Search

The aims of this research were to assess the genetic structure of wild Phaseolus lunatus L. in the Americas and the hypothesis of a relatively recent Andean origin of the species. For this purpose, nuclear and non-coding chloroplast DNA markers were analyzed in a collection of 59 wild Lima bean accessions and six allied species. Twenty-three chloroplast and 28 nuclear DNA haplotypes were identified and shown to be geographically structured. Three highly divergent wild Lima bean gene pools, AI, MI, and MII, with mostly non-overlapping geographic ranges, are proposed. The results support an Andean origin of wild Lima beans during Pleistocene times and an early divergence of the three gene pools at an age that is posterior to completion of the Isthmus of Panama and major Andean orogeny. Gene pools would have evolved and reached their current geographic distribution mainly in isolation and therefore are of high priority for conservation and breeding programs.

Cassava roots are perceived as having relatively low protein content compared with other sources of energy (such as maize and wheat). This perception is in part due to a deficient screening in protein content levels in different cassava germplasm. For the current article, information on protein content of several cassava clones, gathered for a period of about 10 years, has been consolidated. Roots from a total of 149 accessions from the germplasm collection at the International Center for Tropical Agriculture (CIAT) were analyzed for total crude protein content. For 140 clones the evaluations were based on two different root samples and independent quantifications. Results from eight clones were based on three samples, and only for one clone had four different estimations been made. Large differences in protein content (ranging from 0.95% to 6.42%) were observed in the sample analyzed. Results suggest that a considerable proportion of these differences are genetic in nature and therefore that there are excellent possibilities for exploiting these differences and further increasing them by traditional breeding methods.

We examined the effects of salinity on four wild (Phaseolus angustissimus, Phaseolus filiformis, P. microcarpus, and P. vulgaris) and two cultivated (P. acutifolius and P. vulgaris L.) Phaseolus species. Relative growth rate (RGR, g g−1 per day), unit leaf rate (ULR, g m−2 per day), leaf area ratio (LAR, m2 g−1), specific leaf area (SLA, m2 g−1), leaf weight ratio (LWR, g leaf g−1), and rate of ion uptake were calculated for the period between 10 and 20 days after planting. Salinity significantly reduced RGR, ULR, LAR, and SLA whereas LWR showed no definite trend. In all species, except in P. filiformis, ULR, but not LAR, was significantly correlated with RGR, indicating that ULR was an important factor underlying the salinity-induced differences in RGR among species. In P. filiformis, high salinity reduced SLA, and consequently LAR. The significant correlation of SLA and LAR with RGR suggested that growth components affecting leaf area expansion were the primary factors explaining the inhibition of growth in this species. Increasing salinity progressively decreased leaf water vapor conductance. The rate of CO2 assimilation decreased gradually with salinity, showing significant reductions only at the highest salt level (80 mM NaCl). Approximately two-thirds of the reduction in CO2 assimilation rate at high salinity was attributable to reduced stomatal conductance. In P. filiformis, however, neither stomatal conductance nor CO2 assimilation were affected by salt stress. Leaf water and osmotic potentials declined significantly as stress intensified. However, osmotic adjustment permitted the maintenance of positive turgor throughout the growth period. Salinity had a significant effect on tissue concentrations of Na+, K+, Ca2+, and Cl− and on the uptake rate of Na+, K+, Ca2+, and Cl−. Thus, in addition to the toxic effects of high concentrations of Na+ and Cl− in plant tissue, saline-induced changes in mineral nutrient uptake likely contributed to the reduction of plant growth. It appears that salt tolerance in P. filiformis is associated with Na+ exclusion and organ Na+ compartmentation in roots and stems as well as sustained K+ concentration in leaves and better stomatal control through osmotic adjustment. All other Phaseolus species are Na+ excluders, and maintained turgor-driven extension growth by accumulating Cl− (osmotic adjustment), but subsequent weight gain reductions suggest that this led to ion toxicity.