Annals of Botany 2008 102(1):v; doi:10.1093/aob/mcn098
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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
Death and glory
When UK rock group
The Rolling Stones sang ‘
...you can send me dead flowers every morning...’, floral decoration was very far from
their minds. However, dead and dried flowers used as decorations
often last several years. One plant used in this way is
Helichrysum bracteatum, one of the so-called ‘eternal’ flowers.
In this species, the colour is provided by several rows of large,
scarious (but otherwise corolla-like) bracts, as described by
Nishikawa et al. (Kyoto, Japan, pp. 31–37). Bract water
content is low, well below 50 % of the water content of growing
tissues and much more typical of tissues containing high proportions
of dead cells. So, at what stage do the bracts of
H. bracteatum actually die? The authors used DAPI staining combined with fluorescence
microscopy to examine cell nuclei; other aspects of cell structure
and morphology were investigated with a range of microscopic
techniques. Even in the unopened flower bud, loss of nuclei
had started in the bract tip and this loss worked its way down
towards the base of the bract as the bud expanded and then the
flower opened. By the anthesis stage there were no nuclei in
the upper half of the bracts (although cells at the base of
the bract had retained their nuclei) and cells lacked all other
organelles. Both the epidermal and the inner cells were secondarily
thickened (in contrast to bracts and petals of a conventional
flower) and, further, the cell walls showed birefringence, indicative
of orientated cellulose microfibrils. The cells thus have some
features of tracheary elements. However, in the bracts these
features are present in epidermal and parenchymatous cells,
leading the authors to postulate that they represent a new cell
type. Here, then, is a specific form of programmed cell death
associated with floral function and initiated very early in
the life of the flower.
Once more unto the breach
The genus
Cuscuta shows a strong evolutionary commitment to a particular lifestyle:
all 145 species are holoparasitic vines and many of these are
significant pests in agriculture. Further, along with other
genera in the Convolvulaceae, at least some
Cuscuta species
exhibit physical dormancy of seeds, as discussed by
Jayasuriya et al. (Lexington, Kentucky, USA and Taipei, Taiwan, pp. 39–48).
A common feature of seeds with physical dormancy is the presence
in the seed coat of a water gap, the initial point of entry
of water into the seed, but it is not known whether this exists
in
Cuscuta. The authors have therefore investigated this and
other features of dormancy in
C. australis. The majority of
seeds exhibited physical dormancy, which could be broken by
scarifying the seed coat or by dipping the seeds into boiling
water for 10 s or, more ‘naturally’, by incubating
them wet in a temperature regime of 35/20 °C. Seeds
were made sensitive to the latter dormancy-breaking treatment
by dry storage at ambient laboratory temperature for 2 months.
Breakage of dormancy was associated with opening of the hilar
fissure; staining with an aqueous solution of aniline blue demonstrated
that this was the route for water entry. No dye entered dormant
seeds nor non-dormant seeds in which the hilar fissure had been
sealed. The hilar fissure thus functions as a water gap, equivalent
to the bulge adjacent to the micropyle in other members of the
Convolvulaceae. Finally, there was evidence for sensitivity
cycling, regarded as a strategy that enables dormant seeds to
sense favourable conditions for germination and seedling growth.
It is a relatively common phenomenon amongst physiologically
dormant seeds but unusual in physically dormant seeds. Indeed,
the authors state that this is the ‘second example of
a species in Convolvulaceae that can undergo sensitivity cycling
and the first demonstration in the only holoparasitic genus
whose seeds have physical dormancy’.
Parasite prefers to do it on grass
Continuing the parasite
theme, we now consider a hemi-parasite,
Thesium chinensis an
Asian member of the Santalaceae.
Thesium is a widely distributed
Old World genus, all members of which are parasitic or hemiparasitic.
Several members of the genus have a nuisance value as agricultural
weeds and thus it is important to gain knowledge of their ecology
and host range.
Suetsugu et al. (Kyoto, Japan, pp. 49–55) point out that for
T. chinensis such knowledge is very fragmentary.
Further, there is a world of difference in observing proximity
in the field to potential host species and actually knowing
that parasitism is occurring. There is also a difference between
study of interactions with selected plants under controlled
conditions and study of what happens in the wild. The authors
first carried out an association analysis in the field. Of the
38 species recorded at the study site only two (
Eragrostis curvula and
Lespedeza juncea) were present at greater frequency than
expected. When haustorial connections were examined by excavating
around the plants, 22 of the 38 species were shown to be parasitized,
indicating a broad host range such as is seen in many hemiparasites.
However, there was clear evidence that some species were more
effective hosts than others: estimation of haustorium numbers
showed that for many host species numbers differed significantly
from expected. On this basis, grass species (Poaceae) were very
good hosts whilst members of the Rosaceae and Caryophyllaceae
were poor hosts. Other families, such as the Fabaceae contained
both good (
L. juncea) and poor (
Pueraria lobata) hosts. A slightly
puzzling feature of the data is that haustoria on Fabaceae were
larger than on other families. Nevertheless, the results indicate
that
T. chinensis has a clear preference for Poaceae whilst
retaining the ability to utilize a range of other species when
preferred hosts are not in reach.
Seed behaviour: can we blame the mothers?
In cool temperate
regions, an unusually cool, wet summer often results in some
cereal seeds germinating on the parent plant. This is an extreme
case of a more general phenomenon, namely that the conditions
experienced by the parent plant during flowering and seed set
can affect the dormancy and subsequent performance of the seeds.
Although there have been many investigations of this, one group
that has not been studied are Australian native plants. To remedy
this deficiency,
Hoyle et al. (St Lucia, Queensland, Australia and Royal Botanic Gardens, Kew, UK, pp. 93–101) have studied the effects of parental environment on seed dormancy
in
Goodenia fascicularis, a species that has potential for use
in re-vegetation programmes. In
G. fascicularis dormancy is
broken by stratification in warm and damp conditions. Germination
itself has a strong requirement for light. Thus, seeds that
are shed in spring and which lie on or near the soil surface
delay germination until the more favourable autumn conditions
develop. In investigations of the effects of parental growth
conditions plants were grown under two temperature regimes,
39/21 °C and 26/13 °C. In both regimes, some
plants were well-watered while others had a limited water supply.
The effects of temperature on growth were readily apparent:
plants in the cooler environment were taller, with greater above-ground
biomass. They set seed later and produced fewer seeds than in
the warmer environment. However, seeds from the plants in cooler
conditions had a higher level of viability. Seeds from both
sets of plants were, as expected, dormant at maturity; however,
those from the plants grown under warmer conditions were less
dormant in that it took shorter periods of warm stratification
to break dormancy. Water supply also had effect: in both temperature
regimes, the water-restricted plants produced fewer, less-dormant
seeds. Seed biology is thus subtly affected by maternal environment,
exhibiting a physiological fine-tuning rather than a dramatic
change in behaviour.
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- EDITORIAL
- Mike Jackson
Ann Bot 2008 102: 1-2.
[Extract]
[Full Text]
- Characteristic Thickened Cell Walls of the Bracts of the ‘Eternal Flower’ Helichrysum bracteatum
- Kuniko Nishikawa, Hiroaki Ito, Tatsuya Awano, Munetaka Hosokawa, and Susumu Yazawa
Ann Bot 2008 102: 31-37.
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