DIAGNOSIS OF ZINC-DEFICIENCY IN CANOLA BY PLANT ANALYSIS

Canola plants (Brassica napus cv. Eureka) were grown in soil culture w ith seven levels of zinc (Zn) supply (0, 67, 133, 200, 267, 533, and 1 ,067 pg Zn/kg soil) for 39 days. Critical Zn concentrations in young l eaf blades and petioles were established for the diagnosis of Zn defic iency in canola plants during vegetative growth by assessing the relat ionship between the Zn concentration in the leaves and shoot dry matte r on 22 and 39 days after sowing (DAS). Zinc concentrations in leaf bl ades and petioles increased with increasing Zn supply, but Zn concentr ations were always 50% higher in the youngest open leaf (YOL) than in the youngest mature leaf (YML). The relationship between shoot dry mat ter and Zn concentrations in leaf petioles exhibited Piper-Steenbjerg curvature, indicating their unsuitability for Zn-deficiency diagnosis either alone or by inclusion with leaf blades. By contrast, inclusion of leaf mid-ribs with leaf blades did not alter the relationship betwe en shoot dry matter and Zn concentrations, nor the critical Zn concent ration. Critical Zn concentrations in the YOL, YOL+1, and YOL+2 blade on 39 DAS, corresponding with the stem elongation stage, were 15-17, 9 -10, and 7-8 mg Zn/kg dry matter, respectively. In comparison, the cri tical Zn concentration in the YOL+2 leaf blades with mid-ribs was 7-8 mg Zn/kg dry matter. In conclusion, during the vegetative stage up to stem elongation, YOL+2 leaf blades which are also the YML are recommen ded for the diagnosis of Zn deficiency in canola plants with the criti cal Zn concentration being 7-8 mg Zn/kg dry matter.