A comparative study of chlorophyll fluorescence characteristics between early fruiting and late fruiting walnut cultivars in Xinjiang

【Objective】In order to compare the physiological differences between early and late fruiting walnut varieties, and provide theoretical reference for high yield cultivation walnuts under high light and high temperature conditions in Xinjiang, the characteristic of the chlorophyll fluorescence in early and late walnut cultivars in Xinjiang was studied in this experiment.【Methods】Two early fruiting walnut cultivars(‘Xinxin 2#'and‘Xinzaofeng') and two late fruiting walnut cultivars(‘Wuhuo'and‘Hetian 1#')in Xinjiang were used as the materials, the walnut trees were twenty years old and planted according to the conventional cultivation management with spacing 3 m×5 m. All the chlorophyll fluorescence parameters of leaves were measured by the modulation imaging system Imaging-PAM(WALZ, Germany) with a small probe/B IMAG-MINI, and the climatic data were collected using a CIRAS-3 type photosynthetic measurement system( PP-System, USA). The experiment was conducted in June 2015 in the national fruit germplasm collection garden in Luntai, Xinjiang, where it was generally sunny and windless from9:00—19:00. Mature intact leaves were selected for the measurements which were conducted every two hours in simulated natural conditions. For each sample, we measured three leaves and chose three 1 cm2AOI(Area of Interest) for each leaf avoiding the leaf veins. We used the mean value as the result of the sample. The measurements of the minimum fluorescence and the maximum fluorescence were conducted after 20 minutes of dark adaptation. Then the fluorescence kinetic curve program was run with the actiniclight turned on. The related parameters including initial fluorescence(Fo), maximal fluorescence(Fm)and relative apparent electron transport rate(r ETR), the actual photochemical efficiency of PS II(Y(II)),the maximal PS II photochemical efficiency(Fv/Fm) and quantum yield of conditioning energy dissipation(Y(NPQ)) were collected. The fast light response curve(PFD-r ETR) was constructed at the end of the fluorescence kinetic curve using PAR gradients of 1, 20, 50, 100, 200, 400, 600, 800, 900, 1 000, 1 100, 1200, 1 300, 1 400, 1 500 μmol·m-2·s-1. Each gradient took 10 s, and the actual quantum yield and r ETR were measured. The change of r ETR with PAR was called fast light curve. According to the fitted equation, light compensation point(LCP) and light saturation point(LSP) were obtained. In order to analyzes the heterogeneity of chlorophyll fluorescence parameters, 2 spots on the surface of the blade were chosen using Imaging-PAM software Imaging-Win. The two spots were connected with a line in the middle of the blade(between the sixth lateral vein and seventh lateral vein from the base of the leaf), which minimized the interference with the measurement by the veins. The length of the line was set at 6 cm with the main vein as the midline. The value of fluorescence in each pixel of the line was exported, and coefficient of variation(CV) represented the degree of heterogeneity. Using the CIRAS-3 photosynthetic system, the daily variation of environmental parameters was measured together with the changes in chlorophyll fluorescence parameters. The atmospheric temperature(Ta), photon flux density(PARe), and atmospheric CO2concentration(Ca) were collected. Microsoft Excel 2007 and SPSS 17.0 were used to analyze the data.【Results】The fluorescence imaging of different parts showed that the fluctuations of different fluorescence parameters were different. The fluctuation of the maximum quantum yield and the actual photochemical efficiency of PSII was relatively small(less than 5%), and the rest of the fluorescence parameters displayed great variation. Comparison between the early and late fruiting varieties showed that variation in Foin the early fruiting varieties was significantly greater(P<0.05) than that in the late varieties, and the remaining fluorescence parameters showed no significant difference between varieties. The early fruiting varieties‘(Xinxin 2#'‘Xinzaofeng') and late fruiting varieties ‘(Wuhuo'‘Hetian 1#') showed a similar diurnal variation trend in the fluorescence parameters. Further analysis showed that the maximum fluorescence(Fm)(with the significant difference at P<0.01) and quantum yield of conditioning energy dissipation(Y(NPQ))(with the significant difference at P<0.05) of the early fruiting varieties were significantly lower than those of the late fruiting varieties, while the actual photochemical efficiency Y(II) was significantly(P<0.05) higher in the early fruiting varieties than in the late fruiting varieties. The two types of varieties showed no differences in initial fluorescence(Fo) and photochemical efficiency of PSII(Fv/Fm). Also, the light saturation point(LSP) and light compensation point(LCP) were compared in relation to the difference in chlorophyll fluorescence characteristics between the early and late fruiting walnut cultivars. The regression of fast light response curve showed that the‘Xinxin 2#'and‘Xinzaofeng'were typical early fruiting walnuts in Xinjiang, which had a LCP of 20-22 μmol·m-2·s-1and a LSP of 1 179-1 187 μmol·m-2·s-1,while‘Wuhuo'and‘Hetian 1#'were the representative late fruiting walnuts in Xinjiang with a LCP of 19-23 μmol·m-2·s-1and a LSP of 1 056-1 076 μmol·m-2·s-1. The mean value of LCP in the four varieties ranged from 19 μmol·m-2·s-1to 23 μmol·m-2·s-1, and that of LSP from 1 056 μmol·m-2·s-1to 1 192 μmol·m-2·s-1. The LSP of the early fruiting varieties was significantly(P<0.05) higher than that of late varieties.【Conclusion】The difference in chlorophyll fluorescence characteristics between the early and late fruiting walnut varieties was significant, and the need for light of the early fruiting varieties was greater than that of the late fruiting varieties.