Variability of root traits in common bean genotypes at different levels of phosphorus supply and ontogenetic stages.

Selection of common bean (Phaseolus vulgaris L.) cultivars with enhanced root growth would be a strategy for increasing P uptake and grain yield in tropical soils, but the strong plasticity of root traits may compromise their inclusion in breeding programs. The aim of this study was to evaluate the magnitude of thegenotypic variability of root traits in common bean plants at two ontogenetic stages and two soil P levels. Twenty-four common bean genotypes, comprising the four growth habits that exist in the species and two wild genotypes, were grown in 4 kg pots at two levels of applied P (20 and 80 mg kg -1) and harvested at the stages of pod setting and early pod filling. Root area and root length were measured by digital image analysis. Significant genotype × P level and genotype × harvest interactions in analysis of variance indicate that the genotypic variation of root traits depended on soil nutrient availability and the stage at which evaluation was made. Genotypes differed for taproot mass, basal and lateral root mass, root area and root length at both P levels and growth stages; differences in specific root area and length were small. Genotypes with growth habits II (upright indeterminate) and III (prostrate indeterminate) showed better adaptation to limited P supply than genotypes of groups I (determinate) and IV (indeterminate climbing). Between the two harvests, genotypes of groups II and III increased the mass of basal and lateral roots by 40 and 50 %, respectively, whereas genotypes of groups I and IV by only 7 and 19 %. Values of the genotypic coefficient of determination, which estimates the proportion of phenotypic variance resulting from genetic effects, were higher at early pod filling than at pod setting. Correlations between shoot mass and root mass, which could indicate indirect selection of root systems via aboveground biomass, were higher at early pod filling than at pod setting. The results indicate that selection for root traits in common bean genotypes should preferentially be performed at the early pod-filling stage

Morpho-physiological analysis of adaptive responses of common bean (Phaseolus vulgaris L.) to drought stress. Thesis (Ph.D.)

Common bean (Phaseolus vulgaris L.) is the most important food legume in the diet of poor people in the tropics. This legume is cultivated by small farmers and is usually exposed to unfavorable conditions with minimum use of inputs. Drought and low soil fertility, especially phosphorus (P) and nitrogen (N) deficiencies, are major limitations to bean yield in smallholder systems. Beans can derive part of their required N from the atmosphere through symbiotic nitrogen fixation (SNF). Drought stress severely limits SNF ability of plants. Identification of traits associated with drought resistance contributes to improving the process of designing bean genotypes adapted to these conditions. Field studies were conducted at the International Center for Tropical Agriculture (CIAT), Palmira, Colombia to determine the relationship between grain yield and different parameters in elite lines selected for drought resistance over the past decade. The selected traits were effective use of water (EUW), canopy biomass, remobilization of photosynthates to grain (pod partitioning index, harvest index and pod harvest index) and SNF ability. Moreover, in field trials we also validated the use of 15N natural abundance in grain tissue to quantify phenotypic differences in SNF ability for its implementation in breeding programs aiming to improve SNF in common bean. Carbon isotope discrimination (CID) was used for estimation of water use efficiency (WUE) and effective use of water (EUW). A set of 36 bean genotypes belonging to the Middle American gene pool were evaluated under field conditions with two levels of water supply (irrigated and rainfed) over two seasons. Additionally, a greenhouse study was conducted at CIAT using plastic cylinders with soil inserted into PVC pipes, to determine the relationship between grain yield and different root parameters such as total root length, fine root production and visual root growth rate in same group of elite lines under drought stress. Eight bean lines (NCB 280, NCB 226, SEN 56, SCR 2, SCR 16, SMC 141, RCB 593 and BFS 67) were identified as resistant to drought stress. Resistance to terminal drought stress was positively associated with EUW combined with a deeper and vigorous root system, better plant growth, and superior mobilization of photosynthates to pod and seed production, but negatively associated with days to flowering and days to physiological maturity. Based on phenotypic differences in CID, leaf stomatal conductance, canopy biomass and grain yield under drought stress, the tested lines were classified into two groups, water savers and water spenders. These groups also differ in their root characteristics, water spenders with a vigorous and deeper root system and water savers genotypes with a moderate to shallow root system and more presence of fine roots. We used 15N natural abundance method to compare SNF ability estimated from shoot tissue sampled at mid-pod filling growth stage vs. grain tissue sampled at harvest. The results showed a significant positive correlation between nitrogen derived from the atmosphere (Ndfa), estimated using shoot tissue at mid-pod filling, and Ndfa estimated using grain tissue at harvest. The method showed phenotypic variability in SNF ability under both drought and irrigated conditions. A significant reduction in SNF ability was observed under drought stress. We suggest that the method of estimating Ndfa using grain tissue (Ndfa-G) can be applied in bean breeding programs to improve SNF ability. Using this method of Ndfa-G, we identified four bean lines (RCB 593, SEA 15, NCB 226 and BFS 29) that combine greater SNF ability with higher grain yield under drought stress. These lines could serve as potential parents to further improve SNF ability of common bean. Better SNF ability under drought stress was related with superior presence of thick roots. Superior N uptake from the soil was associated with a large root system with more presence of fine roots. Pod harvest index, grain CID and Ndfa using grain tissue could be a useful selection criterion in breeding programs to select for drought resistance in common bean

