Impact of summer shading on net photosynthetic rate of Actinidia chinensis 'Hongyang' and its related eco-physiological factors

Abstract:【Objective】The study explored the diurnal changes in photosynthetic parameters of Actinidia chinensis‘Hongyang'under different shading treatments in summer.【Methods】0% (control) , 35% and 70% shading levels were used in this study, and the treatments began on June 10, 2015. Gas exchange characteristics of the plants were detected on sunny days (July 13 to14 and August 3 to 4, 2015) from 6:00to 18:00 at 1.5 h intervals. Photosynthesis related parameters including P_n, transpiration rate (T_r) , stomatal conductance (G_s) , intercellular CO₂ concentration (C_i) , leaf temperature (Tleaf) and environmental factors including photosynthetic active radiation (PAR) , air temperature (ta) , air CO2concentration (Ca) , and air relative humidity (RH) were automatically measured and recorded with a portable photosynthesis system with an air flow rate of 0.3 L·min^–1and an open system (CI-340, CID Inc., USA) . Central portion of each leaf was used for photosynthesis measurement to ensure sample uniformity. Measurements were replicated 6times (n=6) for each treatment.【Results】(1) A typical double-peak pattern of diurnal changes in Pnwith an obvious midday depression was recorded in the control and the 35% shading treatment. The value of the first peak of Pnoccurred at 9:00 and was the highest. In the control and under 35% shading treatment, this peak value was 11.198 and 14.715 μmol·m^-2·s^-1, respectively, and the value of the second peak occurring at 15:00 was 7.817 and 8.980 μmol·m^-2·s^-1, respectively. In contrast, a single-peak diurnal curveof Pnwas found in the 70% shading treatment, the peak value of which was 10.982 μmol·m^-2·s^-1 appearing at 9:00. The daily average value of Pnwas 7.889, 6.058, and 3.921 μmol·m^-2·s^-1 in the 35% shading treatment, the control and the 70% shading treatment, respectively. However, the value of PNwas significantly higher in the control than in the 35% and 70% shading treatments at 6:00 and 18:00. (2) The94.8% and 99.3% of the variations in Pncould be separately explained by 6 eco-physiological factors, such as PAR, Ca, T_r, G_s, C_i, and Tleafin the control and the 35% shading treatments, respectively. 97.3% of the variation in Pncould be explained by Ca, RH, Gsand Ciunder 70% shading treatment. Further, significant negative effects of PAR, C_i, and Tleafbut significant positive effects of Ca, T_r, and Gson Pnwere found in the control and under the 35% shading treatment. As for the 70% shading treatment, negative factors and the direct positive factors were C_i, RH and Ca, G_s, respectively. According to the decision coefficient, PAR, G_s, Ciand Tleafwere the four inhibitors and Caand Trwere the 2 determinants which affected the Pnof A. chinensis‘Hongyang'in the control. The inhibitors and the determinants were PAR, Tleafand T_r, Gsand Ciunder the 35% shading treatment, respectively, and were C_i, RH and Ca, Gsunder the 70% shading treatment, respectively.【Conclusion】In summer, there was an obvious midday depression of Pnin A. chinensis ‘Hongyang', which was caused by stomatal limitation. The Pncould be effectively improved by35% shading treatment except in the early morning or at dusk. PAR and Tleafwere the major inhibitors affecting the Pnof A. chinensis‘Hongyang', while Trwas its main determinant. We suggest that, for the cultivation of the A. chinensis‘Hongyang', it is better to select an open region with a good air circulation, such as flat terrain or less slope gradient and southern side of hills, and to use moderate shading treatment in day time but avoiding shading in the early morning and at dusk during summer.