Apoplastic & Symplastic Proton Concentrations & Their Significance for Metabolism 111
groups indeed weaken the cell wall (Han 1973; Cleland 1981). During cell
extension and cell wall synthesis, physical as well as biochemical parameters
change. Growing and expanding cells need a flexible skeleton to allow
osmotic swelling and concomitant volume change, whereas differentiated
and mature cells have to be surrounded by a rigid and stiffened wall. For
this reason, growing leaves - or growing parts of a leaf - have a more acidic
pH than nongrowing, mature parts of leaves.
5.3.1.5 Apoplastic pH and Hormone Action
According to the acid growth theory, cell enlargement through water uptake
and osmotic swelling is preceded by IAA-stimulated cell wall loosening (see
above). The hormone is thought to promote acidification of the apoplastic
space via proton extrusion from the protoplasts, leading to a displacement of
Ca (and Mg) ions from the carboxyl groups of the uronides of the middle
lamella (Taiz 1984; Cleland 1971). Acidification of the symplast, on the
other hand, reduces IAA catabolism (cf. Fig. 5.1). Provided that IAA (or
ABA) are moving between the apoplast and the symplast purely by diffusion (Kaiser and Hartung 1981; Cowan et al. 1982; Hartung 1983; Hartung
and Slovik 1991), an external acidification would lead to an increase in
un dissociated IAA and therefore to an influx of the hormone into the cells.
As most of the phytohormones have pK values around 4.7 (GA, IAA, and
ABA are weak acids), accumulation will take place in alkaline compartments where these hormones are trapped. Cowan et al. (1982) were the first
to calculate transport and distribution of ABA on this basis. This excellent
study of Cowan's was the basis of a more detailed computer model established by Hartung and Slovik (1991) some 10 years later. It can be shown
with those models that metabolic or stress-induced pH changes of the
symplast or of the apoplast therefore can lead to a redistribution of slightly
acidic plant hormones, with various consequences on metabolism (Kaiser
and Hartung 1981; Kremer et al. 1987).
5.3.2 The Symplastic pH
5.3.2.1 Photosynthesis and pH
The photosynthetic reactions of higher plants and algae are very sensitive to
changes in the pH of the chloroplast stroma (Heldt et al. 1973; Werdan et
al. 1975; Heber and Heldt 1981). Most of the steps involved in the assimilation of CO2 are catalyzed by enzymes and are therefore strictly pHdependent (Woodrow et al. 1984). The tight regulation of the proton
concentration in the chloroplast therefore plays a dominant role in controlling
photosynthetic carbon reduction.
groups indeed weaken the cell wall (Han 1973; Cleland 1981). During cell
extension and cell wall synthesis, physical as well as biochemical parameters
change. Growing and expanding cells need a flexible skeleton to allow
osmotic swelling and concomitant volume change, whereas differentiated
and mature cells have to be surrounded by a rigid and stiffened wall. For
this reason, growing leaves - or growing parts of a leaf - have a more acidic
pH than nongrowing, mature parts of leaves.
5.3.1.5 Apoplastic pH and Hormone Action
According to the acid growth theory, cell enlargement through water uptake
and osmotic swelling is preceded by IAA-stimulated cell wall loosening (see
above). The hormone is thought to promote acidification of the apoplastic
space via proton extrusion from the protoplasts, leading to a displacement of
Ca (and Mg) ions from the carboxyl groups of the uronides of the middle
lamella (Taiz 1984; Cleland 1971). Acidification of the symplast, on the
other hand, reduces IAA catabolism (cf. Fig. 5.1). Provided that IAA (or
ABA) are moving between the apoplast and the symplast purely by diffusion (Kaiser and Hartung 1981; Cowan et al. 1982; Hartung 1983; Hartung
and Slovik 1991), an external acidification would lead to an increase in
un dissociated IAA and therefore to an influx of the hormone into the cells.
As most of the phytohormones have pK values around 4.7 (GA, IAA, and
ABA are weak acids), accumulation will take place in alkaline compartments where these hormones are trapped. Cowan et al. (1982) were the first
to calculate transport and distribution of ABA on this basis. This excellent
study of Cowan's was the basis of a more detailed computer model established by Hartung and Slovik (1991) some 10 years later. It can be shown
with those models that metabolic or stress-induced pH changes of the
symplast or of the apoplast therefore can lead to a redistribution of slightly
acidic plant hormones, with various consequences on metabolism (Kaiser
and Hartung 1981; Kremer et al. 1987).
5.3.2 The Symplastic pH
5.3.2.1 Photosynthesis and pH
The photosynthetic reactions of higher plants and algae are very sensitive to
changes in the pH of the chloroplast stroma (Heldt et al. 1973; Werdan et
al. 1975; Heber and Heldt 1981). Most of the steps involved in the assimilation of CO2 are catalyzed by enzymes and are therefore strictly pHdependent (Woodrow et al. 1984). The tight regulation of the proton
concentration in the chloroplast therefore plays a dominant role in controlling
photosynthetic carbon reduction.