Mineral nutrient relations in the aquatic carnivorous plant Utricularia australis and its investment in carnivory

Aquatic rootless carnivorous plants usually grow in nutrient-poor waters and take up all nutrients through their shoots, from either water or prey. The carnivorous plant Utricularia australis was sampled from 30 locations in the Trebon basin, Czech Republic, with the aim of investigating the plant's mineral nutrient economy in relation to its carnivorous habit. Relationships were sought between, firstly, mineral nutrient levels in the ambient water together with prey quantity captured in traps and, N, P, K, Na, Ca, and Mg contents of shoot tissues, and, secondly, the proportion of total plant biomass attributable to traps (i.e., investment in carnivory). Even at very oligotrophic sites with low prey capture rates, shoot N and P content was always well above the level below which growth limitation could occur. Plants recycled at least 57% of their N and as much as 81% of their P from senescent shoots, although they lost all of the K, Na, Ca, and Mg from senescing tissues. The P and K content of traps was much greater than that in leaves. Regression analyses revealed much greater uptake of N, P, and K from prey than from the ambient water. The proportion of total biomass invested in traps (range 23-61%) was positively correlated with CO2 concentration, but negatively with shoot N content. It is suggested that shoot N content acts as a key endogenous factor regulating investment in trap biomass through a negative feedback mechanism. Any decline in shoot N content, for whatever reason, stimulates an increase in trap production, resulting in enhanced prey catching and, concomitantly, an increase in shoot N content. A reduction in trap proportion then follows and the cycle starts again.