FURTHER EVIDENCE THAT ZINC IS REQUIRED THROUGHOUT THE ROOT-ZONE FOR OPTIMAL PLANT-GROWTH AND DEVELOPMENT

The effect of variable Zn supply with depth in a soil profile was exam ined in two wheat genotypes differing in their Zn efficiency. 'Gatcher ' (Zn-inefficient) and 'Excalibur' (Zn-efficient) were grown in a low Zn soil in pots with two treatment zones. The upper zone (10 cm) was s upplied with Zn while Zn was either supplied or withheld from the lowe r zone (25 cm). In both genotypes, withholding Zn from the lower zone had no effect on root growth in either the upper or lower zones; neith er did it affect plant appearance prior to booting. However, withholdi ng Zn from the lower zone delayed head emergence in Gatcher by some 10 d and depressed grain yield by 20%. In Excalibur, Zn treatment had no effect on head emergence or grain yield. In Gatcher, withholding Zn f rom the lower zone depressed water usage by 12% during a 60 d period p receding maturity. No effect of Zn treatment on water usage was seen i n Excalibur. Tissue Zn concentrations closely reflected the lower zone Zn treatments in both genotypes. Irrespective of the Zn treatment, Ex calibur had higher Zn concentrations in flag leaves but lower concentr ations in grain than Gatcher. In whole shoots, genotypic differences i n Zn concentration only occurred when Zn was added to the lower zone; Excalibur having almost twice that of Gatcher. Clearly, if Zn is not s upplied to the entire root system there is the potential for impaired root function and plant development, and for reduced grain yield. Appr oaches to managing this problem are