Common bean exhibits low zinc translocation efficiency to grain under zinc enriched field conditions in Malawi

Abstract

Biofortification is a sustainable approach to reduce Zn deficiency in Sub-Saharan Africa, yet the efficiency of common beans (Phaseolus vulgaris L.) in Zn uptake and translocation remain unclear. This study examined genotypic variation in Zn uptake, translocation, and partitioning among common bean lines grown under Zn-enriched field conditions. Eighteen genotypes representing landraces, improved lines, and biofortified varieties were evaluated in a randomized complete block design with four replications. Zinc sulphate (ZnSO₄·7H₂O) was applied at 30 kg Zn ha−1 during the R1 stage. Three biofortified varieties (NUA45, NUA35 and NUA730) were used as controls. Zinc was quantified using ICP–MS and partitioning indices were calculated to assess Zn allocation to grains. Despite Zn enrichment, most genotypes showed limited translocation to grains, with concentrations between 32 and 40 mg Zn kg−1, below the HarvestPlus target of 56 mg Zn kg−1. Only one improved line (SMC 156) reached about 53.9 mg Zn kg−1. Considerable Zn retention occurred in vegetative tissues, and significant genotypic differences were observed in grain Zn concentration, yield, and partitioning efficiency. The findings indicate a physiological bottleneck in Zn mobility and highlight partitioning efficiency as a key trait for breeding Zn-enriched common bean cultivars.