ASPECTS OF RECALCITRANT SEED PHYSIOLOGY1
N.W. PAMMENTER2 AND PATRICIA BERJAK2
School of Life and Environmental Sciences, University of Natal, Durban, South Africa
ABSTRACT - Recalcitrant seeds show wide variation in water content at shedding and post-harvest
physiology, particularly response to desiccation. But all are metabolically active and show germinationassociated changes in storage - it is these properties that underlie the phenomenon of recalcitrance. Three
types of damage on drying are envisaged: mechanical damage associated with reduction of cellular
volume; aqueous-based oxidative degradation, consequent upon unregulated metabolism, which occurs at
intermediate water contents; and biophysical damage to macromolecular structures that occurs on removal
of water at low water contents. The first type of damage probably occurs only in seeds with highly
vacuolated cells, such as Avicennia marina, and so is probably not common. Under normal slow-drying
conditions it is the second type of damage that kills recalcitrant seeds, thus the response to dehydration
depends upon the metabolic activity of the seed and the rate of drying. This makes it difficult to measure
desiccation tolerance - there is no such thing as a “critical water content” that is characteristic of a species
(except the absolute minimum water content that can be tolerated on the most rapid drying possible).
Probably it is the third type of damage that kills tissues that have been dried very rapidly. Desiccation
sensitivity is probably the ancestral state, with tolerance having evolved independently a number of times,
and recalcitrance does place constraints on the regeneration niches open to species producing such seeds.
ADDITIONAL INDEX TERMS: Seed storage, desiccation tolerance, water content.
ASPECTOS DA FISIOLOGIA DE SEMENTES RECALCITRANTES
RESUMO - Sementes recalcitrantes possuem uma ampla variabilidade no conteúdo de água na hora da
dispersão e também muito variabilidade na fisiologia pós-colheita, especialmente com relação à resposta à
dessecação. Mas todas são metabolicamente ativas e mostram alterações associadas à germinação
enquanto estão armazenadas – são estas as propriedades que formam a base do comportamento recalcitrante.
Três tipos de injúria com dessecação são visualizados: injúria mecânica associada à redução do volume
celular; degradação oxidativa em solução aquosa, decorrente do metabolismo desregulada que ocorre em
conteúdos intermediários de água; e dano biofísico a estruturas macromoleculares que ocorre quando se
remove água em conteúdos muito baixos. O primeiro tipo de injúria provavelmente ocorre somente em
sementes com células altamente vacuoladas, como as de Avicennia marina, e não deve ser muito comum.
Em condições normais de secagem lenta, é o segundo tipo de injúria que leva à morte das sementes
recalcitrantes, assim a resposta à desidratação depende da atividade metabólica da semente e da taxa de
dessecamento. Desta maneira é difícil medir a tolerância à dessecação – não existe um “conteúdo crítico
de umidade” que caracteriza a espécie (a não ser o conteúdo mínimo absoluto que possa ser tolerado com
1. Based on an invited lecture presented at the VII Brazilian Plant Physiology Congress, Brasília, July, 1999, in the Coordinated
Session on Seed Physiology
2. School of Life and Environmental Sciences, George Campbell Building, University of Natal, Durban, 4014 South Africa.
E-mail: pammenter@biology.und.ac.za.
57
Aspects of recalcitrant ...
a secagem o mais rápido possível). Provavelmente é o terceiro tipo de injúria que é letal para tecidos que
foram dessecados muito rapidamente. A sensitividade à dessecação provavelmente é o estado ancestral, e
a tolerância evoluiu várias vezes, de maneira independente. Existem limitações quanto aos nichos
ecológicos nos quais as espécies que produzem sementes recalcitrantes podem se regenerar.
TERMOS ADICIONAIS PARA INDEXAÇÃO: Armazenamento de sementes, conteúdo de umidade,
tolerância à dessecação.
