Annals of Botany 2008 101(3):NP; doi:10.1093/aob/mcn004
© The Author 2008. Published by Oxford University Press on behalf of the Annals of Botany Company. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org
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
Seed strategy centred on cycling sensitivity
Plants whose seeds
form long-lived seed banks exhibit a range of strategies to
spread germination potential over a protracted period. One of
these, seasonal cycling in germinability, has been studied by
Jayasuriya et al., at Lexington, USA (pp. 341–352) in
Ipomoea lacunosa, a member of the Convolvulaceae and a troublesome
weed in the USA. It forms seed banks and the seeds may remain
viable for 40 years. Dormancy is physical: the hard seed coat
prevents water uptake. Dormancy may be broken by scarification
of the seed coat or by opening of the water gap. It has been
claimed that the seeds exhibit dormancy cycling but, as the
authors point out, it is impossible to re-impose physical dormancy
once the seed coat has been breached or the water gap opened.
If the seeds do exhibit cycling of germinability, then there
must also be either a reversible physiological aspect to dormancy
or a reversible sensitivity to dormancy break. It is the latter
that the authors' research has demonstrated in
I. lacunosa.
Space does not permit extensive discussion of their work but
the key features are as follows. Incubation of physically dormant
seeds under appropriate conditions at 35/20 °C resulted
in >90 % germination; this figure falling significantly
as temperature decreased, with the exact percentages varying
between seed batches. Physically dormant seeds exposed to dormancy-breaking
incubation on wet sand at 35 °C for 2 h exhibited
>95 % germination even at 25/15 °C. Seeds incubated
in moist conditions at lower temperatures were sensitive to
this dormancy-breaking treatment but seeds incubated dry were
not. Further, seeds in which sensitivity had been induced could
be made insensitive again by incubation under dry conditions.
The data lead the authors to develop a model that shows that
this strategy fits
I. lacunosa very well for its habitat and
lifestyle.
Grass genome evolution – a story with GC bits
I have long been
fascinated by the fact that many eukaryotic organisms have very
much more DNA than they need for coding, and that within some
taxa there is huge variation in the genomic DNA content (the
‘C-value’). Amongst plants there is a thousand-fold
range in C-value. Even within genera there can be striking differences,
as exemplified by
Vicia with its 7-fold range in C-value, the
variation being accounted for by differences in the amount of
non-coding DNA. Thus, acquisition of more non-coding DNA, much
of it represented by different types of repetitive sequences,
is a major part of genome evolution. However, changes in non-coding
DNA can also go in the opposite direction, as shown by some
very neat work by the Czech–Italian team,
marda et al., Brno, Florence and Parvia (pp. 421–433).
They have measured C-values and GC (guanidine cytosine) contents
within the genus
Festuca, in a second genera from which
Festuca diverged, and in a third genera more recently diverged from
the
Festuca lineage. In general, with the exception of diploid
Vulpia species, there was a correlation between genome size
and GC content. Further, when these two characters were used
to assemble a phylogenetic tree, there was a very close fit
to a tree assembled from more specific sequence data. This enabled
the authors to conclude that the initial divergence of the
Festuca lineage involved an increase in genome size and GC content,
characters still seen in the basal fescues. Subsequent and still
ongoing evolution of both broad- and fine-leaved fescues and
divergence of younger taxa from the
Festuca lineage has involved
reductions in genome size and in GC content. The early evolutionary
increases and subsequent deceases in genome size are most probably
indicators of gains and losses of GC-rich mobile DNA elements
known as retro-transposons. Thus, the use of general quantitative
features of plant genomes has provided a good picture of the
dynamic nature of genome evolution.
The flowers that bloom in the spring...
One of the botanical
pleasures of my spring-time visits to West Virginia has been
the sighting of
Trillium erectum and
T. grandiflorum* in the
Kanawha State Forest. These beautiful plants are typical of
those woodland species that sprout from underground organs (e.g.
rhizomes) after winter and flower while the canopy is still
open. This provides the context for the research of
Ida and Kudo at Sapporo, Japan (pp. 435–446),
working with another
Trillium species,
T. apetalon. The authors
are mindful that the trees themselves are affected by environmental
conditions such that the length of the high-irradiance period
from snow-melt to canopy closure is variable from year to year.
