Critical zinc+2 activities for sour orange determined with chelator-buffered nutrient solutions

Chelator-buffered nutrient solutions were used to study the effect of different levels of Zn activity in the rhizosphere on growth and nutritive responses of various tissues of sour orange seedlings. The seedlings were grown for 3 months in a growth chamber in a hydroponic culture containing from 5 to 69 micromolar and 5 to 101 micromolar total Zn in Expts. 1 and 2, respectively. Zn+2 activities were calculated with a computerized chemical equilibrium model (Geochem-PC), and buffered by inclusion of a chelator, diethylenetriamine pentaacetate (DTPA), at 74 and 44 micromolar in excess of the sum of Fe, Mn, Zn, Cu, Ni, and Co in Expts. 1 and 2, respectively. The use of DTPA-buffered solutions proved successful in imposing varying degrees of Zn deficiency. The deficiency was confirmed by leaf symptomatology, leaf chemical analyses, ie., < 16 mg.kg-1 Zn, and responses to foliar sprays and application of Zn to the roots. Growth parameters varied in their sensitivity to Zn deficiency, i.e., root dry weight < leaf number and white root growth < stem dry weight < leaf dry weight < shoot elongation and leaf area. The critical activities, expressed as pZn = -log(Zn+2), were approximately 10.2 +/- 0.2 for root dry weight, 10.1 +/- 0.2 for leaf number and white root growth, 10.0 +/- 0.2 for stem dry weight, 9.9 +/- 0.2 for leaf dry weight, and 9.8 +/- 0.2 for shoot growth and leaf area. Increases in growth were observed in response to Zn applications even in the absence of visible Zn deficiency symptoms. Seedlings containing > 23 mg.kg-1 Zn in leaves did not respond to further additions of Zn to the nutrient solution. Zinc foliar sprays were less effective than Zn applications to the roots in alleviating severe Zn deficiency because foliar-absorbed Zn was not translocated from the top to the roots and thus could not correct Zn deficiency in the roots.