Physiology of genotype x soil fertility effects on yield and accumulation of iron and zinc in the common bean (Phaseolus vulgaris L.) seed

This thesis research focused on evaluating improved common bean ( Phaseolus vulgaris L.) germplasm under intensified fertility regimes, and assessing how assimilate partitioning to the seed impacts seed mineral concentration. In Chapter 2, the agronomic performance of four improved varieties was evaluated on small-landholder farms using locally available manure and typical farming practices. Only one of the improved varieties, K131, yielded significantly more than the local check, Kanyebwa. Use of locally available manure significantly increased yields, but they remained less than 50% of the potential yield of improved varieties as demonstrated by the experiment station. Among resource limited farmers, options for soil fertility improvement are few and thus widespread use of manure requires a shift to mixed production systems. In Chapter 3, the response of common beans to phosphorus intensification was evaluated on acidic ferralsol soils typical of most bean producing areas in Uganda. Application of up to 180 kg P/ha did not significantly increase yields in bean monocrops or offer any yield advantage from a maize/bean intercrop estimated as the land equivalent ratio. Further, available P (Bray 1) did not increase even after three seasons of repeated intensive application of P fertilizer. The large quantities of inorganic P application that would be needed to build soil capital puts this resource out of reach for the majority of subsistence farmers. Complementary use with manure, however, might be a viable solution, coupled with proximal placement of P fertilizer and phased application to ensure synchrony of nutrient uptake with plant needs. In Chapter 4, we evaluated the relationship between yield and seed quality components under a limited supply of photoassimilate during seed filling. Shade treatments decreased seed yield, seed oil concentration and seed starch concentration. Treatment effects on seed iron (Fe) and zinc (Zn) concentrations were positively correlated with seed protein concentration, which has been shown to depend on assimilate supply per seed. Thus, the selection for increased protein concentration may increase the concentration of these important micronutrients. However, the challenge of the inverse relationship between yield and protein concentration may constrain usage of this strategy in breeding.