Copper Supply in Relation to Content and Redistribution of Copper Among Organs of the Wheat Plant*

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Abstract
The relationship of copper supply to the content and movement of copper among organs of wheat plants was examined at seven stages in their growth from seedlings to maturity on a copper deficient sand. In the absence of copper (Cu0), plants became severely copper deficient and produced no grain; development of tillers, leaves, stems, and inflorescences was delayed and growth of roots strongly depressed; leaf senescence was retarded and tiller growth was prolonged. Application of a marginal supply of copper (Cu1) overcame all symptoms and promoted growth and grain production. Increasing copper supply eightfold (Cu2) did not change vegetative or grain production. Copper concentrations in stems, individual leaves, and whole tops were highest and responded most strongly to copper supply when they were young. As they aged, Cu1 and Cu2 leaves lost copper rapidly; the first Cu0 leaves retained their copper and remained healthy for more than 7 weeks even though younger leaves developed severe copper deficiency. In all treatments, loss of copper from the oldest leaf paralleled senescence and the loss of nitrogen. It is suggested that copper does not move out of plant leaves until they lose organic nitrogen compounds. As a result, copper behaves in non-senescent leaves as if it is not mobile in plant phloem. But under conditions favouring senescence, copper is highly mobile: in the present experiment, 67 per cent of the copper present in vegetative organs of the Cu2 primary shoot at flowering moved from them during grain development and this could account for all of the copper found in the grain at maturity. The retention of copper by leaves before senescence, its rapid loss during senescence, and the effect of copper deficiency in delaying senescence resulted in the oldest leaf of severely deficient Cu0 plants in the present experiment having a higher copper concentration than that of copper adequate Cu1 and Cu2 plants. This behaviour could account for the many reports of anomalous C-shaped ‘Piper-Steenbjerg’ curves in the relationship of yield to copper concentrations in plant tops. The coupling of copper movement from leaves to nitrogen movement can also account for the unusually high values reported for critical concentrations of copper in tops of plants given high levels of nitrogen fertilizers. Old organs should not be included in samples for diagnosis of copper deficiency. Only young organs should be used. In the present experiment, the copper concentration of young leaves gave a good indication of the copper status of wheat: a value of 1 μg g−1 in young leaves indicated copper deficiency.