Water use efficiency in a soybean field: influence of plant water stress |
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Authors: | Dennis D Baldocchi Shashi B Verma Norman J Rosenberg |
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Affiliation: | Center for Agricultural Meteorology and Climatology, Institute of Agriculture and Natural Resources, University of Nebraska, Lincoln, NE 68583-0728 U.S.A. |
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Abstract: | Measurements were made in 1980 over a fully-developed soybean (Glycine max (L.) Merrill) canopy at Mead, Nebraska to determine how crop water status influences photosynthesis, evapotranspiration and water use efficiency. Water use efficiency was calculated in terms of the CO2—water flux ratio (CWFR). Micrometeorological techniques were used to measure the exchange rates of CO2 and water vapor above the crop canopy. Crop water status was evaluated by reference to volumetric soil moisture (θv), stomatal resistance (rs), and leaf water potential (ψ) measurements.Stomatal resistance (rs) was independent of ψ when the latter was greater than ?1.1 MPa. rs increased sharply as ψ dropped below this threshold. Canopy CO2 exchange (Fc) decreased logarithmically with increasing rs under strong irradiance. Although Fc was found to be strongly correlated with rs, the influence of low values of ψ and of high air temperature cannot be discounted since these factors affect the enzymatic reactions associated with photosynthesis. Stomatal closure also reduced evapotranspiration and influenced the partitioning of net radiation.Under strong irradiance the CO2 water flux ratio (CWFR) decreased with increasing stomatal resistance. This observation is at variance with predictions of certain early ‘resistance’ models, but substantiates predictions of some recent models in which leaf energy balance considerations are incorporated. |
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