Soybean Radiation Use Efficiency in High-Yield Environments.

Seasonal light capture and radiation-use efficiency (RUE) have not been studied in high yield soybean crops that approach yield potential. In 2015, four producer irrigated soybean fields in Nebraska (USA) were selected for this study. These four fields produced high seed yields during the past three years (>90% of simulated yield potential based on local weather, soil, and management). Light sensors were installed to measure all radiation components (incident, absorbed, transmitted, and reflected) during the entire crop season. Light measurements were taken every second and averaged over 30-minute intervals. Replicated plant samples were collected at 7-d intervals for leaf area index (LAI) and aboveground dry matter (ADM) determination. Energy content of ADM was determined by a bomb calorimeter. Measured seed yield at maturity ranged from 5.2 to 6.3 Mg ha^-1 across the four fields (yield expressed at 133 g H₂O kg^-1 grain). Maximum LAI ranged from 4.2 to 6.4 across fields. Analysis of the fraction of absorbed radiation, in relation to LAI, indicated that soybean crops absorbed ca. 90% of incident solar radiation when LAI ≥4, but light absorption per unit of LAI was different in the ascending versus descending phases of leaf area dynamics. High-yield soybean crops that approach yield potential did not necessarily exhibited a very high RUE, but instead, captured a high fraction of the seasonal solar radiation. On average, absorbed, transmitted and reflected radiation accounted for respective 66, 30, and 4% of total incident solar radiation between emergence and physiological maturity (average:2501 MJ m^-2). Calculated RUE, based on regression analysis of the relationship between ADM and absorbed total radiation, ranged from 0.76 and 0.62 g MJ^-1 across fields and from 1.1 to 1.4% when ADM was expressed on an energy content basis. These estimates fall within the mid-range of RUE values reported in the literature. Interestingly, this study did not find evidence of a reduction in RUE during the seed filling, despite oil and protein synthesis and carbon and nitrogen remobilization from vegetative to reproductive organs. Energy output through biomass and seed yield represented, on average, 0.8 and 0.5 % of the total incident solar radiation during the crop season. These values can be taken as benchmarks for seasonal light capture in high-yield soybean crops that produce near yield potential.