Experiment 1

This experiment examined the effect of the timing of Zn fertilizer application on grain Zn content, relative to plant growth stage and flooding or draining periods. The different levels of Zn fertilizer included in the experiment are: no Zn (Z0), basal Zn (ZB: applied by top dressing into the floodwater one day after transplanting), mid-season Zn (ZM: applied at the beginning of the mid-season drying period), and late-season Zn (ZL applied at the beginning of the late-season drying period). All Zn fertilizer was added at 20 kg ha⁻¹ Zn in the form of reagent-grade zinc sulfate heptahydrate dissolved in 200 mL deionized water and applied into at least 1 cm standing floodwater, spread evenly over the surface of the water in the pot.

Three levels of WM were included in the experiment, namely, continuous flooding (CF), mid-season drying (MSD), and late-season drying (LSD). Under CF, the soils were flooded at a 5-cm depth for the duration of the season. In MSD, the soils were allowed to dry for one week during the late tillering stage and kept flooded at other times; in LSD, the soils were allowed to dry for one week, starting at one week after flowering, and then re-flooded to 5 cm just before harvest. Drying in MSD and LSD treatments referred to in this study means that soils were kept below −10 kPa soil matric potential. Irrigation was done with reverse osmosis water to avoid problems between Zn and Fe. The 12 treatment combinations were arranged in randomized complete block design with 4 replications.