-1); China Agriculture Research System of MOF and MARA (No.CARS-30-Z-02); Hebei Province Key Research and Development Project (No.17226341).
　　Zhaoyuan WANG (1981-), male, P. R. China, associate researcher, devoted to research about fruit tree cultivation and breeding.
　　*Corresponding author.
　　 The texture of fresh fruits and fruit products is a key quality influencing acceptability to consumers［1-3］. Taniwaki developed a texture measurement system based on an acoustic vibration technique［4］. The measurement system is capable of a high sampling rate (80 kHz), and thus enables high-resolution measurement based on the probe speed at 22 mm/s, which is similar to the actual speed of human mastication. The textural properties of fruits are important indicators of quality and include hardness, cohesiveness, springiness, and chewiness［5-7］. Luckett［8］ reported that the attributes affecting texture are usually highly interrelated. Gao et al.［9］ noted that hardness, fracture, springiness, and chewiness could be used as the main indices to evaluate fruit texture, while parameters such as adhesiveness and cohesiveness are responsible for more subtle textural changes. 　　There have been many attempts to investigate and measure the relationship between fruit texture index and light intensity. Light availability is critical to the production of quality fruits, since light is the driving force for leaf photosynthesis, which provides energy for fruit development and sugar accumulation and influences fruit yield and the natural life of a tree［10］. Therefore, improving the light distribution within the tree canopy and the utilization of the available light is expected to improve fruit texture. Wagenmakers［11］ observed that when the light levels on the lower layer of a canopy reached 35%, the distribution of yield increased gradually from top to bottom and from inside to outside. To date, researchers have focused mainly on conventional fruit quality index and relative light intensity. It has been reported that the fruit quality in different layers of a canopy clearly varies and that the quality factors are related to light intensity［12-13］. Xu and Chen［14］ showed that different canopy positions with different relative light intensities resulted in significantly different average fruit weight, firmness, soluble solids content, and anthocyanin content. However, little research has examined the distribution of light intensity and its influence on fruit textural attributes, despite the importance of texture to fruit quality. Texture analyzer is about the character of force with food texture, and the result is sensitive and objective.
　　Therefore, our aim in this study was to investigate the relationships between fruit textural properties and relative light intensity.
　　Materials and Methods
　　Plant materials and experimental site
　　Trials were performed in the Qiuyan peach (Prunus persica L.‘Qiuyan’) orchard of Changling, China (39°45′ N, 119°12′ E). The trees were trained to a central leader form and were 7 years old. Tree spacing was 2 m×4 m and oriented north-south. The experimental site contained a sandy loam soil and benefited from a good irrigation and management system. The fruit trees were pruned in the normal manner in winter and summer.
　　Experimental set-up
　　The test was based on a single tree design repeated three times. Each canopy was divided into different layers and positions using 0.5 m×0.5 m×0.5 m cubes located by bamboo poles according to the method of Wei［12］. Horizontal positions of the canopy were classified as inside (0.5 m from the stem), central (0.5-1.0 m away from the stem), and outside (1.0 m from the stem). The vertical extent of the canopy was divided into six levels (from the base of the tree trunk, respectively 0.5-1.0 m, 1.0-1.5 m, 1.5-2.0 m, 2.0-2.5 m, 2.5-3.0 m and 2.5-3.5 m). 　　Relative light intensity
　　The relative light intensity at different levels and positions of the canopy was measured on typical sunny days during the young fruit period in May, the hard core period in June, and the mature period in July, using a digital illuminometer (Model: TASI-631, China). Measurements were taken at 8 am, 11 am, 2 pm, and 5 pm. Three sets of readings were obtained, each 25 d apart. The values of relative light intensity at different levels and positions were calculated using average values for the four time points on each day and three days for the first and final period of leaf screen formation.
　　Texture parameters
　　Fruits were picked at the time of commercial maturity (on July 28, 2015), and the numbers within each cube were recorded. After harvest, each fruit was weighed with a centigram balance (Model: BL-310, ShuangJie, China) and its textural properties were measured using a texture profile analyzer (Model: CT3-4500 CID, BrookField, USA), according to the method of Li et al.［15］. A TA39 probe was employed, and a 7-mm travel distance and a speed of 10 mm/s in the cycle mode were chosen as operating parameters. Textural properties (hardness, cohesiveness, springiness, and chewiness) were analyzed.
　　Statistical analysis
　　All analyses were carried out in triplicate corresponding to three individual peach fruits, and mean differences were analyzed by one-way ANOVA followed by the post-hoc Tukey HSD test at a significance level of P<0.05. Correlation analysis examined the interrelationships between the study parameters. SPSS 20.0 software was used to analyze the test results (Pearson correlation analysis).
　　Results and Discussion
　　Distribution of light intensity in the canopy
　　We found that the relative light intensity within the central leader forming the crown of the tree increased gradually from inside to outside the canopy and from bottom to top (Fig.1). These results demonstrate that the light intensity distribution in the peach tree canopy has a certain regularity, in agreement with reports by Robinson and Seeleyand Robinson and Lakso［16-17］.
