Abstract1. The response of pineapple plants throughout their life cycle to different nutrient levels of boron in combination with different nutrient levels of calcium was studied in sand cultures. 2. Pineapple plants not receiving boron at the lowest and three highest nutrient levels of calcium, 5, 100, 250, 500 p.p.m., made much less vegetative growth than those grown without boron, but at a low calcium level, 50 p.p.m. Plants in the last group were stout and green as compared with the rather slender and light-green or yellow-green plants of the former. 3. Pineapple plants grown without boron but at the highest calcium level produced the lowest fruit yields as compared with plants grown without boron but at low calcium, 50 p.p.m. These yielded most. 4. In pineapple plants supplied with the highest nutrient concentration of boron, 5 p.p.m., yields increased as the calcium concentration of the substrate increased from 5 to 500 p.p.m. However, yields were greatest from the plants that grew at the second highest calcium concentration, 250 p.p.m. 5. Either deficient or relatively high calcium concentrations, 5, 250, 500 p.p.m., adversely and significantly affected the green weight of plants when no boron was supplied. Low-calcium plants, 50 p.p.m., produced the highest significant green weights at the lowest nutrient level of boron. 6. Increments of calcium above 50 p.p.m. at the lowest nutrient level of boron induced significant reductions in the dry weights of plants, the weights of roots, and number and weight of slips. Deficient calcium induced a similar reduction. 7. At a given boron level, total boron in the plant tissue was largely a function of the boron concentration of the substrate and was not greatly influenced by its calcium concentration. 8. At the lowest level of boron, and at the two highest calcium levels, the content of soluble boron in the tissue was considerably less than at the three lowest calcium concentrations. The least soluble or metabolically active boron was found at the highest calcium level, while the highest content of the soluble fraction of this element was accumulated in the leaves of plants grown at low calcium, 50 p.p.m. 9. The lowest yielding plants grown without boron and at the highest level of calcium, 500 p.p.m., were associated with the lowest leaf-tissue content of soluble or active boron, while the highest yielding plants grown at the same boron level, but with low calcium, 50 p.p.m., were associated with the highest content of soluble or active boron in the meristematic tissue of the leaves. 10. Increments in the calcium level did not have any marked effect on the tissue content of total boron at the highest boron level. However, soluble boron was reduced to some extent in the tissue at the two highest calcium concentrations. 11. Plants grown at the highest level of boron and at the second highest level of calcium, 250 p.p.m., produced heavier yields than plants at the same boron level, but at either lower or higher calcium levels. However, at the same boron level, increments in calcium did not definitely affect the green and dry weights of plants, the fresh and dry weights of roots, or the number and weights of slips. 12. Total calcium of the tissue was mainly determined by the calcium concentration of the substrate and was largely independent of the boron level. On the other hand, soluble calcium was unrelated to total calcium in the tissue, calcium in the substrate, or boron in the tissue.
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