Phenotyping common beans for adaptation to drought

Common beans (Phaseolus vulgaris L.) originated in the New World and are the grain legume of greatest production for direct human consumption. Common bean production is subject to frequent droughts in highland Mexico, in the Pacific coast of Central America, in northeast Brazil, and in eastern and southern Africa from Ethiopia to South Africa. This article reviews efforts to improve common bean for drought tolerance, referring to genetic diversity for drought response, the physiology of drought tolerance mechanisms, and breeding strategies. Different races of common bean respond differently to drought, with race Durango of highland Mexico being a major source of genes. Sister species of P. vulgaris likewise have unique traits, especially P. acutifolius which is well adapted to dryland conditions. Diverse sources of tolerance may have different mechanisms of plant response, implying the need for different methods of phenotyping to recognize the relevant traits. Practical considerations of field management are discussed including: trial planning; water management; and field preparation

Phenotyping common beans for adaptation to drought: protocol for field evaluation

This protocol was provided for the identification of phenotypic differences in drought resistance in common beans (Phaseolus vulgaris L.). It is available in both PDF and photo gallery format with the aim of providing visiting students and researchers with a consultation document they can use to answer questions about our in-house methodologies

The influence of atmospheric carbon dioxide enrichment on belowground growth

Root growth under elevated atmospheric CO{dollar}\sb2{dollar} was examined with the intention of quantifying the effects of elevated CO{dollar}\sb2{dollar} on growth, cell wall characteristics and root carbon status of three herbaceous legumes: Phaseolus vulgaris L., Phaseolus acutifolius Gray and Pisum sativum L. Plants were grown hydroponically in 0.1 strength Hoagland\u27s solution in the UNLV greenhouse at atmospheric CO{dollar}\sb2{dollar} concentrations of ambient (350 {dollar}\mu{dollar}mol mol{dollar}\sp{-1}),{dollar} 1.5x ambient (550 {dollar}\mu{dollar}mol mol{dollar}\sp{-1}){dollar} and 2x ambient (700 {dollar}\mu{dollar}mol mol{dollar}\sp{-1}).{dollar} Root biomass increased under elevated CO{dollar}\sb2{dollar} for P. acutifolius and P. sativum, but not for P. vulgaris. Phaseolus vulgaris roots grew faster during the day under elevated CO{dollar}\sb2{dollar}, P. acutifolius root growth rate increased at night, and P. sativum root growth rate did not change. In spite of these variable changes in growth rates, root:shoot ratios decreased in all three species studied. Measurements of biophysical growth parameters were inconclusive. Starch and ABA concentrations of P. acutifolius roots increased under 2x ambient CO{dollar}\sb2.{dollar} Increased carbon allocation to the roots presumably indicated a strong sink of rapidly elongating cells, possibly directed by ABA. An understanding of the effect of elevated CO{dollar}\sb2{dollar} on belowground growth processes will be necessary to predict the impact of increased industrialization on crop production and ecosystem functioning