INTRODUCTION
PROPERTIES OF WATER IN SEEDS
Orthodox seeds are seeds that acquire
desiccation tolerance during development, can dry
to low water contents (generally less than 5%), and
retain viability in the dry state for predictable
periods. Recalcitrant seeds, on the other hand, are
shed at high water contents, ranging from 0.4 to
4.0 g water per g dry matter (g/g), are sensitive to
desiccation, and are also metabolically active on
shedding.
A characteristic of recalcitrant seeds is
the variability they show among species and within
a species. They vary in the water content at
shedding, the extent of dehydration they tolerate,
their response to drying rate, storage lifespan in the
hydrated state (from a week or two, to two or three
years), and their response to low temperatures (for
some examples see King and Roberts, 1980;
Farrant et al., 1985, 1986, 1989; Hong and Ellis,
1990; Pritchard, 1991; Berjak et al., 1992, 1993;
Finch-Savage, 1992; Tompsett, 1992; Xia et al.,
1992; Tompsett and Pritchard, 1993, 1998). This
means that it is not simply a case of classifying a
seed as orthodox or recalcitrant, but within the
recalcitrant group there is a wide spectrum of
behaviours, from minimally recalcitrant seeds with
a relatively long lifespan and quite tolerant of
desiccation, to maximally recalcitrant, with short
lifespans and very sensitive to desiccation (Farrant
et al., 1988).
Before considering the response of
recalcitrant seeds to the loss of water, we should
consider the properties of water in seed tissue (see
Vertucci, 1990; Vertucci and Farrant, 1995;
Walters, this volume). Based on calorimetric
properties, Vertucci (1990) has identified five types
of water or levels of hydration in seed tissues. The
different hydration levels correspond roughly to
the water contents shown in Table 1, and the water
potentials at the boundaries are also indicated. At
the different hydration levels the tissue water has
different physical properties. At high water
contents and high water potential the water has the
properties of water in dilute solution. As the water
content is decreased the water takes on the
properties of water in concentrated solution, where
the interaction between water and solutes becomes
stronger, and the system deviates from "ideal"
behaviour. On the removal of more water, the
solution becomes so concentrated that it becomes
viscous and has the properties of a glass. Finally, at
very low water contents, those characteristic of
orthodox seeds, all the remaining water is tightly
associated with macromolecular surfaces; its
mobility is reduced and it constitutes the so-called
'bound' water.
At the different hydration levels, because
the thermodynamic properties of water change,
different metabolic processes can take place
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Pammenter and Berjak
TABLE 1 - The properties of water in seed tissue and different hydration levels. The approximate water
content range, water potential corresponding to the boundaries, and the type of metabolic activity that
occurs are also given. Adapted from Vertucci and Farrant (1995).
Hydration
I
II
III
IV
V
<0.08
0.08 - 0.25
0.25 - 0.45
0.45 - 0.70
> 0.70
Level
Water
Content
(g/g)
Water
potential
(MPa)
State of
water
Activity
-150
bound
-11/-14
-3
-1.5
glass/
hydrophilic
glass/
hydrophobic
Concentrated
solution
Dilute
Solution
low level
catabolic
respiration
protein and
nucleic acid
synthesis
cell division:
germination
(Vertucci and Leopold, 1986; Vertucci, 1989;
Vertucci and Farrant, 1995). At the high water
contents, full normal metabolism occurs and seeds
can germinate. In hydration level IV, protein and
nucleic acid synthesis, together with respiration, is
possible, but there is inadequate water for cell
growth and germination. At lower water contents
protein and nucleic acid synthesis are not possible,
but some respiration can be detected. At even
lower water contents only low level catabolic
events occur slowly.
