Calcium requirements of plants

Abstract

Calcium concentrations in the tops and roots of 30 grasses, cereals, legumes, and herbs were examined for plants grown over a wide range of calcium concentrations (0.3 to 1000 µM) which were maintained constant in flowing culture solutions of pH 5.7. When plants were grown continuously in solutions of constant concentration, the relation between the calcium concentration in tops and the yield generally showed two regions. Increasing solution calcium concentrations from 0.3 to 2.5 µM increased the yield greatly, while calcium concentrations in the tops remained constant. By contrast, increasing solution concentrations from 10 to 1000 µM markedly increased concentrations in tops but in most species had relatively small effects on yield. At luxury calcium supply the concentrations in herb and legume tops were very much higher than those of grasses and cereals and were surprisingly similar to those of the same species grown in soil in the field. That calcium concentration in plant tops which remained constant while yield increased substantially is believed to indicate the minimal functional requirement of the tops for calcium. The average functional requirements of herb and legume tops (0.1–0.2% dry weight) were generally much lower than previously reported but were about twice those of grasses and cereals (0.05–0.1%). With the exception of Lupinus spp., no legume or herb had functional calcium requirements as low as those of any grass or cereal. When plants were transferred from 1000 to 0.3 µM calcium concentrations, they developed calcium deficiency symptoms and grew less than plants which had one-third to one-tenth the calcium concentrations in their tops but which had been grown continuously in solutions of low and constant concentration (2.5–10 µM). Excess calcium accumulated in the old leaves during luxury calcium supply but was not mobilized when plants were transferred to calcium-deficient solutions. It is suggested that variation in the conditions of calcium supply under which calcium deficiency develops partly accounts for the wide range of critical values used by various workers to diagnose deficiency in plants.