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Ecology |
2Seed Conservation Department, Royal Botanic Gardens, Kew, Wakehurst Place, Ardingly, West Sussex RH17 6TN, UK; 3Centre National de Semences Forestières, B.P. 2682, Route de Kaya, Ouagadougou 01, Burkina Faso; 4Tanzania Tree Seed Agency, P.O. 373, Morogoro, Tanzania; 5Kenya Forestry Seed Centre, P.O. 20638, Nairobi, Kenya
Received for publication August 7, 2003. Accepted for publication January 13, 2004.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: African drylands • recalcitrant seed • seed drying • seed mass • tropical drylands
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). In contrast, recalcitrant (Type III sensu Pritchard, 2004
), hereafter desiccation-sensitive, seeds are killed by drying the seed/embryo to water contents as high as 20–30% (see Pritchard, 2004
). Because desiccation-sensitive seeds cannot be dried and also progress towards germination when stored fully hydrated, they are difficult to store for anything other than the short term. Consequently, their use in reforestation programs and ex situ conservation is problematic. Therefore, tools for predicting seed storage classification would be of use when handling seeds of species for which seed storage behavior is unknown.
There have been a number of attempts to correlate seed storage behavior with ecological characteristics such as climate and vegetation type and with seed characteristics such as shape, size, and moisture content at shedding. For example, Tompsett (1984
, 1987
) found that in the Araucariaceae, the relative level of desiccation tolerance in seeds was associated with seed and/or embryo size, while in the Dipterocarpaceae the level of desiccation tolerance was associated with seed size and shape; desiccation-sensitive species had seeds with a lower surface area to volume ratio than desiccation-tolerant species. Similarly, Hong and Ellis (1998)
investigated patterns of response to seed desiccation in 40 species of Meliaceae. They found that species with desiccation-sensitive seeds typically occur in moist areas, particularly rainforest, and produce large (>1 g), round seeds, which are shed at high moisture contents. Dickie and Pritchard (2002)
reported that the mean seed mass of 205 desiccation-sensitive species (taken from Hong et al., 1998
) was 3958 mg vs. 329 mg for 839 desiccation-tolerant species. However, in some species, seed size has been a poor predictor of seed response to desiccation. For example, Pritchard et al. (1995)
found that for seven Inga species (Fabaceae), which occur in tropical rainforest, there was no relationship between embryo size and the level of desiccation sensitivity. In addition, Hong and Ellis (1997)
found that within the Aceraceae, seed mass was a poor predictor of response, although there was a link between seed morphology and desiccation tolerance: all flat-seeded species were desiccation-tolerant; some, but not all convex-shaped seeds were desiccation-sensitive.
Desiccation-sensitive seeds are shed at high moisture contents (Tompsett and Kemp, 1996
; Hong and Ellis, 1998
) in a metabolically active state and, in some cases, may be progressing towards germination at the time of seed shedding [e.g., Avicennia marina (Forssk.) Vierh; Berjak et al., 1984
]. Consequently, the period of imbibition required by desiccation-sensitive seeds prior to germination might be expected to be shorter than that of desiccation-tolerant seeds and thus the speed of germination generally faster. For desiccation-sensitive seeds, rapid germination may reduce the period during which seed desiccation can occur. Rapid germination may also have the advantage of reducing seed predation levels. Seed predators can be an important selective force on plant regeneration strategies. For example, vertebrate seed predators are probably the main selective force behind the mast fruiting events of dipterocarp species (many of which have desiccation-sensitive seeds ([Tompsett and Kemp, 1996
]) at irregular, multiyear intervals (Curran and Webb, 2000
). Mast fruiting, coupled with rapid germination, is thought to have evolved in dipterocarps because it reduces the potential window of opportunity for seed predators (Curran and Webb, 2000
). In comparison, desiccation-tolerant seeds may form a soil seed bank and hence be potentially exposed to seed predators for extended periods of time. If predator avoidance through rapid germination is a general feature of desiccation-sensitive seed ecology, it may mean that there is less selection pressure on desiccation-sensitive than desiccation-tolerant seeded species to invest resources in physical defense (i.e., in the form of a thick seed coat or endocarp). Thus on a per-unit-mass basis, desiccation-sensitive seeds may be a more efficient use of resources than desiccation-tolerant seeds. However, there is currently little evidence to support these hypotheses.
