The Biology and Cultivation of Red Australian Aldrovanda vesiculosa

Lubomír Adamec
Academy of Sciences of the Czech Republic • Institute of Botany • Dukelská 145 • CZ-379 82 Trebon, Czech Republic • adamec@butbn.cas.cz

Keywords: cultivation: Aldrovanda.

Aldrovanda vesiculosa L. (Droseraceae) is an attractive but very rare aquatic carnivorous plant from the Old World. In the last five years, Australians have been trying to grow two "red" strains of Aldrovanda endemic to their country (Wilson, 1995; Daly, 1997a, 1997b, 1997c, 1998; Schell, 1997). Among other desirable attributes, these varieties do not require a winter dormancy. In this paper I present notes pertaining to these Australian plants--a more general discussion of Aldrovanda can be found elsewhere (cf. Breckpot, 1997; Adamec, 1997).

In Australia, Aldrovanda is far less common than it once was, and it is designated a rare and threatened plant. In this article I discuss specimens from a poor site at Girraween Lagoon, approximately 30 km southeast of Darwin (Wilson, 1995), and a swamp near Batemans Bay at the East Coast, approximately 110 km southeast of Canberra (S. Jacobs, personal communication). In contrast with most European sites, dense growths of other aquatic plants grow with Aldrovanda at Australian sites (Wilson, 1995; S. Jacobs, personal communication).

Biological particulars

There are four main differences between the European temperate and the Australian (sub)tropical strains of Aldrovanda I have studied: the colors of the plants, their overwintering characteristics, the particulars of their of axillary buds, and their different sensitivities to boron. I discuss the first three below. The details of the boron sensitivities are described in the next section.

Color: The European and Japanese plants are light green and contain a great deal of plumbagin (a sulphur-yellow pigment) but no anthocyanins. The color of Australian plants may be the same green, or slightly rose to deep purple (Figure 1) because they contain anthocyanins as well as plumbagin. Irradiance is the main factor regulating the color of the Australian plants--if the plants are exposed to sunshine for at least two hours a day they become red in summer and rose in winter. If grown indoors, supplemental fluorescent lights help the plants become red.

Overwintering Characters: In contrast with the European and Japanese Aldrovanda, strains from both Australian sites can grow year round under natural light at a daily temperature of about 18°C or higher (at 18-19°C, the growth is very slow and the plants are in the winter growth form). In an indoor aquarium under natural light, the growth rate of east coast plants in June (spring) is approximately 0.7 new leaf whorls per day. The plants average 23-26 cm long, with 31-35 adult leaf whorls. During the winter, the plants are much shorter (3.3-12.4 cm, 10.5-19 whorls) and their growth rates are only approximately 0.2-0.5 whorls per day.

Even though they may be grown continuously, Australian Aldrovanda can be stimulated into producing turions if the plants are subjected to daily temperatures below approximately 18°C (cf. Adamec, 1999). In my outdoor culture, Australian plants started forming turions in early October (autumn), one month later than the temperate strains did. Shoots died very slowly (even at 8-10°C) and were still attached very firmly to the turions. At the end of October, shoots with turions sank to the bottom.

The turions of Australian strains are more weakly dormant than temperate strains are (cf. Adamec, 1999). When plants forming turions were transferred from the Domanínsk_ wetland to an indoor aquarium at 19-24°C (on 27 September), they immediately resumed growth. However, when Australian strains were transferred from the outdoor culture (6-7°C) to indoors (18-20°C) at the end of October they stayed dormant for several weeks. (It is worthwhile to note that these plants demonstrated interesting characteristics. During the first two weeks, big axillary buds formed on the shoots close to the turions, and new plants started growing from these buds. Thus, the dormancy is only confined to main-shoot turions.) In summary, the turions of north Australian plants were more deeply dormant than those from the east coast, and overall, the Australian strains are not as adapted to cold overwintering as the temperate ones are.

Axillary Buds: Long specimens of Australian plants growing both indoors and outdoors in summer frequently form branches from axillary buds (i.e. 1-6 buds per plant). In contrast, temperate strains form axillary branches poorly, and mainly when their apices are damaged.

