Distribution of trace metals in maize (Zea mays L.) root tips under zinc deficiency using laser ablation inductively coupled plasma mass spectrometry

Objectives Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) is an emerging technology to detect content and distribution of elements in plants. We presented the distribution of iron (Fe), manganese (Mn), copper (Cu), and zinc (Zn) in maize root tips under Zn-deficiency, using LA-ICP-MS to reveal the mechanism of transport and enrichment of Zn in crops and the effect of Zn deficiency on the absorption and transport of trace metal elements at the tissue level.

Methods Zea mays L. cv. ZD958 was used as the test crop. The hydroponic culture experiment was conducted using different concentrations of zinc (Zn) treatments. 15-day Zn treated roots were harvested, dried, ground, and homogenized. Part of the dry root samples were digested with HNO₃ and Zn concentration in the digesting solution was determined using ICP-MS with a liquid injection sample system. The rest of the root sample powders was compressed to the discs as the standard for LA-ICP-MS. The good linear correlation (R² = 0.9995) was obtained between the actual Zn concentration of the standard of root discs and the signal intensity detected by LA-ICP-MS, indicating that the self-made root standard discs could be used for LA-ICP-MS quantitative analysis. The tested root tips of maize and the standard discs were placed in the LA sample cell for the measurement. ¹³C was used as an internal standard element to compensate for the difference in the amount of the ablated materials. The quantitative image was obtained to study Zn, Fe, Mn and Cu distribution in the root tips of maize under Zn deficiency.

Results The Zn content in maize roots decreased significantly under Zn deficiency, which was only 27.78% of that in normal roots, while the contents of Mn and Cu increased in Zn-deficient roots. The imaging of LA-ICP-MS showed the Zn content was higher in root tips and increased gradually inward from the epidermis, while the content of Zn decreased in Zn-deficient roots, especially in apex of root tips. In addition, the signal intensity of Fe, Mn and Cu detected by LA-ICP-MS showed qualitative distribution. The signal intensities of Fe, Mn and Cu were higher in the apex of Zn-sufficient root tips, increasing inward from the surface gradually, and which were increased to different degrees in maize roots under Zn-deficiency.

Conclusions The Zn, Fe, Mn, and Cu contents increased from epidermis to the cortex in the root tip of normal maize. However, the contents of Fe, Mn and Cu increased in Zn-deficient root tips.