Allelopathy among Submerged Hydrophytes

Presented as poster at the 9th International Symposium on Aquatic Weeds (Dublin, Ireland), October, 1994.

Aims of the present study:

Introduction:
Submerged and floating hydrophytes play a central role in many aquatic ecosystems. Being the main primary producers in oligotrophic freshwater habitats, the entire food web depends on their vegetation. In addition this vegetation provides refuge from predation for many animal species, including young stages of the larger ones, occupying open waters when adults (Engel, 1988). The understanding of the inter-relationships between the different components of the submerged vegetation is thus very important.

Many of Israel's freshwater habitats have been changed or even destroyed as a result of human interference, in most cases without prior study of the composition and nature of their submerged vegetation. Understanding of the inter-relationships between the different components of this vegetation is essential for a successful restoration of these habitats. It has been established elsewhere that allelopathic influences occur among submerged hydrophytes, that affect their existence and distribution in various aquatic habitats. However, such relationships for most of Israel's submerged species have not been studied. The present study is aimed at doing so.

During this study the secretion, from local hydrophytes, of substances that affect growth of target species has been established both qualitatively and quantitatively, using sensitive hydrophytes and lettuce seedlings as bioassay. Plants proven to secret such substances into the environment will be cultured together with other local hydrophytes, in different compositions, and the relative effects on the latter will be evaluated. The data collected during this study will pour light on the inter-relationships between the different hydrophytes composing the submerged aquatic communities and the processes and factors formative of such communities. In addition, on the basis of these data lists of possible compositions of species capable of successful co-existence, and possible locations within streams: sensitive species upstream and resistant ones downstream, will be prepared. The lists will be highly useful in planning the restoration of aquatic habitats.

Methods:
A) Eight target plants were chosen following field observations of phenomena that may indicate the occurrence of allelopathic inter-relationships (such as the marked dominance of a species in the habitat; absence of epiphytes on certain plant species, etc.). These target plants, i.e., Chara connivens, Ceratophyllum demersum, Myriophyllum spicatum, Najas marina, Potamogeton berchtoldii, P. pectinatus, Ruppia maritima, and Zannichelia palustris, were collected, each from a few sites, and transferred to Tel-Aviv University facilities for experimental studies.

B) Bioassay plants were selected from amongst sensitive Lemna species, clean stocks were prepared (following Landolt, 1989) and cultured in appropriate sterile media (e.g., Hoagland solution). Lettuce seedlings were also used as bioassay.

C) Preliminary experiments were carried out in order to choose the most convenient parameters for quantifying allelopathic effects on submerged hydrophytes. Such parameters are: growth rates, photosynthetic rates, chlorophyll content etc.

D) Target plants are being cultivated in large (200 l) containers, so that secretions with potential allelopathic effects accumulate in the media. Such media are being concentrated by partial removal of water (by vacuuming with a Buchi Rotavapor), and are used in bioassay-plants reaction experiments.

E) Bioassay plants are being cultured in different concentrations of target plants secretions, under controlled conditions. Their reactions are being monitored and recorded.

F) Pairs of target plants will be cultivated in large tanks in order to evaluate their effects on one another.

Results:
Media in which some of the submerged hydrophytes tested had been cultivated, showed marked effect on the growth rates of Lemna fronds, as well as on the germination of lettuce seeds, and the elongation of their rootlets. In these cases growth rates, germination percentages and elongation rates, respectively, were lower in the hydrophytes media than in the controls. Other submerged hydrophytes showed no such effect.

Ceratophyllum demersum, a common dominant species in aquatic habitats in Israel, is one of the plants affecting the growth of Lemna (Fig. 1), as well as the germination of lettuce seeds (Fig. 2), and the elongation of its rootlets (Fig. 3). C. demersum, like most hydrophytes that affect the growth of Lemna, reduces its growth rate by 10-25%.

Chara connivens, however reduces the growth rates of the Lemna much more (Fig. 4). During the first two days after exposure to the experimental conditions, the difference between Lemna growth rate in "C. demersum water" and that in the controls 15%. After three days the growth rates decrease, but the differences between that of treated plans and that of plants in control conditions increased. The effect on growth of concentrated media in which the tested hydrophytes had been cultured was more or less proportional to the level of concentration, the more concentrated the medium, the more prominent its effect.





Fig. 4


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