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Plant Ecology 168: 291–296, 2003. © 2003 Kluwer Academic Publishers. Printed in the Netherlands.
Seed germination during floatation and seedling growth of Carapa guianensis, a tree from flood-prone forests of the Amazon Fábio R. Scarano 1,*, Tânia S. Pereira 2 and Giselle Rôças 1 1
Departamento de Ecologia, IB, CCS, Ilha do Fundão, Universidade Federal do Rio de Janeiro, Caixa Postal 68020, cep 21941-970 Rio de Janeiro, RJ, Brazil; 2Laboratório de Sementes, Instituto de Pesquisas Jardim Botânico do Rio de Janeiro, Rua Pacheco Leão 915, cep 22460-030 Rio de Janeiro, RJ, Brazil; *Author for correspondence (e-mail:
[email protected]) Received 7 November 2001; accepted in revised form 19 January 2002
Key words: Amazon, Flooding, Hydrochory, Meliaceae, Recalcitrance, Seed dormancy, Seed floatation Abstract Carapa guianensis Aubl. (Meliaceae), a hard wood tree from the Brazilian Amazon, has large recalcitrant seeds that can germinate and establish in both flood-free (terra-firme) and flood-prone (várzea) forests. These seeds, although large, can float. This study was designed to experimentally examine seed longevity under floating conditions ex-situ and its effects on subsequent germination and seedling growth. Many seeds germinated while floating, and radicle protrusion occurred from 3 to 42 d after the start of the floating treatment (tap water, room temperature 20–30 °C). Shoots of newly germinated floating seedlings may elongate up to 37.0 cm in 20 d without loss of viability. Epicotyl and first leaf emergence were delayed by floating. Seeds that did not germinate while floating were then placed on vermiculite and watered daily, where many seeds resumed germination. Germination during and after floating was affected by the length of the floating treatment: 88% germinated after 1 mo, 82% germinated after 2 mo and 70% germinated after 2.5 mo. These results indicate that Carapa guianensis has physiological variation regarding dormancy in response to seed floatation. The fact that floatation induces dormancy in recalcitrant seeds of this economically important species can be relevant to initiatives of ex situ storage of seeds. Introduction Flood-prone vegetation in the Brazilian Amazon (see Prance (1979) for terminology) is annually subjected to 6–8 months of flooding. Water levels may be over 10 m deep, submerging entire trees. Thus, plant regeneration in these areas faces at least two major problems: 1) mature fruits and seeds shed during the flood season are likely to fall into water (Kubitzki and Ziburski 1994), thus seeds are subjected to a high degree of risk due to the unpredictability of timing and landing site; and 2) seedlings established during the dry season must withstand, a few months later, full submergence for 6 months or more. Such problems are overcome by a high number of tree species, which often combine efficient water dispersal of seeds with flood- and submergence-tolerance mechanisms of
seedlings and adults (Scarano 1998). This case has prompted a number of studies on seed and seedling ecology, particularly those related to plant establishment, in the seasonally flooded forests of the Amazon (Parolin (2000, 2001); Williamson et al. 1999). Kubitzki and Ziburski (1994) discussed tree regeneration ecology and evolution of hydrochorous dispersal systems by comparing phenology, dispersal and regeneration of various hydrochorous tree species in seasonally flooded forests of the Amazon with their congeners in flood-free sites (terra-firme). In the present study, we focused on aspects related to the germination of a single species, Carapa guianensis, common to both flood-prone and unflooded areas of the Amazon. Fruiting takes place all year in this species (Pennington et al. 1981), meaning that in the flooded forests some seeds are water dispersed while
292 others are dispersed onto the forest floor. Seeds of C. guianensis are usually released when the fruit breaks upon impact with the ground (Pennington et al. 1981) and then float when flooding occurs (McHargue and Hartshorn 1983). This laboratory study of seed germination of C. guianensis examined the effects of the length of floatation on seed viability, germination and seedling growth.