DEVELOPMENT IN ORTHODOX AND
RECALCITRANT SEEDS
Before considering the response of
recalcitrant seeds to water loss, it would be useful
to consider the processes occurring when orthodox
seeds undergo maturation drying. As the seeds dry,
insoluble reserves accumulate, the volume of
water-filled vacuoles is reduced, and protective
molecules are synthesized. These modifications are
followed, or accompanied, by the de-differentiation
of highly structured organelles, particularly
mitochondria, and a general 'switching-off' of
metabolism,
until
finally
the
protective
mechanisms such as vitrification and the
deployment of Late Embryogenic Abundant (LEA)
proteins become operative. Throughout the drying
process, presumably, anti-oxidant mechanisms are
operative. There are limited data from orthodox
seeds, but certainly anti-oxidant activity is
important during drying of desiccation-tolerant
resurrection" plants (reviewed by Oliver and
Bewley, 1997).
To summarise, during the later stages of
development of orthodox seeds the processes of
reserve accumulation, reduction of vacuolar
volume, de-differentiation and shut-down of
metabolism accompany the acquisition of
desiccation tolerance. The question then is whether
these processes are necessary for the acquisition of
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59
Aspects of recalcitrant ...
tolerance, and whether and to what extent they
occur in recalcitrant seeds.
To answer this question we studied the
development of three very different seed types:
Avicennia marina, which is a mangrove, a tropical
wetland species which produces highly
recalcitrant seeds; Aesculus hippocastanum, a
temperate
terrestrial
species
producing
recalcitrant seeds; and Phaseolus vulgaris, an
agricultural species that produces orthodox seeds
(Farrant et al., 1997). We studied these seeds at
three developmental stages: immediately post
histodifferentiation (stage 1), in the middle of
the reserve accumulation phase (stage 2), and at
the end of reserve accumulation (stage 3; P.
vulgaris seeds were still at a high water content at
this stage; it was before maturation drying).
Table 2 shows the desiccation sensitivity
of seeds of the three species at the three
developmental stages (measured as the water
content at which the viability of rapidly dried
excised axes was lost). Avicennia marina is very
sensitive and tolerance did not change from stage 2
to stage 3. Aesculus hippocastanum showed an
increase in tolerance with development, but at
shedding was still sensitive. Phaseolus vulgaris
increased in tolerance with development, and during
maturation drying became even more tolerant,
achieving water contents of less than 0.08 g/g without
viability loss. Thus there is a sequence of
sensitivities: Avicennia > Aesculus > Phaseolus.
Figure 1a shows the contribution of
vacuoles to cell volume for the three stages
averaged over the cotyledon, hypocotyl and root
meristem. We see quite clearly that Avicennia
marina, the most sensitive species, was highly
vacuolated, and this did not change with
development, while the more tolerant species
showed a decline in vacuolation with development.
The inverse pattern is seen with insoluble reserves
(Figure 1b): Avicennia marina, the most sensitive
species, stores its reserves as soluble sugars, while
the more tolerant species accumulated insoluble
reserves during development.
The metabolic activity of the seeds is also
pertinent. In the orthodox Phaseolus vulgaris there
was a decline in the contribution of mitochondria
to cell volume with development, and by the end of
reserve accumulation, mitochondria were virtually
absent (Figure 1c). Although there was a decline in
seeds of Aesculus hippocastanum, by the time
the seeds were shed mitochondria still
contributed significantly to cell volume.
However, it is not only the quantitative
contribution of mitochondria that is important
to metabolic activity, it is also the qualitative
degree of differentiation.
TABLE 2 - Desiccation sensitivity of Avicennia marina, Aesculus hippocastanum and Phaseolus vulgaris
at three stages of development, as measured by the water content (g/g) of isolated embryonic axes at
which viability was zero after drying as rapidly as possibly. [Stage 1: post-histodifferentiation; stage 2: in
the middle of reserve deposition; stage 3: at the end of reserve deposition (prior to maturation drying in
the case of P. vulgaris)].
