The effect of Stratiotes aloides L. and nutrients on the growth rate of Lemna minor L.

We tried to find the reasons for frequently observed low biomass of Lemna minor among floating rosettes of Stratiotes aloides as opposed to the abundance of Spirodela polyrhiza in these stands. The effect of S. aloides on L. minor was analysed in outdoor culture experiments. The duckweed collected from among dense stands of the floating form of S. aloides (test sites) showed a significantly lower growth rate than that from stands free of the water soldier (control sites). Water from the latter stands was significantly richer in nitrate-nitrogen and ammonium-nitrogen, but not in soluble reactive phosphorus suggesting possible nutrient depletion as a reason for observed differences. Growth rates of L. minor were indeed significantly correlated with in situ concentrations of all dissolved nutrients and showed saturation at around 0.18 mg N-NH₄ dm⁻³ and 0.05 mg SRP dm⁻³ above which the growth rates did not increase significantly. We used stepwise regression to test the combined effect of nutrients and the presence/absence of S. aloides on the growth rate of L. minor. The regression showed a negligible effect of ammonium ions and nitrates on the growth rate of the duckweed. At low concentrations of soluble reactive phosphorus the presence of both the S. aloides and SRP concentrations significantly affected the growth of L. minor. Above saturating SRP concentrations none of the two effects was significant. Results of our experiments seem to suggest that the presence of S. aloides exerts a negative effect on the growth of L. minor only at low concentrations of soluble phosphorus. Since the depletion of nutrients in water by the floating form of S. aloides is unlikely, we point to S. polyrhiza as a superior competitor for nutrients and suggest a possible allelopathic effect of the water soldier on L. minor.     

硝态氮和铵态氮对墨兰生长发育的影响

以无土栽培的方法研究不同氮水平的硝态氮和铵态氮对墨兰［Ｃｙｍｂｉｄｉｕｍｓｉｎｅｎｓｅ（Ａｎｄｒ．）Ｗｉｌｌｄ．］生长发育和某些生理特性的影响。１ｍｍｏｌ／Ｌ和１０ｍｍｏｌ／Ｌ的硝态氮和铵态氮处理都促进叶芽和叶片正常生长,前者似乎比后者好一些。５０ｍｍｏｌ／Ｌ的两种形态的氮均使叶片生长缓慢,尤其是铵态氮。１和１０ｍｍｏｌ／Ｌ硝态氮处理则正常开花,而５０ｍｍｏｌ／Ｌ则例外。铵态氮处理各种浓度均不形成花芽。建议墨兰营养生长期施用１ｍｍｏｌ／Ｌ的硝态氮或铵态氮,但生长后期则应施１—１０ｍｍｏｌ／Ｌ硝态氮,以利花芽分化。作者认为,墨兰生长缓慢与光合速率极慢和硝酸还原酶活性极低有关,墨兰开花与否和Ｅ／Ｎ比大小有关。