Evaluation of photosynthetic and production potential in multi-leader-planar-trained apple trees

【Objective】Using the tall- spindle system, which is commonly employed in apple production, as a control, we evaluated the production efficiency, fruit yield and quality of the multi-leader-planar-trained apple trees. This study provides valuable insights for future selection of tree systems in apple orchards and contributes to advancing the mechanization and intelligence of orchard management. 【Methods】The longitudinal and transverse diameters of the fruits were determined using electronic vernier calipers (16FN, MAHR, Germany). The mass of individual fruits was assessed with an electronic balance (E5500S, Sartorius, Germany). Fruit firmness was evaluated utilizing a digital fruit hardness tester (GY-4-J, Tuopu Instrument). The soluble solids content was quantified using a digital display sugar meter (PAL-1, ATAGO, Japan), while the titratable acidity was measured using an acidometer (GMK- 835N, G-WON, Korea). The leaf area was quantified with a Li-3000C portable leaf area meter. Chlorophyll content was assessed using a SPAD-502 meter, while photosynthetic indexes were recorded with a CIRAS-3 device from PP Systems, USA, between 9:00 and 11:00 AM on a sunny day. Light intensity and interception rate were measured with a TSE-1332 meter. Canopy light transmittance is the ratio oflight intensity under the canopy to that above it, and the light interception rate is calculated as (1-canopy light transmittance) × 100%. Before harvesting, the composition of tree branches and spurs was investigated, which were categorized by length: long branches are greater than 30.0 cm in length; medium branches range from 15.0 cm to 30.0 cm; spurs are less than 15.0 cm. The height and length were measured using a tape measure, and the thickness was determined using a vernier caliper.【Results】The average hardness of the tall-spindle trees’fruits in two years was 9.30 kg · cm2 and 9.00 kg · cm2 , respectively, exceeding the 7.10 kg · cm2 and 8.10 kg · cm2 of the multi-leader trees’fruits. The average fruit shape indexes of the tall-spindle trees’fruits in two years were 0.78 and 0.79, respectively, lower than those (0.85 and 0.86) of multi-leader trees’fruits. The average soluble solids contents of tall- spindle trees in the two years were 12.21% and 12.41% , respectively, lower than that of multi- leader trees (13.45% and 13.86% ). Regarding titratable acid, the average values for tall- spindle trees in the two years were 0.31% and 0.30%, respectively, higher than the 0.27% and 0.26% recorded for multi-leader trees. However, the above indicators did not reach significant differences. Although the soluble solids content of multi-leader trees’fruits was higher than those of tall-spindle trees’fruits, the titratable acid content was lower, and no significant differences were detected. This suggested that there was not much difference in fruit flavor between the two types of fruits with high-efficiency-shaped tree systems. By comparing the photosynthetic parameters between the two tree systems, it was found that the net photosynthetic rate (Pn), intercellular carbon dioxide concentration (Ci), and stomatal conductance (Gs) of the multi-leader trees were higher than that of the tall-spindle trees at the same time. The difference was significant only in May, but it was not significant in June and July. The transpiration rate (Tr) of both tree shapes gradually increased from May to July, but there were no significant differences between them in each month. There was no significant difference in water use efficiency (WUE) between the two tree systems at the same time, and the WUE of the multi-leader trees gradually decreased from May to July. However, the total leaf area of tall-spindle trees was significantly higher than that of multi-leader trees. The leaf area index (LAI) with tall-spindle trees was 1.74 in May, 1.86 in June, and 2.02 in July. In contrast, the LAI for multi-leader trees was 1.15 in May, 1.29 in June, and 1.57 in July. This suggested that the tall- spindle leaves had a higher density and photosynthetic potential, which also implied a higher likelihood of severe shading. The SPAD values of the two tree systems gradually increased from May to July, indicating that the chlorophyll content of leaves gradually increased, and there was no significant difference in SPAD values between the two tree systems in the same month. The light interception rate for both tree systems generally exhibited a decreasing trend with increasing height away from the ground. The light interception rate of the tall-spindle trees was higher when the ground height was less than 1 meter, and this rate decreased significantly at a ground height of 3 meters. The difference in light interception rate between the bottom and top of the trees was not as pronounced as that observed in the tall-spindle trees. We also measured the light interception rates of the two tree systems at various distances (0, 20, 50, and 100 cm) from the leader at a height of 1.5 meters. Based on the average values from four time points at each distance, both tree systems displayed a trend where the light interception rate decreased with increasing distance from the leader. Overall, the presence of initial bearing trees resulted in a lower light interception rate for both systems, with multi-leader trees showing an even lower rate compared to tall- spindle trees. For the two tree systems, the leader diameter showed an increase from 2023 to 2024. However, there was no significant difference in tree height. In the case of the tallspindle trees, the number of main branches was approximately 21 in both years. Although the average length of main branches decreased, the data for the two years did not show a significant difference. Forthe multi-leader trees, there were only five upright leaders, and there was no significant difference in leader height between the two years. In 2023, the number of branches with different lengths of the two tree systems was relatively similar. By 2024, the number of long branches and spurs of both tree systems increased, while the number of middle branches decreased. Over the course of two years, multileader trees received approximately 6-7 ton per hectare higher yield than tall- spindle trees.【Conclusion】Although it is difficult to train multi-leader tree systems, the fruit quality has not decreased, and its photosynthetic performance is better than that of tall-spindle trees, and its yield has been improved. The multi-leader trees are more suitable for the future development of orchards in terms of yield, mechanization and labor saving.