Annals of Botany 2008 102(5):NP; doi:10.1093/aob/mcn197
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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
Orchestrating organs? Roots and stems not always in harmony
Roots are often
described as the forgotten parts of a plant, recalling the old
saying ‘
out of sight, out of mind’. And yet roots
play vital roles in plant life, ranging from anchorage to nutrient
absorption. Despite this, we know little about the timing and
co-ordination of annual growth and development of roots in perennial
plants. Thus,
Thibeault-Martel et al. (Québec, pp. 667–674) have studied the annual pattern of root and shoot cambial activity
and xylem formation in two gymnosperms,
Abies balsamea and
Picea mariana. At weekly intervals in the period May to November in
three successive years, wood microcores were collected from
roots and stems and were studied by standard histological techniques.
In any one year, cambial activity showed the same pattern in
roots and stems: cambial cell numbers in the cross-sections
increased from May to June/July and then gradually decreased
until late August/early September. However, initiation of xylem
differentiation was not always strictly co-ordinated between
root and stem. It occurred at the same time in both organs in
2004 and 2005 (although the actual timing differed between the
two years), but in 2006 xylem differentiation in the root started
a week later than in the stem. Wall thickening and the subsequent
formation of tracheary elements was initiated in June with the
stem always preceding the root. The end of xylem formation,
indicated by lignification, occurred at the same time (late
September) in both shoots and roots in 2004 and 2005, but in
2006 was 22 days later in roots than in stems. There is not
a strict linkage between the roots and shoots in respect of
renewal of cambial growth and differentiation. The authors suggest
that this indicates a lack of dependence on auxin transported
basipetally from young shoots: perhaps auxin is already available
in the dormant cells.
Flowers on the floor – rodents rewarded and seeds set
Amongst the very
diverse range of pollination mechanisms, pollination by rodents
has been observed previously in two different South African
ecosystems.
Kleizen et al. (Rondebosch and Pietermaritzburg, SA, pp. 747–755) now postulate that two
Colchicum species,
C. scabromarginatum and
C. coloratum, components of the Succulent Karoo ecosystem,
are also rodent-pollinated. The hypothesis is based on the geophyte
growth habit, floral morphology and colouring, copious nectar
production and nocturnal odour secretion. In the field, insects
were never observed visiting these plants. Hand-pollination
experiments showed that
C. scabromarginatum is self-infertile
while
C. coloratum exhibits low self-fertility. Placing vertebrate-excluding
cages round the plants resulted in a 97 % reduction in
seed set in
C. scabromarginatum and 82 % in
C. coloratum. By
contrast, exclusion of vertebrates from the insect-pollinated
C. hantamense affected seed-set only very slightly. Live-trapping
for four nights in a population of
C. scabromarginatum resulted
in the capture of eight individual rodents, all
Aethomys namaquensis (namaqua rock mouse). In three nights of live-trapping in a
population of
C. coloratum, 28 individual rodents were trapped,
representing three mouse and one gerbil species. Pollen was
present on the snouts and in the faeces of these rodents; for
A. namaquensis, only
C. scabromarginatum pollen was identified.
From the rodents associated with
C. coloratum, almost all of
the pollen belonged to that species, although one other unidentified
type was present in very small amounts. In order to make direct
observations of rodent behaviour, individual rodents were released
into large glass tanks containing flowers of
C. scabromarginatum or
C. coloratum. Flowers were visited at around midnight; pollen
transfer occurred as the rodents lapped nectar. Flowers of
C. hantamense and
Oxalis placed in the same tanks were ignored.
These data confirm the authors' hypothesis that these two
Colchicum species are rodent-pollinated. However, one population of
C. coloratum was also visited by three bird species, which were
observed to carry pollen.
It helps to have P when resisting Al
Plants are subject
to many environmental stresses, several of which interact with
each other. Thus, as discussed by
Sun et al. (Nanjing and Beijing, pp. 795–804),
Al toxicity in acid soils is often compounded by P deficiency,
leading to poor growth and, in crops, greatly reduced yields.
With this in mind, the authors have investigated the interactions
of Al and P in two legumes,
Lespedeza bicolor and
L. cuneata,
the former of which has potential as a forage crop. Here we
focus on just a part of their extensive investigation. In a
hydroponic growth system,
L. bicolor was twice as resistant
to Al as
L. cuneata, indicated by the effects on root elongation.
Interaction between P deficiency and Al toxicity was studied
by exposure on alternate days to P and Al. Measurement of root
Al contents suggested that
L. bicolor was more capable of excluding
Al than
L. cuneata: over a range of external Al concentrations,
roots of
L. cuneata contained nearly twice as much Al as roots
of
L. bicolor. The ability of
L. bicolor to exclude Al was enhanced
by pre-treatment with P, whereas the pre-treatment had no effect
on accumulation of Al in roots of
L. cuneata. However, the ability
of
L. bicolor to exclude Al was not directly related to the
extrusion of chelating acids. Although
L. bicolor roots extruded
malate and citrate while those of
L. cuneata did not, extrusion
in
L. bicolor was decreased after supplying P. There were also
related effects on root morphology: Al inhibited root growth
and especially lateral root formation much more in
L. cuneata than in
L. bicolor; these effects in
L. cuneata were not reversed
by supplying P. In respect of reciprocal interactions, in the
Al-resistant
L. bicolor, Al had little effect on the transport
of P from root to shoot, but strongly inhibited P transport
in
L. cuneata.
Copious carbon fails to influence sink so leaves linger on as usual
In spring ephemerals
of northern temperate woodlands, aerial shoots emerge when conditions
become warm enough, e.g. after the snow has melted; flowering
and seed set are completed before the canopy closes over. During
the same period, the underground perennial organs are replenished
and then the aerial shoots die back. It had been assumed that
shoot senescence was initiated by canopy closure but recently
it has been suggested that completion of sink filling in the
underground perennial organ is the main factor. This has been
tested by
Gutjahr and Lapointe at Québec (pp. 835–843),
working with
Erythronium americanum. The authors point out the
advantages of this plant as a subject of study: non-flowering
individuals consist of a single leaf and a single bulb; the
root system develops during the cold stratification period before
shoot emergence. Non-flowering plants were grown under ambient
(400 ppm) or elevated (1100 ppm) CO
2 concentrations.
As expected, plants grown at 1100 ppm CO
2 exhibited a significantly
higher net assimilation rate and therefore fixed more C than
those grown at 400 ppm. Despite this, there were no differences
in bulb-filling rates, nor in the final size of the bulbs. This
was reflected in the lack of difference in both bulb cell number
and cell size between the two treatments. Similarly, starch
deposition and final starch content were very similar in the
two treatments. Neither were there differences in leaf dry weight
and area, nor in leaf growth period. However, there was one
major difference: both leaves and bulbs of plants grown under
elevated CO
2 exhibited much higher respiration rates than control
plants. Overall then, elevated CO
2 concentrations do not affect
the sink (bulb) because of the over-riding controls of cell
size and cell division, and it remains possible that leaf senescence
is indeed tied to the filling of the sink. It appears that the
extra carbon fixed under elevated CO
2 levels is simply burned
off.
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