Scientific problems to be addressed with overall and specific objectives

Root traits conferring efficient P acquisition can be selected through molecular markers, or more directly through phenotypic screening. Phenotypic analysis of roots in the field is difficult, but in most cases phenotypic screens for specific traits in controlled conditions, typically with young plants, are well correlated with phenotypes observed in the field. Confirmatory trials in the greenhouse or field can and should be conducted to validate results from rapid phenotypic screens. The selection of germplasm for screening is important, and should include a wide sampling if materials including landraces that are more likely to display adaptation to low fertility than are bred lines. The methods involved with these screens and low P confirmatory trials are not commonly known and care must be exercised to avoid erroneous results. The first objective of this activity is to develop standard protocols for the analysis of root phenotypes and their validation, to be used by network participants.

The second objective is to develop a new version of SimRoot (Lynch et al. 1997), a geometric simulation of root growth and architecture, as a tool for the evaluation of root traits for P efficiency. We have used this tool to analyze the effect of root traits on root system performance. SimRoot generates 3-dimensional structures that grow and simulate physiological changes over time, based on measurements of actual root systems. SimRoot permits the exclusion of complicating variables present in real soils and plants, analysis of phenomena that are inaccessible through empirical measurements, and analysis of plant phenotypes not present in nature. We have used SimRoot to demonstrate that root architecture is an important regulator of P acquisition efficiency (Nielsen et al. 1994), of competition among (Ge et al. 2000) and between (Rubio et al. 2003) root systems, to demonstrate morphological synergism among root traits (Ma et al. 2001), to explore the relationships between root fractal properties and soil exploration (Nielsen et al. 1997; Walk et al. 2004), and to model the effect of carbohydrate competition among root classes on soil exploration (Walk et al. 2006). SimRoot will be an excellent tool to examine the relationship of specific root traits, root growth, and soil exploration. At present SimRoot is limited in several ways- it has no user interface, does not include root-shoot interactions, and does not consider water transport in the soil. Inclusion of these processes will permit the evaluation of root traits and their interactions in a modeling context to guide empirical experimentation with actual plants, which in the case of root traits is often laborious. In this activity we will enhance the user-friendliness and scope of this model so that it may be more widely used within and beyond the CRP network.