SVERIGES LANTBRUKSUNIVERSITET - Epsilon Open Archive - SLU

56
MEASUREMENT OF PLANTWATER STATUS AND GAS EXCHANGE
Introduction
As mentioned above plant water status strongly influence plant growth
through its influence on gas exchange and expansion of leaves and roots.
Leaf water deficits cause stomatai closure limiting CO 2 uptake and hence
net photosynthesis. Also plant water deficit may negatively affect the
process of photosynthesis itself. The physiological consequences of water
deficits in plants have been intensively studied and reviewed lately by
Hsiao and Bradford (1983).
The methodology applied for studying plant water status generally
comprises measurement of both water con tent and water potential and its
components of ten associated with measurements of other plant responses
to stress; i.e. changes of growth rate, gas exchange, leaf resistance, stomatai
opening, nutrient uptake, hormon balance etc. Below I will concentrate on
approaches used for water content, water potential and gas exchange
measurements. The subjects have recently been covered by the comprehensive
volume written by Slavik (1974), by the review chapter of Bannister
(1986) and the review papers of Turner (1981 and 1988) and Beadle et al.
(1985).
Water content
A common approach for measuring plant water content is based on
measurement of fresh weight (FW) at time of sampling, dry weight (DW)
determined at 80°C, and turgid weight (TW). TW is obtained by floating
leaves or leaf disks on water at the light compensation point for about 4 hs
(Barrs and Weatherley, 1962). Thus, the following water content
expressions can be obtained:
Absolute water content, W = (FW - DW)/DW
Relative water content, RWC = (FW - DW)/(TW - DW)
Water saturation deficit, WSD = 1 - RWC
Turgid Wt/dry Wt ratio = TW /DW
Also indirect methods of measurements of leaf water content are available,
such as using a source of beta radiation and relating count rates to water
content (Bannister, 1986), or by direct measurement of leaf thickness by
an inexpensive micrometer (Burquez, 1987).
Water potential
Measurement of total water potential has be come routine either by the use
of termocouple psychrometer or pressure chamber. Equipment for the
measurement of water potential by both techniques is commercially
available.