OPCE-STR.vp Acta Bot. Croat. 69 (2), 249–257, 2010 CODEN: ABCRA 25 ISSN 0365–0588 Effect of seed age and soil texture on the germination of some Ludwigia species (Onagraceae) in Nigeria MATTHEW OZIEGBE1*, JULIUS O. FALUYI1, ABIMBOLA OLUWARANTI2 1 Department of Botany, Obafemi Awolowo University, Ile Ife, Nigeria. 2 Department of Crop Production and Protection, Obafemi Awolowo University, Ile Ife, Nigeria. Seed germination in Ludwigia was greatly influenced by seed age and soil type. In Ludwigia abyssinica germination was not influenced by seed age and soil texture. Freshly shed seeds and six month old seeds of Ludwigia decurrens variety B showed a very low percentage germination on all the germination media, and six month old seeds germinated significantly earlier than freshly shed seeds. Some soil types could significantly reduce germination of freshly shed seeds of L. hyssopifolia, L. erecta, L. leptocarpa and L. octovalvis var linearis. Key words: germination, seed, soil, Ludwigia Introduction The genus Ludwigia has existed at least 50 million years and is considered one of the largest and least specialized genera of the family Onagraceae (PENG 1988). The genus is pantropical; it includes some 82 species distributed among 23 sections. Twenty five of these species occur in the Old World, including 8 of its 23 sections. A very diverse assem- blage of Ludwigia species occur in South America, where 45 of the 82 species occur with most primitive species; this may have been the centre of origin for the genus and family the Onagraceae (RAMAMOORTHY and ZARDINI 1987, PENG 1989). In West Africa, they are repre- sented by fourteen species (sixteen taxa) and nine species (eleven taxa) of these are found in Nigeria (WOGU and UGBOROGHO 2000). A few Ludwigia species are predominantly aquatic but all of the species grow in wet places where they concentrate around coastal re- gions, lakes, lagoons, canals, rivers, streams, seas, gutters and water logged areas. The ge- nus contains both herbaceous and woody species. Many aquatic species are phenotypically plastic, such that their growth forms vary under different environmental conditions, which often complicates species identification and has led to a number of fluctuations in their tax- onomic classifications (DUTARTRE et al. 2004). Seed dispersal and germination are phases in the reproductive cycle that are typically of great importance for species fitness. Varia- tions in seed dispersal efficacy or germination percentage are often interpreted as reflecting ACTA BOT. CROAT. 69 (2), 2010 249 * Corresponding author, e-mail: matthewoziegbe@yahoo.com U:\ACTA BOTANICA\Acta-Botan 2-10\293-Oziegbe.vp 11. listopad 2010 13:39:48 Color profile: Disabled Composite 150 lpi at 45 degrees adaptations to specific ecological conditions (VENABLE and LAWLOR 1980, GRIME et al. 1981, MARTIN et al. 1995, NISHITANI and MASUZAWA 1996). Clearly, the characteristics of the microsite occupied by a seed may strongly influence its probability of germination and subsequent survival, including germination capacity (SCHAAL 1980, DOLAN 1984, HENDRIX 1984, STANTON 1985, MARSHALL 1987, NAYLOR 1993), resistance to intra and interspecific competition (WULFF 1986, MAZER 1989, HOUSSARD and ESCARRE 1991), dormancy period (STAMP 1990), distance dispersed with respect to the mother plant (AUGSPURGER and FRANSON 1986) and seedling survival and/or growth (SCHAAL 1980, HOWE and RICHTER 1982, GROSS 1984, STANTON 1984). Ludwigia peruviana seeds can remain dormant for two years, seeds are able to germi- nate while afloat, 20 percent of seeds are dormant, propagation can also be done by young stems; seed production is of 450000 seeds m2, 65000 seeds m2 in soil seed bank and 30000 seeds m2 for seeds that remained on old stems (JACOBS et al. 1994). It has been reported that the capacity of seed germination in Ludwigia species is not known (RUAUX et al. 2009). Fruits of many Ludwigia species