Although it is an over-simplification, desiccation sensitivity is generally accepted as the obvious feature identifying seeds of a species as being recalcitrant. Nevertheless, there are marked differences in the degree of dehydration that recalcitrant seeds of individual species will tolerate, although the lowest water content survived depends on other parameters, especially the rate at which water is lost (see below). Comparisons of published data on individual species are often not helpful, because of the differing conditions under which dehydration was carried out. However, when recalcitrant seeds of three unrelated species, a gymnosperm (Araucaria angustifolia
), a dicotyledonous vine (Landolphia kirkii
) and an herbaceous monocot (Scadoxus membranaceous
), were dehydrated under identical conditions, a similar pattern of ultrastructural events terminating in cell lysis was recorded as occurring at markedly different water concentrations (Farrant et al., 1989
). What was also a significant observation was that the rate at which the seeds of the three species lost water was inversely related to the water concentration at which viability was lost. This highlights an important generalization, first noticed in work with A. marina
(Berjak et al., 1984
), i.e. that the more rapidly water can be lost, the lower is the water content reached before intracellular damage becomes limiting. (The important ramifications of this will be elaborated below.)
Differences in the lowest ‘safe’ water content which recalcitrant seeds will withstand are not confined to disparate genera, but have also been noted for different species of individual genera. For example, Quercus alba
seeds are more desiccation sensitive than those of Q. nigra
(Connor and Bonner, 1996
), and there are differences between seeds of species of Baccaurea
(Normah et al., 1997
). A thought-provoking finding is that seeds of different species of a single genus may be differently categorized, as exemplified by species of Acer
(Hong and Ellis, 1990
), and substantiated for Coffea
spp. by Eira et al. (1999)
who described C. liberica
seeds as being the least desiccation tolerant, while those of C. racemosa
were relatively the most tolerant. Most interesting, though, are the more recent data (explored in more detail below) indicating that seeds of A. hippocastanum
(Daws et al.
) and A. pseudoplatanus
(Daws et al.
) from different provenances differ in their reponse to dehydration. Also relevant in the context of provenance are the findings of Dussert et al. (2000)
that the relative desiccation sensitivity of seeds of different species of Coffea
appears to be related to the mean number of dry months typifying each habitat.
A further characteristic of recalcitrant seeds is the fact that they are metabolically active when they are shed. However, the type and intensity of metabolism differ among recalcitrant seeds of different species, depending on the developmental status and water concentration at shedding. To explain this, it must be appreciated that unlike the situation in orthodox seeds, there is no cessation of metabolism [although the rate may slow down as recalcitrant seeds reach maturity (reviewed by Finch-Savage, 1996)]. Instead, developmental events progress, without any outward signs, into those of germination, without an exogenous water supply (Berjak et al., 1989
). As discussed by those authors, in some cases germination will ensue in a matter of days after shedding; seeds of some species may be poised for immediate germination; while seeds of yet other species are shed with embryos still having to undergo considerable pre-germination development. These differences have marked effects on the degree of dehydration seeds will tolerate, thereby contributing to unpredictable variability. For example, Lin and Chen (1995)
, working with M. thunbergii
, showed that developing seeds lost viability within 30 d when dried at 73 % relative humidity and 25 °C, while those that were mature were able to tolerate a 19 % loss of water (presently recalculated from the data of those authors) before germinability declined. Differing degrees of desiccation sensitivity have been similarly correlated with embryo/seed developmental status for L. kirkii
and Camellia sinensis
(Pammenter et al., 1991
; Berjak et al., 1992
). It appears generally that for recalcitrant seeds of most species, the least desiccation-sensitive stage occurs when the metabolic rate is lowest, which usually (but not invariably) coincides with natural shedding. However, desiccation sensitivity increases markedly as germinative metabolism progresses, macroscopically imperceptibly, to the stage of the onset of mitosis and extensive vacuolation of the embryo cells (Farrant et al., 1986
; Berjak et al., 1989
). This inexorable progress of germinative metabolism – which occurs with no requirement for additional water – sooner or later (depending on the species) will culminate in radicle protrusion, and is one of the major factors hampering short- to medium-term storage of recalcitrant seeds, as will be discussed later.