　　The light intensities in different parts of the canopy volume varied over time: in May, June, and July, respectively, 49.07%, 56.02%, and 58.95% of the canopy received<30% relative light intensity; 18.21%, 26.70%, and 30.09% received 30%-80% relative light intensity; and 32.72%, 17.28%, and 10.96% received >80% relative light intensity (Table1). 　　 Our analysis of the changes in light intensity distribution over time showed that 56.02% and 58.95% of the canopy had a relative light intensity of <30% during June and July, respectively. By contrast, during the young fruit period in May, 32.72% of the canopy volume had a relative light intensity >80%. This supports the practice of increased pruning as the summer advances to provide more light to the inner portions of the tree.
　　Distribution of fruit yield in the canopy
　　High yield is an important aspect of fruit production, and we found a clear relationship between fruit yield and relative light intensity. On the central leader, fruits were mainly distributed in brighter conditions within 1.5-3.0 m of the top of the canopy, and the area of maximum yield was within 2.0-2.5 m of the top; and in the horizontal direction, the order of yields was central>outside>inside (Fig.2). Thus, peach fruit production has a definite connection to light intensity. This agrees with the results of Yue on pears (Pyrus pyrifolia ‘Hwangkumbae’)［18］.
　　Relationships between light intensity and texture
　　We used a texture profile analyzer to determine texture parameters to provide an objective assessment of fruit texture. Different areas and levels within the canopy showed differences in textural properties, including fruit hardness, cohesiveness, springiness, and chewiness. In the horizontal direction, hardness and springiness (two undesirable qualities for peaches) both decreased in the order inside>central>outside; and in the vertical direction, these parameters gradually diminished from top to bottom, and the hardness of the fruits from the bottom layer of the tree was significantly greater than that of fruits from the upper layer. The cohesiveness and chewiness of the fruits (both desirable qualities) showed an opposite relationship with light intensity, decreasing from outside to inside and from top to bottom. Thus, desirable fruit texture qualities were positively associated with the relative light intensity (Table 2). Based on the analysis of these different aspects of fruit texture and the relative light intensity distribution, we found that fruit textural properties have a strong and positive association with the relative intensity of light under which the fruits develop.
　　 Additionally, we found that fruit hardness was positively associated with relative light intensity. This contrasts with the findings of Zhang that the light intensity did not affect fruit firmness, but is consistent with the report of Wanmi peach［19］. He that light conditions influence apple (Malus domestica) fruit color and soluble solid content［20］. The difference between our results and those of Zhang may have to do with characteristics of varieties and tree species, and the underlying mechanism needs further exploration［19］. 　　Regression analysis of relationships between light intensity and texture property
　　We used regression analysis to evaluate the relationship between fruit texture parameters and the relative light intensity in May, June, and July. Taking relative light intensity within each cube as the independent variable, we derived quadratic polynomial regression equations to relate fruit textural properties and relative light intensity (Table 3).
　　The regression equations for hardness, cohesiveness, springiness, and chewiness as functions of relative light intensity in May, June, and July are listed in Table 3. We verified the reliability and efficiency of the equations, confirming that these could be used as predictive equations in experimental research. In combination, these equations indicate that the optimal levels of different fruit textural property parameters require different relative light intensities. The relationships between fruit textural properties and relative light intensity also vary over time. To determine the best relative light intensity for each peach fruit textural property, we obtained derivatives from the regression equations and set them to zero (Table 3). The resulting equations would be expected to identify the optimal relative light intensity levels to maximize any one property among those listed.
　　Light timing also appears to contribute differently to different texture properties. For example, the contribution of relative light intensity to fruit cohesiveness and chewiness was significantly higher in May than in June and July. This suggests that maximizing these properties requires a higher level of accumulated light at the young fruit period than later. By contrast, the effect of relative light intensity on fruit springiness was higher in July than in May and June. This shows that fruit springiness springiness depended on light intensity, not the reverse. Therefore, in the near-mature period, the fruit textural properties could be effectively improved through summer pruning and other technical measures. Our synthetic analysis indicated that, to achieve high yield and optimal texture, the relative light intensity should be >41.83% at all levels and positions of a ‘Qiuyan’ peach canopy with a central leader form.
　　This agrees with the results of He on peaches and Wei on apples, though the recommended minimum relative light intensity is slightly higher［12, 20］. The lowest relative light intensity for peach fruits seems to be higher than that for apples, and the lowest relative light intensity for the central leader form of peach is higher than that for the open central form, suggesting that peaches benefit more than apples from high light intensity. 　　Conclusions
　　We show that the distribution of relative light intensity in the peach canopy affects peach fruit yield. Fruits were distributed in the upper and middle parts of the canopy, within 1.5-3.0 m of the top. Fruit textural properties correlated with relative light intensities in the canopy and gradually worsen from the upper to the lower levels and from the outer to the inner layers of the canopy.