remain on the parent stem when ripened while some are detached immediately at ripening. Some seeds are dispersed freshly from fruits on parent stems in to water as fruits dehisce irregularly while some seed remain on the fruits for sev- eral months before they are dispersed. Fruits of L. adscendens and L. leptocarpa are de- tached from the parent stem immediately they are ripened and remain afloat in water while it takes several weeks and months for their seeds to be dispersed from the fruits. In L. hyssopifolia which possesses both non-endocarp and endocarp seeds, non-endocarp seeds are first dispersed at ripening of fruit while endocarp seeds remain on the parent plant for several weeks and months before they are eventually dispersed into water. Studies have re- ported the germination of some Ludwigia seeds in the field (RUAUX et al. 2009) but there are few studies on seed germination capacity in relation to soil types (WOGU and UGBO- ROGHO 2000). As the fruits of most Ludwigia species remain on the parental plant (i.e. not dispersed), it was important to study the seed aging by comparing fresh seeds with seeds stored over maternal plants for six months and determine their germination on soil types. This study intends to determine the effect seed age and soil types on germination of seven Ludwigia species. Materials and methods Seeds used for this study were collected from Ludwigia species grown in the garden from March 2006 to December 2006 at the Department of Botany, Obafemi Awolowo Universty, Ile Ife. Germination study was carried out on nine Ludwigia seed samples which belonged to seven Ludwigia species: L. abyssinica A. Rich, L. adscendens (Linn) Hara. ssp. diffusa (Forssk.) Raven, L. decurrens var A Walter, L. decurrens var B Walter, L. hyssopifolia (G.Don) Excell, L. erecta (Linn) Hara, L. leptocarpa (Nutt.) Hara, L. octovalvis var linearis (Jacq.) Raven and L. octovalvis var brevisepala. L. decurrens var A has big ovate lanceolate leaves, the stem is prominently winged and 5angled, it initiates flowering within (64–70) days after germination; the number of petals ranges from (4–5). It is mostly found as monotypic stands in most aquatic habitats in Ile–Ife. L. decurrens var B has narrow lanceo- late leaves, the stem is narrowly winged and 4 angled, it initiates flowering within (30–40) days after germination, the number of petals is fixed at (4). It is found growing sparsely among L. decurrens var A and rarely recognized. L. octovalvis var brevisepala has short lin- 250 ACTA BOT. CROAT. 69 (2), 2010 OZIEGBE M., FALUYI J. O., OLUWARANTI A. U:\ACTA BOTANICA\Acta-Botan 2-10\293-Oziegbe.vp 11. listopad 2010 13:39:48 Color profile: Disabled Composite 150 lpi at 45 degrees ear leaves, stem slightly bent, densely villous and the number of petals is fixed at (4). L. octovalvis var linearis has long narrow linear leaves, stem erect, subglabrous and petal num- ber ranges from (4–5). One hundred freshly ripened seeds and six month old seeds of each species were seeded into Petri dishes of diameter 8.00 cm (4 replicates of every treatment) lined with Whatman filter-papers No 1. Also 100 seeds of freshly ripened seeds and six month old seeds of each species were seeded separately on soil surfaces into 8.00cm diameter plastic cups filled with clayey, sandy and loamy soil with four replicates for every treatment. The Petri dishes and cups were watered regularly with distilled water. Germination was re- corded when the radicule emerges from the seeds every day for a period of 30 days for each treatment. Days to initial germination (appearance of radicule) and percentage germination were recorded for each of the treatments. Germinated seeds were removed from the Petri dishes and cups during each count. The experiment was carried out in the screen house of the Department of Botany, Obafemi Awolowo University, Ile-Ife under conditions of natural photoperiod and ambient temperature (20–25 