Stage
A. marina
A. hippocastanum
P. vulgaris
1
0.5
3.5
1.5
2
0.5
0.8
1.5
3
0.5
0.3
0.3
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Pammenter and Berjak
60
a
stage 1
stage 2
stage 3
Area Vacuoles (%)
50
40
30
20
10
0
Area Reserves (%)
60
b
50
40
30
20
10
0
Area Mitochondria (%)
2,0
c
1,5
1,0
0,5
P.
vu
lg
ar
is
A.
hi
pp
oc
as
ta
nu
m
A.
m
ar
in
a
0,0
FIGURE 1 - The contribution of vacuoles (a), insoluble reserves (b) and mitochondria (c) to cells of
seeds of Avicennia marina, Aesculus hippocastum and Phaseolus vulgaris at three developmental stages
(see text for details). Contribution was estimated as the percentage of cell cross-sectional area occupied
by each type of organelle, averaged over the cotyledons, hypocotyl and root meristems.
R. Bras. Fisiol. Veg., 12(Edição Especial):56-69, 2000
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Aspects of recalcitrant ...
Transmission
electron
microscopy
showed that the mitochondria of the root
meristem cells of Aesculus hippocastanum were
highly differentiated and had the appearance
of active mitochondria, while those in Phaseolus
were de-differentiated and appeared inactive
(see Berjak and Pammenter, this volume).
Measured respiration rates support the
ultrastructural observations, remaining high in
the recalcitrant seeds Avicennia marina and
Aesculus
hippocastanum
throughout
development, while the respiration rates of the
orthodox seeds of Phaseolus vulgaris were low,
even though the seeds had not gone through
maturation drying and were still hydrated
(Table 3).
Thus, the most desiccation sensitive
species is the one that remained highly vacuolated,
supporting the concept that drying of highly
vacuolated material leads to mechanical damage.
Also, reduction in metabolic rate and dedifferentiation appear to be prerequisites for
survival of dehydration. A characteristic of all
recalcitrant seeds we have studied is that they are
metabolically active; they show high rates of
respiration relative to orthodox seeds, and have
ultrastructure that is characteristic of metabolically
active tissue.
In summary, orthodox seeds show a
fixed developmental pattern in that, after reserve
accumulation, they acquire desiccation tolerance
and undergo maturation drying. During the
development of most recalcitrant seeds there is
some decline in water content (but mostly
because dry matter accumulates faster than
water, rather than water loss) and some
increase in desiccation tolerance. It has been
suggested that recalcitrant seeds
show
an
indeterminate developmental pattern in that
development is truncated and the seeds are shed
before desiccation tolerance is fully acquired (e.g.
Finch-Savage and Blake, 1994). Certainly, the
lethal water content that one measures depends
upon the developmental status of the seed.
Post-shedding development also has an
effect on desiccation sensitivity. The longer
seeds are stored, the more sensitive they become
(Farrant et al., 1986; Berjak et al., 1992, 1993).
We attribute this observation to the fact that
germination is initiated during storage (thus
metabolism is increasing), and suggest that the
effect of developmental stage on desiccation
sensitivity is related to metabolic rate.
Throughout pre-and post-shedding development,
desiccation sensitivity changes in parallel with
metabolic rate.
TABLE 3 - Respiration rates (nmol O2 (g dry mass)-1 s-1) of cotyledons and embryonic axes of Avicennia
marina, Aesculus hippocastanum and Phaseolus vugaris at three stages of development. [Stage 1: posthistodifferentiation; stage 2: in the middle of reserve deposition; stage 3: at the end of reserve deposition
(prior to maturation drying in the case of P. vulgaris)].
A. marina
A. hippocastanum
P. vulgaris
Stage
Cotyledons
Axes
Cotyledons
Axes
Cotyledons
Axes
1
5.4
-
5.0
5.3
3.5
-
2
4.5
-
3.9
5.0
0.9
1.8
3
4.6
-
3.0
4.7
0.2
1.1
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Pammenter and Berjak
OTHER PROTECTION MECHANISMS
During the final stages of development of
orthodox seeds there is the reduction in the volume
of water-filled vacuoles, the de-differentiation of
organelles and the shut down of metabolism that
are necessary for desiccation tolerance. However,
other processes, involving particularly Late
Embryogenic Abundant (LEA) proteins and
sugars, also occur. LEA proteins accumulate
during the later stages of embryogenesis in
orthodox seeds (Galau et al., 1986, 1991).
