Predawn plant water potential does not necessarily equilibrate with soil water potential under well-watered conditions

Abstract

Predawn leaf water potential (Ψ) and xylem pressure potential (Ψ) are expected to be in equilibrium with the soil water potential (soil Ψ) around roots of well-watered plants. We surveyed 21 plant species (desert, chaparral, and coastal salt marsh species, as well as two temperate tree and two crop species) for departures from this expectation and for two potential mechanisms explaining the departures. We measured soil Ψ, leaf Ψ, and xylem Ψ in the glasshouse under well-watered conditions that eliminated soil moisture heterogeneity and ensured good soil-root hydraulic continuity. Most species failed to equilibrate fully with soil Ψ, depending on whether leaf Ψ or xylem Ψ was used as the measure of predawn plant water potential. The contribution of nighttime transpiration to predawn disequilibrium was assessed by comparing plants with bagged canopies (enclosed overnight in plastic bags to eliminate transpiration) to plants with unbagged canopies. Nighttime transpiration significantly reduced predawn xylem Ψ for 16 of 21 species and the magnitude of this contribution to predawn disequilibrium was large (0.50-0.87 MPa) in four woody species: Atriplex confertifolia, Batis maritima, Larrea tridentata, and Sarcobatus vermiculatus. The contribution of nighttime transpiration to predawn disequilibrium was not more prevalent in mesic compared with xeric or desert phreatophytic compared with non-phreatophytic species. Even with bagging that eliminated nighttime transpiration, plants did not necessarily equilibrate with soil Ψ. Plant xylem Ψ or leaf Ψ were significantly more negative than soil Ψ for 15 of 15 species where soil Ψ was measured. Predawn disequilibrium based on leaf Ψ was of large magnitude (0.50-2.34 MPa) for seven of those 15 species, predominately halophytes and Larrea tridentata. A portion of the discrepancy between leaf and soil Ψ is consistent with the putative mechanism of high concentrations of leaf apoplastic solutes as previously modeled for a halophyte, but an additional portion remains unexplained. Predawn leaf Ψ and xylem Ψ may not reflect soil Ψ, particularly for woody plants and halophytes, even under well-watered conditions.

Reduced nighttime transpiration is a relevant breeding target for high water-use efficiency in grapevine

“…High transpiration rate at night, by contrast, prevents overnight equilibration between plant and soil water potentials (38,39). In line with this interpretation, a significant, negative correlation was observed between the genotypic values of E n and predawn leaf water potential (Ψ pd ; SI Appendix, Table S4).…”

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.…”

Hydraulic redistribution in a Douglas‐fir forest: lessons from system manipulations

“…The Ψ soil threshold for HR was reached approximately when upper Ψ soil approached predawn Ψ leaf , and thus the upper soil had become an effective competitor for water with the above-ground portion of the tree. A disequilibrium between Ψ soil and predawn Ψ leaf is not uncommon (Donovan, Linton & Richards 2001;Donovan, Richards & Linton 2003;Bucci et al 2004) and will certainly influence HR. If the rate of HR is regulated by competing water potential differences in the plant and soil systems, we should be able to alter the rates of HR by altering these Ψ driving forces.…”