Boron Toxicity in Hevea brasiliensis

IT has recently been found in Malaya that the rubber tree, Hevea brasiliensis , when growing on certain soils, can accumulate sufficient boron to cause the appearance of boron toxicity symptoms on the leaves. The soils on which evidence of boron toxicity has been found are the recently deposited alluvial clay soils on the west coast of Malaya, on which approximately 20 per cent of the rubber in Malaya is grown, and certain coarse sandy loams derived in situ from a very acid granite where only a shallow layer of top soil overlies the decomposing parent material. The main leaf symptoms of boron toxicity on rubber, common to many plants, are the development of necrotic spots in the interveinal areas, particularly around the leaf margin, and of a tip and marginal necrosis which may take the form of a narrow regular band or may result in a large area of the leaf, at the distal end, becoming necrotic (Fig. 1).