Indices to screen for grain yield and grain-zinc mass concentrations in aerobic rice at different soil-Zn levels

Zinc is an important micronutrient for both crop growth and human nutrition. In rice production, yields are often reduced and Zn mass concentrations in the grains are often low when Zn is in short supply to the crop. This may result in malnutrition of people dependent on a rice-based diet. Plant breeding to enhance low-Zn tolerance might result in higher yields and nutritional quality but requires effective selection criteria embedded in physiological insight into the Zn husbandry of the crop and applicable in field evaluation of advanced breeding material or in screening of existing varieties. Using existing and newly developed low-Zn tolerance indices, this study presents the results of screening experiments carried out in high- and low-Zn soils. Sixteen accessions of aerobic rice were grown under greenhouse conditions to conceptualize the indices and 14 under field conditions to validate the indices. As the differences in soil-Zn levels in these experiments did not result in differences in grain yield, literature data were used from experiments where the soil-Zn level did have an effect on grain yield, to further check the validity of the indices. Several indices were applied to evaluate the genotypic low-Zn tolerance performance in attaining (relatively) high grain yield, high grain-Zn mass concentration, or both. The results indicate that the grain-Zn mass concentration efficiency index is different from the grain yield efficiency index and that the low-Zn tolerance indices identified superior genotypes best. Amongst the indices tested, the low-Zn tolerance index for grain yield and the low-Zn tolerance index for grain-Zn mass concentration were closely correlated with grain yield and grain-Zn mass concentration, respectively. Therefore, the low-Zn tolerance index for grain yield was effective in screening for high stability and high potential of grain yield, and the low-Zn tolerance index for grain-Zn mass concentration was effective for grain-Zn mass concentration under low and high soil-Zn conditions. Genotypic differences in yield and grain-Zn mass concentration were shown to be unrelated and therefore deserve separate attention in breeding programmes. Combining the low-Zn tolerance index for grain yield and the low-Zn tolerance index for grain-Zn mass concentration in a single low-Zn tolerance index was considered but did not appear to be superior to using the two indices separately