Annals of Botany 2008 101(4):NP; doi:10.1093/aob/mcn025
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John Bryant takes a closer look at some of this month's Original Articles
J. A. Bryant, Professor
University of Exeter, UK
E-mail j.a.bryant{at}exeter.ac.uk
Plants brace themselves for cold shock
We already know that
the cell wall is dynamically involved in many aspects of cell
physiology. Following the work of a research team from Warsaw,
B
onie and Rzeszów in Poland
(Solecka et al., pp. 521–530),
it has become apparent that changes in cell wall biochemistry
are implicated in low-temperature acclimation. In common with
many species, oilseed rape (
Brassica napus) may be protected
against freezing injury by prior exposure to cold, but not freezing,
conditions (chilling) over a period of days or weeks. Thus,
plants grown at 2 °C for 3 weeks showed some resistance
to subsequent freezing temperatures as low as –10 °C.
This frost tolerance was, however, rapidly lost if the plants
were transferred to 12 °C. Amongst the obvious changes associated
with cold acclimation was a significant reduction in leaf expansion,
accompanied by increased tensile stiffness. These changes were
correlated with changes in cell wall metabolism. Cell walls
made up a greater proportion of the total leaf dry weight, with
pectins constituting a greater proportion of the cell walls.
The latter feature would not necessarily be expected to increase
cell wall stiffness. However, pectin methylesterase activity
was increased by chilling in association with a much lower level
of pectin methylation, making the pectin polysaccharides more
available for cross-linking, thus contributing to wall stiffness.
All these changes except one were reversed when plants were
‘de-acclimated’ at 12 °C. These data certainly
provide a clear indication that changes in cell wall metabolism
and, especially, in the synthesis and modification of pectin
are involved in cold acclimation. However, there remains one
puzzle: despite the decrease in cell stiffness, the one feature
that was not reversed by warming to 12 °C was the increased
specific activity of pectin methylesterase (although it did
decline as a proportion of cell wall weight). So, as the authors
rightly say, these interesting and significant results pave
the way for further important research.
Water-repellent lichens have the solution... to the problem of SO2 pollution
It is widely stated
that lichens are very susceptible to SO
2 damage. It is certainly
true that many lichen species disappeared from areas polluted
by smoke and acid rain, but this only gives half the story.
A number of lichen species are actually tolerant of SO
2 and
have been seen to increase in frequency in polluted areas. According
to
Hauck et al. at Göttingen, Germany (pp. 531–539),
the basis of this tolerance lies in the hydrophobicity of the
thallus surface. Their initial observation was that the very
SO
2-tolerant species
Lecanora conzaeoides has a very hydrophobic
(‘super-hydrophobic’) surface. The observation was
followed by relatively simple but very informative experiments:
lichens ranging from susceptible to tolerant were air-dried.
Water droplets (500 µm in diameter, the size of an
average rain drop) were placed on the thallus surface and the
droplet contact angles were measured to within ±1°.
High contact angles (90° and above) indicate hydrophobicity.
At the other end of the scale, contact angles of less than 50°
could not be measured because, as expected, water spread over
the thallus surface more readily on very hydrophilic thalli.
When contact angles were compared with known SO
2 tolerances
a very clear correlation emerged. All but one of the super-hydrophobic
species (contact angles
120°) were highly tolerant while,
with one exception, the very hydrophilic species all exhibited
low tolerance. It seems therefore that it is the inability of
SO
2 to enter the thallus in solution that confers tolerance
on the hydrophobic species (which, for the same reason, are
also tolerant of heavy metals). Interestingly, thallus hydrophobicity
almost certainly originally evolved to prevent waterlogging
of the thallus (which in turn inhibits photosynthesis of the
algal symbiont) in lichens that grow in wetter habitats. Thus,
tolerance of SO
2 and of heavy metals is a beneficial side-effect
of the adaptation to wet places that adds further selective
advantage in polluted areas.
Size does not always matter much
Plant functional
traits, as discussed by
Kahmen and Poschlod (Regensburg, Germany, pp. 541–548),
are defined as ‘the biological characteristics of plants
that both respond to and determine the dominant processes in
an ecosystem’. My interpretation of this is that they
represent the interface between the systems' biology of the
plant and the wider systems of which the plants are a component.
