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

Problems 143
log Rate constant (h -1 )
-0.4
-0.6
-0.8
-1.0
-1.2
-1.4
-1.6
-1.8
FeIDHA
CaCl 2 + FeEDTA
0 5 10 15 20 25
Concentration (x10 3 mol/L)
FeEDTA
Fig. 5.13 Penetration at 100% humidity of 59 FeIDHA and 59 FeEDTA across Populus CM at concentrations
increasing from 0.002 to 0.02moll −1 , and penetration of 45 CaCl2 in the presence of
non-radioactive FeEDTA. Glucopon 215 CSUP was added to the donor solutions. Temperature
was 20 ◦ C. The same set of CM was used for all experiments. (Redrawn from data of Schönherr
et al. 2005)
blockage of aqueous pores in the Populus CM, serving as polar path of transport for
both Fe chelates and Ca salts, and as a consequence rates of penetration decrease.
Leaf age greatly affected penetration of Fe chelates. Highest rates of penetration
were observed with young unfurling leaves, whereas penetration into fully expanded
leaves of different species was hardly measurable. These results show that young
growing leaves should be sprayed, and humidity must be 100%. Hence, spraying
in the evening is recommended when due to decreasing temperatures the dew point
may be reached. Fe chelates are destroyed by UV radiation, which requires spraying
after sun set (Schönherr et al. 2005).
Problems
1. Why is the cuticular permeability of polar compounds very low?
2. Why is the diffusion of ionic solutes in the lipophilic cutin and wax phase
impossible?
3. Which are preferential sites in the leaf surface where aqueous pores are located?
4. What is the half-time of cuticular penetration t 1/2 of a Ca 2+ salt having a rate
constant k of −0.0001s −1 ?
5. How does relative humidity affect the penetration of a salt across the cuticle?