Studies on photosynthetic characters and their relationship with yield in two late season walnut cultivars

Abstract:【Objective】The purpose of the study was to explore the relationship between photosyntheticcharacters and productivity in two late season walnut cultivars in order to provide theoretical reference forhigh yielding cultivation. The changes in the photosynthesis, chlorophyll fluorescence and yield per plantin the two cultivars were studied in this experiment.【Methods】The two late season walnut cultivars, ʻQin⁃gxiangʼ and ʻJinlong1ʼ, were used as the materials. The trees grown at a space of 5 m×6 m were ten yearsold and cultivated according to the conventional management. The soil was sandy loam, and water and fer⁃tilizer management was poor in orchard. Determination of changes in photosynthetic and fluorescence pa⁃rameters, fruit yield and kernel yield per plant was under normal circumstances. Photosynthetic and environmental parameters were determined using a portable photosynthetic analyzer (Li-6400, USA) and achlorophyll fluorescence analyzer (PAM-2100, Germany). The experiment was conducted in July 2016 ina walnut orchard in south mountain of Chengxian, Longnan. The measurements were carried out on July 1and 2, July 11 and 12 and July 21 and 22. Three grafted walnut seedlings were selected for each cultivar,and the fifth or sixth functional leaves of the middle compound leaves from three new branches in thesame growth direction of the outer canopy of each tree were measured. The diurnal variation of photosyn⁃thesis was measured at one hour interval from 8 O’clock to 18 O’clock, and the parameters measured in⁃cluded net photosynthetic rate (Pn, μmol·m-2·s-1), intercellular CO2 concentration (Ci, μmol·mol-1), stomatalconductance (Gs, mol·m-2·s-1), and transpiration rate (Tr, mmol·m-2·s-1). Besides, the environmental param⁃eters including atmospheric temperature (ta,℃), air CO2 concentration (Ca, μmol·mol-1), atmospheric rela⁃tive humidity (RH, %) and light intensity (PAR, μmol·m-2·s-1), and water use efficiency (WUE, mmol·mol-1)were also collected. The light response curve of Pn was constructed by measuring Pn at 2 000, 1 800, 1600, 1 400, 1 000, 500, 200, 150, 100, 50, 20 and 0 μmol·m-2·s-1. SPSS software was used to obtain theregressed light response curve. According to the regression equation, light compensation point (LCP) andsaturation point (LSP), maximum photosynthetic rate (Pmax), and apparent quantum yield (AQY) could beobtained. The diurnal variations of fluorescence parameters were measured at one hour interval from 8 O’clock to 18 O’clock at the same time with the measurement of photosynthetic parameters. The fluores⁃cence parameters included PSⅡ maximum photochemical efficiency (Fv/Fm), PSⅡphoton yield (Yield),photochemical quenching (qP), non-photochemical quenching (qN) and so on. The number of fruit andyield in each investigated tree were collected when walnut were ripe. 30 fresh walnuts were randomly collected for calculation of the average single fruit weight after fully dried in the middle of September in thesame year. Finally, walnut yield and kernel yield per plant were calculated. All the data were analyzedusing Excel 2010 and SPSS 21.0.【Results】The diurnal changes in Pn in both walnut cultivars exhibited adouble peak curve with a clear“mid-day rest”around 13 o’clock. The LSP, LCP, AQY, and Pmax in‘Jin⁃long1’and‘Qingxiang’were 1 612.5 and 1 716.7 μmol·m^-2·s^-1, 37.32 and 49.27 μmol·m^-2·s^-1, 0.031 8and 0.037 6, and 11.40 and 18.68 μmol·m^-2·s^-1, respectively. The data indicated late season walnut treesneed strong light and was insensitive to weak light, and therefore should be planted in low density. Thevariety of ʻQingxiangʼ showed a higher photosynthetic capacity. F_v/F_m, Yield, and qP were low, while qNwas high around 13 O’clock, when photosynthetic efficiency decreased with significant photoinhibitionbut no significant damage on photosynthetic system. F_v/F_m, qP, qN and Yield in ʻQingxiangʼ were generally higher than in‘Jinlong1’throughout the day, suggesting that the former had a better light conversionefficiency and stress tolerance and thus more suitable for planting in Longnan mountain. The yield perplant and yield per hectare were also significantly different between cultivars, and productivity of ʻQingxiangʼ was 1.77 times that of ʻJinlong1ʼ. Pn, F_v/F_m, Yield, qP, qN and yield per plant of the two late seasonwalnut cultivars had strong correlations.【Conclusion】The mid-day decrease in Pn in both cultivars wascaused by non-stomatal limitation under extreme environmental conditions. Photosynthetic capacity andplant productivity were significantly positively correlated. Their requirement for strong light with low sen⁃sitivity to low light indicated that they were suitable for sparse planting.