We hypothesized that adventitious roots may improve crop adaptation to low-phosphorus soils by enhancing topsoil foraging. In a tropical field study, phosphorus stress stimulated adventitious rooting in two phosphorus-efficient genotypes of common bean (Phaseolus vulgaris L.) but not in two phosphorus-inefficient genotypes. Although phosphorus availability had no consistent effects on the length or biomass of whole root systems, it had differential effects on adventitious, basal, and taproots within root systems in a genotype-dependent manner, resulting in increased allocation to adventitious roots in efficient genotypes. Adventitious roots had greater length per unit biomass than other root types, especially under phosphorus stress. Adventitious roots had less construction cost than basal roots, despite having similar tissue nitrogen content. Phosphorus stress reduced lateral root density, and adventitious roots had less lateral root density than basal roots. Lateral roots formed further from the root tip in adventitious roots compared with basal roots, especially under phosphorus stress. Field results were confirmed in controlled environments in solid and liquid media. Stimulation of adventitious rooting by phosphorus stress tended to be greater in wild genotypes than in cultivated genotypes. We propose that adventitious rooting is a useful adaptation to low phosphorus availability, because adventitious roots explore topsoil horizons more efficiently than other root types.

Low phosphorus availability is a primary constraint to crop production in developing countries. Adventitious roots play an important role in phosphorus acquisition, as they are localized near the soil surface where phosphorus is relatively abundant. A population of recombinant inbred lines of Phaseolus vulgaris L. (G2333/G19839) was screened under high‐ and low‐phosphorus conditions in the greenhouse and field. We observed phenotypic variation and transgressive segregation for adventitious root traits in both environments. Allometric analysis revealed that although the taproot and basal roots are closely linked to shoot growth, recombinant inbred line (RILs) differ substantially in biomass allocation for adventitious roots. A linkage map with 149 genetic markers and a total cumulative map length of 1175 cM was used to identify a total of 19 QTL across 8 of the 11 linkage groups. Together these quantitative trait loci (QTL) accounted for 19 to 61% of the total phenotypic variation for adventitious root traits in the field and 18 to 39% under greenhouse conditions. Two major QTL for adventitious rooting under low phosphorus conditions in the field were observed on linkage groups B2 and B9 that together accounted for 61% of the observed phenotypic variation. We conclude that adventitious rooting under low phosphorus is a feasible target for bean breeding.

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A study was undertaken to estimate the relative degree of genetic diversity among lines of common bean (Phaseolus vulgaris L.) selected in Central America for BGMV resistance vs. lines that had no selection. Genetic distance (GD) based on the presence or absence of random ampHlied polymorphic DNA (RAPD) bands was estimated among breeding lines and cultivars of red and black beans. Another six black-seeded cultivars from Brazil and Argentina were included for comparison. The multidimensional scaling plots revealed that the red and black beans in this study formed distinct clusters with no overlap. Based on consistency of marker frequencies within vs. between populations, reds and blacks were confirmed to be distinct populations. The mean RAPD marker diversity measured among lines selected for resistance was significantly less than that among unselected lines in reds (0.140 vs. 0.224) and blacks (0.170 vs. 0.223). Coel~cients of parentage (CP) were also utilized to estimate the relationships among selected and unselected lines. Selected lines displayed significantly larger CP than unselected lines in both reds (0.29 vs. 0.06***) and blacks (0.13 vs. 0.05"**), reflecting closer genetic relationships and reduced diversity. Therefore, both molecular markers and CP indicatted that selection for resistance had reduced genetic variability in both red and black beans. When CP were correlated with GD for reds, blacks, and all genotypes, correlations of-0.35"**,-0.38"**, and-0.36*** resulted, respectively. In spite of low correlation values, GD and CP presented similar tendencies. The choice of GD or CP as a measure of genetic relationship may depend on the quantity and quality of pedigree information available.

This paper reports the changes on growth, photosynthesis, water relations, soluble carbohydrate, and ion accumulation, for two salt-tolerant and two salt-sensitive Phaseolus species grown under increasing salinity (0, 60 and 90 mM NaCl). After 20 days exposure to salt, biomass was reduced in all species to a similar extent (about 56%), with the effect of salinity on relative growth rate (RGR) confined largely to the first week. RGR of salt-tolerant species was reduced by salinity due to leaf area ratio (LAR) reduction rather than a decline in photosynthetic capacity, whereas unit leaf rate and LAR were the key factors in determining RGR on salt-sensitive species. Photosynthetic rate and stomatal conductance decreased gradually with salinity, showing significant reductions only in salt-sensitive species at the highest salt level. There was little difference between species in the effect of salinity on water relations, as indicated by their positive turgor. Osmotic adjustment occurred in all species and depended on higher K + , Na + , and Cl − accumulation. Despite some changes in soluble carbohydrate accumulation induced by salt stress, no consistent contributions in osmotic adjustment could be found in this study. Therefore, we suggest that tolerance to salt stress is largely unrelated to carbohydrate accumulation in Phaseolus species.