oC). Soil samples used were collected from Obafemi Awolowo University campus located within (latitude 7o 30 N to 7o 35 N and longi- tude 4o 30 and 4o 35o E). Soil textures were determined at Department of Soil Science, Obafemi Awolowo University, Ile Ife, using a mechanical analysis method. Duncan’s multi- ple range test (DMRT) was used to compare sets of means obtained for germination regimes, at a probability level of 0.05, using system analysis software (SAS) version 9.2. Results The study on Ludwigia species reveals that percentage germinations of six months seed were very high and varied significantly from the very low percentage germinations ob- served in freshly shed seeds on the three types of soils with the exception of L. abyssinica, L. hyssopifolia, L. erecta and L. leptocarpa (Tabs. 1, 2). The soil media also significantly reduced the percentage germination of freshly shed seeds of (L. hyssopifolia, L. erecta, L. leptocarpa and L. octovalvis var linearis) when compared to their control. In L. adscen- dens, L. decurrens variety A, L. decurrens variety B, L. hyssopifolia, L. erecta and L. octovalvis var linearis six months old seeds germinated significantly earlier than freshly shed seeds; but in L. abyssinica and L. octovalvis var brevisepala there was no significant difference in days to initial germination between six month old seeds and freshly shed seeds on any of the germination media (Tab. 2). Ludwigia abyssinica Percentage germination of new seeds and six month seeds of L. abyssinica were very high in all the germination media with no significant differences in the percentage germina- tions (Tab. 1). Seeds germinated very early in all the media and there was no significant dif- ference in days to initial germination in any of the media (Tab. 2). Ludwigia adscendens Very high percentage germination in six month seeds of L. adscendens was significantly different from the very low percentage germination of new seeds on all the germination media. Seed age affected days to initial germination, six month old seeds germinated sig- ACTA BOT. CROAT. 69 (2), 2010 251 GERMINATION OF LUDWIGIA SPECIES (ONAGRACEAE) U:\ACTA BOTANICA\Acta-Botan 2-10\293-Oziegbe.vp 11. listopad 2010 13:39:48 Color profile: Disabled Composite 150 lpi at 45 degrees nificantly earlier than freshly shed seeds but the soil types had no significant effect on days to initial germination (Tab. 2). Ludwigia decurrens The high percentage germination (93.4) of six month seeds observed in the control of L. decurrens var A is significantly different from the very low percentage germination of freshly shed seeds in control (5.20), loamy (1.60), clayey (1.40) and sandy (13.4) soils; there were significant differences in percentage germinations of six month seeds on the three types of soils (Tab. 1). Seed age affected days to initial germination: six month old seeds germinated significantly earlier than freshly shed seeds but the soil types had no sig- nificant effect on days to initial germination (Tab. 2). The percentage germination of L. decurrens var B was very low in six month and freshly shed seeds in all germination media; freshly shed seeds did not germinate on the control and loamy soil, with no significant dif- ference in percentage germination observed on the other freshly shed seeds media and six month seeds media (clayey and sandy); highest percentage germination (39.60) of six month seed was observed in the control, which is significantly different from the 34.20 ob- served on loamy soil (Tab. 1). Age and soil type affected days to initial germination, and six month seeds germinated significantly earlier than freshly shed seeds (Tab. 2). Ludwigia hyssopifolia There was no significant difference in the high percentage germination observed in six month and freshly shed seeds of the control; six month seeds in the soil media showed a sig- nificantly higher percentage germination than freshly shed