Seed desiccation sensitivity is potentially a high-risk regeneration strategy for plants because a prolonged dry spell at the time of seed shedding could result in the death of an entire annual cohort of seeds. This presumably explains the typically large size of desiccation-sensitive seeds, their high frequency in aseasonal tropical forests (Tweddle et al., 2003
), and their convex shape, all of which will reduce the likelihood of seed drying (Tompsett, 1992
). Consequently, there is a general perception that, in the tropics, desiccation-sensitive species do not occur in arid or savannah habitats (e.g., Murdoch and Ellis, 2000
). However, such species do occur naturally in the tropical drylands (e.g., Gaméné et al., 1999
; Danthu et al., 2000
; Tweddle et al., 2003
) although little is known about their regeneration strategies.
The possible ecological advantages and disadvantages of seed desiccation sensitivity have received little study and are currently unclear (Pammenter and Berjak, 2000
). It has been proposed that desiccation tolerance may be the ancestral state in seeds, with desiccation sensitivity being a derived trait (Farnsworth, 2000
; Oliver et al., 2000
; Dickie and Pritchard, 2002
). This implies that the loss of desiccation tolerance provides a fitness advantage relative to desiccation-tolerant species. In dryland environments, where rainfall events can be sporadic and unreliable even during the wettest month(s), species with desiccation-sensitive seeds would be expected to be comparatively infrequent. When they do occur, the following adaptations to minimize seed mortality and increase fitness seem likely: (1) shedding at the time of maximum annual rainfall, (2) large seed mass to reduce the rate of seed drying, and (3) rapid germination to reduce the likelihood of seed dehydration.
Here we report the results of a study on the desiccation tolerance and germination characteristics of 10 dryland species predominantly from Burkina Faso, Kenya, and Tanzania. We then extend this data set with information on the seed storage behavior of 70 African dryland tree species to explore the relationship between seed mass, rainfall at the time of seed shedding, and seed desiccation tolerance and test our propositions. Furthermore, we discuss the results in relation to the possible ecological consequences of the sensitivity of seeds to desiccation for tropical dryland trees.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Seed desiccation tolerance
For dehydration, six or seven aliquots of seeds were placed in polythene bags with an equal mass of freshly regenerated silica gel desiccant. Each aliquot contained enough seed to determine moisture content and germination percentage. The number of seeds per aliquot necessarily varied between species, depending on the quantity of seed available from each collection. The bags were held in an incubator at 26°C and periodic reweighing of the seeds, separated from the silica gel, allowed target masses and hence moisture contents to be achieved. Maximum drying times varied from 6 d, for Sclerocarya birrea, to 35 d for Syzygium cumini dependent on the time required for the seeds to reach 3–7% moisture content. Individual seed moisture contents were determined gravimetrically after drying for 17 h at 103°C (ISTA, 1999
) on the following quantities of seeds: 10–15 Lannea microcarpa; 20 Ximenia americana; 25 Kigelia africana, Khaya senegalensis, Vitellaria paradoxa, Sclerocarya birrea, Syzygium cumini; 25–30 Trichilia emetica, Dovyalis caffra; and 20–40 Strychnos cocculoides.
Seeds held with an equal volume of moist vermiculite (expanded mica) in inflated polythene bags served as the controls for the desiccation experiment. The bags, held at 26°C, were regularly ventilated. Treatment times and the number of individual seeds used for moisture content determination were the same as those used for the desiccation treatment.
After desiccation, seeds were sown for germination on 1% (m/v) agar in water, either in 9 cm diameter Petri dishes or 175 x 115 x 75 mm clear polystyrene (sandwich) boxes. These germination tests were conducted in an incubator at 26°C with a 12-h photoperiod provided by white fluorescent lights. Seeds were scored as germinated when the radicle had protruded through the seed coat by 2 mm. Replication levels were as follows: 5 x 20 Lannea microcarpa, Syzygium cumini, Vitellaria paradoxa; 2 x 20 Sclerocarya birrea, Ximenia americana, Strychnos cocculoides, Trichilia emetica; and 4 x 25 Dovyalis caffra, Khaya senegalensis, Kigelia africana. Seeds of Sclerocarya birrea have a stony endocarp with an operculum, removal of which facilitates germination; opercula were removed after desiccation (for method, see von Teichman et al., 1986
).