　　Through analysis of the ratio of different relative light intensity at different periods of fruit growth and development in canopy volume, we found that the leaf curtain formed in May resulted in a high proportion of the canopy having a relative light intensity of <30%. Therefore, further trimming is advisable as the canopy grows.
　　Regression equations for the relationships between relative light intensity distribution and fruit textural properties showed that the hardness, cohesiveness, springiness, and chewiness of individual fruits were positively correlated with the relative light intensity in which each fruit was grown. Finally, we showed that high quality peaches at all levels and positions of ‘Qiuyan’ peaches grown with a central leader form relied on a relative light intensity of above 41.83%.
　　References
　　［1］ ZHANG LF, CHEN FS, YANG HS, et al. Changes in firmness, pectin content and nanostructure of two crisp peach cultivars after storage［J］. LWT-Food Science and Technology, 2010(43): 26-32.
　　［2］ LABAKYABC P, GROSMAIREB L, RICCIA J, et al. Innovative non-destructive sorting technique for juicy stone fruits: textural properties of fresh mangos and purees［J］. Food and Bioproducts Processing, 2020(123): 188-98.
　　［3］ VAN BS, SILA DN, DUVETTER T, et al. Pectins in processed fruits and vegetables: Part III texture engineering［J］. Comprehensive Reviews in Food Science and Food Safety, 2009(8): 105-117.
　　［4］ TANIWAKI M, HANADA T, SAKURAI N. Device for acoustic measurement of food texture using a piezoelectric sensor ［J］. Food Research International, 2006(39): 1099-105.
　　［5］ HARKER FR, REDGWELL RJ, HALLETT IC, et al. Texture of fresh fruit［J］. Horticultural Reviews, 1997(20): 121-124.
　　［6］ SZCZESNIAK AS. Texture is a sensory property［J］. Food Quality and Preference, 2002(13): 215-125.
　　［7］ PREDIERI S, RAGAZZINI P, RONDELLI R. Sensory evaluation and peach fruit quality［J］. Acta Hortic, 2006(713): 429-434.
　　［8］ CURTIS RL, HAN SS. Consumer attitudes toward texture and other food attributes［J］. Journal of Texture Studies, 2015, 46(1): 46-57. 　　［9］ GAO H, JIA Y, WEI J, et al. Studies on the post-harvested fruit texture changes of ‘Yali’ and ‘Jingbaili’ pears by using texture analyzer［J］. Acta Hortic.Sin., 2012(39): 1359-1364.
　　［10］ ZHAN L, HU H, PANG L, et al. Light exposure during storage preserving soluble sugar and l-ascorbic acid content of minimally processed romaine lettuce (Lactuca sativa L. var. longifolia) ［J］. Food Chem, 2013(136): 273-278.
　　［11］ WAGENMAKERS PS. Effects of light and temperature on potential apple production［J］. Acta Horticulture, 1996(416): 191-197.
　　［12］ SHU YW, SU SC, MA LY, et al. Effects of canopy microclimate on fruit yield and quality of Camellia oleifera［J］. Scientia Horticulturae, 2018(235): 132-141.
　　［13］ WEI QP, LU RQ, ZHANG XC, et al. Relationships between distribution of relative light intensity and yield and quality in different tree canopy shapes for ‘Fuji’ apple［J］. Acta Hortic. Sin, 2004(31): 291-296.
　　［14］ XU SL, CHEN XQ. Light distribution and setting fruit position on quality of "Xiang" pear trees trained in hedgerow［J］. Shanxi Fruits, 2004(3): 3-5.
　　［15］ LI YH， WANG ZY，CHANG RF, et al. Characteristics of canopy structure and its relationship with fruit texture of main-trunk-type and open-central-type peach tree［J］. Acta Agricuiturae Jiangxi, 2017, 29(3): 66-69.
　　［16］ ROBINSON TC, SEELEY EJ. Effect of light environment and spur age on Delicious apple fruit size and quality［J］. J Am Soc Hortic Sci, 1983(70): 855-861.
　　［17］ ROBINSON TL, LAKSO AN. Modifying apple area canopies for improved production efficiency［J］. Hort Science, 1991(26): 1005-1012.
　　［18］ YUE YL, WEI QP, ZHANG JX, et al. Relationship between the distribution of relative light intensity and fruit quality of trellis trained ‘Hwangkumbae’［J］. Acta Hortic Sin, 2008(35): 625-630.
　　［19］ ZHANG XM, LIU Z, WANG H, et al. Relationships between distribution of relative light intensity and yield and quality in different tree canopy Shapes of ‘Yue Yanghong’ apple［J］. Southwest China Journal of Agricultural Sciences, 2014(1): 625-630.
　　［20］ HE FL, WANG F, WEI QP, et al. Relationship between distribution of relative light intensity in canopy and yield and quality of peach fruit［J］. Sci Agric Sin, 2008(41): 502-507.