COS 79-5 - The implications of isohydric and anisohydric regulation of leaf water potential under current and future precipitation regimes

Wednesday, August 10, 2011: 2:50 PM
12B, Austin Convention Center
Jennifer A. Plaut, Biology, University of New Mexico, Albuquerque, NM, Nathan G. McDowell, Earth and Environmental Sciences, Los Alamos National Laboratory, Los Alamos, NM and William T. Pockman, Department of Biology, University of New Mexico, Albuquerque, NM

Plants regulate transpiration and leaf water potential (Ψl) to avoid tissue dessication and hydraulic failure. Two broad strategies of Ψl regulation have been described and, more recently, hypothesized to play a mechanistic role in drought-related vegetation mortality. Isohydric species maintain nearly constant Ψl despite declining soil water potential (Ψs) during drought, while anisohydric species exhibit proportional decreases in Ψl as the soil dries. This distinction has been made for a handful of species, but a broad review of the phenomena has so far been lacking.

Drawing upon published data, we characterized species based on the relationship between the daily drop in leaf or stem water potential (DΨ) and the predawn leaf water potential (Ψpd) or Ψs experienced by the plant. Papers were included in this analysis if pre-dawn (Ψpd) and midday (Ψmd) plant water potentials were available for at least 5 measurement dates representing a range of Ψpd of at least 0.25 MPa. We included non-porous, diffuse-porous, and ring-porous woody species as well as annual herbs and grasses. Other data gathered for each species included xylem cavitation resistance, degree of known xylem refilling ability, and the leaf area:sapwood area ratio (Al:As). Data from field and greenhouse experiments were used.  


In general, isohydric plants did exhibit a faster decline in DΨ as Ψpd dropped than did anisohydric plants. However, there was no absolute cutoff in slope which indicated Ψl regulation strategy and for some species the strategy was only determined upon observation of the original timeseries. Most of the previous classifications from the literature agreed with those arrived at in this study. Anisohydric species were overall more resistant to xylem cavitation than were isohydric species. They also had lower minimum Ψpd and Ψmd, suggesting that they may be better-adapted to more xeric environments. Preliminary results do not reveal a pattern regarding Al:As or xylem refilling capability, though the latter characteristic has not been widely described.   

Isohydric and anisohydric Ψl regulation may be somewhat artificial designations based on the soil moisture conditions most commonly observed. Some anisohydric species might actually behave in an isohydric manner if pushed to a sufficient level of drought. The terms “isohydric” and “anisohydric” are still useful for functional comparisons, especially in light of projected climate change effects on precipitation regimes.

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