Methods Study species Carapa guianensis Aubl. (Meliaceae; see Pennington et al. (1981) for nomenclature) is a canopy tree species present in swamps and periodically flooded lands from Belize to the Brazilian Amazon, and from sealevel to 1400 m a.s.l. In the Brazilian Amazon, it is found in the seasonal várzea forests that are flooded by regular annual cycles of the nutrient-rich whitewater rivers, and in terra-firme upland forests (Pennington et al. 1981). The species is also found in the estuarine várzea forests (Scarano et al. 1994) that are often flooded twice daily because of the tide that temporarily blocks the flow of the rivers. Its heavy durable wood is used in furniture manufacture and general construction, and its seed oil in soap production (Mors and Rizzini 1966). Experiments Germination during transportation from native sites of C. guianensis to the laboratory in Rio de Janeiro Botanic Gardens was an obstacle for conducting the experiments described here. Thus, we decided to use seeds collected from the ground underneath parent trees growing in the Rio de Janeiro Botanic Gardens. Parent trees were grown from seeds collected in their their natural habitat in the Brazilian Amazon and planted in the Botanic Gardens some 90 years before the experiments. Unfortunately, there are no available records of the precise date of planting and of the type of habitat where these seeds were collected. Seeds for these experiments were collected during the months of November 1992 and September 1993. Experiments began immediately after harvest. In November 1992, 50 control seeds were placed in trays filled with vermiculite and watered daily with tap water. Three groups of 50 seeds were submitted to 1, 2 and 2.5 mo floating periods respectively, by placing them in plas-
tic basins (20.0 cm depth × 40.0 cm diameter) threequarters full of tap water. The water was changed weekly. From the second month onwards, until the end of the experiment 2 wk later, water was changed daily because seeds deteriorated rapidly. During the floating period, radicle and shoot emergence were observed and recorded. Germination was assessed as radicle protrusion. After the floating period, seeds were placed on vermiculite. Germination in vermiculite or basins was carried out in open air at temperature ranging from 20 °C to 30 °C. Seeds received an average light intensity of 10 mol m −2 s −1 for 12 h per day. The length of shoots of germinated control seeds of C. guianensis was measured over a period of 10 d after shoot emergence. The length of shoots of floating seeds was measured weekly during the floating period. In September 1993, 50 control seeds were grown on vermiculite as above, to observe the timing (median, maximum and minimum) of emergence of radicles, epicotyls and first leaves. Timing of each of these phases also was observed for seeds in three floating treatments described above. At the end of 12 wk, ten seedlings grown on vermiculite and ten that had produced a 20-cm long or longer epicotyl under floating conditions were selected for measurements of root, seed and epicotyl dry weight. Ten newly collected seeds were used as controls. Plant material was oven dried at 70 °C for 24 h prior to weighing. Shapiro-Wilks’ W test of normality (Zar 1996) was used for data analysis, but the data did not show a normal distribution after transformations. Thus, we used Notched Box Plot, a non-parametric analysis, to verify the distribution and differences among the groups. Systat 7.0.1 software (SYSTAT 1992) was used for the latter analysis. Statistica 5.1 was used for the normality tests.
Results In the first experiment in 1992, control seeds took 3 to 28 d to germinate, whereas floating seeds germinated from 3 to 42 d after the start of the floating treatment. The second experiment conducted the following year showed that floating delayed the onset of germination as well as epicotyl emergence and first leaf emergence. Epicotyl emergence and first leaf emergence were delayed by 17.5 and 22 d, respectively (Table 1).
