Temporal dynamics in wheat grain zinc distribution: is sink limitation the key?

BACKGROUND AND AIMS Enhancing the zinc (Zn) concentration in wheat (Triticum aestivum) grain is a breeding objective in order to improve human Zn nutrition. At enhanced plant Zn uptake, grain Zn levels do not increase proportionally and within the grain the endosperm Zn levels remain below grain Zn levels. This study analysed the temporal dynamics of Zn concentrations in grain tissues during grain filling to find major bottlenecks. METHODS Plants of two cultivars were grown at 1 and 5 mg Zn kg(-1) soil. Individual panicles were harvested 7, 14, 24 or 34 d after their flowering or at maturity and seeds were dissected into constituting tissues, which were analysed for Zn and other minerals. KEY RESULTS The Zn concentration of the crease was found to increase five- to nine-fold between 7 and 34 d after anthesis, while that of the endosperm decreased by 7 and 45 % when grown at 1 or 5 mg Zn kg(-1), respectively. The Zn turnover rate (d(-1)) in the crease tissues was either independent of the Zn application level or higher at the lower Zn application level, and the Zn concentration increased in the crease tissues with time during grain filling while the turnover rate gradually decreased. CONCLUSIONS There is significant within-seed control over Zn entering the seed endosperm. While the seed crease Zn concentration can be raised to very high levels by increasing external Zn supply, the endosperm Zn concentrations will not increase correspondingly. The limited transfer of Zn beyond the crease requires more research to provide further insight into the rate-determining processes and their location along the pathway from crease to the deeper endosperm.