The ratio of root length to root dry weight (specific root length SRL) varies widely among plant species. Since water and nutrient uptake is influenced more by root length than mass, species of high SRL may appear to invest their root biomass more efficiently than species of low SRL. A more complete cost-analysis, however, must include the life-span of the root. Species that construct fine roots (high SRL) may tend to proliferate roots in favorable volumes of soil and shed roots in unfavorable soil volumes more readily than species with coarse roots (low SRL). Thus, the short-term efficiency of high SRL roots may be offset by higher costs associated with higher root turnover rates. This proposal tests the relationship of SRL to root plasticity and root life- span by using a microvideo camera inserted into transparent tubes buried in the ground in the field. A citrus rootstock trial, with genetically diverse root systems grafted to genetically identical shoots, will serve as the model plant system. This study will provide information on factors that influence root death and turnover as well as insights into how interspecific differences in root characteristics may influence nutrient and water acquisition, root turnover, and, ultimately, plant productivity. In plant breeding, selection based on specific root traits is seldom undertaken. While the citrus rootstock trial serves as an excellent model system with which to examine basic relationships between root characteristics and root turnover rates, the information gained also has direct practical applications, since the rootstocks in the study are all of economic importance. Additionally, this information will be valuable for ecosystem models that are being used to help estimate global climate change, since root turnover is a major source of carbon dioxide efflux into the atmosphere.