Effect of temperature on germination
The effect of temperature on germination total and rate was assessed for nine species (i.e., excluding Sclerocarya birrea) by sowing 2 x 20 Lannea microcarpa, Syzygium cumini, Trichilia emetica, Vitellaria paradoxa; and 2 x 25 Dovyalis caffra, Strychnos cocculoides, Ximenia Americana, Kigelia africana, Khaya senegalensis seeds on the surface of 1% agar in water in sandwich boxes and sown at 11°, 16°, 21°, 26°, 31°, and 36°C. Estimates of the time (t) at each temperature for cumulative germination to reach 50% of maximum germination were interpolated from germination progress curves. Subsequently, the reciprocal (1/t) was plotted against temperature. One-way ANOVA followed by Fisher's LSD test was used to assess whether there were significant differences in germination rate between the nine species.
Determination of resource allocation to defense
For each species, a minimum of 25 seeds (dispersal unit) was dissected into their component parts: endocarp (Lannea microcarpa and Sclerocarya birrea), testa, and embryo/ endosperm. These component parts were subsequently dried at 103°C for 17 h (ISTA, 1999
) followed by mass determinations. To calculate the allocation to defense, the ratio of the mass of covering structures (endocarp and testa) to the mass of the total dispersal unit was determined (Grubb and Burslem, 1998
).
Analysis of species distribution, climate, and seed characteristics
In addition to the 10 species investigated here, information on seed mass, desiccation sensitivity, and timing of seed shedding was obtained from the scientific literature for a further 70 species. These species had a wide taxonomic coverage being drawn from 27 different families. Including the 10 species from this study, the data set included 80 species from 33 different families.
The following data sources on plant distribution and seed shedding times were used: Botswana, Tietema et al., 1992
; Davies and Pritchard, 1998
; Kenya, Teel, 1984
; Albrecht, 1993
; Beentje, 1994
; Senegal, Danthu et al., 2000
; and Tanzania Mbuya et al., 1994
. For each species in turn, mean monthly rainfall, either in the month of collection (those species that were investigated in detail here) or months of known dispersal, was collated from meteorological stations within its range for a 30-yr record (Meteorological Office, 1983
). If there was more than one station within the species' range, an average data set was constructed by averaging the values for mean monthly rainfall across the months of seed shedding and the different meteorological stations. No stations were included if they fell outside the altitudinal range of the species. Species were excluded from the analysis if the shedding period lasted for 6 mo or more and if their distribution fell within an area with an average annual precipitation of >1200 mm/yr (i.e., were not dryland habitats; Middleton and Thomas, 1997
).
Seed masses were experimentally determined here for 10 species and data for other species came from various sources (Teel, 1984
; Tietema et al., 1992
; Albrecht, 1993
; Mbuya et al., 1994
; Venter and Venter, 1996
; Davies and Pritchard, 1998
; Hong et al., 1998
; Danthu et al., 2000
; Seedbank Database, Royal Botanic Gardens, Kew, personal communication). For the 10 study species, oven-dried seed masses were used in the analysis. However, masses obtained from the literature were generally for either air-dried or fresh seed. Thus, while not completely comparable, seed masses for the 10 species will generally be underestimates compared to seed mass values from the literature. When a range of seed masses was obtained from the literature, the mid-point of the range was used in the analysis. Mean rainfall in the month of seed shedding was plotted against seed mass (log10 scale).
Seed storage category was assigned for the 10 species on the basis of desiccation tolerance alone: desiccation-tolerant if the seeds survived drying to approximately 5% moisture content and desiccation-sensitive if the seeds had lost most of their viability after dehydration to 20% moisture content. Information on the other 70 species was taken from Hong et al. (1998)
, and all family names were validated against the Angiosperm Phylogeny Group (APG, 1998).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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The effects of drying on seed germination fell into three categories (Fig. 1A and B). In group 1, Lannea microcarpa and Sclerocarya birrea seeds exhibited an approximately 25% increase and then a decrease in germination from initial germination values of 65% and 30%, respectively, and germination peaked after drying to 3–5% moisture content. In group 2, little or no change in germination was seen in Kigelia africana, Khaya senegalensis, Dovyalis caffra, Stychnos cocculoides, and Ximenia americana seeds as drying progressed to 4–6% moisture content. In group 3, Trichilia emetica, Syzygium cumini, and Vitellaria paradoxa seeds had a progressive decrease in germination primarily between approximately 35 and 15% moisture content (Fig. 1). In group 3 seeds, changes in viability for the control seeds, i.e., held in vermiculite, were much lower than after the drying process (cf. Table 2, Fig. 1). For the controls overall, seed moisture contents generally changed less than 3%, the exception being Ximenia americana. However, for this species, very little seed viability was lost during this period (Table 2). Slightly larger viability decreases were observed in four species (11–42%), whilst in the other six species viability of the controls increased slightly (0– 15%) (Table 2).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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), seeds of both species imbibed during the germination test. In the case of Sclerocarya birrea, the potential physical constraint to germination, the cap above the embryo, was removed before the seeds were sown, suggesting that if seed dormancy had been altered by drying, the dormancy was most likely physiological, rather than physical, in nature. In support of this proposition, there is some evidence that seeds of this species can after-ripen in the dry state (von Teichman et al., 1986
). Perhaps the approximately 2-wk period in the drying experiment at 26°C constituted a mild after-ripening treatment in both species' seed. Even though the seeds of these species exhibited considerable desiccation tolerance, ultra-desiccation to 
2% moisture content did reduce germination slightly. Seeds of Dovyalis caffra, Kigelia africana, Khaya senegalensis, Strychnos cocculoides, and Ximenia americana were similarly desiccation-tolerant to low moisture contents (Fig. 1). This response in Ximenia americana is of interest as the species has been categorized previously as recalcitrant (Hong et al., 1998
).
