Effects of covering with light-diffusing film on photosynthetic characteristics of leaves at different node positions and fruit quality of grapes un- der protection

Abstract:【Objective】The aim of this study was to explore the effects of covering light-diffusing film on light environment in greenhouse and photosynthetic characteristics of leaves at different node posi-tions as well as fruit quality, in order to provide reference for application of light diffusing film.【Methods】In this study, 4-year-old Zaoheibao grapevines were adopted as experimental materials, and the ef-fects of changing light environment on growth and quality of grape berries through covering with light-diffusing (as treatment) or with ordinary PO film (as control) were examined. Diurnal variations of light intensity at lower leaf (LL, 0.80-0.90 m above ground), middle leaf (ML, 1.20-1.30 m above ground) and upper leaf (HL, 1.60-1.70 m above ground) positions of the eastward and westward sides were mea-sured at intervals of one hour from 08:00 to 18:00 using a hand-held photons instrument (MQ-100) in three successive clear days. Diurnal variations of leaf net photosynthetic rate (Pn) and dark respiration rate (Rd) at LL, ML and HL of eastward side and westward side were measured once every two hours from 08:00 to 18:00 using a LI-6400XT photosynthesis measurement system. Chlorophyll fluorescence parameters including minimum fluorescence in dark (Fo), maximum fluorescence in dark (Fm), and po- tential quantum efficiency of PSⅡ (Fv/Fm) at LL, ML and HL in the treatment and the control were mea- sured using the LI-6400XT equipped with an integrated fluorescence chamber head (LI-6400-40). The response curves of Pn to photosynthetically active radiation (PAR) and intercellular CO2 concentration (Ci) of LL, ML and HL were measured using the LI-6400XT with its standard 6 cm 2 leaf chamber and red-blue LED light source (LI-6400-02B). The Pn-PAR response curves were obtained at ambient CO2 concentration of (400±20) μmol · mol^{- 1} with PAR set in a decreasing series of 1500, 1200, 1000, 800,500, 200, 100, 80, 50, 20 and 0 μmol·m^-2·s-1. The Pn-Ci response curves were measured under light in-tensity of 1200 μmol·m-2·s^-1 (saturated PAR derived from Pn-PAR response curves) with CO2 concentra-tion set at 400, 300, 200, 100, 50, 400, 400, 600, 800, 1000, 1200, 1500 and 2000 μmol·mol^-1. The leaf traits such as length, width and relative chlorophyll content (reflected by SPAD value) of LL, ML and HL were also measured for correlation analysis. During the fruit harvest season, fruit quality characters including single fruit weight, fruit shape index (vertical diameter/cross diameter of the fruit), soluble solid content, vitamin C content, soluble sugar content, total acidity as well as total calcium, total potas-sium and crude protein content in the fruit were determined.【Results】Compared with the control, de-creased light intensity at HL and increased light intensity at ML of shaded side (i.e. westward side in the morning and eastward side in the afternoon) in treatment were observed, and light environment was im- proved significantly at LL in the whole day with daily mean vale of light intensity increased by 29.1%-35.4% in the treatment. Pn of HL in the control was significantly lower than that in the treatment at noon and in the afternoon due to the photoinhibition induced by oversaturated light intensity at midday. Pn of ML at the shaded sides in the treatment was significantly higher than that in control. Pn of LL in the treatment was increased significantly in the whole day with daily maximum value and daily mean value increased by 18.3% and 59.8%, respectively, compared with the control (p＜0.05). Leaf Fo, Fm and Fv/Fm in the treatment were higher than in the control across different node positions (HL, ML and LL).Leaf maximum Pn under saturated light intensity (Pnmax) and leaf maximum Pn in the Pn- Ci response curves (Pnmax, c) in the treatment increased by 2.2% (mean value of HL, ML and LL at eastward side)- 9.0% (mean value of HL, ML and LL at westward side) and 30.2% (mean value of HL, ML and LL at eastward side)-68.7% (mean value of HL, ML and LL at westward side) (p＜0.05), respectively than those in control. The value of leaf length multiplied by leaf width in the treatment was significantly higher than that in the control across different node positions and a sharp increase occurred as node posi-tion became lower. Relative chlorophyll content (SPAD value) of HL was not significantly different be-tween the treatment and the control. However, those of ML and LL were significantly higher in the treat-ment than in the control. Except for fruit shape indices which showed no significant difference, other fruit quality characters such as single fruit weight, soluble solids content, vitamin C content, soluble sugar content as well as total calcium, total potassium and crude protein content in grape fruit increased significantly while total acidity decreased significantly in the treatment.【Conclusion】Accordingly, in greenhouse covered with light-diffusing film, light environment of the middle and lower leaves was im-proved greatly, leading to increased leaf photosynthetic rate and leaf size and relative chlorophyll con-tent, and thus improved fruit quality.