Drought limits crop growth and yield in soybean. Rapid and effective methods of screening large numbers of soybean lines for drought tolerance are urgently needed. Two experiments were conducted to evaluate the effects of drought in soybean during reproductive stages. In the first experiment five genotypes from maturity groups 2 through 5 were tested under well-irrigated and drought conditions. Beginning at R5, leaf samples were taken for nitrogen concentration analysis. Pictures were taken across the top of each plot to determine the intensity of greenness using the Dark Green Color Index (DGCI). Aerial photographs were also taken to determine aerial DGCI values. Leaf nitrogen concentration decreased as plants approached maturity and was closely related to ground DGCI. Additionally, ground DGCI and aerial DGCI values followed similar trends. The aerial DGCI measurements had advantages over ground DGCI measurements in that it allowed discernment between both water treatments. This opens up the possibility of using aerial DGCI to screen genotypes that senesce more slowly under drought.

In the second experiment, the effects of drought in soybean were evaluated by aerial infrared image analysis, carbon isotope discrimination (Δ13C) and oxygen isotope composition (δ18O). Five fast-and five slow-wilting genotypes derived from a cross of Benning × PI416937 were evaluated under three water treatments that included a full and two deficit-irrigation treatments of increasing severity (deficit 1, and 2). After canopy closure, aerial infrared images were taken to determine the relative canopy temperature. Soybean leaves sampled at late R5 and seed at harvest were collected to measure Δ13C (leaf and seed) and δ18O (seed) as surrogate measurements for water use efficiency (WUE) and transpiration, respectively. As water availability decreased, the Δ13C values from leaf and seed generally decreased (i.e., higher WUE). In contrast, the δ18O values and relative canopy temperature generally increased with increasing drought stress. Moreover, slow-wilting genotypes generally had lower Δ13C, δ18O and canopy temperature than fast-wilting genotypes. However, δ18O values were not consistent over years. The results from these two experiments indicate that the determination of DGCI, Δ13C, and canopy temperature were promising tools for rapid characterization of drought-related traits in soybean.