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

154 6 Diffusion of Non-Electrolytes
Amount in receiver (µmol)
2.5
2.0
1.5
1.0
0.5
0.0
Lycopersicon F.
Capsicum F.
Solanum F
Olea
0 10 20 30 40 50 60 70 80
Time (h)
Ficus
Citrus
Nerium
Fig. 6.2 Time course of 2,4-D diffusion across CM of various plant species at 25 ◦ C. The steady
state flow was extrapolated to the time axis, which yields the extrapolated hold-up time (te). Fruit
CM were obtained from tomato, pepper and egg plants; all others were isolated from astomatous
adaxial leaves. (Redrawn from Riederer and Schönherr 1985)
Steady state flow rates and the hold-up times (te) varied greatly among species.
The amount diffused was greatest with fruit CM, and te was short. With leaf CM,
slopes are considerably smaller and hold-up times are longer (Fig. 6.2). Permeance
calculated from these data range from 2.72 ×10 −8 (pepper) to 1 ×10 −10 m s −1
(Ficus), that is, among species they differed by a factor of 272. With P known, the
steady state fluxes caused by a given concentration can be calculated. For instance,
if donor concentration is 1 × 10 −3 mol l −1 , the steady state flux in 24 h would be
2.35 × 10 −3 mol m −2 s −1 and 8.64 × 10 −6 mol m −2 s −1 across CM of pepper fruit
and Ficus leaf, respectively. At the same driving force and time interval, 272 times
more 2,4-D penetrates into a pepper fruit than in a Ficus leaf. It should be realised
that this represents a rather small number of species, and that with more species
included variability might likely be larger.
In the laboratory, growth regulators and other biologically active materials can be
applied to leaves under controlled conditions, and the steady state can be maintained
for a long time. Applying the same substance at constant concentration for the same
time period, the dose delivered to leaves, stems or fruits of different species, genotypes
or mutants differs if permeance is not the same. If P are not known, this type
of experiment is likely to lead to wrong conclusions in dose–response experiments.
By reference to Fig. 6.2 it is clear that penetration experiments with only one time
point are not a good practice. Permeance can still be calculated using (6.6), but holdup
time is not known and must be assumed to be zero. With Citrus CM, 28µmol 2,4-
D penetrated in 24 h and extrapolated hold-up time was 7.9 h. Had only one sample
Clivia