293 Table 1. Number of days for epicotyl emergence (in days after radicle protrusion) and leaf emergence (in days after epicotyl emergence) of floating seeds of Carapa guianensis Aubl. compared to controls (median, maximum and minimum values). Seeds
Vermiculite-Germinated
Floating-Germinated
Epicotyl emergence
Med = 7.0 (n = 55) Max = 21.0 Min = 4.0 Med = 8.0 (n = 52) Max = 17.0 Min = 5.0
Med = 24.5 (n = 52) Max = 56.0 Min = 7.0 Med = 30.0 (n = 13) Max = 51.0 Min = 24.0
Leaf emergence
Table 2. Seed germination of Carapa guianensis. Control seeds were sown on vermiculite immediately after harvest. Three floating treatments were applied regarding floating duration: 1, 2, and 2.5 mo. Columns are: n = number of seeds used in each treatment and control; germination during floatation = number (%) of seeds that germinated while floating; germination after floatation = number (%) of seeds that germinated after the floating treatment, when planted on a vermiculite substrate; total germination = addition of the two previous columns; rotten seeds = seeds that rot and died during the experiment. n
Control Treatments 1 mo floating 2 mo floating 2.5 mo floating
germination
rotten seeds
during floatation
after floatation
total
50
–
–
50 (100%)
0 (0%)
50 50 50
17 (34%) 29 (58%) 29 (58%)
27 (54%) 12 (24%) 6 (12%)
44 (88%) 41 (82%) 35 (70%)
6 (12%) 9 (18%) 15 (30%)
Table 2 shows that not all seeds of Carapa guianensis were able to germinate while floating. After 1 mo, 34% (17 out of 50) of the floating seeds were germinated. After 2 mo, 58% (29 out of 50) of the floating seeds germinated. Germination did not increase after 2 additional weeks of floating. At the end of a single month, 100% of the control seeds germinated in vermiculite and continued to develop. Table 2 also illustrates the performance of seeds that did not germinate while floating and then were transferred to a daily-watered vermiculite substrate. Germination under these conditions decreased with the length of the previous floating period. The reduction in germination percentage was particularly abrupt between 2 and 2.5 mo floating. These results indicated that some C. guianensis seeds did not germinate while floating and post-floating seed death may depend upon the length of the floating period. Figure 1 shows that seeds subjected to up to 2 mo of floating did not have impaired growth when returned to dry conditions, even if the radicle elongated while floating. The elongation of epicotyls on vermiculite was significantly impaired only for seeds previously subjected to 2.5 mo of floating or for seeds which epicotyl emergence took place while floating.
Figure 1. Height (cm) of Carapa guianensis’s epicotyl after emergence. C = controls (a) Two developmental stages: seeds with no radicle protrusion and with radicle protrusion while floating; 1, 2 and 3 = number of months of floatation; r = seeds which showed radicle protrusion while floating; (b) Three developmental stages: seedlings with roots formed while floating (I); seedlings with roots and epicotyl emerged while floating (II); and seedlings with roots, epicotyl and leaves formed while floating (III). Notched box-plot was used to evaluate the differences among the treatments. Each box contains 50% of all data. Vertical lines (whiskers) indicate the first and the fourth 25% (quartiles) of the box. The notch in each box represents a 95% confidence interval of the median, which is shown by the narrowest point of the box. Different letters indicate significant differences (p < 0.05).
294 Discussion
Figure 2. Dry weight (g) of Carapa guianensis. (a) seed; (b) root and (c) shoot dry weight of controls and plants which have undergone 3 months floating. The notched box-plots showed that the responses between the treatments were statistically different in all cases (n = 10; p < 0.05).
Elongation of epicotyls during floating was significantly less than for seeds grown on vermiculite, irrespective of whether or not the seeds/seedlings in the latter case were submitted to a previous floating treatment (Figure 2). The attached seeds of the floating propagules showed a dry weight equal to that of the control seeds, whereas the seeds attached to the 90day-old vermiculite grown seedlings showed less than 50% of the dry weight of the control seeds. The ratio of root plus shoot to attached seed dry weight was 0.03:1 for the floating propagules and 0.71:1 for the vermiculite grown seedlings.
