Transpiration and the Ascent

Measurement of Transpiration and Evaporation 225
plants grow larger. In long-term experiments, corrections also need to be made
for the increasing fresh weight of rapidly growing plants by harvesting samples
plants occasionally and weighing them. Estimates of transpiration over short
periods of time may also involve errors caused by a decrease in water content
resulting from the lag of absorption behind transpiration shown in Fig. 6.1. A
variation of this method is to grow the plants in nutrient solution and measure
the volume or weight of solution removed by the roots during a given period of
time. Harris (1971), Kitano and Eguchi (1992a), and Simonneau et al. (1993)
described apparatus for simultaneously measuring water absorption, transpiration,
relative water content, and growth. Some data are presented in Fig. 7.20.
Extrapolation of data from potted plants to plants in the field is questionable,
but the phytometer method is useful for comparing plants of various species
under the same conditions and for measuring the effects of environmental factors
and experimental treatments. The desire to estimate the transpiration of
plants too large to be easily accommodated in phytometers and the need for
rapid measurements resulted in the development of other methods of measuring
or estimating the rate of transpiration.
Cut-Shoot Method. This sometimes is called the rapid weighing method
and was formerly used extensively because of its convenience and applicability
to large plants in the field. Shoots or single leaves are detached and suspended
on a special balance as near their original location as possible and the change in
weight over time is recorded. Such measurements can last for only a few minutes
because the transpiration rate tends to decrease with decreasing leaf water content.
Also there often is a transient increase in the transpiration rate a minute or
two after detachment, the Iwanoff effect, caused by the sudden increase in stomatal
opening after release of tension by detachment from the plant. Furthermore,
the detached leaf or stem no longer is in competition with the remainder
of the shoot for water. The method was discussed by Franco and Magalhaes
(1965), Slavik (1974) and others, but is seldom used today.
Sometimes stems of plants are detached under water and are placed in containers,
and water uptake is measured over time. However, water uptake often
is reduced by plugging of the cut stem surface with air or debris or by microbial
activity and the stem must be recut frequently. Ladefoged (1963) and Roberts
(1977) made useful measurements of transpiration of forest trees up to 16 m in
height by cutting them off under a stream of water and placing the bases of the
trunks in containers of water while their tops were supported in their normal
position in the canopy. Knight et al. (1981) also measured transpiration of
lodgepole pine trees up to 30 cm in diameter by this method. Although absorption
slowed after 2 or 3 days, it could be restored by recutting the base of the
trunk. Myers et al. (1987) reported that cutting Eucalyptus stems under water
increased leaf water potential, but decreased leaf conductance.