Diagnostic indices for sulphur status of subterranean clover

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Abstract
The variations in total sulphur, sulphate sulphur, and total nitrogen/total sulphur ratio, (N/S)t, in subterranean clover (Trifolium subterraneum L.) with age of plant, nutrient status, and plant part were studied in a glasshouse experiment on a sulphur-deficient soil. The concentrations of total sulphur and sulphate sulphur in the plant increased with increasing sulphur supply and decreased with plant age or with supplemental nitrogen. Overall, the laminae contained a higher concentration of total sulphur than the petioles; when the optimal concentration for growth was present, the concentration in the laminae was approximately twice that in the petioles. In sulphur-deficient plants there was little difference between the sulphate concentrations in laminae and petioles. In young sulphur-sufficient plants, higher concentrations were found in the petioles than in the laminae but this was reversed for the older plants. With increasing sulphur supply there were bigger relative increases in sulphate than in total sulphur and the magnitude of both these fractions changed more in petioles than in laminae. In general, the (N/S)t ratio increased with age of plant or supplemental nitrogen, and decreased with increasing sulphur supply. The changes with age were smaller in the laminae than in the petioles. Petioles had a higher ratio than laminae in sulphur-deficient clover, and a lower ratio where sulphur was adequate. Critical concentrations of total and sulphate sulphur in the tissue were lowered by extra nitrogen, fell markedly as the plant aged, but were not much affected by the maturity of the plant part. The (N/S)t ratio changes less with all factors than the other two indices, and thus promises fewer problems in interpretation.