S.AfrJ.Bot., 1993,59(4)

Seed germination requirements of Alectra sessiliflora

C.A. van der Merwe Department of Botany, University of Stellenbosch, Stellen­bosch, 7600 Republic of South Africa

Received 23 March 1993; revised 19 May 1993

Alectra sessiliflora (Thunb.) seeds do not germinate spon­taneously. This species is host-dependent since it requires a chemical stimulant from the host roots in order to germinate. Results with GR-7, a synthetic germination stimulant, show­ed that the seeds are totally dependent on a stimulant for germination. It was found that preconditioning of the seed as well as the germination temperature play an important role during germination.

Sade van Alectra sessiliflora (Thunb.) ontkiem nie spontaan nie. Hulle benodig 'n chemiese stimulant wat deur die gas­heerplant se wortels afgeskei word. Resultate met GR-7, 'n sintetiese ontkiemingstimulant, het getoon dat die sade totaal afhanklik van 'n stimulant vir ontkieming is. Ontkiemings­temperatuur en vooraf-kondisionering van die saad speel 'n belangrike rol tydens die ontkieming van die saad.

Keywords: Alectra sessiliflora, germination temperature, pre­conditioning, stimulant.

Alectra sessiliflora (Thunb.) is an angiospermous root para­site belonging to the family Scrophulariaceae. It parasitizes a wide range of hosts, mainly members of the Poaceae, such as Eleusine, Eragrostis and Imperata (Visser 1981).

The seeds of A. sessiliflora are extremely small. As small seeds contain very little reserves, contact with a suitable host must be established as soon as germination has occur­red. Dependence on stimulation for germination has been demonstrated for other root parasite members, viz. A. vogelii (Botha 1948; Visser 1975; Visser et al. 1987), Lathraea and Tozzia (Heinricher 1917), Striga asiatica (= S. lutea) (Saun­ders 1933; Brown & Edwards 1944) and S. gesnerioides (Brown 1965; Edwards 1972; Musselman 1980).

Very little information exists about the germination of A. sessiliflora. This study was undertaken to determine the effect of a germination stimulant as well as the effect of germination temperature and preconditioning of the seed on its germination.

Seeds were harvested from plants growing in the Pieters­burg district. The host was Eragrostis plana. After cleaning, the seeds were stored in sealed containers at 3 ± 2°C.

The synthetic analogue of strigol, GR-7 (after G. Rose­bury), which has been shown to stimulate the germination of Striga (Johnson et al. 1976) and Alectra (Visser & Johnson 1982), was used in all experiments. Stimulant concentrations of 0, 0.001, 0.01, 0.1 and 1.0 mg.dm~3 were used.

Seeds were surface-sterilized in 70% (v/v) ethanol for 2 min, succeeded by 0.5% (m/v) NaOCI for 10 min. They were then thoroughly washed in distilled water and dried before use. All equipment was surface-sterilized before use.

Preconditioning of seed was carried out in plastic con­tainers on a layer of coarse, moist, sterilized quartz sand.

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Between 15 and 45 seeds were placed on Whatman GF/A glass fibre discs (10 mm diameter). These discs were placed on a layer of filter paper which was placed on the sand. Containers were sealed to prevent desiccation and were placed in an incubator at 30°C in the dark. Preconditioning times were chosen arbitrarily as 0, 6, 12, 18, 24 and 36 days in the dark.

After preconditioning, discs with seeds were removed from the containers and placed in Teflon germination trays. Stimulant solutions were added, the trays were sealed in glass tubes (Visser 1975) and finally placed in darkened incubators at 10, 20 and 30°C, respectively. After 5 days, germinated seeds were counted under a dissecting micro­scope. A seed was considered germinated when the radicle had penetrated the testa.

Five replications were used per treatment. Results were statistically analysed and the standard errors calculated.

No germination was obtained in the absence of GR-7. Optimal germination occurred at a stimulant concentration of 0.1 mg.dm~3. There was no significant change in ger­mination percentage with a further increase in stimulant concentration.

In the case of Orobanche crenata, germination is in­hibited by high concentrations of the natural stimulant. This phenomenon is ascribed to the possible presence of germi­nation inhibitors in the root exudate (Mallet 1973). The possibility that higher GR-7 concentrations than those used in this investigation will inhibit the germination of seed of A. sessiliflora was, however, not investigated.

From Figure 1 it is evident that the highest germination percentage (87.4%) is reached at 20°C. Maximum germina­tion at 20°C is reached after only 6 days of preconditioning, whilst germination at 30 and 10°C reaches a maximum after 12 and 18 days, respectively (Figure 1). Although germina­tion is possible without any preconditioning (50% germina­tion at 20°C), a period of preconditioning is a prerequisite for optimal germination in A. sessiliflora.

A decrease in germination at 10°C occurred after periods of preconditioning longer than 18 days. At 20 and 30°C, this decrease occurred after 24 days. This decrease is in contrast to the hypothesis of Brown and Edwards (1946), who stated that a compound with stimulating action is pro­duced and accumulates during preconditioning. This could lead to a situation where spontaneous germination would

100

80

l c 0 60 '"§ c §

40 OJ (9

20

0 12 18 24 36

Preconditioning (days)

Figure 1 The effect of preconditioning on germination at differ­ent temperatures of seed of A . sessilijIora in the presence of 1 mg dm~3 GR-7. Vertical bars represent standard errors.

