Novel nutrient solutions for zinc nutrition research: buffering free zinc²⁺ with synthetic chelators and P with hydroxyapatite

Abstract

Chelator-buffered nutrient solutions, in which computed free Zn²⁺ activities are buffered at ≤10^-10.0 M by including an excess of a synthetic chelator such as EDTA, have recently shown promise as a means of precisely regulating Zn nutritional status. A further refinement that would eliminate the confounding effect of high (and often phytotoxic) shoot P concentrations in solution-grown, Zn-deficient plants is also desirable. Several crop species were grown in 120-L of HEDTA-buffered solutions that contained just 10+-1 μM P. Critical free Zn²⁺ activities ranged from ≈10 to 60 pM, and relative yields as low as 32% of control were achieved. Concentrations of P in the older leaves were very high (up to 46 mg g^-1) at low (Zn²⁺), suggesting that P toxicity can occur even without the high P concentrations (about 1 mM) typically used in Hoagland-type solutions. A second study was undertaken to better simulate soil conditions, wherein diffusion of P from the solid phase to the root is rate-limiting. Commercial hydroxyapatite (HAP) was enclosed in a pouch constructed of dialysis tubing, such that dissolution and diffusion occurred in response to plant depletion of P. Maize (Zea mays L.) and wheat (Triticum aestivum L.) can be supplied with P at adequate levels using this approach, and acutely Zn-deficient plants did not hyperaccumulate P. However, two dicots tested were too P-inefficient to grow normally with HAP as the sole P source.