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 (PN) 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 s^-1). Thus, K⁺ uptake by shoots was not strictly correlated with PN. 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. This revised version was published online in July 2006 with corrections to the Cover Date.