Differences in grain zinc are not correlated with root uptake and grain translocation of zinc in wild emmer and durum wheat genotypes

BACKGROUND AND AIMS: Cereal-based foods fall short of providing adequate dietary zinc (Zn) to human beings. Developing new genotypes with high genetic capacity for root uptake and grain deposition of Zn is an important challenge. There is a large genetic variation for grain Zn concentration among and between wheat species, especially within wild emmer wheat (Triticum turgidum ssp. dicoccoides) that can be exploited in order to understand the physiological mechanisms contributing to grain Zn accumulation. METHODS: Eight different wild emmer genotypes and two durum wheat (Triticum durum) cultivars were used to investigate root uptake, root-to-shoot translocation and remobilization (i.e., retranslocation) from flag leaves into grains of ⁶⁵ZnSO₄-treated plants. The initial seed Zn concentrations of wild emmer wheat and durum genotypes used in the experiments were different, ranging from 45 to 73 mg kg⁻¹ and from 35 to 40 mg kg⁻¹, respectively. Plants were grown in nutrient solution for the experiments investigating root uptake and shoot transport of Zn by using ⁶⁵Zn labeled ZnSO₄ and in soil medium for the experiments studying shoot and grain Zn concentrations and ⁶⁵Zn translocation from flag leaves into grains. The treatment of flag leaves with ⁶⁵Zn was realized by immersion of flag leaves into ⁶⁵ZnSO₄ solution for 15 seconds and for 5 times during the anthesis and early milk stages. RESULTS: Wild emmer and durum wheat genotypes expressed highly significant differences in root uptake and root-to-shoot translocation of ⁶⁵Zn and translocation of ⁶⁵Zn from flag leaves into grains. However, none of these parameters showed a significant correlation either with the initial seed Zn concentrations at sowing or the grain Zn concentrations at harvest. The durum wheat cultivars with higher grain yield had lower concentration of Zn both in seeds at sowing or in grains at harvest, while wild emmer genotypes with lower grain yield capacity had higher concentration of Zn both in seeds at sowing or in grains at harvest. The concentration or content (total amount) of Zn in shoot during the early growth stage also did not correlate with the initial seed Zn concentrations. CONCLUSIONS: Differences in grain Zn concentration of wild emmer and cultivated wheats could not be explained by root Zn uptake and Zn translocation from flag leaf into grains during seedling and reproductive growth stages, respectively. It seems that there are additional key factors affecting the expression of genetic variation for grain Zn accumulation.