PHOSPHORUS DEFICIENCY IN CITRUS

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Abstract
Leaves of trees in phosphate-deficient sand cultures contained .05-.07% of P in the dry matter; those of trees in soil (low in nitrate N) contained several times this amt. Shoots and trunk and root bark of the phosphate-deficient cultures contained less P than leaves or young roots. Lemon fruits of phosphate-deficient cultures contained .06% of P in the dry matter, approximating that found in the leaves and roots. The shoots and bark are the first portions to have their organic P fraction reduced to a trace when P is omitted from the culture solution. Organic P within the tissues may be used by citrus when conditions for obtaining inorganic P become desperate. The % of phospholipid P is highest in the leaves even after a prolonged period of P starvation. Symptoms of P deficiency in citrus leaves are variable and may consist of fading of the chlorophyll, burning of the blade in various locations, reduction in leaf size, change in leaf form, or change to a dull brownish-green color without luster. P deficiency brought about changes in the shape and contents of the cells of lemon leaves. There was increased terminal shoot growth, accompanied by an almost total absence of lateral shoots. In severe cases the leaves not only burn but feel brittle; they droop at the petiole in a direction parallel to the shoot axis and later abscise. The growth of Valencia orange and pomelo leafy-twig cuttings was improved by using 105 p.p.m. of phosphate in the unaerated culture solution for only a few days at intervals of several weeks. Organic sources of P in nonaerated and aerated solution cultures may be of use to lemon cuttings when the supply of P becomes deficient.
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