These proteins are similar to the
dehydrins produced on drying of vegetative
tissue. In fact, dehydrins are a subset of LEAs.
These proteins are thought to have a
protective role in the dry state, or certainly to
contribute in some way to desiccation tolerance
(reviewed by Oliver and Bewley, 1997;
Kermode, 1997), although details of their mode
of action are not yet clear.
LEAs have been shown to be present in
quite a wide range of recalcitrant seeds, mostly
temperate (Bradford and Chandler, 1992; FinchSavage et al., 1994a), but also some tropical
species (Gee et al., 1994; Farrant et al., 1996).
However, they are not universally present in all
recalcitrant seeds; they are certainly absent from
the axes of several tropical wetland species
(Farrant et al., 1996). It is possible that the
presence of LEAs can contribute to the slightly
greater desiccation tolerance and chilling tolerance
of some recalcitrant species.
If sucrose and oligosaccharides such as
raffinose are present in the correct ratios, then a
glass can be formed as water is lost. This state is
highly viscous, which should slow down any
chemical processes occurring in the seed, thus
providing protection on desiccation (Williams and
Leopold, 1989; Leopold et al., 1994). Sugars,
possibly in the appropriate mass ratios, occur
in many recalcitrant seeds (Farrant et al., 1993;
Finch-Savage and Blake, 1994; Lin and Huang,
1994; Steadman et al., 1996), and it is possible that
glasses may form on dehydration. However, these
glasses, if they do form, are obviously not effective
protectants, as dehydration does kill recalcitrant
seeds.
To summarise, some species of
recalcitrant seeds do have the appropriate sugars
and some have dehydrins. However, it should be
pointed out that these mechanisms are thought to
operate and to provide protection at low water
contents. On dehydration recalcitrant seeds die at
water contents higher than those at which these
protective measures are thought to operate.
Whatever the role of LEAs and sugars may be in
orthodox seeds, they are not pertinent to
recalcitrant seeds.
It is now relatively well established that
treatments which disturb normal metabolism can
generate free radicals, and protection against
these free radicals by anti-oxidant systems is
essential for survival. Highly active anti-oxidant
systems are certainly very important in the
protection mechanisms of resurrection plants,
both on drying and during rehydration (Oliver
and Bewley, 1997). Although not a lot of work
has been done, these mechanisms are present in
orthodox seeds.
During the drying of recalcitrant seeds,
stable free radicals do accumulate (Hendry et al.,
1992; Finch-Savage et al., 1993, 1994b). There is
evidence that anti-oxidant systems also fail
(Hendry et al., 1992; Finch-Savage et al., 1993), or
they may not be present at sufficient levels.
Oxidative damage probably does occur during
drying.
There have been some new suggestions
concerning the mechanisms of desiccation
tolerance (Hoekstra et al., 1997; Golovina et al.,
1998). These authors postulate the existence of
amphipathic molecules that become integrated into
the membrane as water is lost. It is suggested that
these molecules lower the water content at which
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63
Aspects of recalcitrant ...
lipid phase changes occur and thus possibly
stabilize the integral proteins in the membranes.
This is recent work and so far no information is
available with regard to the presence of these
molecules in recalcitrant seeds.
It is apparent that there is a suite, a
collection, of mechanisms that are thought to
protect against desiccation damage. Some of these
protective mechanisms probably also operate
during rehydration, as well as during drying. The
different protective mechanisms seem to operate in
different water content ranges. All these
mechanisms must be operational for desiccation
tolerance. Recalcitrant seeds lack some or all of
these mechanisms. It is not apparent whether there
is any relationship between the degree of
recalcitrance of a seed and the extent to which it
has or expresses some of these mechanisms.
