Magnitude and Mechanisms of Disequilibrium Between Predawn Plant and Soil Water Potentials

Abstract

Predawn plant water potential (Ψw, measured with leaf psychrometers) and surrogate measurements made with the pressure chamber (termed Ψpc here) are used to infer comparative ecological performance, based on the expectation that these plant potentials reflect the wettest soil Ψw accessed by roots. There is growing evidence, however, that some species exhibit substantial predawn disequilibrium (PDD), defined as plant Ψw or Ψpc at predawn substantially more negative than the Ψw of soil accessed by roots. In the western Great Basin desert, the magnitude of PDD calculated as soil Ψw minus predawn leaf Ψw was as large as 1.4 and 2.7 MPa for two codominant shrub species, Chrysothamnus nauseosus and Sarcobatus vermiculatus, respectively. The magnitude of PDD calculated as soil Ψw minus predawn Ψpc was smaller, up to 0.6 and 2.1 MPa for Chrysothamnus and Sarcobatus, respectively. For both species, mechanisms contributing to PDD included nighttime transpiration and putative leaf apoplastic solutes, but not hydraulic conductance limitations. Hydraulic lift also occurred in both species and likely contributed to PDD for Sarcobatus. Finding large magnitude PDD in field populations emphasizes that species differences in predawn plant Ψw or Ψpc do not necessarily reflect differences in accessible soil Ψw and rooting depth, nor does a low predawn plant Ψw or Ψpc value necessarily mean that soil Ψw is also low. Mechanisms contributing to PDD affect relationships between plants and soil resources, as well as the potential for plant–plant interactions. Corresponding Editor: F. C. Meinzer

Comparative Drought Responses of Quercus ilex L. and Pinus sylvestris L. in a Montane Forest Undergoing a Vegetation Shift

“…Hence, we may question whether ΨPD is truly in equilibrium with soil water potential in our system, since we would expect the lowest ΨPD measurements in the relatively shallow rooted P. sylvestris trees. One possible reason for this disequilibrium could be nocturnal sap flow [57], which has been demonstrated in Q. ilex trees in our study site [32]. However, P. sylvestris at our site also show some nocturnal sap flow during nights with high VPD [38], making it an unlikely explanation for the differential ΨPD patterns across species.…”

Diurnal and seasonal variation in root xylem embolism in neotropical savanna woody species: impact on stomatal control of plant water status

“…The disequilibrium between plant and Y soil early in the season was probably attributable to nocturnal transpiration or incomplete overnight rehydration of the above-ground portion of the trees (Bucci et al 2005). It has been shown that nocturnal transpiration can prevent overnight equilibration between plant and Y soil (Donovan, Linton & Richards 2001, Donovan, Richards & Linton 2003Bucci et al 2004a). Later in the season, Y soil in the morning fell below that of B. crassa and B. salicifolius roots, the species with the shallowest roots (Fig.…”

Soil water energetic status and cowpea beans irrigated with saline water

“…Donovan et al (2001) observed disequilibrium in the predawn hours between plant and soil water potentials in 16 of the 21 plant species tested. Donovan et al (2003) also reported disequilibrium between plant and soil water potentials in plants from the genera Chrysothamnus and Sarcobatus. James et al (2006), working with plants from the genus Sascorbatus under saline conditions, observed disequilibrium of great magnitude between the plant and the soil.…”