Carapa guianensis has a set of ecological characteristics that appear to favour adequate seed dispersal, germination and plant establishment during both wet and dry season in flood-prone forests, as well as in unflooded forests. Its large seeds, like many other large-seeded tropical rain forest species (VázquezYañes and Orozco-Segovia 1990), typically lack dormancy. They germinate and grow rapidly when shed during the unflooded season in a flooded forest, to an extent that shoots of unflooded seeds reached ca. 60 cm in 10 days after the start of epicotyl emergence. McHargue and Hartshorn (1983) claimed that fast epicotyl growth favours seedling establishment and survival by avoiding full submersion and its consequent induction of anoxic stress during the following wet season. Although tall epicotyls may survive in areas where flooding is shallow, as in many estuarine várzeas, this is unlikely to be the case in areas where flooding is up to 10 m deep. In such cases, flood-tolerance strategies at the metabolic level are likely to maintain seedling life during full submersion (Scarano et al. 1994). It has been shown that C. guianensis seeds are dispersed by rodents during the dry season in the flooded forests or in unflooded forests (Henriques and Sousa 1989; Pennington et al. 1981). During the wet season, however, seeds float and are water-dispersed. In the case of floatation, our results showed that the seed responses were two-fold: they either germinated while floating, growing both shoots and roots, or germination was inhibited. The viability of germinated seeds in our experiments depended on the length of the floating period, and death was possibly due to lack of essential nutrients or exhaustion of stored reserves provided by the endosperm (see Manasse (1990)). Under natural conditions, the viability of germinated seeds may also depend on the physical, chemical and biological characteristics of the flood-water. In nature, the ability of a plant to keep roots and shoots alive until the following dry season may favour plant anchoring and rapid establishment when water levels drop. In case of root and shoot death, our laboratory observations showed that a return to well-drained conditions can still favour the emergence of a new root and shoot from the seed. Slow release of endosperm nutrient content to roots and shoot during floatation of germinated seeds may result in sufficient nutrient availability in the endosperm after flooding,
295 which possibly allows the emergence of new root and shoot. Alternatively, some seeds interrupted germination while floating, which was resumed when they were transferred to an unflooded treatment. This induction of physiological dormancy in some seeds of C. guianensis under floating conditions, delays germination until conditions are more favourable and is ecologically advantageous in flood-prone habitats (Scarano et al. 1997). However, it appeared from the present experiments that the efficiency of quiescence in maintaining seed viability while floating would diminish as the length of the floating period increased (see also Scarano (1998)). Seed deterioration was faster in laboratory conditions after 2 mo floating in tap water. Under natural conditions, as argued above for the germinated propagules, seed deterioration during floatation would also depend on the characteristics of the flood-water. Seed polymorphism in regard to dormancy is thus a feature of C. guianensis, which in nature may allow a display of distinct establishment strategies according to season and to habitat. Ecological plasticity in this respect may account for its wide geographic distribution. Indeed, levels of gene flow between distinct populations throughout its broad geographic range are reportedly high probably due to successful seed water dispersal (Hall et al. 1994). The results showing that some seeds of C. guianensis can stand a 2 mo floating period and remain viable suggest that further experiments on “floating storage” should provide useful information for medium- to long-term ex situ storage of the recalcitrant seeds of this species. The difficulties involved in the long-term storage of recalcitrant seeds have been highlighted by various authors, and freezing and/or desiccation are some of the most common treatments to try and induce dormancy in such seeds (Berjak et al. 1984; Chin and Roberts 1980). Given the economic importance of this species (Hammer and Johns 1993; Mors and Rizzini 1966) and its current use in reafforestation initiatives in the Amazon (Bauch and Dunisch 2000; Smith et al. (1998a, 1998b)), further efforts regarding seed conservation might be relevant.
Acknowledgements We thank David Biesboer for valuable comments on the manuscript and linguistic advice, Isolde D. K. Ferraz for comments on an earlier version, Ana Paula M. Cruz for technical assistance and CNPq (Brazilian Research Council) for research grants.
