It is argued that isohydric plants do not tolerate more intensive and frequent droughts (less negative soil water potentials) use stored reserves to survive in drought events? Similarly anisohydric plants tolerate more intensive and frequent drought (more negative soil water potentials) and keep growing.
There are questions about stress resistance and sensing. I think the balance between suction and stomatal conductance is one of reactions for plants that are glycophytes under both iso-and aniso- as bean and tomato. Avoidance and tolerance are in fact coming from ephemerals and CAMs respectively, but CAMs have to face a great energy loss. Would then limit the meaning to stomatal reactions
Isohydric plants are often identified (even if this does not mean a perfect correspondence between the two categories) as “drought avoiders”, and anisohydric plants as “drought tolerant”.
The reason why isohydric plants are usually identified with drought avoiders is that they tend to avoid the potentially dangerous effects related to an excessive reduction in water potential, through a faster stomatal closure they perform when drought stress starts to occur. The price to pay for this are reduced growth performances in comparison with anisohydric plants, which continue exchanging gases at higher rates even in conditions of water stress. The benefit is, for example, the reduced risk of experiencing potentially lethal phenomena of drought-induced cavitation.
Drought tolerant, anisohydric plants, behave in the opposite way, keeping their stomata open for longer during drought conditions, and so photosynthesizing at higher rates while markedly reducing their water potential.
In this sense, if we “anthropize” plant responses, we can say say that isohydric plants have a more cautious behaviour, while anisohydric plants display a daring behaviour. And, therefore, also your definition of “conservative” vs. “opportunistic” plants can make sense, although I would say that it can have a more colloquial and educational rather than scientific significance.
Each kind of response has its pro- and contra-. Several Authors, anyway, demonstrated that anisohydric plants tend to present higher survival rates than isohydric ones in conditions of severe and prolonged drought stress (see for example Baltzer et al., 2008; Kursar et al., 2009; McDowell et al., 2008).
Baltzer, J.L., S.J. Davies, S. Bunyavejchewin and N.S.M. Noor 2008. The role of desiccation tolerance in determining tree species distributions along the Malay–Thai Peninsula. Functional Ecology. 22:221-231.
Kursar, T.A., B.M.J. Engelbrecht, A. Burke, M.T. Tyree, B. EI Omari and J.P. Giraldo 2009. Tolerance to low leaf water status of tropical tree seedlings is related to drought performance and distribution. Functional Ecology. 23:93–102.
McDowell, N., W.T. Pockman, C.D. Allen, D.D. Breshears, N. Cobb, T. Kolb, J. Plaut, J. Sperry, A. West, D.G. Williams and E.A. Yepez 2008. Mechanisms of plant survival and mortality during drought: why do some plants survive while others succumb to drought? New Phytologist. 178:719-739.
Paolo Zuccarini, Thank you very much for your detailed and specific answer with interesting references. Sharing of your expertise is highly appreciated.
you can look at http://era.daf.qld.gov.au/3052/1/Iso-anisohydric_vegetable_report_PR09-4248-sec.pdf
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