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take place, which is not true for A. sessiliflora. The reason for the decrease in germination with pro­

longed preconditioning is not clear. A possibility is that the seed reserves could become depleted during long periods of preconditioning. Another possibility is that the seeds move into a state of secondary (wet) dormancy owing to the pro­longed preconditioning, as first reported by Parker and Reid (1979). This might possibly be a self-protecting mechanism to ensure that seeds in the conditioned state will not use up all their reserves before germination occurs.

In conclusion, the process of preconditioning, as it appears in A. sessiliflora, is not yet fully understood. The need for such a process appears to be an established feature for the parasitic members of the family Scrophulariaceae. The confirmation of the dependence on a stimulant for germination is, however, of great importance in the study of the chemical control of this weed.

Acknowledgements The author thanks Prof. O.T. de Villiers, University of Stel­lenbosch, for his valuable comments.

References BafHA, PJ. 1948. The parasitism of Alectra vogelii Benth. with

special reference to the germination of its seeds. II. S. Afr. Bot. 14: 63 - 80.

BROWN, R. 1965. The germination of angiospermous parasite seeds. In: Encyclopedia of plant physiology, Vol. 15/2, ed. W. Ruhland, pp. 925 - 932. Springer-Verlag, Berlin.

BROWN, R. & EDWARDS, M. 1944. The germination of the seed of Striga lutea. I. Host influence and the progress of germination. Ann. Bot. 8: 131 - 148.

BROWN, R. & EDWARDS, M. 1946. The germination of the seed of Striga lutea. II. The effect of the time of treatment and of concentration of the host stimulant. Ann. Bot. 10: 133 - 144.

EDWARDS, W.G.H. 1972. Orobanche and other plant parasite factors . In: Phytochemical ecology, ed. J.B. Harborne, pp. 235 - 248. Academic Press, New York.

HEINRICHER, E. 1917. Zur Physiologie der schmarotzenden Rhinantheen, besonders der Halbparasitischen. Naturwissen· schaften 8: 113 - 119.

JOHNSON, AW., ROSEBURY, G. & PARKER, C. 1976. A novel approach to Striga and Orobanche control using syn­thetic germination stimulants. Weed Res. 16: 223 - 227.

MALLET, A.1. 1973. Studies in the chemistry of the Orobanche crenata germination factor present in the roots of Vicia faba and other host plants. In: Proc. EuT. Weed Res. Council Symp. Parasitic Weeds.

MUSSELMAN, LJ. 1980. The biology of Striga, Orobanche, and other root-parasitic weeds. A. Rev. Phytopathol. 18: 463 - 489.

PARKER, C. & REID, D.C. 1979. Germination requirements of Striga species. Proc. 2nd Int. Symp. Parasitic Weeds, pp. 202 -210. North Carolina State University, Raleigh, NC.

SAUNDERS, AR. 1933. Studies in phanerogamic parasitism with particular reference to Striga lute a Lour. Dept. Agric. Union S. Afr. Bull. 128: 1 - 56.

VISSER, J.H. 1975. Germination stimulants of Alectra vogelii Benth. seed. Z. PflPhysiol. 74: 464 - 469.

VISSER, J.H. 1981. South African parasitic flowering plants. Juta, Cape Town.

VISSER, J.H., HERB, R. & SCHILDKNECHT, H. 1987. Re­covery and preliminary chromatographic investigation of ger­mination stimulants produced by Vigna unguiculata Walp. cv. Saunders Upright. I. Pl. Physiol. 129: 375 - 381.

VISSER, J.H. & JOHNSON AW. 1982. The effect of certain strigol analogues on the seed germination of Alectra. S. Afr. 1. Bot. 1: 75 - 76.

S.-Afr.Tydskr.Plantk., 1993, 59(4)

A note on Amphisbetema (Dasyaceae, Rhodophyta), from Natal, previously unknown in Africa

R.E. Norris

National Botanical Institute, Kirstenbosch, Cape Town, Re­public of South Africa Present address: Department of Botany, University of Hawai'i at Manoa, 3190 Maile Way, Honolulu, Hawai'i 96822-2279, USA; and "Botanical Research Institute of Texas, 509 Pecan Street, Fort Worth, Texas 76102-4060, USA

Received 17 February 1993; revised 7 May 1993

Keywords: Amphisbetema, Dasyaceae, Ceramiales, Rhodo­phyta, taxonomy.

" Address to be used for correspondence.

Amphisbetema Weber-van Bosse (1913), an erect plant with polysiphonous structure and dorsiventral branching, was described as a monotypic genus of uncertain position, the name having been derived from the Greek root meaning 'controversial', although Weber-van Bosse suspected it to be a member of the Rhodomelaceae where members of the Dasyaceae were placed at that time. Subsequent studies on the Dasyaceae (Kylin 1956; Parsons 1975) gave brief mention of Amphisbetema, accepting it in the family but noting in particular that further investigations on repro­ductive structures were necessary before it could be related to other genera of that family.

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1 Figure 1 The three plants in the Nat 5292 collection. The plant in the lower left of the figure is the one on which pre­sumed immature tetrasporangial stichidia were found.