Detecting Sorghum Plant and Head Features from Multispectral UAV Imagery.

In plant breeding, unmanned aerial vehicles (UAVs) carrying multispectral cameras have demonstrated increasing utility for high-throughput phenotyping (HTP) to aid the interpretation of genotype and environment effects on morphological, biochemical, and physiological traits. A key constraint remains the reduced resolution and quality extracted from "stitched" mosaics generated from UAV missions across large areas. This can be addressed by generating high-quality reflectance data from a single nadir image per plot. In this study, a pipeline was developed to derive reflectance data from raw multispectral UAV images that preserve the original high spatial and spectral resolutions and to use these for phenotyping applications. Sequential steps involved (i) imagery calibration, (ii) spectral band alignment, (iii) backward calculation, (iv) plot segmentation, and (v) application. Each step was designed and optimised to estimate the number of plants and count sorghum heads within each breeding plot. Using a derived nadir image of each plot, the coefficients of determination were 0.90 and 0.86 for estimates of the number of sorghum plants and heads, respectively. Furthermore, the reflectance information acquired from the different spectral bands showed appreciably high discriminative ability for sorghum head colours (i.e., red and white). Deployment of this pipeline allowed accurate segmentation of crop organs at the canopy level across many diverse field plots with minimal training needed from machine learning approaches.

Multi-Spectral Imaging from an Unmanned Aerial Vehicle Enables the Assessment of Seasonal Leaf Area Dynamics of Sorghum Breeding Lines.

Genetic improvement in sorghum breeding programs requires the assessment of adaptation traits in small-plot breeding trials across multiple environments. Many of these phenotypic assessments are made by manual measurement or visual scoring, both of which are time consuming and expensive. This limits trial size and the potential for genetic gain. In addition, these methods are typically restricted to point estimates of particular traits, such as leaf senescence or flowering and do not capture the dynamic nature of crop growth. In water-limited environments in particular, information on leaf area development over time would provide valuable insight into water use and adaptation to water scarcity during specific phenological stages of crop development. Current methods to estimate plant leaf area index (LAI) involve destructive sampling and are not practical in breeding. Unmanned aerial vehicles (UAV) and proximal-sensing technologies open new opportunities to assess these traits multiple times in large small-plot trials. We analyzed vegetation-specific crop indices obtained from a narrowband multi-spectral camera on board a UAV platform flown over a small pilot trial with 30 plots (10 genotypes randomized within 3 blocks). Due to variable emergence we were able to assess the utility of these vegetation indices to estimate canopy cover and LAI over a large range of plant densities. We found good correlations between the Normalized Difference Vegetation Index (NDVI) and the Enhanced Vegetation Index (EVI) with plant number per plot, canopy cover and LAI both during the vegetative growth phase (pre-anthesis) and at maximum canopy cover shortly after anthesis. We also analyzed the utility of time-sequence data to assess the senescence pattern of sorghum genotypes known as fast (senescent) or slow senescing (stay-green) types. The Normalized Difference Red Edge (NDRE) index which estimates leaf chlorophyll content was most useful in characterizing the leaf area dynamics/senescence patterns of contrasting genotypes. These methods to monitor dynamics of green and senesced leaf area are suitable for out-scaling to enhance phenotyping of additional crop canopy characteristics and likely crop yield responses among genotypes across large fields and multiple dates.

The Physics of Toppling and Regaining Balance during a Pirouette.

One of the most common movements in dance is a turn around a vertical axis with one supporting foot on the floor--a pirouette. If the pirouette is not performed with the body on balance, it is not considered successful. Dancers are often taught to perform successful pirouettes by beginning the movement on balance and then keeping the body in that configuration, as opposed to correcting for an imbalance with small adjustments during the turn. Many, even advanced, dancers have significant difficulty performing more than two or three turns in a pirouette before losing balance, despite continued trial and error efforts to improve. To describe the mechanics of toppling and control of toppling during a pirouette, a theoretical model of a dancer in standard pirouette position was created, and an experimental study of real dancers performing pirouettes was conducted. Body segment parameters for the model (mass, length, etc.) were based on anatomical data and adjusted for sex, total body mass, and height. The principal moments of inertia were determined for several hypothetical dancers, and rigid body equations of motion numerically solved to express topple angle vs. time. When dancers reach too large a topple angle, they are forced to compensate by either hopping on the supporting foot in an attempt to regain balance or terminating the turn. The angle at which dancers lose stability and feel inclined to hop (θmax) was determined experimentally through a video analysis of nine intermediate to advanced ballet dancers' pirouettes (8 female, 1 male; 16 ± 2.3 years of age). The dancers hopped on the supporting foot after the body reached an average angle of 9.3 ± 1.9° from the vertical. With an average spin rate of 1.7 rev/s, it was found that a "rigid body" dancer (male or female) would need to begin the pirouette displaced less than one degree from the vertical in order to perform more than a double pirouette before reaching θmax. The results of this study demonstrate the difficulty of achieving many rotations when the body is held rigidly, whereas dancers may have success in consistently performing more pirouettes if they are taught strategies for regaining balance while turning.