Submitted to: Plant and Soil
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/9/2001
Publication Date: N/A
Interpretive Summary: Considerable research has shown that most plant processes including transpiration respond similarly to soil drying. That is, there is little effect on the specific plant process until the soil drys to a certain point (the threshold). Research has shown that as atmospheric vapor deficit increases, transpiration also increases. Our research was conducted to determine if the increased transpiration demand caused by increased vapor pressure deficit would change the soil water threshold at which transpiration begins to decline. To examine this, we grew maize plants in out-door environmentally controlled chambers in which we set the vapor pressure deficit to specific levels. We then measured the soil water content at which transpiration began to decline. The results of the experiment indicated that the soil water content at which transpiration declined was stable across a wide range of vapor pressure deficits.
Technical Abstract: The decline in plant transpiration has been widely observed to occur within a fairly stable range of threshold values of fraction transpirable soil water (FTSW), usually 0.3 to 0.4. However, the stability of this function has not been compared at various levels of atmospheric vapor pressure deficit (VPD). Soil hydraulic conductivity is likely to be involved in determining the threshold where water supply is limiting. Thus it was hypothesized that at a high VPD resulting in increased transpiration rates, the FTSW threshold for the decline of transpiration rates as a result of drying soil would be increased. This study was undertaken in controlled environment chambers with two maize (Zea mays L.) hybrids so as to subject plants to four VPD levels (1.1, 2.0, 2.9, and 3.6 kPa) during a soil drying experiment. In contrast to the original hypothesis, there was little (0.05 FTSW) change in the threshold FTSW in response to increased VPD for either hybrid. In fact, over the narrow 0.31 to 0.38 FTSW range observed, the two hybrids showed opposite trends in FTSW threshold as VPD increased. These results supported the view that the FTSW threshold for the decline in transpiration with drying soil is stable, showing little sensitivity to changes in VPD.