Germination is an obvious feature that contributes to plant
community composition whilst, in turn, germination success can
be affected by features of the ecosystem such as grazing. The
authors selected two functional traits relevant to germination,
namely seed mass (0·5–2·0 mg, ‘large’,
or <0·5 mg, ‘small’) and germination
season (autumn or spring). The range of necessary combinations
was covered by use of seeds from eight species. The habitat
was semi-natural dry grassland subject to three management techniques
– mowing, grazing and ‘abandonment’. Seeds
were set to germinate in early autumn and seedling establishment
was assayed in late autumn and in the following spring and early
summer. Several clear trends were seen. Large-seeded species
were more successful than small-seeded species. For both groups,
most seedlings were established in mown plots and fewest in
abandoned plots, although the effect of management system on
small-seeded species was very small. This lack of a major effect
of management system on small-seeded species confirms some of
the authors' earlier data (Kahmen S, Poschlod P. 2004.
Journal of Vegetation Science 15: 21–32) but is contrary to the
‘general idea’ (as the authors put it) that establishment
from small seeds is much more successful in more open habitats,
such as the grazed or mown plots in these experiments, than
in more closed habitats. Mowing very much favoured autumn-germinating
species but there was no difference between the two groups in
the other management systems. Overall, this study gives a clear
indication of how ‘experimental testing of functional
trait responses’ can help our understanding of ecosystem
dynamics.
Reality of Rhinanthus parasitic potential is punishing for Phleum photosynthesis
Rhinanthus minor (yellow rattle) is an interesting representative of a group
of hemi-parasitic or facultatively parasitic species in the
Scrophulariaceae. Although this phenomenon has been known for
many years, we have little information about the effect of hemi-parasites
on host photosynthesis compared with the impact of obligate
parasites. This deficiency has been addressed by
Cameron et al. (Aberdeen, UK and Montpellier, France, pp. 573–578).
Seedlings of
R. minor were grown in the same pots as either
seedlings of the grass
Phleum bertolonii (a good host) or of
the forb,
Plantago lanceolata (a poor host). In
Phleum, host
biomass was much reduced by the parasite, whereas there was
no effect on
Plantago biomass. This difference was reflected
in the effects of parasitism on aspects of photosynthesis. In
Phleum, there were significant reductions in steady-state quantum
yield and chlorophyll content, and a non-significant reduction
in Rubisco content. These reductions were taken to reflect,
at least partly, the ability of the parasite to withdraw major
nutrients from its host. In
Plantago, the parasite had little
or no effect on these parameters. Nevertheless, in both host
species, maximum photosynthetic quantum yield was slightly reduced
by parasitism (but not significantly so in the good host,
Phleum).
Thus, even in the absence of a vascular connection with
Plantago,
the parasite was able to exert some effect on this poor host.
Perhaps the most telling differences were in the parasite itself:
photosynthetic quantum yields (maximum and steady state) and
Rubisco content were lower (much lower for steady-state yield)
when
Plantago was the host than when
Phleum was the host, leading
to very large differences in parasite biomass. Intriguingly,
however, the increased parasite biomass on
Phleum was insufficient
to account for the reduction in biomass inflicted on its host.
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Related articles in Ann Bot:
- Are Pectins Involved in Cold Acclimation and De-acclimation of Winter Oil-seed Rape Plants?
- Danuta Solecka, Jacek Zebrowski, and Alina Kacperska
Ann Bot 2008 101: 521-530.
[Abstract]
[Full Text]
- Surface Hydrophobicity Causes SO2 Tolerance in Lichens
- Markus Hauck, Sascha-René Jürgens, Martin Brinkmann, and Stephan Herminghaus
Ann Bot 2008 101: 531-539.
[Abstract]
[Full Text]
- Does Germination Success Differ with Respect to Seed Mass and Germination Season? Experimental Testing of Plant Functional Trait Responses to Grassland Management
- S. Kahmen and P. Poschlod
Ann Bot 2008 101: 541-548.
[Abstract]
[Full Text]
- Suppression of Host Photosynthesis by the Parasitic Plant Rhinanthus minor
- Duncan D. Cameron, Jean-Michelle Geniez, Wendy E. Seel, and Louis J. Irving
Ann Bot 2008 101: 573-578.
[Abstract]
[Full Text]
