Thursday, January 8, 2009

Drought and isohydry

I recently had the chance to read "Mechanisms of plant survival and mortality during drought: why do some plants survive while others succumb to drought?", which is a Tansley review in New Phytologist by Nate McDowell and others. One thing I really liked about their paper was the dichotomy between two potential strategies for coping with drought. The anisohydry strategy has plants experiencing lower leaf water potential as soils dry, but maintaining photosynthesis and transpiration throughout the drought. These plants are typical "low-water" species with all the anatomical traits that resist cavitation. The other strategy is to avoid drought through stomatal closure, maintaining more constant leaf water potentials. This is termed an isohydric strategy.

The latter strategy is interesting in that "drought-avoidance" is usually considered to be associated with little ability to withstand drought causing plants to die back or complete a generation until drought is relieved. Instead of the succulent strategy, where plants store water, the isohydry strategy requires plants to store carbon.

The paper is intriguing and I have a few questions about the generality of the isohydry strategy. Here are two:

1) It isn't clear that many species with low psi-crit's store a lot of C to endure drought. For example, I can't think of many herbaceous grassland species that use stored carbon to maintain a plant after leaves have closed their stomata.

2) After plants close their stomata, as soils continue to dry, plant water still comes under increasing tension, unless plants can hydraulically isolate themselves from the soil. The authors allude to this, but I'm not sure I understand how viable a strategy it is to close your stomates and wait for rain. Are plants more likely to run out of carbon before they cavitate? 

A lot of plants have big carbon stores that we don't know when they are used. This might explain some of those patterns.

Good job to the authors on the paper. It'll be interesting to see the ideas developed more and the hypotheses tested in other systems.

1 comment:

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