Annals of Botany 2009 104(6):v; doi:10.1093/aob/mcp264
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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
Submergence scenarios I – Rumex reaches up
While walking in
the north-west Highlands of Scotland in late May, I came across
several small, shallow, clear-water pools. The range of terrestrial
plants growing in these pools indicated that they were temporary,
probably resulting from snow melt. Interestingly, while the
grasses appeared similar to specimens growing on drier ground,
some of the submerged dicots had elongated petioles, bringing
the leaf-blades to the water surface. It is this phenomenon
that has been investigated by
Chen et al. (Nijmegen and Utrecht, pp. 1057–1067).
They were particularly interested to know the extent of genetic
variation within populations of
Rumex palustris. Eight genotypes
were selected from each of 12 populations; the populations were
from either river flood plains subject to periodic and often
deep flooding or from stagnant water. At equivalent developmental
stages, plants were completely submerged for 17 days; control
plants were not flooded. The major response in all populations
was petiole elongation (up to 6-fold longer); laminas also elongated
but to a much lesser extent. Leaves of flooded plants were thus
longer and narrower. Laminas were also thinner and less dense.
Comparison of the 3rd, 4th and 5th oldest leaves showed that
the most recently formed leaves elongated the most. In both
extent and timing of this response, there was considerable variation
between genotypes within each population. Thus, maximum flooding-induced
petiole elongation varied between 50 and 90 mm; most genotypes
completed the majority of their elongation growth in the first
7 days of flooding but some showed the greatest elongation in
the last 10 days. However, contrary to the authors' expectations,
there was no correlation between habitat and the extent or timing
of the response. The difference between habitats in the selective
pressure operating on these traits is thus not large. Indeed,
it may well be masked by other selective pressures imposed by
a range of different environmental factors.
Patrolling pollinators sense scents
Many orchids achieve
pollination by sexual deception. Male insects, often of a single
species, are attracted to the flower as if to a female insect
and attempt to copulate with it. Several
Ophrys species provide
good examples of this, with flowers apparently providing a strong
visual cue for the visiting insect. However, although the visual
cues are indeed spectacular they are far from being the whole
story: scent is also very important and may involve emission
of female pheromone-like signalling molecules. The relative
importance of these two types of attraction has been studied
in
Ophrys arachnitiformis by
Vereecken and Schiestl (Brussels and Zurich, pp. 1077–1084).
This orchid, native to the Mediterranean regions of western
Europe, is early-flowering and is pollinated by male plasterer
bees (
Colletes cunicularius). In respect of visual cues, it
is interesting that there are two colour morphs, one with a
green perianth and one with a white perianth. The relative frequency
of the two morphs varies considerably between different populations.
The authors analysed floral scents by GC-MS, showing that the
scent is attributable to a series of straight-chain alkanes
and associated alkenes. There was no difference between colour
morphs in respect of total amounts or relative abundance of
these compounds. Further, pollinators were attracted specifically
to beads coated with these compounds but not to control beads.
‘Decorating’ the beads with either green or white
perianths did not result in any greater frequency of pollinator
visits to the beads. Finally, flowers from which the floral
scent compounds had been removed by washing with hexane did
not attract pollinators. These results thus lead clearly to
the conclusion that it is floral scent that determines pollinator
specificity in
O. arachnitiformis. The pressures that have driven
the selection of other floral traits remain unknown, as do the
interactions of traits and ‘their changes in composition
over evolutionary time’.
SUC2 mutant's seed set poses puzzling problems
In the early days
of my botanical career, phloem transport of sucrose was a controversial
topic. Discussions often became heated; argument and counter-argument
were exchanged with some vehemence. The situation is much calmer
now. At least three phloem-loading mechanisms have been identified
and there is general consensus about sucrose transport in the
sieve tubes. Of those phloem-loading mechanisms, that involving
the sucrose/protein symporter (the SUC2 protein) is the major
one. However,
Srivastava et al., at Denton, Texas (pp. 1121–1128) have shown that
Arabidopsis thaliana plants with a T-DNA insertion
in intron-2 of the
SUC-2 gene can still complete their life
cycle. Mutant plants contained transcripts of the first two
exons of
SUC-2 but no functional protein was produced. Indeed,
protein produced from the truncated mRNA was unable to catalyse
sucrose uptake into yeast cells, again indicating that this
mutation (
SUC2-4) is a null mutation. Plants harbouring the
mutation were extremely stunted and ‘debilitated’
with much smaller cells than wild-type. They took twice as long
as wild-type to come into flower, although flowering took place
at the same developmental phase as in wild-type. Not all plants
produced seed and even in those that did, the number of seeds
was only a small fraction of the number produced by wild-type
plants. Nevertheless, many of the seeds set on mutant plants
were viable (69 % mean percentage germination, c.f. 98 %
in wild-type). These results raise several questions. Why have
not plants harbouring other mutant alleles of
SUC-2 produced
seeds? The authors suggest differences in growth conditions
as an explanation. How, in the absence of this major sucrose
symporter, did the plants achieve any significant growth and,
having done so, how did they channel fixed carbon to the developing
seed? The authors discuss these questions at some length. I
simply comment that perhaps the phloem story is not as clear
as I implied earlier.
Submergence scenarios II – Lotus lies low
The flooding theme
is continued here but the focus shifts to a forage crop species,
Lotus tenuis. As described by
Manzur et al., at Buenos Aires (pp. 1163–1169),
this perennial legume is flood-tolerant and the authors have
investigated its responses to different degrees of waterlogging
and submergence. Six-month-old plants of a commercial cultivar
were subjected to four treatments: control, totally waterlogged
soil (2–5 mm standing water), partial submergence
(60 mm water) and total submergence; plant responses were
monitored over a period of 30 days. The results were very clear:
at all levels from biochemistry to growth patterns, the plant
can alter its responses according how bad the situation actually
is. For example, in waterlogged soil the porosity of roots,
and to a lesser extent of shoots, increased, presumably because
of aerenchyma formation. Shoot number, shoot growth and plant
biomass accumulation showed little or no effect of this treatment.
In partially submerged plants, root and shoot porosity increased
further. Overall biomass accumulation and shoot numbers were
lower than in controls but shoots were on average 30 %
longer, thus bringing leaves above the water surface. This might
lead us to think that shoot elongation would occur to an even
greater extent in completely submerged plants. However, in these
plants, there was no shoot elongation; shoot numbers did not
increase and there was an overall decrease in biomass; shoot
and root porosities were similar to or less than waterlogged
plants. Further, unlike plants in the other three treatments,
submerged plants showed significant loss of starch and soluble
carbohydrates. Thus plants respond to total submergence by ‘sitting
it out’ in a non-growing state, utilizing the carbohydrates
stored in the crown and surviving on limited energy available
from mainly anaerobic respiration. I end with two questions:
how do plants perceive the extent of submergence and how does
that perception lead to the responses discussed here?
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