Tissue mineral nutrient content in turions of aquatic plants: does it represent a storage function?

Turions are frost resistant, vegetative dormant organs produced by aquatic plants. Tissue N, P, K, Ca, and Mg content was estimated in turions of 5 rooting, non-carnivorous plant species of the genera Caldesia, Hydrocharis, and Potamogeton and in 7 rootless carnivorous plant species of the genera Aldrovanda and Utricularia, and compared with concentrations of these nutrients in ambient waters either in the field or in culture. Mean turion content of N in all species was 1.87 % DW, P 0.26 %, K 0.65 %, Ca 0.15 %, and of Mg 0.19 % DW. Of all nutrients, only turion K content differed significantly between carnivorous and non-carnivorous species (0.48 +/- 0.03 vs. 1.04 +/- 0.10 % DW). For all plant species and sites (n = 18), turion nutrient content did not depend on nutrient concentrations in the ambient water at the time of turion initiation. Results of a literature survey conducted for the 12 plant species revealed that mean turion nutrient content was moderately (N, P) or markedly (K, Ca, Mg) lower than shoot or leaf nutrient content for the same 12 species. In conclusion, turions of aquatic plants also represent storage organs for mineral nutrients (N, P) though their storage function is presumably less distinct than in the case of carbohydrates.