Annals of Botany 2008 101(9):NP; doi:10.1093/aob/mcn079
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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
The answer is blowing in the wind
Plants that colonize
new and/or transient habitats may find themselves ‘out
on their own’ with very few individuals of the same species
in the vicinity. Opportunities for outbreeding are then very
restricted and it is therefore not surprising that many colonizing
and pioneer species are self-fertile. Indeed, so common is this
correlation that it is often assumed that all such species are
mainly inbreeders. However, it is dangerous to make unsupported
assumptions of this kind, as nicely shown by
Friedman and Barrett (Toronto, Canada, pp. 1303–1309) in respect of the North American annual
Ambrosia artemisiifolia.
The authors' experiments were models of efficiency and clarity.
Firstly, they grew plants in arrays of differing densities.
Planting density had some effect on the amount of pollen received:
plants grown at the highest density received the most pollen.
However, even at the lowest densities, enough pollen was received
to achieve good seed set. Study of multi-locus allozyme markers
in the progeny showed that at all planting densities
A. artemisiifolia behaved as an obligate outbreeder; outcrossing rates were at
or very close to 1·0. The predominance of outbreeding
was confirmed by the very poor seed set observed in isolated
plants. Secondly, when plants were pollinated by hand, stigmatic
surfaces were receptive to both self and non-self pollen. However,
the self pollen either failed to germinate or, if it did germinate,
the pollen tube did not penetrate very far down the style. Indeed,
the stylar tissue exhibited a typical self-incompatibility reaction,
namely the synthesis of callose, a β1-3 glucan. So, how
does an obligate outbreeder act as a colonizing species? The
authors suggest two main factors. The first is the ability to
deposit seed banks, providing a long-lived potential source
of colonizing individuals. The second is the production of very
large amounts of wind-blown pollen, maximizing the chance of
outbreeding except for completely isolated plants.
Salt stimulates Suaeda seeds
The requirement for
cold to break seed dormancy is common in species that inhabit
cool temperate regions. However, low temperature is not the
only environmental feature encountered by seeds. This is clearly
exemplified by the salt-marsh plant
Suaeda maritima, as discussed
by
Wetson et al. (Universities of Sussex, UK and Catania, Italy, pp. 1319–1327).
In a typical winter dormancy period of 20 weeks the seeds certainly
do experience low temperatures but they are also exposed to
varying levels of salinity and hypoxia. The question here is,
how do these other factors affect dormancy? From a very extensive
study we can focus on only a selection of the results. Seeds
pre-treated dry at 4 °C and then set to germinate on filter
paper soaked in 50 % artificial sea-water (ASW) exhibited a
minimum temperature for germination of 15 °C. All subsequent
germination experiments were carried out under a day/night temperature
regime of 15/5 °C, equivalent to conditions during the germination
season in the plant's local habitat. Pre-treatment conditions
were then compared. The previously used conditions, dry/4 °C,
led to approx. 45 % germination; seeds pre-treated dry at 17
°C did not germinate. Low temperature is thus important.
However, dry exposure to –18 °C for 20 weeks killed
the seeds, although a 2-week exposure to this temperature late
in the dormancy period led to some seeds germinating. Salinity
also had a major effect: pre-treatment at 4 °C in ASW caused
nearly 100 % germination whereas hydration with distilled H
2O
at 4 °C did not break dormancy. Further, seeds stored dry
for 12 weeks at 4 or 17 °C and then transferred to ASW at
the same temperatures for 2 weeks exhibited 40–50 % germination;
thus a relatively short-term exposure to damp saline conditions
is enough for at a least partial dormancy breakage even at 17
°C. The salt effect was mediated osmotically rather than
via specific ions; polyethylene glycol at equivalent osmotic
potentials having the same effects as ASW.
Attractive bodies energize ants
Relationships between
plants and animals are many and varied. Pollination and seed
dispersal are obvious examples but there are others. Thus,
Buono et al. of Bel Horizonte, Brazil (pp. 1341–1348) discuss mutualistic relationships in which ants help to protect
plants from herbivory by other invertebrate species, i.e. the
employment of one type of animal to ward off attacks by other
animals. At their most developed, plant–ant relationships
involve ants living in or on the plants; such plants are known
as myrmecophytes. The ants obtain much of their nutrition from
food bodies (FBs) in which proteins, lipids and carbohydrates
are deposited. In many of these species, FBs are modified trichomes
but they can also be derived from other cell types, including
epidermis and parenchyma. There are also forms of plant–ant
mutualism in which ‘plants... offer food to ants but not
shelter’; these plants are classified as myrmecophyles.
The production of FBs is known to occur in several angiosperm
families but had not been previously seen in the Rhamnaceae.
However, Buono
et al.'s observations on
Hovenia dulcis, a tree
that is native to Asia, add that family to the list. The authors
observed multi-cellular FBs on the abaxial face, especially
alongside the midrib and second- and third-order veins, of leaves
in trees of all ages from post-seedling (20 cm high) to
immediately before the reproductive phase. Ants of the genera
Camponotus and
Crematogaster were observed visiting the plants,
breaking the FBs from the leaves and transporting them away.
FBs in this species can be derived from several tissues, including
epidermis and parenchyma. The epidermis of the FB does not store
nutrients but may act as a protective layer for the nutrient-storing
FB parenchyma cells. Analysis of the FBs showed that the main
storage compounds are starch and lipids, the latter mainly consisting
of glycerin esters of fatty acids, giving a very ‘energy-rich’
mix for the ants.
The proof of the postulate is in the eating
An obvious disadvantage
of being rooted to the spot is the inability to run away from
predators. Plants have thus evolved many different forms of
defence against herbivores. The role of tough leaves in defence
has been studied by a large multinational team working in tropical
lowland rain forests in several different locations who have
produced two comprehensive and informative papers
(Dominy et al., pp. 1363–1377 and Grubb et al., pp. 1379–1389).
Space does not permit a discussion of all their results and
so we focus on the relationship between herbivory and leaf toughness
in shade-tolerant monocots and dicots. Leaf toughness was measured
as punch-strength using a penetrometer or as fracture-resistance
using automated scissors. Herbivory was assessed by measuring
loss of leaf area and by direct presentation of leaves to potential
predators. Monocot leaves were tougher than dicot leaves at
all stages of development; indeed it was especially noted that
monocot leaves can be tough during the expansion phase, a phenomenon
not seen in dicots. Further, the authors invite us to broaden
our mental picture of ‘tough leaves’. Although it
is true that in some monocot groups, such as palms, tough leaves
fit the stereotype of being stiff with relatively low water
content, others (mainly seen in the Zingerales) have large,
non-stiff leaves with high water content that quickly roll up
in dry conditions. There is a strong negative correlation between
toughness (measured by either technique) and the extent of herbivory.
This is reflected in the field observations made by the authors:
monocots were much less prone to herbivory than dicots; for
both groups, losses were mainly confined to the leaf expansion
phase. However, the extent to which leaf toughness is involved
in protection is variable and the authors challenge the botanical
community to undertake further studies of herbivory in relation
to monocot and dicot abundance in lowland tropical rain forests.
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Related articles in Ann Bot:
- High Outcrossing in the Annual Colonizing Species Ambrosia artemisiifolia (Asteraceae)
- Jannice Friedman and Spencer C. H. Barrett
Ann Bot 2008 101: 1303-1309.
[Abstract]
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- Do Conditions During Dormancy Influence Germination of Suaeda maritima?
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Ann Bot 2008 101: 1341-1348.
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