558
E.-D. Schulze and M.M. Caldwell
thought. Even the apparent direct response to humidity appears to be
regulated by local epidermal water stress which is created by water loss from
the inner side of the guard.cell complex. This is an apparent contradiction
of the observation that large variations in leaf water potential can occur
without stomatal effects (Schulze 1993). This leads to questions of internal
plant controls, which include mineral nutrition and regulation by plant
hormones.
One key factor in this complex appears to be the regulation of cellular
and xylem pH (Pfanz, Chap. 5). The pH gradient causes changes in the
distribution and protonation of abscisic acid, which is probably the most
potent stress hormone and which directly affects stomatal function. The pH
regulation is dependent on many factors such as nutrition and carbon and
acid metabolism. Losch and Schulze (Chap. 9) show that water stress at the
root level will cause roots to produce ABA which moves through the xylem
and reaches the leaf, where (1) it may be recirculated into the phloem, or
(2) it may enter into the mesophyll cells depending on the pH gradient,
where it is stored or metabolized in the light, or (3) it reaches the stomata
and interacts with the ion channels of the guard cell plasmalemma. Plant
nutrition is involved because the balance in uptake of anions and cations will
determine the xylem pH and thus determine the protonation of ABA and
movement into mesophyll cells. Apart from its effects on stomatal function,
ABA emerges as a very potent phytohormone which affects extension
growth (Schulze 1993).
Although it was largely the ecophysiology of photosynthesis that initiated
this line of research, it has revealed an aspect of whole-plant physiology that
formerly was not recognized, namely root-shoot signals that coordinate
shoot and root function in addition to carbohydrates. It is just such research
that demonstrates that photosynthesis can no longer be studied in isolation
when in the context of the whole plant. Photosynthesis is a highly interactive
process which involves not only water but also nutrient relations and
hormonal signals.
26.5 Photosynthetic Performance of Different Plant Groups
The extensive data set of plant photosynthetic behaviour under a broad
range of conditions has enabled us to recognize plant patterns and specific
features of different plant groups.
Aquatic plants are much more diverse than terrestrial plants in terms of
light-harvesting mechanisms, which has been apparent from their diverse
pigment systems. Aquatic plants have evolved a COz-concentrating mechanism which is different from that observed in terrestrial plants (Raven,
Chap. 15). This mechanism is not known in terrestrial plants, except in
E.-D. Schulze and M.M. Caldwell
thought. Even the apparent direct response to humidity appears to be
regulated by local epidermal water stress which is created by water loss from
the inner side of the guard.cell complex. This is an apparent contradiction
of the observation that large variations in leaf water potential can occur
without stomatal effects (Schulze 1993). This leads to questions of internal
plant controls, which include mineral nutrition and regulation by plant
hormones.
One key factor in this complex appears to be the regulation of cellular
and xylem pH (Pfanz, Chap. 5). The pH gradient causes changes in the
distribution and protonation of abscisic acid, which is probably the most
potent stress hormone and which directly affects stomatal function. The pH
regulation is dependent on many factors such as nutrition and carbon and
acid metabolism. Losch and Schulze (Chap. 9) show that water stress at the
root level will cause roots to produce ABA which moves through the xylem
and reaches the leaf, where (1) it may be recirculated into the phloem, or
(2) it may enter into the mesophyll cells depending on the pH gradient,
where it is stored or metabolized in the light, or (3) it reaches the stomata
and interacts with the ion channels of the guard cell plasmalemma. Plant
nutrition is involved because the balance in uptake of anions and cations will
determine the xylem pH and thus determine the protonation of ABA and
movement into mesophyll cells. Apart from its effects on stomatal function,
ABA emerges as a very potent phytohormone which affects extension
growth (Schulze 1993).
Although it was largely the ecophysiology of photosynthesis that initiated
this line of research, it has revealed an aspect of whole-plant physiology that
formerly was not recognized, namely root-shoot signals that coordinate
shoot and root function in addition to carbohydrates. It is just such research
that demonstrates that photosynthesis can no longer be studied in isolation
when in the context of the whole plant. Photosynthesis is a highly interactive
process which involves not only water but also nutrient relations and
hormonal signals.
26.5 Photosynthetic Performance of Different Plant Groups
The extensive data set of plant photosynthetic behaviour under a broad
range of conditions has enabled us to recognize plant patterns and specific
features of different plant groups.
Aquatic plants are much more diverse than terrestrial plants in terms of
light-harvesting mechanisms, which has been apparent from their diverse
pigment systems. Aquatic plants have evolved a COz-concentrating mechanism which is different from that observed in terrestrial plants (Raven,
Chap. 15). This mechanism is not known in terrestrial plants, except in