Thus, in their work on plants in the field, the authors subjected
some plants to artificial shading to mimic a shorter period
of canopy openness. As expected, maximum photosynthesis occurred
during the high irradiance period. Use of
13C showed that photosynthate
was initially allocated mainly to the shoot and leaves, promoting
growth, and then to the rhizome, laying down provision for over-wintering
and for the next year's shoot and flowers. Translocation to
the developing fruit did not occur until after canopy closure
when the photosynthetic rate was very low. In plants that were
shaded during the normal flowering and active growth period,
mimicking early canopy closure, the switch between allocation
to rhizome and allocation to fruit occurred early, as if the
reduction in irradiance/canopy closure acts as a signal for
changing the main sink for photosynthate. Seed production was
reduced in the early-shaded plants. It was also clear that early
shading reduced flower production in the following year, presumably
because of reduced allocation to the reserves in the rhizome.
Overall, this strategy of prioritizing the shoot and then the
rhizome over seed production ensures that the existing plant
is more likely to survive while retaining some potential for
propagation by seeds that can be expanded in years when canopy
closure is delayed.
* For descriptions, see West Virginia Wildlife Magazine http://www.wvdnr.gov/Wildlife/Magazine/Archive/05spring/elegance_spring_trilliums.shtm
Photosynthesis saved by specific Si siting
It is well known
that grasses, especially the bamboos, accumulate silica in their
leaves. Indeed, the highest recorded Si concentrations are seen
in the leaves of the bamboo,
Sasa veitchii, as discussed by
Motomura et al., at Sendai, Japan (pp. 463–468). Silicon
is accumulated via uptake of silicic acid from the soil and
transported in the transpiration stream to the leaves. A question
then arises: do these extensive deposits of Si interfere with
other physiological processes such as photosynthesis? To answer
this question, the authors measured Si accumulation during the
3-year life time of individual leaves. Si is accumulated in
all three seasons over a period from spring through to autumn,
reaching up to 15 % of dry weight in year 1, up to 25 %
in year 2 and up to an astonishing 41 % in year 3. Relationships
between Si content and photosynthesis are complex. In year 1,
photosynthetic capacity (
Pmax) increases as Si content goes
up, but in years 2 and 3
Pmax declines markedly when plotted
against Si content. However, this does not necessarily mean
that Si inhibits photosynthesis. Photosynthesis expressed as
a function of N-content (PNUE) fell dramatically in year 1,
whether expressed over time or in relation to Si; this was a
result of the significant increase in N content. PNUE was approximately
constant in year 2 and fell again in year 3, while N content
fell. Nitrogen content thus also affects photosynthesis and
the authors conclude that Si accumulation only inhibits photosynthesis
in year 3, i.e. in the later stages of the leaf's life. The
changing effects of Si probably reflect its deposition pattern
in the leaves: in years 1 and 2, Si seems to be directed to
the epidermal cells; CO
2 diffusion remaining unimpeded. However,
when the Si concentration exceeds 25 % it is also deposited
in the chlorenchyma; CO
2 diffusion is now impeded and photosynthesis
inhibited thereby.
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Related articles in Ann Bot:
- Cycling of Sensitivity to Physical Dormancy-break in Seeds of Ipomoea lacunosa (Convolvulaceae) and Ecological Significance
- K. M. G. G. Jayasuriya, J. M. Baskin, and C. C. Baskin
Ann Bot 2008 101: 341-352.
[Abstract]
[Full Text]
- Genome Size and GC Content Evolution of Festuca: Ancestral Expansion and Subsequent Reduction
- Petr Smarda, Petr Bures, Lucie Horová, Bruno Foggi, and Graziano Rossi
Ann Bot 2008 101: 421-433.
[Abstract]
[Full Text]
- Timing of Canopy Closure Influences Carbon Translocation and Seed Production of an Understorey Herb, Trillium apetalon (Trilliaceae)
- Takashi Y. Ida and Gaku Kudo
Ann Bot 2008 101: 435-446.
[Abstract]
[Full Text]
- Relationships Between Photosynthetic Activity and Silica Accumulation with Ages of Leaf in Sasa veitchii (Poaceae, Bambusoideae)
- Hiroyuki Motomura, Kouki Hikosaka, and Mitsuo Suzuki
Ann Bot 2008 101: 463-468.
[Abstract]
[Full Text]
