Effects Of Fruit Load On Spatial Heterogeneity Of Leaf Photosynthetic Capacity Within Canopy In Apple Trees

The heterogeneity of canopy spatial structure regulates canopy microclimate,especially light,and thus affects the spatial heterogeneity of leaf photosynthetic capacity.Fruit load has significant effect on the synthesis and transport of photosynthetic products in the leaf,which in turn regulates the photosynthetic heterogeneity of canopy leaves with co-occurring light availability.Thus,the coordinated regulation of canopy structure and fruit load can maximize the total amount of canopy photosynthetic products and the amount of carbon allocated to fruit.However,the effects of fruit load and light on capacity and spatial heterogeneity of photosynthesis in apple leaves are not clear.In this study,the effects of fruit load and light on photosynthetic capacity,nitrogen content and spatial heterogeneity of non-structural carbohydrates in apple canopy leaves were investigated use 7-year-old Royal Gala/M9-T337 apple trees under fruit load with 0（L）,4（M）and 6（H）fruits per trunk cross-sectional area（cm²）,respectively.The results would support precise fruit load control and canopy structure optimization.The main results were as follows:1.Leaf length,leaf width and intercellular CO₂concentration of apple leaves had coefficients of variation lower than 20%,while specific leaf weight,net photosynthetic rate,nitrogen content per unit area and non-structural carbohydrates were higher than 20%.The coefficients of variation for leaf length,leaf width,net photosynthetic rate,stomatal conductance,transpiration rate and water use efficiency were34.93%,62.69%,80.59%,73.39%,59.81%and 63.58%higher for L-treated leaves than for H-treated leaves,respectively.The coefficients of variation for specific leaf weight,intercellular CO₂concentration and fructose content were 35.34%,62.63%and 22.88%higher in the H treatment than in the M treatment,respectively.The coefficient of variation for sucrose content was 34.04%higher in the L treatment than in the M treatment,while the coefficients of variation for sorbitol and glucose content were 43.18%and17.64%higher in the H treatment than in the L treatment,respectively.2.The photosynthetic capacity of the sun leaves was significantly higher than that of the shade leaves for all fruit load,while the photosynthetic conductance limitation of the L shade leaves was significantly higher than that of the stomatal and biochemical limitations,while the dominant photosynthetic limitation of the L sun leaves and the shade and sun leaves of H and M was the photosynthetic biochemical limitation.Nitrogen content per unit area was significantly higher in sun leaves than in shade leaves regardless of the fruit load treatment,while chlorophyll content per unit area was not significantly different,and the ratio of chlorophyll to N content per unit area was significantly higher in shade leaves than in sun leaves except for L.The proportion of N allocated to the H carboxylation system,the proportion of N allocated to photosynthetic components and the photosynthetic N use efficiency were all significantly higher in sun leaves than in shade leaves.3.The net photosynthetic rate and stomatal conductance of the corresponding H leaves were significantly higher than those of M and L during the day,irrespective of whether the leaves were positive on the east or west side of the canopy or negative at the bottom,and all showed the highest values between12:00-15:00,with no significant differences at night.Starch accumulation on the east side of the canopy was between 9:00 and 18:00,on the west side of the canopy it extended to 21:00,while on the shade leaves it only accumulated between 9:00 and 12:00.Sucrose accumulated from 9:00-21:00 in both the eastern and western canopy positive leaves and from 9:00-15:00 in the negative leaves.sorbitol accumulated from6:00-21:00 in both the eastern and western canopy positive and negative leaves,and decreased at 12:00 in all cases.Fructose accumulated from 9:00-12:00 on the eastern side of the canopy,and from 9:00-15:00 on the western side,and from 6:00-15:00 on the western side,respectively:The glucose accumulation time was 6:00-12:00 and 15:00-21:00 for the eastern side of the canopy,6:00-12:00 and 18:00-21:00 for the western side of the canopy,and 6:00-15:00 for the shaded leaves.In conclusion,the physiological activity of the leaves of Royal Gala/M9-T337 apple trees with high loads is higher,which can effectively use light energy,and the photosynthetic capacity of the sun leaves is higher than that of the shade leaves.Adjusting the spatial structure of the canopy can be used to optimise the overall photosynthetic carbon gain and photosynthetic nitrogen use efficiency of the plant.Fruit load,meteorological factors and circadian rhythms have a synergistic effect on the spatial heterogeneity of canopy photosynthetic capacity,and there is an interaction between the photosynthetic capacity of apple leaves and the carbon demand and storage capacity of the canopy.The results of this project have important theoretical and practical implications for the precise regulation of fruit load and canopy optimisation in dwarf apples.