Soil-water stress as it affects the plant-water status and crop production is becoming increasingly important. When soil-water stress is severe, partial closure of the stomates causes a repartitioning of the incident energy, often resulting in increased temperatures. The effect of soil-water stress on air temperatures within a soybean canopy was studied and the increased air temperatures related to decreased evapotranspiration and plantwater stress. One treatment of soybeans was trickle-irrigated when the matric potential at the 15-cm depth was equal to --0.2 bar. Air-temperature profiles were measured in both irrigated and nonirrigated field-grown soybeans, using calibrated thermistors. Evapotranspiration was measured using a portable chamber. Plant-water status was evaluated indirectly through the use of LVDT's (linear variable displacement transducers). The measured stem-diameter changes were related to the air temperature differences in the irrigated and nonirrigated canopies. The data showed as soil-water stress became more severe, canopy air temperatures within nonirrigated soybeans increased above those within the irrigated soybean canopy. The above-canopy minus the within-canopy temperature difference between the irrigated and nonirrigated plots increased during peak radiation with little difference at night. When the plant-wilt symptoms indicated severe stress, evapotranspiration decreased 40--70% when within-canopy air temperatures increased. This increase in the canopy air temperature was related to stem-diameter shrinkage and may be an indirect measure of the plant-water status under field conditions.
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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 (TL) and ambient temperatures (TA) were highly correlated, and no significant improvement in the correlation resulted from treating row spacing or diurnal periods separately. Both TL and ΔT (TL – TA) were highly correlated with pressure chamber determination of xylem pressure potential (ΨX). Parallel leaf diffusive resistance (R5) 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 Ψx was observed, but mean seasonal midday Ψx 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.
An experiment was conducted in SPAR systems at Florence. S.C, to obtain a data set for use in the simulation of the effect of drying soil on photosynthetic rates in cotton. The plant water status was monitored using leaf water potential and stem diameter measurements. Reductions were noted in apparent photosynthesis rales after only 5 days of soil drying, and as anticipated, there was uniform displacement of the diurnal cycle of leaf water potential, and corresponding decreases in transpiration and CO; uptake. The photosynthesis-light response curves indicated that an average twofold reduction in photosynthesis rates occurred for solar radiation greater than 250 W/nr. Stein diameter change (from a nonstress pre-sunrise value) and integrated stem stress were found to be good indicators of maximum daily plant water stress. The integrated stem stress gave a measure of the duration of the stress along with its magnitude. A simulation method for predicting leaf water potential from stem diameter measurements was used to show that the magnitude and duration of plant water stress increased uniformly during the experiment. This increase was representative of the decreased rates of photosynthesis measured. These data will be used in the simulation of cotton growth and yield.
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