Hydraulic failure and repair are not routine in trees

摘要

Trees can pull tons of water up to 100 m above ground, whereas even the best engineered suction pumps can manage only 10 m at most. Beyond this height, the pull of gravity exceeds atmospheric pressure. The pressure at the top of the water column then becomes negative, and pumps drain by a process of vaporization called cavitation. So how do plants, particularly tall trees, cope with cavitation? Do they have an astonishingly high resistance to this process or are they routinely exposed to cavitation events and possess remarkable repair capacities? These questions were first asked when the mechanism of sap ascent in trees was discovered in the late nineteenth century (Brown 2013), but were not answered until reliable methods for measuring cavitation were introduced a century later. Early studies of plant hydraulics suggested that cavitation occurred only in conditions of severe drought (Fig. 1a). However, many studies carried out in the last decade have called this understanding of tree physiology and ecology into question. Diurnal cycles of cavitation and repair have been reported, even in well watered plants, suggesting that plants routinely face cavitation and recover easily from it. Writing in Plant Cell and Environment, Wheeler et al. (2013) provided compelling new experimental evidence that trees are much more resistant to cavitation than previously thought, demonstrating that this change of paradigm should now be reconsidered.