110
H. Pfanz
0.6
A Picea-IWF
0.5
'2
• Armoracia·IWF
c
E
.2 c
0.4
Q.
E :s
::>
e
III
Q.
C
0> 0.3
0
0
~
c -
.. 0'"
0>
>- 15
0.2
)(
0
E
2.
0.1
0.0
0
2 3 4
5 6
7 8 9 10
pH of incubation medium
0.020
'2 0.015
c ·E
0
:;; c:
Q. .E l!l
::> e
III
Q.
S ~
0.010
c -
.. '"
0>0
~15
o E 0.005
2.
0.000 L..-..L..-+---L---I'--.1.-...J..... ...... ~=-~
o 1 2 3 4 5 6 7 8 9 10
pH of incubation medium
Fig. 5.1. pH dependence of the IAA oxidation of apoplastic peroxidases from different
leaf sources. Upper panel (6) = Picea abies; (_) = Armoracia lapathifolia. Lower panel
(\7) = Fagus sylvatica; (e) = Hordeum vulgare
in the ratio of IAAsympl' to IAAapopl' (Kaiser and Hartung 1981; Cowan et
al. 1982). To our knowledge, no evidence has been brought forward that
ABA is also sequestered in the extracellular space.
5.3.1.4 Apoplastic pH and Growth
One theory for cell expansion is based on extracellular acidification with a
concomitant cell wall loosening by cation/proton exchange from the carboxyl
groups of the uronic acids. IAA has been postulated to catalyze the apoplastic acidification (Taiz 1984) and is thus directly involved in cellular
growth. Nevertheless, it is still a matter of dispute whether or not proton
extrusion and the concomitant loosening of the Ca 2 + from the carboxyl