seeds on the soil media (Tab. 1). Seed age and soil type affected days to initial germination; six month old seeds germinated significantly earlier than all other species throughout the experiment (Tab. 2). Ludwigia erecta There was no significant difference in percentage germination of six month seeds and freshly shed seeds in the control; the high percentage germination of six month seeds var- ied significantly from the low percentage germination of new seeds on the three soil media. Seed age affected days to initial germination, six month old seeds germinated significantly earlier than freshly shed seeds. Soil type had no significant effect on days to germination (Tab. 2). Ludwigia leptocarpa There was no significant difference in percentage germination of six month seeds of control, loamy, clayey soils and the control of freshly shed seeds, which varied signifi- cantly from the low percentage germination of freshly shed seeds in loamy, clayey and sandy soils (Table1). Seed age and soil type affected days to initial germination, and six months old seeds germinated significantly earlier than freshly shed seeds (Tab. 2). Ludwigia octovalvis In L. octovalvis var linearis the high percentage germination (88.60) in six month seeds of the control is significantly different from the 74.20 percentage germination in the freshly 252 ACTA BOT. CROAT. 69 (2), 2010 OZIEGBE M., FALUYI J. O., OLUWARANTI A. U:\ACTA BOTANICA\Acta-Botan 2-10\293-Oziegbe.vp 11. listopad 2010 13:39:49 Color profile: Disabled Composite 150 lpi at 45 degrees ACTA BOT. CROAT. 69 (2), 2010 253 GERMINATION OF LUDWIGIA SPECIES (ONAGRACEAE) Tab. 1. Media growth mean values of percentage germination of freshly shed seeds and six month old seeds on germination media. FRESHLY SHED SEEDS SIX MONTHS OLD SEEDS CON- TROL LOAMY CLAYEY SANDY CON- TROL LOAMY CLAYEY SANDY Ludwigia abyssinica 94.40a 90.60a 94.20a 94.20a 92.20a 93.40a 92.80a 92.80a L. adscedens 1.80c 2.40c 1.00c 1.00c 90.80a 90.00a 89.20a 85.20b L. decurrens var A 5.20f 1.60f 1.40f 13.40f 93.40a 63.20b 38.00c 27.40d L. decurrens var B 0.00c 0.00c 1.00c 1.00c 39.60a 34.20b 1.60c 1.20c L. hyssopifolia 93.20a 5.20d 3.40d 32.40c 96.00a 91.60b 91.20b 90.20b L. erecta 94.20a 16.00e 28.00d 44.00c 94.00a 94.80a 93.80a 86.60b L. leptocarpa 89.40a 33.80d 34.80d 59.60c 84.40a 86.80a 82.40a 71.80b L. octovalvis var linearis 74.20b 5.40e 11.80d 15.20d 88.60a 85.80a 77.40b 68.60c L. octovalvis var brevisepala 32.20c 37.40c 35.20c 34.80c 91.40a 82.40b 85.60a 83.20b Means in each row followed by the same letter(s) are not significantly different at 5% level of significance Tab. 2. Media growth mean values of days to initial germination of freshly shed seeds and six month old seeds on germination media. FRESHLY SHED SEEDS SIX MONTHS OLD SEEDS CON- TROL LOAMY CLAYEY SANDY CON- TROL LOAMY CLAYEY SANDY Ludwigia abyssinica 4.20a 4.20a 4.0a 4.40a 4.00a 4.00a 4.20a 4.00a L. adscedens 17.6a 16.60a 17.00a 17.60a 3.80b 4.20b 4.40b 4.80b L. decurrens var A 12.60a 12.60a 12.80a 12.40a 3.40b 4.00b 4.20b 3.80b L. decurrens var B 0.00d 0.00d 7.40a 7.60a 4.60c 4.60c 6.60b 6.80b L. hyssopifolia 10.20c 11.60b 12.60a 12.80a 3.20ef 4.00d 3.80ed 3.00f L. erecta 6.40a 6.40a 6.40a 6.60a 4.00b 3.80b 3.80b 4.00b L. leptocarpa 5.40bc 6.00ba 6.60a 6.60a 4.40d 4.80dc 4.40d 5.40bc L. octovalvis var linearis 6.00b 6.60ba 6.60ba 6.80a 3.40d 4.40c 4.00dc 4.20c L. octovalvis var brevisepala 3.60a 3.60a 3.80a 3.80a 3.60a 3.60a 3.80a 3.80a Means in each row followed by the same letter(s) are not significantly different at 5% level of significance U:\ACTA BOTANICA\Acta-Botan 2-10\293-Oziegbe.vp 11. listopad 2010 13:39:49 Color profile: Disabled Composite 150 lpi at 45 degrees shed seeds of the control; high percentage germinations (85.80, 77.40 and 68.60) in six month seed on the soil media also varied significantly from the very low percentage germi- nations (5.40, 11.80 and 15.20) in freshly shed seeds of the soil