SELECTIVE ABSORPTION OF CATIONS BY HIGHER PLANTS

Abstract

Some 20 phanerogams representing different ecological types were cultivated in complete nutrient solns. containing several cations in equivalent amts. After approx. 2 months of growth the cation composition of the plants was detd. using Lundegardh''s method of quantitative spectral analysis. The cations most closely studied in this way were Na, K, Rb, Mg, Ca, Sr, and Mn. Li and Cs were also studied but less extensively; Cu only cursorily. The results obtained were as follows: The differences between the plant spp. cultivated in a given soln. are very unequal in regard to the different cations. They are very great in the case of Na and Mn contents; the maximum values are about 20-60 times greater than the minimum. They are considerable in the case of the Li, Mg, Ca, and Sr contents; the maximum values are about 3-5 times greater than the minimum. For K, Rb, and Cs, on the other hand, only moderate differences were observed; the maximum values were 2-3 times greater than the minimum. The cause of variation of different cations in such unequal magnitudes is not known. The differences observed are, for the most part, truly specific. Single plant species are constantly (irrespective of the year of cultivation and composition of the culture solns.) rich in certain cations and other spp. as constantly rich in other cations. All halo-phytes cultivated (Salicornia herbacea, Plantago maritima, Atriplex litorale, A. hortense) were very rich in Na; but Aster tripolium contained only moderate quantities. Fagopyrum esculentum, Zea mays, and Helianthus annuus were distinguished by their unusually large exclusion of Na. All Chenopodiacean plants, as well as Fagopyrum, were remarkably rich in Mg. Ca and Sr were absorbed in greatest amts. by Helianthus, Fagopyrum, and Sinapis alba; Avena, Zea, and Spinacia absorbed these 2 cations in the least amts. All plants studied absorb Rb and Cs with almost the same rapidity as K. They also take up Sr almost as readily as Ca. All the other cations studied show no distinct mutual correlation in their absorption rates by the plants. The specific differences between the absorption characteristics of different plant spp. are so great that it is not possible to arrange the absorption of the cations studied in any certain order which would be valid for all plant species investigated. The following data are, however, valid for the majority of the species studied. K, Rb, and Cs are, in general, the cations most copiously accumulated. The next place is in most cases occupied either by Ca and Sr or, about as frequently, by Mg, Li, Na, and Mn; the latter were taken up by most plant species to a distinctly less extent than all of the 6 cations so far mentioned. The absorption of Na amounted generally to about 1/50 to 1/2 of the simultaneously absorbed K. K and Rb ions and Ca and Sr ions depress the absorption of each other more than do any other cations studied in this respect. Rb or Sr, in the presence of an excess of K or Ca, respectively, do not obey the general rule that the absorption ratio of a given ion increases when the conc, of that ion in the medium decreases. On the contrary, the absorption ratios of Rb and Sr remain constant if only the total conc, of K + Rb, respective to Ca + Sr, is held constant. These findings are explained by assuming that the ions K and Rb, and Ca and Sr, behave in the salt absorption of plants somewhat as identical ions, or as 2 isotopes of the same element. The complexity of the selective salt absorption of higher plants is stressed.