Relations between K+ uptake and photosynthetic uptake of inorganic carbon by aquatic plants

The uptake of K+ by the leafy shoots of four submersed higher aquatic plants (Elodea canadensis, Ranunculus aquatilis, R. trichophyllus, and Callitriche hamulata) with different HCO3- affinity was measured in successive 2-h periods under the conditions of high or low photosynthetic rates (i.e. at pH 7.5 or 10). At pH 7.5 the uptake of K+ by species with the higher HCO3- affinity (E. canadensis, R. trichophyllus) was significantly faster than that by species with a lower HCO3- affinity (R. aquatilis, C. hamulata). In the former group of species, the K+ uptake rate at pH 7.5 was 1.7 - 3.5 times higher than at pH 10. At pH 10, the soft-water species, R. aquatilis, had the lowest net photosynthetic rate (P-N) of the three HCO3- users but, in contrast to the relative hard-water species, R. trichophyllus, showed a small K+ efflux (47 nmol kg(-1) d(-1)). Thus, K+ uptake by shoots was not strictly correlated with P-N. A significant K+ efflux (73 - 86 nmol kg(-1) s(-1)) occurred from all HCO3- users in darkness. The relatively low K+ uptake by the strict CO2 user, C. hamulata, was quite independent of PN and light or darkness. It may be suggested that uptake of K+ by shoots of submersed plants depends on their HCO3- affinity.