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

6.2 Steady State Penetration 175
effect on rates was shown to be caused by stomatal opening which increased permeability
of cuticular ledges (Schönherr and Bukovac 1978). Alar is an electrolyte,
and at pH 5 it is neutral because negative and positive charges are present in equal
amounts (Schönherr and Bukovac 1972b). The role of stomata in foliar penetration
of ionic compounds is treated comprehensively in Chap. 5.
Alar penetration into bean leaves using the leaf disk method with attached wells
is a good example to demonstrate the merits of the method. The main advantages
are the facts that permeability of large leaves can be measured, and it is possible to
test if permeability of lower and upper leaf surfaces differ. With submerged leaves
this is not possible. Kirsch et al. (1997) have used this method to compare solute
permeability of isolated CM with non-isolated cuticles (Sect. 6.5). It is a somewhat
laborious undertaking, since sampling is destructive and when rates of penetration
(that is the time course of penetration) are studied, different leaf disks must be used
for each time interval. This increases variability, and a large number of leaf disks
(25 and more) must be used for representative sampling and for testing linearity. We
have shown above why this is absolutely essential.
Autoradiography makes it possible to study distribution of radioactive labels,
provided the leaves are freeze-dried quickly to avoid redistribution and metabolism.
Autoradiographs of selected bean leaf disks show that in the presence of Tween
20 more succinic acid-2,2-dimethylhydrazide penetrated and radio-label was much
more uniform (Fig. 6.14). Sometimes the label spread along the veins and reached
the cut edges. In later experiments with CaCl2, this was avoided by placing the leaf
disks on stainless steel washers rather than directly on moist filter paper (Fig. 5.4b).
Permeances for non-ionised 2,4-D, salicylic acid and benzoic acid were measured
using the upper, astomatous leaf surfaces or CM obtained from Prunus
laurocerasus, Ginkgo biloba and Juglans regia leaves (Kirsch et al. 1997). With
leaf disks and CM, penetration plots were linear and permeances could be calculated
from slopes. Permeances measured with leaf disks and CM did not differ
significantly with all three species and compounds. Clearly, enzymatic isolation of
cuticles did not affect permeability of cuticles.
Fig. 6.14 Autoradiographs of bean leaf disks after penetration of 14 C labelled succinic acid-2,2dimethylhydrazide
(Alar) in light for 12 h. Penetration without (a) and with 0.1% Tween 20 (b) as
wetting agent