Rate of calcium absorption by plant roots and its relation to growth

Abstract

Rates of calcium absorption per unit root weight were measured for each of three successive weeks for 30 grasses, cereals, legumes, and herbs grown in flowing culture solutions of pH 5.7 with a wide range of calcium concentrations (0.3 to 1000 µM) which were maintained constant. Plants did not absorb calcium from 0.3 µM solutions and absorbed little from 0.8 µM solutions. Increasing solution concentrations from 0.8 to 10 µM increased the average rates of calcium absorption almost linearly in all plant groups: increasing solution concentrations to 100 or 1000 µM increased rates of calcium absorption but with decreasing effectiveness. Rates of absorption generally remained constant during the experiment, except that in grasses and cereals at 100 and 1000 µM calcium they tended to fall with time. At concentrations from 2.5 to 1000 µM, the average rates of calcium absorption by roots of legumes and herbs were generally very much higher than those by equal weights of grass and cereal roots. Individual cultivars within each plant group tended to follow the average behaviour, so that almost all legumes and herbs absorbed calcium very much faster (up to 15 times) than grasses and cereals at all solution concentrations above 2.5 µM. The relationship between rates of calcium absorption and plant growth is discussed. Since excess calcium accumulated in plants is not readily mobilized to developing tissues, plants become calcium-deficient as soon as the rate of calcium absorption falls below the functional requirements of the tissues. Absorption rater of 0.5 µg atom in grasses and cereals, and in herbs and legumes of 1 µg atom per gram fresh roots per day are suggested as minimal to maintain relative growth rates of c. 9% per day. In standard culture solutions legumes and herbs would deplete calcium from solution and hence develop calcium deficiency more quickly than cereals and grasses. By contrast, in flowing culture solutions with low calcium concentrations maintained constant, high rates of calcium absorption by legumes and herbs sometimes offset their high tissue requirements for calcium; this explains why some legumes and herbs are able to grow at lower calcium concentrations than some grasses and cereals. It is suggested that conditions of calcium supply under which deficiency develops in solutions and in soils should be taken into account when assessing the relative susceptibility of plant species to calcium deficiency.