Isotope tracing and microscale mapping to investigate the fate of foliar applied zinc in wheat under drought stress

Limited soil moisture negatively affects the zinc (Zn) bioavailability to roots by impacting Zn diffusion (mobility) which may severely diminish Zn accumulation in edible parts of plants such as grains. Therefore, soil moisture drought poses a critical threat to Zn nutrition of plants and agronomic Zn biofortification strategies that aim at enhancing grain Zn concentrations. Foliar Zn application represents an alternative solution by bypassing adverse soil conditions and increasing grain Zn concentrations. While foliar Zn application has been shown to enhance grain Zn concentration under drought stress, it remains unclear how soil moisture drought influences foliar Zn uptake and translocation processes in crops. Furthermore, although the agronomic effectiveness of foliar-applied Zn has been well-studied using isotope tracers, similar investigations under soil moisture drought are lacking. Addressing this gap will contribute to optimization of foliar Zn application strategies, especially under stressful environmental conditions.

This ongoing study aims to quantify the agronomic effectiveness of foliar-applied Zn in wheat (Triticum aestivum) under conditions with and without soil moisture drought using stable isotope tracing. A pot experiment was conducted by using two wheat cultivars, Katya and Diavel, differing in drought stress tolerance. Plants were subjected to soil moisture drought in a controlled greenhouse environment, and a ⁶⁶Zn labeled Zn sulfate fertilizer was applied to the flag leaves at flowering stage. In this study, yield parameters were already determined, and Zn concentrations in different plant tissues will be measured in ICP-OES. Leaf properties, such as trichome and stomatal density, were also analyzed using a scanning electron microscope. By measuring ⁶⁶Zn: ⁶⁴Zn isotope ratios in different bulk plant tissues using a high resolution ICP-MS, we precisely quantified the transfer of foliar-applied Zn to grains and other plant parts. Additionally, ⁶⁶Zn: ⁶⁴Zn isotope ratios within the flag leaves were mapped using laser ablation (LA)-ICP- time-of-flight mass spectrometry (TOFMS), providing detailed insights into the fate of foliar applied Zn at the microscale.

Preliminary results indicate distinct responses to drought stress between the two cultivars. Under drought stress, Katya exhibited higher drought tolerance than Diavel, evidenced by its lower leaf water potential. Traits of Katya such as greater root biomass, shorter leaf length, higher trichome density, and lower stomatal density than Diavel further supported this observation LA-ICP-TOFMS results revealed that drought increased the transfer of foliar-applied Zn within the flag leaves in Diavel. In contrast, drought reduced the transfer of Zn within the flag leaves in Katya. Moreover, source tracing with stable isotopes revealed that drought reduced the transfer of foliar applied Zn to grains, particularly in Katya. These preliminary findings suggest that drought stress modifies the mobility and partitioning of foliar-applied Zn, with cultivar-specific traits playing a crucial role. At the conference, we will present the complete dataset and discuss the implications of our findings for improving Zn biofortification strategies in wheat under drought stress.