Abstract

Ferskvannsplanter: Rike på natrium (Na), og viktige for saltbalansen hos store beitedyr. Blyttia 56:120-125.
Aquatic macrophytes: Rich in sodium (Na), and important to the salt balance of large herbivores.

Whereas both sodium (Na) and potassium (K) are essential elements to animals, plants are considered to depend on K only. Both groups have evolved elaborate mechanisms to discriminate between Na and K uptake. Yet analyses from Norway (Tab. 1. and Tab. 2) of dry weight content of Na in water plants and land plants show very different values. Mean value of Na content (mmol Na per kg dry weight) in aquatic macrophytes (as defined by Rørslett 1991) is 193.3 (n=44). For other species of vascular plants from moist habitats the value is 74.6 (n=46), while for all groups of plants, from mesic to xeric habitas, it is 16.7 (n=346).
In old Norwegian literature there is some scattered information on the use of water plants as fodder. There are folk names for Menyanthes trifoliata referring to the salty taste of the plant. An old clerical note from 1594 states that according to local farmers Ranunculus peltatus was a popular fodder, and "as salty as seaweed".
Large herbivores living on a salt poor diet have a set of physical mechanisms to economise Na utilisation, and to reduce Na excretion. Yet salt deficiency is quite common. In periods of growth or milk production, physical salt stress can be extreme. In such situations, water plants rich in Na are valuable fodder. There are several references to seasonal, selective foraging on water plants by moose. Whether this has any structuring effect on the limnic vegetation is uncertain. Physiologically the high content of Na found in water plants is best explained as an effect of a decrease in the discrimination of K⁺/Na⁺ uptake by the roots in an O₂ poor medium, i.e. in water, or in the soil at the bottom of ponds and lakes.

Anders Often & Hans Staaland, Norges landbrukshøgskole, Institutt for biologi og naturforvaltning, Postboks 5014, N-1432 Ås

Blyttia 56(2)
Blyttias hjemmeside
Lagt ut 26.1.1999 Jan Wesenberg