IDENTIFICATION OF AN INDEX TISSUE TO PREDICT ZINC STATUS OF WHEAT

The critical concentration of Zn in wheat tissues for the prediction o f Zn response and diagnosis of Zn deficiency was examined in a glassho use experiment with wheat (Triticum aestivum, line QT 4118) grown to a nthesis in two Vertisols at Zn application rates of 0, 1.25, 2.5, 5, 1 0, 15 and 30 kg ha-1 equivalent as ZnSO4 7H2O. The wheat tissues exami ned were the youngest mature leaf blade (YMB), the leaf immediately be low the youngest mature leaf blade (YMB-1), the older leaves, the ear, the stem and the whole tops. The minimum Zn concentration required in a tissue at 0.90 relative yield, referred to as the critical Zn conce ntration, was determined using the Cate-Nelson graphical and statistic al models, the Mitscherlich equation and a two-intersecting straight l ines model. The Zn status of wheat was best defined by the Zn concentr ation in the YMB. Although the critical Zn concentration of the YMB di d not vary much with the method of estimation, the Cate-Nelson statist ical procedure explained a higher percentage of the variation in Zn co ncentration in the YMB and relative yield than the Mitscherlich and th e two intersecting straight lines models. The critical concentration o f Zn in the YMB was 16.0 mg kg-1 dry matter. It is concluded that dete rmination of Zn concentration in the YMB is the best procedure for eva luating the Zn status of wheat plants.