No abstract

The bioavailability of Fe from 24 select genotypes of bean (Phaseolus vulgaris L.) seeds containing a range of concentrations of Fe, myo-inositol pentaphosphate plus phytic acid (IP5+IP6), and tannins was studied using a rat model. Bean accessions, selected from field trials for their variations in Fe, phytate, and tannin seed concentrations, were grown in a greenhouse in nutrient solutions radiolabeled with (59)Fe. Mature seeds were autoclaved and lyophilized. Test meals (containing 1 g of dried bean, 0.5 g of sucrose, and 1 g of basal Fe-deficient diet) were fed to marginally Fe-depleted weanling rats over a 3-h period; rats were radioassayed in a gamma-spectrometer immediately after feeding and daily thereafter for the next 10 d. Radioiron retention data were used to calculate percent Fe absorption (i.e., Fe bioavailability) from the meals. Seed Fe concentrations ranged from 52 to 157 microg g(-)(1) dry weight. There was a tendency to also select for higher Zn concentrations in the beans when selecting for high Fe concentrations. The Fe bioavailability to rats from test meals depended on the genotype and varied from 53% to 76% of the total Fe. Bean genotypes with higher seed Fe concentrations resulted in increased amounts of bioavailable Fe to rats. There was no significant correlation between the Fe concentration in different bean genotypes and Fe bioavailability to rats attributable to variations in IP5+IP6 or tannins, even though these antinutrients varied widely (i.e., from 19.6 to 29.2 micromol of IP5+IP6 g(-)(1) and from 0.35 to 2.65 mg of tannins g(-)(1)) in the test meals. Other unknown seed factors (i.e., antinutrients or promoter substances) may be contributing factors affecting Fe bioavailability from bean seeds.

A whole-body radioassay procedure was used to assess the bioavailability to rats of zinc (Zn) in seeds of 18 genotypes of beans (Phaseolus vulgaris L) that were grown hydroponically. Dry beans that were labelled intrinsically with 65 Zn were added to test meals fed to rats that were marginally Znde®cient. The amount of Zn in the seeds varied between genotypes and ranged from 26.7 to 62.4 mg g À1 (from 0.41 to 0.95 mmol g À1 ) dry weight (DW). Similarly, the amount of iron (Fe) in the beans varied nearly twofold (from 52.3 to 96.3 mg g À1 DW), and Zn and Fe concentrations were positively correlated. Concentrations of myo-inositolhexaphosphate (IP6) plus myo-inositolpentaphosphate (IP5) varied from 18.1 to 27.3 mmol g À1 DW. Cultivars with white-coloured seeds contained relatively small amounts of tannins varying from 0.12 to 0.16 mg g À1 DW (determined as catechin equivalents) compared to those with coloured seed coats (up to 2.58 mg g À1 DW). All rats readily ate the test meals so that Zn intake varied directly with seed-Zn concentration. As indicated by 65 Zn absorption, the bioavailability to rats of Zn in the seeds varied between genotypes and ranged from about 78 to 95% of the total Zn in the seeds. The bioavailability of Zn to marginally Zn-de®cient rats was not affected markedly by either IP5 IP6 or tannin in the dry beans. These results demonstrate that the concentration of Zn in dry beans can be increased through traditional plant-breeding techniques and that this may result in signi®cant increases in the amount of bioavailable Zn in the beans. Increasing the amount of Zn in beans may contribute signi®cantly to improving the Zn status of individuals dependent on beans as a staple food.

A rat model was used to determine the bioavailability of iron and zinc in bean seeds and rice grain from enriched genotypes of these globally important staple foods. Seed and grain from the genotypes tested (intrinsically radiolabelled with either 59 Fe or 65 Zn) were cooked, homogenized in water, and lyophilized to dryness. The dried, radiolabelled powder was fed to young male rats in single meals. Bioavailability was calculated from the amount of radiolabelled iron and zinc retained in the rats over a 10-day period as determined each day by whole-body gamma spectrometry assay. The data collected demonstrate that increasing the amount of iron or zinc in enriched rice grain and bean seed significantly increases the amount of iron or zinc bioavailable to rats. Although a rat model is not ideal for determining iron and zinc bioavailability to humans, because rats are much more efficient at absorbing iron and zinc from plant foods than humans, rats can be used to give relative estimates of bioavailable iron and zinc in plant foods. These estimates can be used to rank promising genotypes of staple foods for use in later feeding trials with humans, greatly reducing the numbers of genotypes that would have to be tested in humans without use of the rat model. Ultimately, because of the complexities of determining the bioavailability to humans of iron and zinc in plant foods, human feeding trials performed under free-living conditions should be conducted with the most promising genotypes before these genotypes are released for distribution to breeding programmes worldwide

The number of Phaseolus vulgaris germplasm accessions numbers more than 30,000. While the large numbers of accessions increase the probability of preserving genetic variability they simultaneously limit the efficient and routine utilization of this resource. From the approximately 4000 P. vulgaris accessions in the C.I.A.T. whole collection that were collected in Mexico, a core collection of 400 accessions was developed based on variation for agronomic performance, ecological adaptation, and seed characteristics. Random samples of 90 accessions each were drawn from the core and whole collections and evaluated for 224 polymorphic RAPD bands. Based on analysis of the RAPD data there were no significant differences in genetic diversity between the two samples. The correlation of marker frequency for the two samples was 0.984 confirming that the two samples represent the same population.