THE IMPORTANCE OF DEHYDRATION
RATE
Dehydration rate is perhaps one of the
most confusing issues in recalcitrant seed
physiology. When whole seeds are dried relatively
slowly - over several days - viability is generally
lost at embryonic axis water contents in the range
of 1.0 - 0.5 g water/g dry mass (50 - 35% wet mass
basis). However, if isolated embryonic axes are
dried rapidly (minutes to hours) they survive to
much lower water contents - 0.45 to 0.25 g/g - and
possibly even lower (e.g. Pammenter et al., 1991).
Thus the effect of dehydration depends upon the
rate of dehydration. This makes it extremely
difficult to assess or measure desiccation tolerance
- the response depends upon how the tissue is
dried. However, this approach could be criticised
because of the experimental protocol; the effect
could be due to different drying rates, or it could
be due to the fact that different tissues were used to
achieve the different drying rates (embryonic axes
for rapid drying and whole seeds for slow drying);
thus for rapid drying the embryonic axis has been
removed from the influence of the rest of the seed.
To resolve this question we undertook
studies on whole seeds of Ekebergia capensis
(Pammenter et al., 1998). This species produces
seeds about the size of a peanut with a hard
endocarp. If the endocarp is removed the seeds
can be dried by burying in silica gel to an axis
water content of around 0.2 g/g (dry mass basis)
in about 48 h (Figure 2a). If the endocarp is left
intact it takes about 10 d for axis water content
to reduce to 0.5 g/g (Figure 2b). We also
manipulated
the
drying
technique
to
achieve what we describe as an intermediate
drying rate.
The response of final germination to
water content using the three drying treatments is
seen in Figure 3. Seeds that were dried slowly
started to lose viability at an axis water content of
about 1.3 g/g and were all dead at 0.6 g/g. Seeds
that were dried rapidly retained 90% viability to a
water content of 0.5 g/g, and were all dead at 0.2
g/g. Seeds dried at an intermediate rate showed an
intermediate response to desiccation. The rate of
drying does have a marked effect on the water
content tolerated; the effect is not one of removing
the axis from the seed. Why should this be the
case?
The cells of embryonic axes of the seeds
dried at different rates showed very different
ultrastructural responses to dehydration (see Berjak
and Pammenter, this volume). In seeds that were
dried slowly to the extent that considerable
viability loss occurred, there was advanced
degradation of membrane structures, particularly
in the plastids, and an abnormal appearance of
the lipid bodies, particularly a darkening of the
surface. Seeds that had been dried rapidly to a
slightly lower water content, but still were mostly
viable, generally showed well preserved
membranes and nuclei. Although membrane
damage did ultimately occur on rapid drying, there
R. Bras. Fisiol. Veg., 12(Edição Especial):56-69, 2000
64
Pammenter and Berjak
was very marked degradation at high water
contents in slowly dried material. This
ultrastructural evidence suggests that different
deteriorative processes may be occurring at the
different drying rates.
To understand the effect of drying rate
we have to appreciate that some damaging
processes are aqueous based. Slowly dried
material will spend a long time at the
intermediate water contents where damage from
these aqueous based processes can accumulate.
However, if material is dried rapidly, only a
short time is spent at these intermediate water
contents, so little damage accumulates. Thus the
faster the drying, the less damage that
accumulates and the lower the water content that
can be tolerated. However, there is a lower limit
- no matter how fast recalcitrant seeds are dried,
they cannot tolerate the low water contents
typical of orthodox seeds.
2,0
Axis water content (g/g)
a
1,5
1,0
0,5
0,0
0
20
40
60
80
Drying time (h)
Axis water content (g/g)
2,5
b
slow
intermediate
2,0
1,5
1,0
0,5
0,0
0
2
4
6
8
10
Drying time (d)
FIGURE 2 -The drying time course of embryonic axes of Ekebergia capensis when whole seeds were
dried either rapidly (4a) or at an intermediate or slow rate (4b).
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Aspects of recalcitrant ...