References Bauch J. and Dunisch O. 2000. Comparison of growth dynamics and wood characteristics of plantation-grown and primary forest Carapa guianensis in Central Amazonia. IAWA Journal 21: 321–333. Berjak P., Dini M. and Pammenter N.W. 1984. Possible mechanisms underlying the differing dehydration responses in recalcitrant and orthodox seeds: desiccation-associated subcellular changes in propagules of Avicennia marina. Seed Science & Technology 12: 365–384. Chin H.F. and Roberts E.H. 1980. Recalcitrant Crop Seeds. Tropical Press SDN.BDH, Kuala Lumpur, Malaysia. Hall P., Orrell L.C. and Bawa K.S. 1994. Genetic diversity and mating system in a tropical tree, Carapa guianensis (Meliaceae). American Journal of Botany 81: 1104–1111. Hammer M.L.A. and Johns E.A. 1993. Tapping an Amazonian plethora - 4 medicinal-plants of Marajo Island, Pará (Brazil). Journal of Ethnopharmacology 40: 53–75. Henriques R.P.B. and Sousa E.C.E.G. 1989. Population structure, dispersion and microhabitat regeneration of Carapa guianensis in northeastern Brazil. Biotropica 21: 204–209. Kubitzki K. and Ziburski A. 1994. Seed dispersal in flood plain forests of Amazonia. Biotropica 26: 30–43. Manasse R.S. 1990. Seed size and habitat effects on the water-dispersed perennial, Crinum erubescens (Amaryllidaceae). American Journal of Botany 77: 1336–1341. McHargue L.A. and Hartshorn G.S. 1983. Seed and seedling ecology of Carapa guianensis. Turrialba 33: 399–404. Mors W.B. and Rizzini C.T. 1966. Useful Plants of Brazil. HoldenDay Inc, San Francisco. Parolin P. 2000. Seed mass in Amazonian floodplain forests with contrasting nutrient supplies. Journal of Tropical Ecology 16: 417–428. Parolin P. 2001. Seed germination and early establishment of 12 tree species from nutrient-rich and nutrient-poor Central Amazonian floodplains. Aquatic Botany 70: 89–103. Pennington T.D., Styles B.T. and Taylor D.A.H. 1981. Meliaceae. Flora Neotropica Monograph 28. New York Botanical Garden Press, New York. Prance G.T. 1979. Notes on the vegetation of Amazon III. The terminology of Amazonian forest types subject to inundation. Brittonia 31: 26–38. Scarano F.R. 1998. A comparison of dispersal, germination and establishment of woody plants subjected to distinct flooding regimes in Brazilian flood-prone forests and estuarine vegetation. In: Scarano F.R. and Franco A.C. (eds), Ecophysiological Strat-
296 egies of Xerophytic and Amphibious Plants in the Neotropics. Oecologia Brasiliensis Series. Vol. 4. PPGE-UFRJ, Rio de Janeiro, Brazil, pp. 177–193. Scarano F.R., Cattânio J.H. and Crawford R.M.M. 1994. Root carbohydrate storage in young saplings of an Amazonian tidal várzea forest before the onset of the wet season. Acta Botanica Brasilica 8: 129–139. Scarano F.R., Ribeiro K.T., Moraes L.F.D. and Lima H.C. 1997. Plant establishment on flooded and unflooded patches of a freshwater swamp forest of southeastern Brazil. Journal of Tropical Ecology 14: 793–803. Smith C.K., Gholz H.L. and Oliveira F.D. 1998a. Fine litter chemistry, early-stage decay, and nitrogen dynamics under plantations and primary forest in lowland Amazonia. Soil Biology & Biochemistry 30: 2159–2169. Smith C.K., Gholz H.L. and Oliveira F.D. 1998b. Soil nitrogen dynamics and plant-induced soil changes under plantations and primary forest in lowland Amazonia, Brazil. Plant and Soil 200: 193–204.
SYSTAT 1992. SYSTAT for Windows: graphics. Version. SYSTAT Inc., Evanston. Vázquez-Yañes C. and Orozco-Segovia A. 1990. Seed dormancy in the tropical rain forest. In: Bawa K.S. and Hadley M. (eds), Reproductive Ecology of Tropical Forest Plants. Man and the Biosphere Series. Vol. 7. Unesco, Paris, pp. 247–259. Williamson G.B., Costa F. and Vera C.V.M. 1999. Dispersal of Amazonian trees: hydrochory in Swartzia polyphylla. Biotropica 31: 460–465. Zar J.H. 1996. Biostatistical Analysis. 3rd edn. Prentice Hall, New Jersey.
