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

4.3 Ion Exchange Capacity 71
has an exchange capacity of 0.5eqkg −1 (data not shown). Between pH 9 and 12, the
MX changed colour from yellow/orange to red. The variety “Traveller” (Fig. 4.2)
does not turn red when ripe, and isolated cuticles have an opaque white appearance.
At pH 9 exchange capacity was around 0.5eqkg −1 , and above pH 9 it increased
only slightly.
Titration of MX with base results in titration curves indicative of three distinctly
different fixed groups whose acid strengths are influenced by pH, the nature of
counter ions and concentration of neutral salts. This is typical of weak acid groups
such as carboxyl and phenolic hydroxyl groups. Based on acid strengths, the first
group titrating up to pH 6 was assigned to carboxyl groups of pectins and polypeptides,
the second group appeared to be donated by non-esterified carboxyl groups of
hydroxyfatty acids, and the third group most likely was phenolic in nature (Schönherr
and Bukovac 1973). This picture is complicated by the presence of nitrogen in
isolated cuticles. Schönherr and Huber (1977) determined amino acids liberated by
acid hydrolysis (6 N HCl for 12 h at 110 ◦ C). Since amino acid amounts and compositions
of apricot leaf MX were the same when isolated with enzyme or Zn/ZCl2, the
polypeptides were not contaminants from the enzymatic isolation. They are located
inside the polymer, and were not removed by washing or by the recycling procedure
used to condition isolated cuticles.
Basic and acidic amino acids can contribute both acidic and basic fixed charges.
Below the isoelectric point, cuticles are positively charged because of the presence
of basic amino acids (Table 4.2). Histidine, lysine and arginine are basic amino
acids, and Kb values of their free amino groups are 6.0, 10.53 and 12.48 respectively.
Below the isoelectric points of cuticles all of them are protonated, and they lose their
charges only at pH values above 8 (histidine) or higher. Asparaginic and glutamic
acid are acidic amino acids, and the pKa of their free carboxyl groups are 3.65 and
4.25 respectively. They are ionised above the isoelectric points, and contribute to
exchange capacities of all three ionisable groups (Table 4.2).
Upon acid hydrolysis the tomato fruit MX lost 13% of its weight and all of its
nitrogen. Oxygen content decreased from 22.7% to 19.7%. Water droplets spread on
Table 4.2 Total Exchange capacity (E C ) of selected fruit and leaf MX membranes at pH 6, 8 and
9, and contribution of amino acids (AA) and pectin to E C
Species E C E C Basic AA Acidic AA Pectin Total AA
(meqkg −1 ) (meqkg −1 ) (meqkg −1 ) (meqkg −1 ) (meqkg −1 ) (mMolkg −1 )
pH 6.0 pH 8–9
Capsicum fruit 230 580 (pH 8) 21 46 184 256
Lycopers. fruit 200 500 (pH 9) 13 40 160 190
Citrus aurantium 133 250 (pH 8) 12 22 308 107
Prunus armeniaca 120 290 (pH 8) 19 33 87 152
Schefflera actinoph. 80 400 (pH 8) 14 27 53 139
Pyrus communis – – 26 54 – 266
E C of pectins was estimated by subtracting E C of acidic amino acids from the total E C at pH 6
(data taken from Schönherr and Bukovac 1973; and Schönherr and Huber 1977)