media. The situation is quite similar in L. octovalvis var brevisepala in which a high percentage germination (91.40) of old seeds in the control is significantly different from low percentage germination (32.20) in the control of freshly shed seeds; high percentage germinations (82.40, 85.60 and 83.20) in six month seeds on soil media also varied significantly from the very low percentage ger- minations (37.40, 35.20 and 34.80) in freshly shed seeds on the soil media (Tab. 1). Seed age and soil type affected days to initial germination in L. octovalvis var linearis, and six month old seeds germinated significantly earlier than all other species throughout the experiment with the exception of L. hyssopifolia. (Tab. 2). Discussion The very high percentage germination shown by six month old seeds as compared to freshly shed seeds of Ludwigia adscendens, L. decurrens var A and B and L. leptocarpa re- veals that freshly shed seeds of these species exhibit some degree of dormancy and the dor- mancy was relieved at six months, because the seeds germinated early showing a very high percentage germination. The high percentage germination of six month old seeds and freshly shed seeds of L. abyssinica, L. hysopifolia, L. leptocarpa, and L. octovalvis in the control indicate there is no dormancy. The very low percentage germination of freshly shed seeds can be regarded as primary dormancy according to (KARSSEN 1982); this is a state in which freshly shed seeds do not germinate under any set of environmental condition until dormancy is relieved. Six month old seeds are expected to have higher desiccation than freshly shed seeds. Depending on the weed species, desiccation of seeds either improves germination or does not affect germination at all (KARSSEN et al. 1988). Reduced germina- tion at high densities may be a population maintenance mechanism. A weed species can produce seeds copiously under favourable conditions, while having only a few germinating at any time, thus maintaining a seed reservoir over extended periods (PALMBLAD 1968, MUOGHALU and CHUBA 2005). Higher percentage germination in dense arrays will result in greater competition. It is also possible that when several seeds germinate together the com- bined force of several roots growing simultaneously may help them to penetrate a hard soil. Many plants however have specific requirements for germination and subsequent estab- lishment (YAN 1976). Soil type might not be a critical factor in the distribution of six month old seeds of Ludwigia species because they showed very high percentage germination on all the three types of soils except in L. decurrens var A and B in which percentage germination was very low (Tab. 1). The significantly very low percentage germination of freshly shed seeds of (L. hyssopifolia, L. erecta, L. leptocarpa and L. octovalvis var linearis) on the three soils indi- cates that the higher percentage of freshly shed seeds of these Ludwigia species will not germinate immediately on dispersal in their natural habitat. Seed dormancy observed in some Ludwigia species will ensure that they survive adverse conditions in their environ- ment as dormant seeds only germinate when the environmental conditions favour the sur- vival of their seedlings. But the very high percentage germination of freshly shed seeds of L. abyssinica indicates that the seeds will germinate readily on three types of soils and the 254 ACTA BOT. CROAT. 