Water and Solute Permeability of Plant Cuticles: Measurement and ...

9.7 Measuring Water Permeability 267
temperature equilibrium and steady state are established before weighing is started.
Depending on water permeability, chambers are weighed at intervals of hours or
even days to get a significant loss of mass. These intervals are too long for detecting
extrapolated hold-up times in the range of 4–15 min (Table 4.10).
When we used the cup method, the CM were inserted such that the morphological
inner side of the CM was in contact with water while the outer side was exposed
to dry air. In this arrangement, driving force was maximum (∆aw = 1.0; ∆Cwv =
23.05g m 3 at 25 ◦ C). With most plant species, swelling of cuticles and permeance
depend on partial vapour pressure (Fig. 4.6). When in equilibrium with silica gel the
limiting skin is not or very little swollen, and the cup method results in minimum
permeances. Humidity in the box does not have to be zero. If constant humidity
saturated salt solutions (cf. Greenspan 1977; Kolthoff et al. 1969; Appendix B) or
glycerol/water mixtures (Slavik 1974) are used instead of silica gel, humidity can be
varied and effects of humidity on permeance can be studied. We have not tried this,
however. The cup method has been used extensively because it is simple, cheap,
and very accurate, and large sample sizes can be handled (Geyer and Schönherr
1990; Mérida et al. 1981; Riederer and Schönherr 1990; Schönherr and Bauer 1992;
Schreiber and Riederer 1996a, b).
The cup method resembles the situation in nature, except that in the field humidity
is rarely close to zero. It has some other limitations. Water fluxes at 100%
humidity cannot be measured, because driving force is zero and no net flux occurs.
In this situation, tritiated water (THO) was used as tracer (Schönherr and Schmidt
1979). An aqueous buffer containing THO served as donor in contact with the morphological
inner surface of the CM. Air of constant humidity obtained by the dew
point method was blown over the outer surface of the CM. The moisture containing
the THO which had penetrated was trapped in a dry-ice cold trap (−70 ◦ C),
and radioactivity in the cold trap was assayed by scintillation counting (Figs. 4.6
and 4.8). Schreiber et al. (2001) also studied the effect of water activity of the
receiver using a slightly different approach. The stainless steel transpiration chamber,
with THO as donor solution facing the morphological inner side of the cuticle,
was attached upside down to a scintillation vial (Fig. 9.3). Humidity of the receiver
(the scintillation vial) facing the morphological outer side of the CM was varied
using 100µl glycerol (2% RH), pure water (100% RH) or mixtures of glycerol and
water having humidities between 2 and 100% RH (Slavik 1974). The surface of
the CM was in contact with the equilibrium vapour pressures of the glycerol/water
mixtures, but not with the solutions themselves, which were on the bottom of the
vials (500µl). THO which penetrated the cuticle was absorbed in the glycerol, water
or glycerol/water mixtures, and after adding scintillation cocktail, radioactivity was
determined by liquid scintillation counting.
The effect of cations and pH on water permeability of CM or MX membranes
was studied using the system buffer/CM or MX/buffer and tritiated water as tracer
(Schönherr 1976a, b). If a fine capillary is attached to the receiver containing an
osmotic solute, this system can be used to measure osmotic water transport in MX
membranes (Fig. 4.9), but it is not sufficiently sensitive for CM (Schönherr 1976a;
Schönherr and Mérida 1981).