Influence of leaf water content on the C3–CAM transition in Mesembryanthemum crystallinum
Changes in leaf water content, night-time accumulation of malic (Δ-malate) and citric acid (Δ-citrate) and phosphoenolpyruvate carboxylase (PEPC, EC 4 . 1 . 1 . 31) activity were followed for 60 d after germination in well watered and salt-stressed plants of the facultatively halophytic ephemeral Me...
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| Published in: | The New phytologist 1997-07, Vol.136 (3), p.425-432 |
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| Main Authors: | , |
| Format: | Article |
| Language: | eng |
| Subjects: | |
| Online Access: | Get full text |
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| Summary: | Changes in leaf water content, night-time accumulation of malic
(Δ-malate) and citric acid (Δ-citrate) and
phosphoenolpyruvate carboxylase (PEPC,
EC 4 . 1 . 1 . 31) activity were
followed for 60 d after germination in well
watered and salt-stressed plants of the facultatively halophytic
ephemeral Mesembryanthemum crystallinum L. To
separate the effects of development, salt stress and water deficit on
crassulacean acid metabolism (CAM) induction
plants were stressed initially 10 d after germination and then successively
at 1-wk intervals (five sets). Related to
dry mass or organic matter (i.e. dry mass corrected for the mass of
inorganic ions) water content started to decrease
during the late embryonal phase of the life cycle. Water content on a dry
mass basis was always lower in salt-stressed than in well watered individuals.
However, on an organic matter basis no difference was detectable. This
indicated that salt treatment did not reduce leaf water content but falsified
the basis (dry mass). Increases in leaf
succulence and in pressure potential prevented long-term water deficit in well
watered and in salt-stressed plants.
Instead, these changes displayed enhanced vacuolisation, which is an essential
prerequisite for the development
of CAM. The end of that differentiation process might allow the initiation of
nocturnal malic acid accumulation
in a threshold response. At the onset of each salt treatment, short-term water
deficits occurred due to an
incomplete osmotic adaptation independent of plant age. As Δ-malate
only appeared when plants were c. 35 d old
this water deficit was unlikely to be a decisive CAM-inducing factor.
About 2 wk after germination water content
began to decline during the light periods in plants of all treatments.
This pattern disappeared again when CAM
had been fully established. Daytime transpirational water loss is therefore
unlikely to be the decisive factor because
it failed to induce the metabolic shift in young plants. Environmental
stress (e.g. salt or drought) can therefore only
induce Δ-malate when leaf and plant differentiation has
reached a certain stage. |
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| ISSN: | 0028-646X 1469-8137 |