Characteristics of photosynthesis and vertical canopy architecture of citrus trees under two labor-saving cultivation modes using unmanned aerial vehicle (UAV)-based LiDAR data in citrus orchards

Abstract

Analyzing and comparing the effects of labor-saving cultivation modes on photosynthesis, as well as studying their vertical canopy architecture, can improve the tree structure of high-quality and high-yield citrus and selection of labor-saving cultivation modes. The photosynthesis of 1080 leaves of two labor-saving cultivation modes (wide-row and narrow-plant mode and fenced mode) comparing with the traditional mode were measured, and nitrogen content of all leaves and photosynthetic nitrogen use efficiency (PNUE) were determined. Unmanned aerial vehicle (UAV)-based light detection and ranging (LiDAR) data were used to assess the vertical architecture of three citrus cultivation modes. Results showed that for the wide-row and narrow-plant and traditional modes leaf photosynthetic CO₂ assimilation rate, stomatal conductance, and transpiration rate of the upper layer were significantly higher than those of the middle layer, and values of the middle layer were markedly higher than those of the lower layer. In the fenced mode, a significant difference in photosynthetic factors between the upper and middle layers was not observed. A vertical canopy distribution had a more significant effect on PNUE in the traditional mode. Leaves in the fenced mode had distinct photosynthetic advantages and higher PNUE. UAV-based LiDAR data effectively revealed the differences in the vertical canopy architecture of citrus trees by enabling calculating the density and height percentile of the LiDAR point cloud. The point cloud densities of three cultivation modes were significantly different for all LiDAR density slices, especially at higher canopy heights. The labor-saving modes, particularly the fenced mode, had significantly higher height percentile data.

Acknowledgements

This work was supported by the National Key Research and Development Plan (no. 2019YFD1000104) and a project supported by the Fundamental Research Funds for the Central University (no. 2662020YLPY020). This study was also supported by two National Natural Fund Projects (no. 31901963 and 31972356) and an earmarked fund for CARS 26.

Author contributions

Y.-Y.D. obtained LiDAR data using a UAV and extracted and analyzed the corresponding parameters of the point cloud data. X.-Y.L., L.-H., J.-X.Z., and W.-B.Z. measured all photosynthetic and nitrogen contents. J.-Z.Z. and C.-G.H. found appropriate citrus orchards and helped determine the experimental design. Y.-Z.L. checked the experimental data and revised the manuscript accordingly. Q.-Q.H and Y.-H.Z helped to determinte leaf nitrogen cotents and draw figures. Y.-N.F. coordinated the laboratory instruments and assisted with nitrogen determination. J.-J.Z. designed the experiment, analyzed the photosynthetic factors and nitrogen content-related experimental data, and wrote the manuscript. All the authors have read and agreed to the published version of the manuscript.

Data availability

The data used in this study are available from the corresponding author upon reasonable request.

Conflict of interest

The authors declare no conflict of interest.

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