In outdoor experiments, both strains of Australian Aldrovanda grew and proliferated well from early June to late October 1998 (after Adamec & Tich_, 1997). Plants from north Australia grew vigorously in a nylon enclosure in a shallow dystrophic wetland near Trebon, where Polish Aldrovanda had been grown previously (Adamec, 1995). While the doubling time of apices (15.5-26.0 days) was the same as for the Polish Aldrovanda, the Australian plants branched much more frequently--branches were formed every 3.3-4.5 leaf whorls, as opposed to approximately every 6 whorls as in the Polish plants. Branch proliferation almost stopped in late August and the plants shortened gradually to ca 1.8 cm.

How to grow Australian Aldrovanda

To grow Australian Aldrovanda successfully you must modify the methods that are appropriate for temperate strains (see Adamec, 1997; also Wilson, 1995; Daly, 1997a, 1997b, 1997c, 1998; Schell, 1997). In summary, the Australian strains can be grown outdoors in big containers or small aquaria as normal (Figure 2), from May to October (i.e. spring to autumn), and then transferred to indoor aquaria. Turions can probably overwinter in water in a refrigerator at 3-5°C. It is convenient to grow them indoors in small aquaria (3-20 liter) under natural light at temperatures exceeding 18°C for the whole year. It is the best if the aquaria stand close to an east or southeast oriented window so the plants are irradiated by direct morning sunlight for a few hours each day. This should be supplemented by diffuse light for the rest of the day. A 10 Watt fluorescent lamp can help. Australian Aldrovanda tolerate less irradiance but higher temperatures (optimum 25-29°C; maximum 34°C) than do temperate strains. In order to reduce the growth of filamentous algae, the aquarium wall facing the sun should be shaded by a sheet of paper from about 2 cm below the water surface to the bottom. During the summer, the aquarium should be shaded with a sheet of fine paper to keep it cool.

In indoor aquaria, Australian Aldrovanda flower richly in summer at 20-29°C. The flower stalks of the Australian ones were red (Figure 1). No flower set seeds, but they do in the wild (S. Jacobs, D. Wilson, personal communication).

Robust sedge or reed litter (i.e., dry dead leaves or straw collected in late winter) is the required substrate. The optimum amount of dry litter is approximately 4+0.5 g per three-liter aquarium. A dose of fresh litter lasts for 2-3 months. The water pH should be 6.0-7.5. A high CO₂ concentration >0.1 mmol/l is necessary for vigorous growth of Aldrovanda, and you may wish to add CO₂ to the water (see the article about CO₂ generators in this issue, on page xx).

As described for temperate Aldrovanda (Adamec, 1997) the cultivation water should be rather poor in N and P to control algal growth. In one aquarium, very low concentrations were measured (e.g., in µg/l: PO₄-P, 10.2; NH₄⁺-N, 0.0; NO₃^--N, 11.8; NO₂^--N, 6.5). You may add 1-2 small water snails (e.g. Planorbis) to a three-liter aquarium, but too many foul the water (Underwood, 1991).

In a previous paper (Adamec, 1997), I described how a disorder commonly exhibited by cultivated Aldrovanda was determined to be a boron deficiency. It has been confirmed recently that an addition of 0.05 ml/l of complete microelement solution (see Table I) once or twice a year cures the plants. I have often observed this disorder in the Australian strains, but these plants are rather sensitive to boron--adding 0.5 mg/l of H₃BO₃ can damage them. However, the addition of 0.03-0.05 ml/l of the new, complete microelement solution will cure the plants of this disorder. Symptoms of iron deficiency (yellowish but healthy apices) are suppressed by an addition of 1-2 mg/l of FeSO₄.7H2O or 2-3 mg/l of Fe-EDTA.

Table I. Composition of the Gaffron microelement solution and final concentrations of salts and elements after an addition of 0.05 ml of the stock solution to one liter of water.

Acknowledgements: I am very grateful to Dr. Surrey Jacobs and Mr. Denis Daly for sending me the Australian plants of Aldrovanda and providing interesting data.

References

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