Other confounding issues include the fact that axes and storage tissues seldom (if ever) have the same water concentrations, as shown for A. hippocastanum
(Tompsett and Pritchard, 1993
). Working with Araucaria hunsteinii
, Pritchard et al. (1995)
reported that in these gymnospermous seeds too, there is uneven water distribution between the component tissues. Most frequently, axes are at higher water contents, and are more desiccation sensitive, than the cotyledons, e.g. as shown for Q. robur
(Finch-Savage et al., 1992
), M. thunbergii
(Lin and Chen, 1995
) and T. cacao
(Li and Sun, 1999
), and for seeds of numerous screened African species [e.g. Dovyalis caffra
(Erdey and Berjak, 2004); E. capensis
(Erdey et al., 2004
); and Warburgia salutaris
(Kioko et al., 2004
)]. However, to confound the issue, in seeds of Castanea sativa
, cotyledons have been reported to be more sensitive to dehydration than the axes (Leprince et al., 1999
A further contribution to the variability among seeds of individual species is that their characteristics differ both intra- and interseasonally. Intraseasonal variation includes differing water contents of the component tissues of ostensibly mature seeds depending on the time of harvest and, even when harvested simultaneously, there are usually marked differences in axis water contents among individual seeds (Berjak and Pammenter, 1997). An additional feature that has been consistently observed for a variety of species is the poor quality of seeds produced late in the season, which are more often than not severely fungally infected. In this regard, an enhanced rate of deterioration upon dehydration has been reported for late-harvested seeds of Machilus kusanoi
(Chien and Lin, 1997
). It has also been observed that late-season fruits of A. marina
and Syzygium cordatum
have a tendency either to abort or not to abscise.
It is probable that at least the poor quality of late-season seeds may be explained in terms of the cumulative heat sum during development: Daws et al. (2004b) monitored A. hippocastanum seed development along a latitudinal gradient, and reported that the greater the cumulative heat sum, the more robust, further developed and less desiccation sensitive were the seeds. Although those observations were made along a North–South gradient in Europe over the flowering/fruiting season, a similar interpretation for poor seed quality can be applied to fruits and seeds produced in the latter part of the season in non-equatorial zones. Temperatures decline as the summer wanes and, accompanied by shortening day-lengths, results in a sub-optimal heat sum to late-developing fruits. This is proposed to influence fruit and seed development negatively, resulting in their poor quality, which includes lowered resistance to fungal establishment. (This proposal is based on the assumption that the late-season fruits are derived from flowers produced late in the season, which our casual observation suggests to be the case for A. marina.)
Interseasonal variation among seeds of the same species may be similarly rationalized, but in some cases there are remarkable differences. For example, C. sinensis
seeds harvested in consecutive seasons from the same provenance showed embryonic axis water concentrations as disparate as 2·0 ± 0·3 to 4·4 ± 2·4 g g−1
for harvests in different years (Berjak et al., 1996
). As mentioned above, recalcitrant seeds generally will entrain germination at the shedding water content, and thus will germinate under storage conditions not allowing dehydration. However, in one particular season, Q. robur
seeds harvested from the same tree as previously and subsequently had lower than usual water contents, and did not germinate in storage (Finch-Savage et al., 1993
; Finch-Savage, 1996), constituting a marked example of interseasonal differences. Similarly, Pritchard et al. (1999)
have recorded interseasonal differences in germination capacity after a period of dormancy-breaking chilling for seeds of A. hippocastanum
, which those authors ascribed to mean temperature during seed filling.
While interseasonal differences in heat sum may be a feature of temperate climatic zones, they are less marked in the tropics. Consequently, differing effects of seed dehydration on neotropical rain forest species in Mexico (del Carmen Rodriguez et al.
, 2000) and interseasonal differences in a variety of traits of Euterpe edulis
seeds from Brazil (Martins et al., 2000
) are rather more difficult to explain. However, in the case of nine species of Coffea
all originating within tropical Africa, Dussert et al. (2000)
found no significant correlation between the duration of seed development and the level of desiccation tolerance, but were able to demonstrate a significant inverse correlation between desiccation sensitivity and the mean number of dry months after seed-shed in the various habitats.
Daws et al. (2004a) have made an interesting observation that could explain some of the intraharvest variability in desiccation sensitivity often observed. They showed that in a collection of Vitellaria paradoxa seeds, the fresh mass of which varied from 4 to 10 g, the smaller seeds dried faster than the larger ones, and only the larger seeds survived the drying. This survival was not because of greater tolerance to desiccation, but because the large seeds took longer to dry and so were at a higher water content (above the lethal limit under the conditions used) when compared with the smaller seeds at particular sampling intervals. The apparent variability in sensitivity within a seed lot may be due largely to the variation in seed size within the sample.