[MCN] Plant physiology: How drought kills trees

[Lance Olsen]

PNAS Early Edition doi: 10.1073/pnas.1522569113

Revealing catastrophic failure of leaf networks under stress
Timothy J. Brodribb, Diane Bienaimé, and Philippe Marmottant

Keywords
embolism drought xylem vein leaf

Significance
Water sustains photosynthesis and growth of land 
plants, but it must be transported from the soil 
to leaves under high tension. Drying soil leads 
to an increase in water tension, exposing plants 
to the problem of breakage of the water column, 
causing embolisms that cut off water supply, 
leading to tissue death during drought. The 
ability of leaves to resist embolism formation is 
a key adaptive axis in plant evolution, and yet 
the process itself has never been visualized in 
the leaf venation. We describe a new optical 
method that allows the evolution and spread of 
embolism in the entire leaf network to be mapped, 
thus revealing general rules in the sequence of 
leaf vein transport failure.

Abstract
http://www.pnas.org/content/early/2016/04/06/1522569113.abstract

The intricate patterns of veins that adorn the 
leaves of land plants are among the most 
important networks in biology. Water flows in 
these leaf irrigation networks under tension and 
is vulnerable to embolism-forming cavitations, 
which cut off water supply, ultimately causing 
leaf death. Understanding the ways in which 
plants structure their vein supply network to 
protect against embolism-induced failure has 
enormous ecological and evolutionary 
implications, but until now there has been no way 
of observing dynamic failure in natural leaf 
networks. Here we use a new optical method that 
allows the initiation and spread of embolism 
bubbles in the leaf network to be visualized. 
Examining embolism-induced failure of 
architecturally diverse leaf networks, we found 
that conservative rules described the progression 
of hydraulic failure within veins. The most 
fundamental rule was that within an individual 
venation network, susceptibility to embolism 
always increased proportionally with the size of 
veins, and initial nucleation always occurred in 
the largest vein. Beyond this general framework, 
considerable diversity in the pattern of network 
failure was found between species, related to 
differences in vein network topology. The 
highest-risk network was found in a fern species, 
where single events caused massive disruption to 
leaf water supply, whereas safer networks in 
angiosperm leaves contained veins with composite 
properties, allowing a staged failure of water 
supply. These results reveal how the size 
structure of leaf venation is a critical 
determinant of the spread of embolism damage to 
leaves during drought.

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"A new area of study is the field that some of us 
are beginning to call social traps. The term 
refers to situations in society that contain 
traps formally like a fish trap, where men or 
whole societies get themselves started in some 
direction or some set of relationships that later 
prove to be unpleasant or lethal and that they 
see no easy way to back out of or to avoid."

John Platt. Social Traps. American Psychologist, August 1973
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