1999 Volume 44 Issue 1 Pages 19-28
The effects of riverside eutrophication on plant growth were examined using Persicaria thunbergii, a typical annual species abundant in riverbanks in Japan. The ecological characteristics of this species were studied under three different conditions, viz. cultivated under experimental nutrient gardients in glasshouse, directly planted on riverbanks, and planted in pots and placed in streams. In the glasshouse, the biomass and leaf area of this species lineraly increased with increasing nutrient levels (Fig. 1). The relative growth rate (RGR) and leaf area ratio (LAR) also increased in a similar fashion, and RGR gained a maximum of 0.96mg mg-1 week-1, though the net assimilation rate (NAR) did not show any particular trend (Fig. 2). The higher LAR was attributed to the higher allocation ratios to the leaves (Fig. 3), which resulted in enhanced RGR at higher nutrient levels. Under field conditions, the biomass and top: root ratio of P. thunbergii planted on the riverbanks of mid- and lower streams were higher than those in the upper stream area devoid of eutrophication (Fig. 4, 6). Morphological indices (MI) of the plants grown in the mid-and lower streams were higher (MI>1) than those in the plants grown in the upper stream (MI<1), which may be due to horizontal expansion of the shoots of the plants grown at higher nutrient levels (Fig. 5). The individuals placed in the mid-and lower streams, however, died in some areas due to frequent flooding, and did not show any apparent growth response to nutrient levels. These findings indicated that P. thunbergii does not reach a normal growth under frequent flooding. It is concluded that the changes in biomass, leaf area and top: root ratio of P. thunbergii are quantitative features referred to nutrient levels except for the flooding conditions, and therefore this species can serve as an indicator plant to assess the effects of riverside eutrophication.
