Appraisal of zinc fertilization methods and application timing for grain zinc biofortification in rice

Agronomic biofortification can serve as an important strategy to minimize widespread Zn malnutrition in humans, yet its success depends on many factors including right fertilization method. The present research aimed to evaluate the impact of various Zn fertilization techniques on productivity, Zn concentration and bioavailability in rice grains of NIA-Mehran under the agro-climatic conditions of Tandojam, Sindh, Pakistan. The experiment was arranged in a randomized complete block design (RCBD), with each treatment replicated four times. The treatments included: Control (No Zn), Soil Zn (5 kg Zn ha-1 at 2 weeks after transplanting + 5 kg Zn ha-1 at flowering stage), Soil + Foliar application (5 kg Zn ha-1 at 2 weeks after transplanting in soil + 0.3% Zn through foliar at flowering stage) and Foliar Zn (0.3% Zn at booting, flowering and milky stage). The supplementation of Zn significantly (P< 0.05) influenced most of the growth and yield attributes, the concentration of Zn and phytic acid (PA) in brown rice grains, and Zn bioavailability indices (PA: Zn molar ratio and Total Daily Absorbed Zn [TAZ]). The supplementation of Zn resulted in increment of number of productive tillers to 12.5%, panicle length to 8.1%, thousand grain weight to 6.2%, grain yield to 26.2%, straw yield to 15.2%, Zn concentration to 54.7%, and reduction in PA concentration to 17.9%. A reduction in PA: Zn molar ratio to 45.8% and increment in TAZ values to 45.0% in brown rice grains was also noticed as a function of Zn fertilization. Among various Zn supplementations, foliar fertilization technique produced highest Zn concentration (52.0 ± 0.91 mg kg-1) with higher TAZ values (2.0 ± 0.03 Zn day-1). The required level of PA: Zn (<18) that ensures maximum Zn bioavailability was successfully obtained in rice grains of all Zn fertilized treatments (16.2, 16.3, 14.2). The daily Zn requirement for humans (3 mg Zn day-1) was covered up to 66.7% with ingestion of brown rice grains through foliar application of Zn. Based on these findings, it is suggested that the foliar Zn application should be practiced for selected rice variety (NIA-Mehran) under the agroclimatic conditions of Tandojam, as it significantly increased Zn concentration and its bioavailability in brown rice grains. The consumption of brown rice grains will be an effective tactic to minimize the widespread human Zn malnutrition.