69 (2), 2010 OZIEGBE M., FALUYI J. O., OLUWARANTI A. U:\ACTA BOTANICA\Acta-Botan 2-10\293-Oziegbe.vp 11. listopad 2010 13:39:49 Color profile: Disabled Composite 150 lpi at 45 degrees plant is not likely to have a soil seed bank. Influence of soil factor on germination of seeds is based on transmittance of light through the soil, which includes particle size, moisture content, particle colour and presence of organic matter (TESTER and MORRIS 1987). Studies have noted the decrease in soil transmittance with decreased particle size (BLISS and SMITH 1985). However, there have been no detailed measurements of the penetration of light through soil mixtures of widely different particle sizes, as would often be found in field sit- uations. Depending on the soil type, moisture content either increases or decreases the light transmittance of the soil. When sand is saturated the transmittance will increase, whereas saturation of clay and loam decreases the transmittance of light. This difference is probably attributed to a reduction in the reflection of light by the soil particles. When the particles are translucent, as in sand, transmission can increase through the particles; but in dark soil, re- duced reflection only leads to increased absorption by the particles (BLISS and SMITH 1985). The darker particles are thought to absorb the light. Another explanation is the increased re- flection by particles of the paler coloured soils, whereas the reflection in dark soils is lower (TESTER and MORRIS 1987). Six month old seeds are likely to contribute more to plant pop- ulation in Ludwigia species, in which they germinated significantly earlier than freshly shed seeds. Seed age and soil type have been found to affect germination in the Ludwigia species studied. Six month old seeds of Ludwigia species are more likely to contribute to plant pop- ulations than the freshly shed seeds germination media investigated. Retention of fruits and seeds on Ludwigia stems at maturity might be a strategy for maintaining plant population density. There is a need for further investigation of variations because of the very signifi- cant higher percentage germination than freshly shed seeds shown by L. species on growth media as well as seed germination at one year to determine seed contribution to plant popu- lations at this age. References AUGSPURGER, C. K., FRANSON, S. E., 1986: Wind dispersal of artificial fruits varying in mass, area, and morphology. Ecology 68, 27–42. BLISS, D., SMITH, H., 1985: Penetration of light into soil and its role in the control of seed germination. Plant Cell and Environment 8, 475–483. DOLAN, R. W., 1984: The effect of seed size and maternal source on individual size in a population of Ludwigia lectocarpa (Onagraceae). American Journal of Botany 71, 1302–1307. DUTARTRE, A., DANDELOT, S., HAURY, J., LAMBER, E., LE GOFF, P., MENOZZI, M-J., 2004: Les jussies: caractérisation des relations entre sites, populations et activités humaines. In: Implications pour la gestion. Rapport intermédiaire programme INVABIO, CEMA- GREF, Bordeaux, 44. GRIME, J. P., MASON, G., CURTIS, A. V., RODMAN, J., BAND, S. R., MOWFORTH, M. A. G., NEAL, A. M., SHAW, S., 1981: A comparative study of germination characteristics in a local flora. Journal of Ecology 69, 1017–1059. GROSS, K. L., 1984: Effects of seed size and growth form on seedling establishment of six monocarpic perennial plants. Journal of Ecology 72, 369–387. ACTA BOT. CROAT. 69 (2), 2010 255 GERMINATION OF LUDWIGIA SPECIES (ONAGRACEAE) U:\ACTA BOTANICA\Acta-Botan 2-10\293-Oziegbe.vp 11. listopad 2010 13:39:49 Color profile: Disabled Composite 150 lpi at 45 degrees HENDRIX, S. D., 1984: Variation in seed mass and its effects on germination in Pastinaca sativa L. (Umbelliferae). American Journal of Botany 71, 795–802. HOUSSARD, C., ESCARRE, J., 1991: The effects of seed weight on growth and competitive ability of Rumex acetosella from two successional old-fields. Oecologia 86, 236–242. HOWE, H. F., RITCHER, W., 1982: Effects of seed size on seedling size in Virola surinamen- sis: a within and between tree analysis. Oecologia 53, 347–351. JACOBS, S. W. L., PERRETT, F., JACOBS, B. J., 1994: Ludwigia peruviana (Onagraceae) in the Botany wetlands near Sydney, Australia. Australian Journal of Marine and Freshwater Research 45, 1481–1490. KARSSEN, C. M., 1982: Seasonal patterns of dormancy in weed seeds. In: KHAN, A. A. (ed.), The physiology