Lettuce Field UAV Time Series Summer 2019

Flower opening and closure are traits of reproductive importance in all angiosperms because they determine the success of self- and cross-pollination. Different accessions of cultivated and wild lettuce were observed to flower at different times of day. An F6 recombinant inbred line population (RIL) had been derived from accessions of L. serriola accession Armenian999 x L. sativa landrace PI251246 that originated from different environments and differed markedly for daily floral opening time. The 236 RILs, both parental lines, and two controls, L. sativa cv. Salinas and L. serriola accession UC96US23, were grown in summer 2019 at the Department of Plant Sciences field facility in Davis, CA. The experiment had two complete randomized blocks, each consisting of 240 plots to accommodate the 240 genotypes. Within each block, eight individuals of each RIL or parent were planted into one 10 ft x 1 row plot.

Seven ground control points were set up in the field, four near the corners and three along the field’s East-West centerline. GPS coordinates, with an accuracy within a few centimeters, were recorded using a handheld data collector (Trimble Geo 7x Series). These coordinates were used in processing drone images to ensure that images collected at different times and dates aligned perfectly with one another.

A MicaSense RedEdge multi-spectral camera was mounted on a DJI Matrice100 drone. The camera captured images at five wavelengths: blue (475 nm center, 20 nm bandwidth), green (560 nm center, 20 nm bandwidth), red (668 nm center, 10 nm bandwidth), red edge (717 nm center, 10 nm bandwidth), and near-infrared (840 nm center, 40 nm bandwidth). In this study, only the blue, green, and red wavelengths were used for flower identification. The drone was flown over the experimental field at 9 am, 11 am, 1 pm and 3 pm on July 1st, 2019, and 10 am, 12 pm, 2 pm and 4 pm on July 9th, 2019. A DJI GS Pro app was used to plan and execute the flight. The drone flew at 15 meters above ground, and images were taken at a frequency that ensured 85% front- and side-overlaps between each pair of adjacent images. A MicaSense calibration panel was used for automated adjustment of the reflectance spectra. Raw images from the camera were stitched and processed with the Pix4DMapper Pro photogrammetry software to generate orthomosaic maps of surface reflectance at 1 cm spatial resolution. On average, 2,309 raw images were generated per time point, and 2,181 raw images were used to assemble each five-spectrum field map. With the reconstructed maps, the borders of individual plots were manually determined using the software ArcMap.

花的开放与闭合是所有被子植物（angiosperms）的重要繁殖性状，因其直接决定自花授粉与异花授粉的成败。研究观察到，栽培莴苣与野生莴苣的不同种质（accession）在一天中的不同时段开花。本研究使用的F6代重组自交系群体（recombinant inbred line, RIL），源自两个来源环境迥异且每日开花时间差异显著的种质：L. serriola种质Armenian999与L. sativa地方品种PI251246。本研究共种植236个RIL株系、两个亲本，以及两个对照材料：L. sativa品种Salinas与L. serriola种质UC96US23，种植试验于2019年夏季在美国加利福尼亚州戴维斯市植物科学系田间试验基地开展。
试验采用完全随机区组设计，共设置2个区组，每个区组包含240个小区以容纳全部240份基因型材料。每个区组内，每个RIL株系或亲本的8个单株被种植于1个长10英尺、宽1英尺的单行小区中。
试验田内共设置7个地面控制点：4个布设于田块四角附近，3个沿田块东西向中心线布置。研究采用手持数据采集器（Trimble Geo 7x Series）记录GPS坐标，定位精度可达厘米级。这些坐标用于后续无人机影像的配准处理，确保不同时段、不同日期采集的影像能够完全对齐。
本研究搭载MicaSense RedEdge多光谱相机于DJI Matrice100无人机平台。该相机可同时采集5个波段的影像：蓝光波段（中心波长475 nm，带宽20 nm）、绿光波段（中心波长560 nm，带宽20 nm）、红光波段（中心波长668 nm，带宽10 nm）、红边波段（中心波长717 nm，带宽10 nm）以及近红外波段（中心波长840 nm，带宽40 nm）。本研究仅使用蓝、绿、红三个波段的影像进行花器官识别。
无人机分别于2019年7月1日的9:00、11:00、13:00、15:00，以及2019年7月9日的10:00、12:00、14:00、16:00飞越试验田开展数据采集。飞行任务由DJI GS Pro应用程序规划并执行，无人机飞行高度设置为距地面15米，影像采集频率确保相邻影像间的航向重叠度与旁向重叠度均达到85%。研究采用MicaSense校准板对反射光谱进行自动校正。
相机采集的原始影像通过Pix4DMapper Pro摄影测量软件进行拼接与处理，最终生成空间分辨率为1厘米的地表反射率正射影像图。每个采集时段平均生成2309张原始影像，而每张五波段田间影像图则由2181张原始影像拼接而成。借助重建得到的正射影像，研究人员通过ArcMap软件手动划定每个小区的边界。