Snap beans: Present status in the developing world and bibliography of research (1919-1987)

The present status of snap beans in the developing world is reviewed in the leading paper accompanying this publication. Socioeconomic aspects are mainly discussed. Production, marketing, and consumption features are described and the future potential for the crop is outlined. Constraints to production increases are discussed and the opportunities for research and development of the crop are defined. The bibliography contains 922 references to research on snap beans. Most of the documents (761) are available at CIAT. References are organized by disciplines and are complemented with author and subject indexes. The institutions identified by the Bean Information Center as carrying out research on snap beans are listed in the enclosed preliminary directory, which also includes the names of the snap bean researchers recorded in the CIAT database and the participants in the workshop on snap bean breeding held at CIAT in 1987. (CIAT)Se revisa la situacion actual de la habichuela en el mundo en desarrollo en el articulo principal que acompana esta bibliografia. Se discuten principalmente los aspectos socioeconomicos. Se describen los aspectos de produccion, mercadeo y consumo y se esboza el potencial futuro para el cultivo. Se discuten las limitaciones para el incremento de la produccion y se definen las oportunidades de investigacion y desarrollo del cultivo. La bibliografia contiene 922 referencias de investigacion sobre habichuela. La mayoria de los documentos (761) se encuentran disponibles en el CIAT. Las referencias estan organizadas por disciplinas y complementadas con indices de autores y materias. En el directorio preliminar adjunto se enumeran las instituciones identificadas por el Centro de Informacion sobre Frijol que estan realizando investigacion en habichuela. Tambien se incluyen los nombres de los investigadores de habichuela registrados en la base de datos del CIAT y los participantes en la reunion sobre mejoramiento de habichuela realizado en CIAT en 1987. (CIAT

Characterization of nitrogen-fixing bacteria from Phaseolus vulgaris L. in Kenya

Phaseolus vulgaris (common bean) is an important food crop in Sub-Saharan Africa. Low soil nitrogen limits the productivity of P. vulgaris in Kenya and a greater exploitation of symbiotic nitrogen fixation, resulting from interactions with rhizobia, has the potential to improve yields. To enable the increased use of the symbiosis in Kenyan agriculture in the future, studies in this thesis examined the genetic diversity of rhizobia that nodulate P. vulgaris in the central and western parts of Kenya, their nitrogen-fixing capabilities, and their competitiveness against Rhizobium tropici CIAT 899, a leading commercial inoculant strain for P. vulgaris. Lastly, studies investigated the relative importance of the genotypes of resident soil rhizobia, soil rhizobial population densities, inocula densities, and levels of soil nitrogen, in determining nodule occupancy by R. tropici CIAT 899 inoculated onto P. vulgaris. Phylogenetic studies using 16S rRNA and recA genes indicated at least five species of Rhizobium viz., R. sophoriradicis, R. phaseoli, R. leucaenae, R. paranaense and R. etli nodulate P. vulgaris in Kenya. In addition to the five species, strains that likely belong to new species in the genus Rhizobium also widely nodulate P. vulgaris in Kenyan soils. In glasshouse studies, recovered strains were variably effective on Kenyan cultivars of P. vulgaris and 11 fixed as much nitrogen as R. tropici CIAT 899. From the 11, strains such as NAK 227, NAK 288, NAK 214 and NAK 157 were also highly competitive in liquid co-inoculation assays, carried out with the aid of gusA and celB marker genes, and are potential future inoculants for P. vulgaris in Kenya. The genotype of the rhizobia in the soil was found to be the primary determinant of the nodule occupancy achieved by the inoculant strain, a finding that conflicts previous reports that indicated nodule occupancy was mainly determined by soil rhizobial densities. The rhizobial genotypes varied in their rhizosphere competence and in their ability to preferentially nodulate the host, suggesting these two traits are important for the successful colonization of P. vulgaris nodules by rhizobia. It is anticipated that future studies will leverage on the results in this thesis, to develop locally-targeted inoculation solutions that optimize nitrogen fixation in P. vulgaris in Kenya, and to elucidate the molecular basis for preferential nodulation in P. vulgaris