Comparison of leaf traits and diurnal variation of photosynthetic rates between red- and green-leafed walnuts

Abstract:【Objective】In this research, red- and green-leafed walnuts‘( Zhonglin 1’) were used as the materials for comparing the photosynthetic parameters between two types of walnuts under field condi- tions. The purpose of the study was to explore the differences in photosynthetic characteristics between red- and green-leafed walnuts, so as to provide a reference for the development and utilization of red- leafed walnut resource. Therefore, in this experiment, the diurnal variation of photosynthetic parameters and the difference in photosynthetic pigment contents were studied.【Methods】4-year-old red- and green-leafed walnut trees were grown at a space of 2 m×3 m in north-south row and cultivated with conventional management. Total chlorophyll, carotenoid and anthocyanin contents in leaves during fruit set- ting period (April 25, 2019) and seed enrichment period (June 27, 2019) were determined using UV-visi- ble spectrophotometer (L5, Shanghai). Determination of photosynthetic parameters was carried out dur- ing the walnut fruit growth period (July 6, 2019). Photosynthetic parameters were determined under nat- ural conditions with a portable photosynthetic analyzer (Li-6400 XT, USA). The healthy and consistent leaves from the middle compound leaves on the branches were selected as the samples. The photosyn- thetic parameters were measured at two hours interval from 6:00 to 18:00, including net photosynthetic rate (Pn, μmol·m-2·s-1), transpiration rate (Tr, mmol·m-2·s-1), intercellular CO2 concentration (Ci, μmol·mol-1),stomatal conductance (Gs, mol·m-2 ·s-1), atmosphere relative humidity (RH, %) and water use efficiency (WUE, mmol·mol-1). All the test data were analyzed using Microsoft Excel 2010 and Origin pro 8【. Re- sults】The photosynthetic activity of red-leafed walnut was lower than that of green-leafed one. The diur- nal changes of net photosynthetic rate of green-leafed walnut showed an obvious“bimodal curve”and the“photosynthetic noon break”appeared at 12:00. The diurnal variation of net photosynthetic rate of red-leafed walnut was irregular with a clear“photosynthetic noon break”around 14 o’clock. The Pmaxingreen-leafedwalnutandred-leafedwalnutwere15.3and12.67μmol·m-2·s-1,respectively.TheGS in both leaves exhibited a bimodal curve and kept consistent with changes in net photosynthetic rate. TheCi in both leaves had the same trend. They all decreased first, then rised slowly, and then decreased again after 14:00. Because the Gs of red-leafed walnut was lower than that of green-leafed walnut, the Trwas lower than that of green-leafed walnut. The Tr in both leaves still showed a bimodal curve, and the highest peak appeared at 14:00. The total chlorophyll and carotenoid contents in leaves of green-leafed walnut were higher than those of red-leafed walnut, while the contents of anthocyanin in leaves of the red-leafed walnut were higher than those of green-leafed walnut during fruit setting period, and the con- tent of anthocyanins of red-leafed walnut decreased rapidly during fruit growth period, but still higher than those of green-leafed walnuts. Many studies have shown that the photosynthetic efficiency of color- ful plants is lower than green- leafed plants. However, the photosynthetic capacity of some colorful plants did not decreased, but instead was higher than green-leafed tree species. The photosynthetic effi- ciency of red-leafed walnut was lower than that of green-leafed walnut in this test. This may be due to the presence of anthocyanins. It is possible that anthocyanins absorbed and dissipated a part of the light energy, resulting in a decrease in the light energy available for photosynthesis.【Conclusion】The photo- synthetic capacity of red-leafed walnut was weaker than that of green-leafed walnut, which may be related to the presence of anthocyanins in the leaves of red-leafed walnut. When growing red-leafed wal- nuts, we should create conditions conducive to photosynthesis as much as possible, such as reasonable pruning, excellent ventilation and light transmission; appropriate irrigation in summer to increase the humidity in walnut orchards, reducing temperature and transpiration, shortening the time of photosyn- thetic noon-break, and improving photosynthetic levels.