Tepary bean

Tepary bean (Phaseolus acutifolius) is an important food legume originating from South
America and the South-western parts of the United States. The crop is produced in many
countries worldwide including South Africa. It is highly tolerant to drought and the seed
contains a wide range of vitamins, minerals and protein of high nutritional quality. The genetic
base of tepary bean is narrow but can be widened by chemical mutagenesis. However, there
are no reports on the impact of chemical mutagenesis on the root nodulation and seed storage
proteins in tepary bean. Therefore, this study was designed to examine root nodulation
attributes and seed storage proteins of three tepary bean genotypes in the early mutagenic
generations (M2 to M4) derived through treatment with varying doses (0.0, 0.5, 1.0, 1.5 and 2.0
v/v) of ethyl methanesulfonate (EMS). The experiment on root nodulation attributes was laid
out as a 3 x 5 x 3 (genotypes x EMS doses x mutant generations) factorial design replicated
three times. At harvest, shoot height (SHT), primary root length (PRL), dry weights (shoot, root
and nodule), number of nodules per plant (NNP) and grain yield components such as the
number of pods per plant (NPP) and number of seeds per pod (NSP) were measured. Highly
significant (P?0.01) dose effects were observed for SHT, PRL, shoot dry weight (SDW) and
root dry weight (RDW). Highly significant (P?0.01) interaction effects of mutant generation x
genotype x dose were observed for NSP. A highly significant (P?0.01) positive linear
relationship was observed between the NNP and nodule dry weight (NDW). Increase in the
PRL suggested that tepary bean mutants could be important in drought tolerance. EMS
treatment led to an enhanced partitioning of dry matter (assimilates) to the shoots and roots.
There was a three fold increase in most of the root nodulation traits at the 0.5% EMS dose.The
Kjeldahl method was used for crude protein determination whereas the sodium dodecyl
sulphate – polyacrylamide gel electrophoresis (SDS PAGE) was utilized in determining the
protein banding patterns of the bean. There were highly significant (P?0.01) differences
among the genotypes in crude protein accumulation. Highly significant (P?0.01) mutant
generation x genotype x dose were observed for seed protein accumulation. ‘Genotype 3’
attained the highest protein content (24.23%) at 1.5% EMS dose in the M4 generation. EMS
doses ?0.5% positively stimulated protein accumulation in all genotypes but high EMS doses
(2.0%) depressed protein content. There were significant variations in seed storage protein
profiles among the genotypes and mutant generations. ‘Genotype 6’ showed a distinct
15.0kDa protein fragment which was absent in the majority of the remaining genotypes. The
presence of distinct protein subunits in the three genotypes could be used in varietal
identification. The results demonstrated that chemical mutagenesis using EMS could induce
significant variations in both the agronomic and nutritional traits of tepary bean.