DIAGNOSIS OF ZINC-DEFICIENCY IN SEEDLINGS OF A TROPICAL EUCALYPT (EUCALYPTUS-UROPHYLLA BLAKE,S.T.)

A glasshouse experiment was undertaken to establish the internal Zn re quirement for shoot growth of Eucalyptus urophylla, a fast-growing com mercial plantation species widely planted in tropical regions of the w orld. A Zn-deficient sand was supplemented with ten rates of Zn and se edlings were harvested after three months. In Zn-deficient plants the new growth was dwarfed with small, necrotic leaves and short internode s. Foliar Zn concentrations declined markedly with leaf age in both Zn -deficient and Zn-adequate plants. The critical Zn concentration for t he diagnosis of Zn deficiency also fell with leaf age. Zinc concentrat ions in the youngest fully expanded leaf ranged from 8-11 mu g Zn g(-1 ) dry weight in plants with severe symptoms to 30-37 mu g Zn g(-1) dry weight in non-deficient plants. The critical Zn concentration for the diagnosis of Zn deficiency at 90% of maximum shoot growth in the same leaf was 21 mu g Zn g(-1) dry weight. This value is nearly twice that reported for several other species of eucalypts and may indicate a hi gher internal demand for Zn in tropical than in temperate eucalypts.