Calcium in the Vacuoles and Cell Walls of Plant Tissue )

Abstract

Summary Current knowledge of Ca uptake and modes of transport in higher plants were discussed. The relationships of Ca both within the cell and in relation to metabolic movement along transport paths throughout the plant were considered. It was stressed that Ca flows into the plant for the most part passively with the water flow of transpiration. The deposition of this often surplus amount of Ca is accomplished by various means in different physiological types of plant. The following results of the author’s research group were presented in a coordinated form: 1. Ca found outside the cytoplasm is partly in the vacuoles and partly in the cell walls. In the latter case, it is not only bound to the anionic constituents of the cell walls (pectins), but may also be present, in some plant species, in considerable quantities as CaCO 3 . 2. Ca may be present in different fractions and different conditions in the vacuoles: a) Plants which produce oxalate will always contain Ca-oxalate. When there is more oxalate than Ca, then all of the Ca in the vacuole will be in the insoluble form of Ca-oxalate except for a very small quantity of free Ca 2+ ions which can exist in the presence of excess oxalate (4.2 • 10 −5 mol/l at the most). Plants where this occurs are well established and follow taxonomic groupings. Some of these “oxalate types” are stimulated to oxalate production by an influx of Ca so that dissolved Ca will never be stored. b) Ca is present in dissolved form to a lesser extent than other cations such as K and Mg in the majority of plant families and species. However, Poaceae and Cyperaceae have an especially large surplus of dissolved K in comparison to dissolved Ca. c) Ca is stored in appreciably large amounts in the vacuoles of a group of plants where certain plant families predominate. This “calciotrophic type” has a K/Ca ratio below 1 in the water soluble fraction of its leaves. 3. Current analytical methods for determination of water-soluble Ca do not reveal what fraction is present as free Ca 2+ ions. Investigation with an ion sensitive electrode showed only part of the Ca to be in this form: a) Several Brassicaceae contain an oversaturated, but stabile solution of CaSO 4 which is only partly ionized. b) The vacuoles of most Boraginaceae have pectin type poly-anions that Ca binds to. c) Generally if the cell sap contains ions of hydroxy-carbon acids such as malate and citrate, an appreciable part of Ca may be complexed by them. 4. In vacuoles of plant types mentioned in sub 2.b., ionized Ca 2+ may range from a few mmol to about 20mmol/l. The total of dissolved Ca may be more than double these values. In calciotrophic types (2.c.), relations in press saps allow us to infer that the vacuoles may have up to 140mmol/l dissolved Ca and more than 50mmol/l ionized Ca 2+ . The special types mentioned in sub. 3.a. may contain 50−100mmol/l dissolved Ca and 10−15mmol/l ionized Ca. The Boraginaceae (3.b.) have been estimated to contain 2−37mmol/l dissolved Ca and 0.25 − 1 mmol/1 ionized Ca 5. The consequences of these observations for our concept of the compartmentalization of Ca within the plant cell and the transport modes between cell compartments were discussed.