The level of desiccation sensitivity (i.e., range of moisture contents over which viability is apparently lost and the moisture content at which 50% remained viable) displayed by Trichilia emetica, Syzygium cumini, and Vitellaria paradoxa seeds is typical of recalcitrant, Type III seeds, i.e., 20–30% moisture content (Pritchard, 1991
, 2004
; Tompsett and Kemp, 1996
).
Predictors of seed response to desiccation
The nine desiccation-sensitive species identified in our survey of 80 species were all comparatively large, with a seed mass greater than 500 mg (Fig. 4). This is in agreement with previous studies of tree species which have found that desiccation-sensitive species typically have large seeds. For example, Dickie and Pritchard (2002)
found that the average seed mass of desiccation-sensitive seeds in Hong et al. (1998)
was 3958 mg. While this figure seems high in comparison to the minimum mass in this study, it should be remembered that the masses in Hong et al. (1998)
were not necessarily oven-dried masses, as were those from this study and those drawn from Danthu et al. (2000)
. Seed size alone, however, was not a reliable predictor of seed storage behavior. For example, there are a number of large (>1000 mg) seeds in the data set with desiccation-tolerant seeds (Fig. 4).
Seed moisture content at shedding has been suggested as a reliable predictor of tolerance to desiccation (Hong and Ellis, 1998
). However, in this study this was not the case. While all three desiccation-sensitive species were received at high (>38%) moisture contents, a number of the desiccation-tolerant species were also received at high moisture contents (e.g., Dovyalis caffra; Table 1). Many desiccation-tolerant seeds are borne in fleshy fruits and are at high moisture contents at shedding, though the seeds themselves are not particularly large, e.g., papaya (Wood et al., 2000
). Of the 10 species in this study, nine are borne in fleshy fruits and Khaya senegalensis has wind-dispersed seeds. Although Sclerocarya birrea had a low (9%) moisture content on receipt, seeds were at a moisture content of 72% when collected; seeds were sun-dried before shipping to the UK (Were, 1997
). Thus, while desiccation-sensitive seeds will, of necessity, be shed at high moisture contents, this cannot be used reliably as a marker of seed response to desiccation.
All nine desiccation-sensitive species in Fig. 4 were shed when monthly rainfall was in excess of 60 mm. Species with desiccation-sensitive seeds have seeds that are shed wet and will suffer desiccation-induced mortality, suggesting a need to time seed shed to coincide with high rainfall. Interestingly, the three desiccation-sensitive species identified in this study (Fig. 1) not only timed seed shed to when rainfall is in excess of 60 mm, but also coincident with the peak in annual rainfall (Fig. 3). Presumably, this time is the least risky period for seed shed and seedling establishment. In comparison, seed shed in the desiccation-tolerant species is not necessarily coincident with high rainfall (Figs. 3 and 4). In the case where seeds are shed during a dry period, desiccation tolerance may enable seeds to accumulate in the soil seed bank and wait until the onset of prolonged rainfall before germination occurs.
Ecological implications of seed desiccation sensitivity
Large seed size can be ecologically advantageous. Seedlings from large seeds are larger (Boot, 1996
), have a higher probability of surviving in low light (Leishman and Westoby, 1994a
) and drought (Leishman and Westoby, 1994b
) conditions, and are more likely to recover from herbivory (Harms and Dalling, 1997
) than seedlings from small seeds. However, large seeds can be highly attractive to vertebrate seed predators. Vertebrate seed predators may have provided the selective force for mast fruiting events, which lead to predator satiation, observed in dipterocarp species (Curran and Webb, 2000
), many of which have desiccation-sensitive seeds (Tompsett and Kemp, 1996
). Curran and Webb (2000)
also reported that in combination with mast fruiting, dipterocarp seeds "escape" predation through rapid germination; seedlings are less attractive to predators than seeds.
