Photosynthetic CO₂ affinity of the aquatic carnivorous plant Utricularia australis (Lentibulariaceae) and its investment in carnivory

Abstract

Aquatic carnivorous plants usually grow in shallow dystrophic waters poor in inorganic N and P. Utricularia australis was chosen as a model plant for its prolific distribution and great ecological plasticity. The photosynthetic CO 2 compensation point and factors associated with investment in carnivory and capture of prey were measured in 17 U. australis micropopulations in Trˇebonˇbasin, Czech Republic, together with water chemistry factors at these sites differing greatly in their trophic level, water hardness, and prey availability. Apical shoot growth rate was estimated at some oligotrophic sites. The micropopulations differed greatly in the proportion of traps with animal prey (2.7-70%, mean 26%), trap proportion to total biomass (1.4-42%, mean 26%), mean trap biomass (0.7-63 lg trap À1 , mean 19 lg), and maximum trap size (1-3 mm, mean 2.0 mm). CO 2 compensation points ranged from 0.7 to 6.1 lM (mean 2.6 lM). A weak HCO 3 À use (compensation point 0.51 mM) was found in plants growing in alkaline water. Trap biomass proportion did not correlate significantly with prey capture and CO 2 compensation points with ambient [CO 2 ]. A very rapid apical growth (2.5-4.2 new nodes day À1 ) occurred in sand pits. Thus, HCO 3 À use in U. australis can be induced by growing at very high pH. CO 2 compensation points resembled those known in other aquatic non-carnivorous plants. They did not reflect carnivory. In spite of very rapid apical shoot growth, the relative growth rate of U. australis can be zero in oligotrophic habitats without prey.

The smallest but fastest

“…In wide-spread U. australis, traps usually represent only about one-third of dry biomass of mature shoot segments as a structural investment in carnivory (i.e., structural cost), 6,7,31 but the total trap R D amounted to 67% of the total shoot R D . If the net photosynthetic rate of traps of aquatic Utricularia (per unit fresh weight) reached only 7-10% of the values for leaves or shoots even under optimum conditions, then this combination means that traps represent a very high energetic (metabolic) cost for the plants.…”

“…6,14,17,18,31 Starting with the novel study by Richards, 14 it has therefore been assumed that a mutualistic interaction between the plant and trap commensal community prevails over catching animal prey for mineral nutrient uptake in these barren waters. [17][18][19]38 Trap commensal microorganisms are not strictly specialized to living inside the traps.…”

“…In other words, it would take between 40-70 days for an empty trap to accumulate the measured N or between 15-23 days for them to accumulate the P if there were only spontaneous firing and water recirculation mechanisms occurring. By the application of the model data for the relationship between trap volume and biomass and for shoot N and P content in U. australis, 6,17,28,31 one can further estimate that the total N and P amount inside the traps 17 represents only about 3.5% of total plant N and 1.2% of P. What conclusions can be drawn from these model data? (A) The calculated N and P input rates from the ambient water due to both spontaneous firings and hypothetical water recirculation are so low that each trap without prey can only reach these nutrient levels after a long time, which is comparable with or even longer than the trap life-span.…”

“…(D) Utricularia species commonly grow in very oligotrophic, barren waters with very low prey availability. [4][5][6]14,17,18,31 It can be inferred that the N and P uptake affinity of Utricularia shoots is very high (the limits for uptake should be below 1 μM for NH 4 + and 0.1 μM for phosphate), while that for N-and P-containing substances in the traps without prey can be very low or zero. 17,28 (E) If the trap microorganisms are beneficial for the plant, the ecological benefit can occur only in traps with captured prey to facilitate prey C supply to the trap fluid is undoubtedly an important additional maintenance (energetic) cost of traps.…”

“…Their entirely rootless shoots are mostly linear and, under favorable conditions, they exhibit very rapid apical shoot growth of 3-4 leaf nodes per day while their shoot bases decay at this same high rate. [5][6][7] Although the Utricularia traps are the smallest among those of carnivorous plants, they are arguably the most sophisticated and intricate ones. They have always fascinated scientists.…”