Photosynthetically Active Radiation, CO2 Uptake, and Stomatal Diffusive Resistance Profiles Within Soybean Canopies

Profiles of PPFD (photosynthetic photon flux density), CO₂ uptake and stomatal diffusive resistance were measured in field-grown canopies of Hark and Rampage soybeans [Glycine max (L.) Merr.]. These studies were conducted to examine and quantify the relationships between these parameters over the daylight period for discrete canopy stratum and the complete canopy. Measurements were made in the field with soybeans grown on a Huntsville silt loam. CO₂ uptake was measured with a field portable ¹⁴CO₂ leaf exposure chamber, PPFD with a quantum sensor attached perpendicularly to the chamber and stomatal resistance with a diffusion porometer. Each of the measurements was made within four strata of both canopies at various growth stages and under varying atmospheric conditions. Canopy distributions of CO₂ uptake showed that most photosynthate w;is produced in the upper 20% of the canopies. This is approximately one-third of the total canopy LAI (leaf area index) and represents the level of 90% interception of incident photosynthetically active radiation. Regression analyses of the relationship between PPFD and CO₂ uptake produced a linear model, and the slope of the line was dependent upon the day. The variation between days was caused by environmental conditions, which reduced CO₂ uptake by increased stomatal diffusive resistance. These reductions in CO₂ uptake were caused by soil moisture stress and cold temperature shock. Hark exhibited the greater CO₂ uptake rates and also the largest reduction in uptake with increased stomatal diffusive resistance. Stomatal diffusive resistance profiles showed an increase with canopy depth in both cultivars; this was attributed to low PPFD. Major photosynthetic production by soybean canopies occurs in the upper portion of the canopy and further understanding of the relationship between canopy behavior and the environment may lead to increased crop production.