Table 1 suggests that desiccation-sensitive seeds have thinner seed coats/endocarps than the desiccation-tolerant species studied. This is of particular note because the allocation to seed coat has been reported to increase with seed size for species in the Asteraceae (Fenner, 1983
) and for a range of Carex spp. (Schütz, 2000
). Thus, larger seeds are better defended than small seeds. Such adaptation may facilitate animal dispersal, by the seeds' resistance to digestion. Viewed in this context, the three desiccation-sensitive species have a surprisingly small investment to defense compared to smaller, desiccation-tolerant seeds. This result is only based on three desiccation-sensitive species, but it nonetheless suggests that desiccation-sensitive seeds may be highly susceptible to herbivory, which is presumably minimized by rapid germination (Fig. 2). Large seeded, desiccation-tolerant species may reduce the problem of seed predation by having thick seed coats or endocarps, e.g., Sclerocarya birrea (Table 1) and Hyphaene petersiana Klotzsch ex Mart. (seed coat ratio of 0.68; M. I. Daws, unpublished data). However, this means that per unit mass, desiccation-sensitive seeds may be a more cost-effective, efficient use of resources for large-seeded species. While temperate desiccation-sensitive tree species are presumably operating under a different set of ecological constraints, they may also have a comparatively low investment in seed coat (e.g., seed coat/seed mass ratios of 0.19, 0.20, and 0.19 for Aesculus hippocastanum L., Castanea sativa Mill., and Quercus robur L., respectively [M. I. Daws, unpublished data]). However, more research is required to test these propositions.
Rapid germination of the desiccation-sensitive species not only minimizes the window of opportunity for seed predation but also contributes to rapid access to soil water at depth. This may be of particular benefit in dryland environments where rainfall is sporadic. Species with desiccation-sensitive seeds may be able to germinate rapidly as a consequence of being shed at high moisture contents and metabolically active, thus reducing the need for imbibition prior to the start of metabolism. However, some species with desiccation-tolerant seeds also have seeds that are shed at high moisture contents. Doussi and Thanos (2002)
reported that for Mediterranean climates where rainfall is sporadic and unpredictable, it is advantageous for seeds to germinate slowly, at the individual or population level. In either case, the risk of drought- or desiccation-induced mortality once seeds have either started to germinate or are at the early seedling stage would be reduced. This may explain the comparatively slow germination of the desiccation-tolerant species that were shed at high moisture contents. However, for desiccation-sensitive seeds, the overriding concern is to germinate rapidly because delayed germination will, presumably, almost certainly result in desiccation-induced seed death. Species with desiccation-sensitive seeds are unable to spread the risk associated with unpredictable rainfall after seed shed. An additional protective adaptation in such species would be a temporal spread of seed-dispersal times, thereby reducing the risk of losing an entire annual cohort of seeds to a dry spell after shedding. Field studies to investigate inter-annual recruitment success and the phenology of seed dispersal in dryland species with desiccation-sensitive seeds would clarify the importance of such ecological adaptations to the survival of such species.
Overall, it appears that desiccation-tolerant seeds may spread the risk of germination in an unpredictable environment by germinating comparatively slowly. However, this strategy may widen the window of opportunity for seed predation to occur, and this is reflected in greater allocation to seed defenses. In contrast, desiccation-sensitive species directly commit relatively less resource to mitigate against seed predation, preferring to narrow the window of opportunity for seed predation by faster germination. Nonetheless, species that produce desiccation-sensitive seeds have the disadvantage that the entire annual cohort of seeds could, potentially, be killed by desiccation after seed shedding. While this strategy is a more efficient use of resources, only large-seeded species are able to adopt this strategy (Fig. 4). The risk of seed desiccation would be too great for small-seeded desiccation-sensitive species, unless the species was adapted to permanently moist environments, e.g., some aquatic species (Hay et al., 2000
). This is reflected in the fact that desiccation sensitivity, while having some potential advantages, is comparatively infrequent in dryland environments (nine of 80 species in this study had desiccation sensitive seeds). While this work identifies some possible benefits of producing desiccation-sensitive seeds, further investigations are warranted on the regeneration ecology of species possessing such seeds.
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FOOTNOTES |
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6 E-mail: h.pritchard{at}rbgkew.org.uk
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