and biochemistry of seed development, dormancy and germination, 243–270. Elsevier Biomedical Press, Amsterdam KARSSEN, C. M., DERKX, M. P. M., POST, B. J., 1988: Study of seasonal variation in dor- mancy of Spergula arvensis L. seeds in a condensed annual temperature cycle. Weed Research 28, 449–457. MARSHALL, D. L., 1987: Effects of seed size on seedling success in three species of Sesba- nia (Fabaceae). American Journal of Botany 73, 457–464. MARTIN, A., GRZESKOWIAK, V., PUECH, S., 1995: Germination variability in three species in disturbed Mediterranean environments. Acta Oecologica 16, 479–490. MAZER, S. J., 1989: Ecological, taxonomic, and life history correlates of seed mass among Indiana Dune angiosperms. Ecological Monographs 59, 153–175. MUOGHALU, J. I., CHUBA, D. K., 2005: Seed germination and reproductive strategies of Tithonia diversifolia (Hemls.) Gray and Tithonia Rotundifolia (P. M.) Blake. Applied Ecology and Environmental Research 1, 39–46. NAYLOR, R. E. L., 1993: The effect of parent plant nutrition on seed size, viability and vig- our, and on germination of wheat and triticale at different temperatures. Annals of Ap- plied Biology 123, 379–390. NISHITANI, S., MASUZAWA, T., 1996: Germination characteristics of two species of Poly- gonum in relation to their altitudinal distribution on Mt. Fuji, Japan. Arctic and Alpine Research 28, 104–110. PALMBLAD, I. G., 1968: Competition in experimental populations of weeds with emphasis on the regulation of population size. Ecology 49, 26–34. PENG, C. I., 1988: The Biosystematics of Ludwigia sect. Microcarpium (Onagraceae). An- nals of the Missouri Botanical Garden 75, 970–1009. PENG, C. I., 1989: The systematics and evolution of Ludwigia section Microcarpium (Onagraceae). Annals of the Missouri Botanical Garden 76, 221–302. RAMAMOORTHY, T. P., ZARDINI, E., 1987: The systematics and evolution of Ludwigia sect. Myrtocarpus sensu lato (Onagraceae). Monographs in Systematic Botany from the Missouri Botanical Garden 19, 1–120. RUAUX. B., SABINE, G., JACQUES, H., JEAN-PIERRE, B., 2009: Sexual reproduction of two alien invasive Ludwigia (Onagraceae) on the middle Loire River, France. Aquatic Bot- any 90, 143–148. 256 ACTA BOT. CROAT. 69 (2), 2010 OZIEGBE M., FALUYI J. O., OLUWARANTI A. U:\ACTA BOTANICA\Acta-Botan 2-10\293-Oziegbe.vp 11. listopad 2010 13:39:49 Color profile: Disabled Composite 150 lpi at 45 degrees SCHAAL, B. A., 1980: Reproductive capacity and seed size in Lupinus texensis. American Journal of Botany 67, 703–709. STAMP, N. E., 1990: Production and effect of seed size in a grassland annual (Erodium brachycarpum, Geraniaceae). American Journal of Botany 77, 874–882. STANTON, M. L., 1984: Seed variation in wild radish: effect of seed size on components of seedling and adult fitness. Ecology 65, 1105–1112. STANTON, M. L., 1985: Seed size and emergence time within a stand of wild radish (Raphanus raphanistrum L.): the establishment of a fitness hierarchy. Oecologia 67, 524–531. TESTER, M., MORRIS, C., 1987: The penetration of light through soil. Plant Cell and Envi- ronment 10, 281–286. VENABLE, D. L., LAWLOR, L., 1980: Delayed germination and dispersal in desert annuals: escape in space and time. Oecologia 46, 272–282. WOGU, A., UGBOROGHO, R. E., 2000: Seed morphology, germination and seedling charac- ters in Ludwigia species (Onagraceae) in Nigeria as aids to identification. Seed Science Technology 28, 657–697. WULFF, R. D., 1986: Seed size variation in Desmodium paniculatum, 3. Effects on repro- ductive yield and competitive ability. Journal of Ecology 74, 115–121. YAN, B. L., 1976: Density dependent seed germination strategies in colonizing versus non – colonizing plant species. Journal of Ecology 64, 375–380. ACTA BOT. CROAT. 69 (2), 2010 257 GERMINATION OF LUDWIGIA SPECIES (ONAGRACEAE) U:\ACTA BOTANICA\Acta-Botan 2-10\293-Oziegbe.vp 11. listopad 2010 13:39:49 Color profile: Disabled Composite 150 lpi at 45 degrees