Internal Coordination of Plant Responses to Drought and Evaporational Demand
197
sapwood area, Meinzer and Grantz (1990) concluded that a coordination
exists between the vapor phase conductance and the liquid flow conductivity
of the sugarcane plant. They speculate about root metabolites which adjust
vapor exchange rates to the water supply capacity of the plant. This capacity
is determined not only by the ability for water uptake of the roots but also
by the transport efficiency of the xylem pathway through the shoots. The
coordination between root water gain, transpiring leaf area, stomatal control
of transpiration, and an appropriate structure of the vessel system will be
important for plant water balance.
9.5 Xylem Conductivity and Leaf Conductance
In the sugarcane plants studied by Meinzer et al. (1992), shoot hydraulic
conductivity per leaf area ("specific conductivity") decreases in a similar
manner as unit-area-related leaf conductance and transpiration once a
critical leaf area of 0.2 m 2 plane 1 is exceeded. The parallel change of the
three parameters prevents steep water potential gradients along the stalks,
which could cause cavitation of vessel elements. The possibility of vessel
embolism can result from the strong tension under which water exists in the
xylem ducts according to the well-established cohesion theory of sap ascent
in plants (Dixon 1914). Cavitation occurs when one of the continuous
strands of water through the conducting elements becomes interrupted
by the sudden expansion of a submicroscopic vapor bubble produced by
nucleation at hydrophobic cracks or by meniscal failure at a pore in the cell
wall. Such an event is accompanied by the acoustic emission of energy which
results from the abrupt enlargement of the gas volume within the conducting
element. It can be monitored by microphones and a suitable signal amplification, both in the low-frequency, audible range and also at ultrasonic
frequencies (Milburn and Johnson 1966; Tyree and Dixon 1983). Tyree and
Sperry (1989) emphasized that cavitation is a common phenomenon in
nature, and occurs particularly often with winter freezing and under water
stress. Since each cavitation of a tracheid or a vessel will block the water
transport through this particular strand, it will additionally aggravate the
risk of drought damage. Following cavitation, fewer functional vessels have
to sustain the transpiration stream, which comes under even stronger tension. A repair of embolism is possible only at very high water potentials
after rain or by root pressure during overnight replenishment of the plant
water reserves. Tyree and Sperry (1989) and Field and Holbrook (1989)
proposed the hypothesis that plants very often operate on the verge of a
catastrophic breakdown of the water transport system caused by a runaway
embolism. Stomatal regulation may function just to keep transpirational
flow at a rate which allows sufficient carbon gain (Cowan 1982) but prevents
breakdown of the plant's hydraulic system. A feedback regulation respond-
197
sapwood area, Meinzer and Grantz (1990) concluded that a coordination
exists between the vapor phase conductance and the liquid flow conductivity
of the sugarcane plant. They speculate about root metabolites which adjust
vapor exchange rates to the water supply capacity of the plant. This capacity
is determined not only by the ability for water uptake of the roots but also
by the transport efficiency of the xylem pathway through the shoots. The
coordination between root water gain, transpiring leaf area, stomatal control
of transpiration, and an appropriate structure of the vessel system will be
important for plant water balance.
9.5 Xylem Conductivity and Leaf Conductance
In the sugarcane plants studied by Meinzer et al. (1992), shoot hydraulic
conductivity per leaf area ("specific conductivity") decreases in a similar
manner as unit-area-related leaf conductance and transpiration once a
critical leaf area of 0.2 m 2 plane 1 is exceeded. The parallel change of the
three parameters prevents steep water potential gradients along the stalks,
which could cause cavitation of vessel elements. The possibility of vessel
embolism can result from the strong tension under which water exists in the
xylem ducts according to the well-established cohesion theory of sap ascent
in plants (Dixon 1914). Cavitation occurs when one of the continuous
strands of water through the conducting elements becomes interrupted
by the sudden expansion of a submicroscopic vapor bubble produced by
nucleation at hydrophobic cracks or by meniscal failure at a pore in the cell
wall. Such an event is accompanied by the acoustic emission of energy which
results from the abrupt enlargement of the gas volume within the conducting
element. It can be monitored by microphones and a suitable signal amplification, both in the low-frequency, audible range and also at ultrasonic
frequencies (Milburn and Johnson 1966; Tyree and Dixon 1983). Tyree and
Sperry (1989) emphasized that cavitation is a common phenomenon in
nature, and occurs particularly often with winter freezing and under water
stress. Since each cavitation of a tracheid or a vessel will block the water
transport through this particular strand, it will additionally aggravate the
risk of drought damage. Following cavitation, fewer functional vessels have
to sustain the transpiration stream, which comes under even stronger tension. A repair of embolism is possible only at very high water potentials
after rain or by root pressure during overnight replenishment of the plant
water reserves. Tyree and Sperry (1989) and Field and Holbrook (1989)
proposed the hypothesis that plants very often operate on the verge of a
catastrophic breakdown of the water transport system caused by a runaway
embolism. Stomatal regulation may function just to keep transpirational
flow at a rate which allows sufficient carbon gain (Cowan 1982) but prevents
breakdown of the plant's hydraulic system. A feedback regulation respond-