Spatial heterogeneity of photosynthetic characteristics in Chinese chestnut canopy

  Objective  Canopy is an important part of the interaction between trees and external environment. The study on spatial variation law of photosynthesis in different parts of Chinese chestnut canopy provides a theoretical basis for revealing Chinese chestnut canopy productivity.

  Method  The 8-year-old Chinese chestnut trees were used as tested materials, the Li-6400 portable photosynthesis measurement system was used to determine the diurnal changes of photosynthesis in different canopies and positions of the Chinese chestnut canopy in July, fruit collection was carried out to determine the economic traits of nuts in September. Variance analysis, path analysis and regression analysis were done on the base of indexes.

  Result  (1) The daily dynamics of photosynthetic rate (P_n) in the upper east, middle east, south middle, and lower south showed a unimodal curve distribution; the rest of the region showed a bimodal curve, and the areas showing a bimodal curve distribution all presented a “midday depression” phenomenon at 13:00. The daily dynamics of water use efficiency (WUE) in each canopy area presented a bimodal curve. (2) In the vertical direction of the canopy, the deficient vapor pressure (V_pdL) and intercellular CO₂ concentration (C_i) were shown as “lower > middle > upper”. The maximum values were 2.13 kPa, 274.93 μmol/mol, and the minimum values were 1.95 kPa, 258.75 μmol/mol; photosynthetic rate, stomatal conductance (G_s), transpiration rate (T_r), photosynthetically active radiation (PAR), WUE and carboxylation efficiency (V_c) presented a “upper > middle > lower” phenomenon. The SPAD and specific leaf area (SLA) had significant differences in the vertical canopy (P < 0.05), and both showed “lower > middle > upper”. (3) In different directions in the canopy, the strongest photosynthetic capacity was in the east, but the photosynthetic indexes were not significantly different compared with the vertical canopy. The SPAD only had significantly difference in the east, west and north directions. The SLA was not significantly different in varied directions. (4) Path analysis showed that the main factors affecting P_n were T_r, WUE, V_c, V_pdL, G_s, air temperature (T_a) and C_i, only T_a had an inhibitory effect on the P_n; T_r and V_pdL were the physiological and environmental factors that had the greatest influence on Chinese chestnut photosynthesis.（5）In the vertical canopy, the P_n was significantly or extremely significantly and positively correlated with the single fruit mass, bur nut rate and yield per unit area, and the correlation coefficients were 0.872, 0.965 and 0.958, respectively. In the horizontal canopy, the P_n was significantly positively correlated with the single fruit mass and yield per unit area, and the correlation coefficients were 0.777 and 0.487, respectively. A comprehensive analysis of the horizontal and vertical canopy showed that the P_n was significantly or extremely significantly and positively correlated with the single fruit mass, bur nut rate and yield per unit area, with correlation coefficients of 0.600, 0.669 and 0.532, respectively.

  Conclusion  The photosynthesis of Chinese chestnut has obvious spatial heterogeneity, and the difference between photosynthetic index and fruit yield is the result of synthesis of vertical and horizontal canopy. In Chinese chestnut production, pruning should be reasonable to improve the efficiency for solar energy utilization in the lower and middle part of canopy, so as to increase the fruit yield.