The role of root size versus root efficiency in phosphorus acquisition in rice.

In rice, genotypic differences in phosphorus (P) uptake from P-deficient soils are generally proportional to differences in root biomass or surface area (RSA). It is not known to what extent genotypic variation for root efficiency (RE) exists or contributes to P uptake. We evaluated 196 rice accessions under P deficiency and detected wide variation for root biomass which was significantly associated with plant performance. However, at a given root size, up to 3-fold variation in total biomass existed, indicating that genotypes differed in how efficiently their root system acquired P to support overall plant growth. This was subsequently confirmed, identifying a traditional genotype, DJ123, with 2.5-fold higher RE (32.5 µg P cm(-2) RSA) compared with the popular modern cultivar IR64. A P depletion experiment indicated that RE could not be explained by P uptake kinetics since even IR64 depleted P to <20nM. A genome-wide association study identified loci associated with RE, and in most cases the more common marker type improved RE. This may indicate that modern rice cultivars lost the ability for efficient P uptake, possibly because they were selected under highly fertile conditions. One association detected on chromosome 11 that was present in a small group of seven accessions (including DJ123) improved RE above the level already present in many traditional rice accessions. This subspecies is known to harbor genes enhancing stress tolerance, and DJ123 may thus serve as a donor of RE traits and genes that modern cultivars seem to have lost.