Germination (%)
100
80
60
40
rapid
slow
intermediate
20
0
0,0
0,5
1,0
1,5
2,0
2,5
Axis water content (g/g)
FIGURE 3 - Final germination of seeds of Ekebergia capensis dried at different rates to a range of axis
water contents.
We visualise two types of damage that seed desiccation response as a character state fails can occur during drying. As a consequence of both tolerance and sensitivity occur in all major
disturbed metabolism, free radical-mediated clades of extant seed plants. However, it is
oxidation processes can proceed. These reactions are probable that sensitivity is the ancestral state, but
aqueous-based and occur at intermediate water tolerance evolved independently a number of times
contents. The protection mechanisms against this
(von Teichman and van Wyk, 1994; Pammenter and
type of damage are the switching-off of metabolism,
de-differentiation of organelles, and efficient anti- Berjak, 2000). This raises the interesting question as
oxidant systems. The second type of damage we to whether the mechanisms of tolerance are
envisage is due to the biophysical disruptions caused necessarily the same in all seed species.
by the removal of water from macromolecular and
ECOLOGICAL CONSIDERATIONS
membrane surfaces. These occur at low water
contents. Protection mechanisms against this type of
A characteristic of recalcitrant seeds is
damage include LEAs, vitrification by sugars,
and possibly the amphipathic molecules their limited lifespan, and this will place
suggested to migrate between cytosol and constraints on the range of environmental
membranes on dehydration and rehydration.
conditions in which regeneration via seeds can
occur. However, when we try to get detailed
EVOLUTIONARY STATUS OF
information on the seed ecology of species
producing recalcitrant seeds, we have a major
RECALCITRANT SEEDS
problem – in the past ecologists and seed scientists
In
vegetative
tissue
desiccation have not often communicated. Ecologists study the
sensitivity is probably the ancestral state, but composition of soil seed banks, the lifespan of
tolerance evolved independently a number of seeds in these banks, and environmental conditions
times, including more than once in the initiating germination, but often have no
angiosperms (Oliver and Bewley, 1997). Any information as to the water content of the seeds
attempt at a similar phylogenetic analysis using of interest. Seed physiologists, on the other
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Pammenter and Berjak
hand, have a good knowledge of only a few
species, mostly economically important ones
(either crop species or weedy invaders), and only
occasionally become involved in ecological
considerations.
Recalcitrant seeds are common in mesic
tropical forests. These conditions generally would
be favourable for germination and seedling
establishment, and so there has been no pressure
driving the evolution of desiccation tolerance, or
the characteristic has been secondarily lost. Seed
ecologists will tell us that seeds of climax species
of these ecosystems are not often found in the soil
seed bank – their lifespans are low, whereas seeds
of gap specialists are well represented in the soil
seed bank.
The seeds of climax species generally
show no dormancy, germinate rapidly (unless they
have a hard covering), are absent from the soil seed
bank and these species persist as a seedling bank.
They are generally large and not wind dispersed.
These are the characteristics that one would expect
from recalcitrant seeds, and it is highly probable
that most of these seed species are indeed
recalcitrant, although this has been confirmed for
only a limited number of species.
Although recalcitrant seeds are common
in mesic tropical forests, they are by no means
confined to these systems. Species producing
recalcitrant seeds do occur in more seasonal
habitats, and a number of strategies have evolved
to permit successful seedling establishment. Many
temperate recalcitrant seeds are shed in autumn
and over-winter as seeds. A necessary prerequisite
for this, of course, is chilling tolerance. Because of
the low temperatures, rates of germination will be
slow, and will not be completed before the arrival
of spring. In fact some species such as Aesculus
hippocastanum actually have a chilling
requirement for germination (Pritchard et al.,
1996). Low temperatures and general dampness
will also slow water loss, reducing the risk of death
by desiccation. As a generalisation, recalcitrant
species of temperate regions are generally more
desiccation tolerant and have longer life spans than
those of tropical origin.
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