Effects of foliar rare earth fertilizer on photosynthesis, flowering and fruiting in Litchi chinensis

Abstract:【Objective】In this experiment, the effects of foliar rare earth fertilizer on the photosynthesis, flowering, fruit setting and fruit quality of litchi were studied to provide references for increasing effi- ciency in litchi industry.【Methods】Guiwei and Nuomici litchi trees grown in the orchard of South Chi- na Agricultural University were selected as test materials. The flowering rate in 2019 was extremely low due to the warm winter in 2018. The unfruiting Guiwei and Nuomici trees were sprayed with rare earth fertilizer on 29th April. Two weeks after the treatment, the photosynthetic parameters were ana- lyzed, and the activities of key photosynthetic enzymes were determined. Single tree was used as one replicate and 5 biological replicates were set up. In late October of 2019, another Nuomici and Guiwei trees with relatively consistent tree vigor and development state were sprayed with rare earth fertilizer, and leaf photosynthetic parameters were measured at two weeks after spraying. Rare earth element was sprayed again after photosynthesis measurement, and the flowering rate was investigated in April, 2020. Single tree was used as one replicate and 8 biological replicates were set up by using random block de- sign. Jingganghongnuo was selected to investigate the effects of rare earth element on the fruit set and fruit quality of litchi. Rare earth fertilizer was sprayed twice at male flowering and two weeks after an- thesis around mid-March and early April in 2020, respectively. Eight trees of each treatment and thirty clusters from each tree were analyzed. The clusters were used to investigate the fruit retention rate. The yield was calculated, and fruits were harvested to determine the fruit quality at maturity. Li6400 porta- ble photosynthesis meter (Li-Cor, USA) was used to measure leaf photosynthetic parameters. Sixteen mature shoots with similar growth status were randomly selected from different directions of the cano- py. In the summer of 2019, the photosynthesis of autumn shoots of last year and spring shoots of current year were determined, respectively. In the winter of 2020, autumn shoots of that year were selected to measure the photosynthetic parameter. Leaf SPAD value was measured by SPAD-502 Plus chlorophyll meter. The numbers of terminal shoots and flowering shoots in the selected branches were recorded. The flowering rate was calculated using a formula (flowering rate/% = number of flowering shoots/num- ber of terminal shoots × 100).【Results】The net CO2 assimilation of newly mature leaves from spring shoots was much higher than that on old leaves of autumn shoots in the previous year. Accordingly the stomatal conductance of the old autumn leaves in the previous year was significantly less than that of the new mature spring leaves in the current year. The net CO2 assimilation rate of mature leaves signifi- cantly increased after spraying with amino acid rare earth fertilizer (GM) and rare earth fertilizer (XT), especially on the old autumn leaves. At the same time, the treatments also increased the stomatal con- ductance and transpiration rate of the leaves, and the effect on the old leaves was more significant than that on the newly mature leaves. The increase of net photosynthetic rate may be related to the increase of chlorophyll content or photosynthetic enzyme activity. In this study, the leaf chlorophyll level and photosynthetic key enzyme activity with rare earth treatment and control were measured. The results showed that there were no significant differences in SPAD value, Rubisco, GAPDH and PRK activities between the two treatments, indicating that rare earth elements did not increase net photosynthetic rate by increasing photosynthetic pigment content or photosynthetic key enzyme activities. Foliar spraying with rare earth elements in winter significantly increased the net photosynthetic rate of mature leaves on autumn shoots, and also significantly increased the percentage of flowering shoots. The percentage of flowering shoots was only(37.1±6.9)% and(29.2±7.9)%, while that of treatments was(64.8±5.8)% and(49.0±6.7)% in Guiwei and Nuomici, respectively. The increment was more than 50%. There was no significant difference in the average number of fruits per cluster between the treatment and the con- trol at 10th day after anthesis, but with the development of fruits, the average fruit number of the clus- ters from trees treated with foliar rare earth fertilizer was significantly higher than that of the control. The average yield per tree was (53.0±6.2) kg with the treated trees, but only (33.9±5.6) kg with the con- trol trees. The increment was around 60%. Compared with the control, the average fruit size with the treatment was 22.5 g, which was slightly smaller than that of the control (23.6 g), but it did not reach a significant level. The smaller fruit size may be related to the significant increase of yield. There was al- so no difference in fruit color parameters and fruit recovery rate. The total soluble solids content in fruits with treated trees was 16.2%, which was significantly less than that of the control (16.8%). How- ever, sucrose, glucose, fructose and thus total sugar contents in the aril with treatment and control dis- played no significant difference.【Conclusion】Rare earth elements can enhance the flowering and fruit setting rate of litchi and thus increase the yield by improving net CO2 assimilation rate mainly due to an increase in stomatal conductance.