Costs and Benefits of Carnivory in Plants: Insights from the Photosynthetic Performance of Four Carnivorous Plants in a Subarctic Environment

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Abstract
We measured photosynthetic performance in four subarctic carnivorous plants, Pinguicula alpina, P. villosa, P. vulgaris and Drosera rotundifolia, in order to test if there is a cost of combining photosynthetic and trapping devices into the same organ (leaves). We compared these data with published results on photosynthetic rates in subarctic non-carnivorous plants. In P. vulgaris, an experiment of prey addition and removal further tested the existence of a short-term benefit of increased nutrient gain from prey in terms of photosynthetic efficiency. Leaf area-based photosynthetic rates ( P a) ranged 2.0-3.0 μmol CO2 m-2 s-1, dry mass-based photosynthetic rates ( P w) 42-69 nmol CO2 g-1 s-1, and photosynthetic nitrogen use efficiency (PNUE) 29-45 μmol CO2 mol N-1 s-1. In general, P a and P w of carnivorous plants increased with leaf nitrogen content. When each species was analysed separately, those relationships were weak (P. alpina and P. villosa) or non-significant (P. vulgaris and D. rotundifolia). The photosynthetic rate of carnivorous plants was lower than that of other subarctic growth forms. In addition, P w for a given leaf nitrogen content was significantly lower in carnivorous plants than in non-carnivorous ones. No change in P a, P w or PNUE occurred as a result of prey capture manipulation, but treatments differed only slightly in nutrient content. P w and PNUE showed a trend to be higher in reproductive P. alpina plants as compared to vegetative ones. In P. vulgaris, however, an increased leaf respiration was found in reproductive plants.

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