Soybean Water Status and Canopy Microclimate Relationships at Four Row Spacings¹

Abstract

Abstract There is little field data relating plant water status and canopy microclimate of determinate soybean [ Glycine max (L.) Merr.] cultivars grown in humid environments. Also, few observations of these parameters have been reported for the period prior to complete canopy closure. A field study was conducted in South Carolina in 1979 to determine these basic relationships for two determinate soybean cultivars, Davis (Group VI) and Coker 338 (Group VIII) grown in 1.02, 0.76, 0.51 and 0.36‐m row spacings. The experiments were conducted on a Norfolk loamy sand (fine‐loamy, siliceous, thermic, Typic Paleudult). Thermistor‐determined leaf temperatures (T L ) and ambient temperatures (T A ) were highly correlated, and no significant improvement in the correlation resulted from treating row spacing or diurnal periods separately. Both T L and ΔT (T L – T A ) were highly correlated with pressure chamber determination of xylem pressure potential (Ψ X ). Parallel leaf diffusive resistance (R 5 ) was not highly correlated with any of the canopy microclimate or water status parameters observed. Atmospheric vapor pressure deficit (VPD) was correlated with ΔT. Xylem pressure potential was very highly correlated with leaf vapor pressure deficit (LVPD). No row spacing effect on Ψ x was observed, but mean seasonal midday Ψ x was 0.10 MPa lower for Coker 338 than for Davis (P≤0.001) and osmotic potential was 0.20 MPa lower for Coker 338 than for Davis (P≤0.01). The authors propose that the slope of ΔT vs. VPD may be influenced by high prevailing relative humidity and heating of the canopy from exposed soil between rows in the period prior to complete canopy coverage.