Analysis of drone-assisted liquid pollinating effects in chestnut trees

【Objective】To explore the optimal formulations of stabilizer and pollen suspension, flight parameters, and pollination period for drone-assisted liquid pollination of chestnuts, so as to provide a reference for the application of drone liquid pollination technology in chestnut orchards.【Methods】Using Castanea mollissima Yanshan Zaofeng as the research object (maternal parent) and Yanzi as the pollen donor, DJI T50 drone was used for assisted liquid pollination. First, solutions with different ratios of xanthan gum to pollen were prepared, and the pollen concentration was measured at 0, 30, and 60 minutes to screen the optimal stabilizer ratio for pollen. Subsequently, an orthogonal experiment was designed to prepare pollen suspensions, and the pollen viability was measured at 24 and 36 hours respectively to screen the optimal suspension formula. Meanwhile, Principal Component Analysis (PCA) was used to screen the optimal flight parameters of the drone for assisted pollination and the best pollination period. Additionally, the prepared solutions were placed in a 25 ℃ constant- temperature chamber to evaluate the stability changes at the above-mentioned time points. For the pollen suspension orthogonal experiment, three factors (sucrose, borax, calcium chloride) with three levels for each were set according to different concentrations. After incubating the suspensions at 34 ℃, the pollen germination rates were measured at 24 and 36 hours. For the optimization of drone parameters, an orthogonal experiment with four factors (flight speed, flight height above the canopy, atomized droplet size, spray volume) and three levels for each was conducted to determine the Number Median Diameter (NMD), droplet spectrum width, droplet coverage rate, droplet density, and droplet deposition amount. Pollination tests were carried out using the T50 drone at three flowering stages: early full bloom (S1), mid full bloom (S2), and late full bloom (S3). The key indicators measured included fruit set rate, bracteate rate, nut-to-bur weight ratio, yield per fruiting branch, single fruit weight, individual bract weight, number of grains per bract, and number of bracts per fruiting branch, and PCA was further used to determine the optimal pollination period based on fruit yield improvement.【Results】The suspension stability of stabilizers W14, W12, W15, W10, W11, and W4 all exceeded 90%, with no significant differences among them. The pollen concentration of stabilizer W11 (xanthan gum 1.5 g·L-1 + pollen 1.5 g·L-1 ) was the highest at both 0 and 60 minutes, while stabilizer W12 (xanthan gum 1.5 g ·L- 1 + pollen 2 g ·L- 1 ) had the highest pollen concentration at 30 minutes. At 0, 30, and 60 minutes, the pollen concentration of W11 remained above 100 000 grains·mL-1 , showing high stability with less material consumption and cost-effectiveness, and its stability rate reached 91.77%. The F5 formulation (sucrose 150 g·L-1 , borax 0.2 g·L-1 , calcium chloride 0.2 g ·L- 1 ) achieved the highest pollen germination rate in the pollen suspension, reaching 23.70% at 24 hours and 27.79% at 36 hours. The flight parameters of the drone had significant effects on NMD, droplet density, coverage rate, deposition amount, and droplet spectrum width. Among them, treatment T2 (drone flight speed 1 m/s, flight height above the canopy 2 m, atomized particle size 150 μm, spray volume 55 L·hm-2 ) had the best and significant effect on the NMD of the middle and upper canopy layers; treatment T8 (flight speed 3 m/s, flight height above the canopy 2 m, atomized particle size 100 μm, spray volume 65 L·hm-2 ) had the best effect on the droplet spectrum width of the middle and upper canopy layers, but the difference was not significant; treatment T7 (flight speed 3 m/s, flight height above the canopy 1 m, atomized particle size 130 μm, spray volume 55 L· hm-2 ) had the optimal effect on the droplet density, droplet coverage rate, and droplet deposition amount of the middle and upper canopy layers, with significant differences. PCA results showed that the cumulative contribution rate of the first two principal components was the highest, reaching 91.834%. Among them, the first principal component accounted for 63.637% of the variance (eigenvalue=3.182), and the second principal component accounted for 28.197% of the variance (eigenvalue=1.410). Comprehensive evaluation and ranking based on these components indicated that the spraying effect of drone-assisted pollination was the best under treatment T7. Drone-assisted pollination at S1, S2, and S3 stages all effectively improved the economic traits of chestnuts. Among these stages, S1 had the highest values in most indicators across all treatments, including a bracteate rate of 100%, a nut-to-bur weight ratio of 40.37%, a yield per fruiting branch of 38.71 g, 2.73 grains per bract, and 1.84 fruiting bracts per branch. S2 had the highest fruit set rate of 93.63%, while S3 had the largest single fruit weight (8.53 g) and individual bract weight (58.00 g). PCA results showed that the cumulative contribution rate of the first two principal components was 94.461%. The first principal component explained 66.160% of the variance (eigenvalue = 5.293), which was mainly determined by the nut-to- bur weight ratio, individual bract weight, number of grains per bract, number of bracts per fruiting branch, and yield per fruiting branch. The second principal compo- nent explained 28.301% of the variance (eigenvalue = 2.264), which was mainly driven by the fruit set rate, bracteate rate, and single fruit weight. Among the three stages, S1 had the most significant improvement effect on fruit yield.【Conclusion】The optimal ratios of stabilizer and suspension for chestnut pollen are as follows: xanthan gum 1.5 g ·L-1 , pollen 1.5 g ·L-1 , sucrose 150 g ·L-1 , borax 0.2 g ·L-1 , and calcium chloride 0.2 g·L-1 . When using the DJI T50 drone for assisted liquid pollination at the early full bloom stage (S1) with a flight speed of 3 m/s, a height of 1 m above the canopy, an atomized droplet size of 130 μm, and a spray volume of 55 L· hm-2 , the effect of improving chestnut fruit yield is the most significant.