The implications of isohydric and anisohydric regulation of leaf water potential under current and future precipitation regimes

Background/Question/Methods

Plants regulate transpiration and leaf water potential (Ψ_l) to avoid tissue dessication and hydraulic failure. Two broad strategies of Ψ_l regulation have been described and, more recently, hypothesized to play a mechanistic role in drought-related vegetation mortality. Isohydric species maintain nearly constant Ψ_l despite declining soil water potential (Ψ_s) during drought, while anisohydric species exhibit proportional decreases in Ψ_l 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 (Ψ_pd) or Ψ_s experienced by the plant. Papers were included in this analysis if pre-dawn (Ψ_pd) and midday (Ψ_md) plant water potentials were available for at least 5 measurement dates representing a range of Ψ_pd 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 (A_l:A_s). Data from field and greenhouse experiments were used.  

Results/Conclusions

In general, isohydric plants did exhibit a faster decline in DΨ as Ψ_pd dropped than did anisohydric plants. However, there was no absolute cutoff in slope which indicated Ψ_l 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 Ψ_pd and Ψ_md, suggesting that they may be better-adapted to more xeric environments. Preliminary results do not reveal a pattern regarding A_l:A_s or xylem refilling capability, though the latter characteristic has not been widely described.   

Isohydric and anisohydric Ψ_l 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.