With about half of the world’s people dependent on rice as their main food source, improving the nutritional value of rice could have major impact on human health. The first step toward breeding rice cultivars with enhanced element composition (ionomics) is to understand the genetic diversity available to breeders in germplasm collections.  The element × element and plant trait × element relationships observed therein can implicate mechanisms of mineral uptake, transport, and grain accumulation. 

A core subset of 1640 accessions from among the 17,000+ rice accessions in the USDA National Small Grains Collection was grown over two years, two replications/year in Beaumont, TX, under both flooded and unflooded field conditions to impact soil redox and nutrient availability.  ICP-MS was used to analyze the harvested brown rice for grain concentration of Mg, P, K, S, Ca, Mn, Fe, Co, Ni, Cu, Zn, As, Rb, Sr, Mo, and Cd.

Fifteen repeated check-plots per replication documented that environmental variance was low compared to genetic variance.  Wide differences (from 2x to 100x) in grain concentration were seen for all 16 elements; unflooded fields generally provided greater ionomic variance than did flooded fields.  Statistically significant element × element correlations included P×K (r=0.56), P×Mg (r=0.66), and Sr×Ca (r=0.64). K and Mg were more directly correlated with P than each other (r=0.35).  Phosphorus enhances root growth, potentially contributing to the observed elemental relationships.  Due to chemical similarity, Sr and Ca follow the same routes of plant uptake and transport.  All element histograms were skewed with more accessions having low grain concentration than those having notably high concentrations.  Accessions high in specific elements were sometimes found to have similar genetic or geographic origins.  For example, genotypes high in Ca, Mg, P, and K were more likely to be of the japonica than of the indica subspecies.  Other elements were more associated with tropical vs. temperate origins, e.g., high Cu and As were less prevalent among the temperate japonicas than among the tropical japonicas or indicas.  Four of the five lines highest in Mo-concentration originated from Malaysia, suggesting they share a heritable mechanism underlying their high Mo-concentration.  Accessions exhibiting extreme grain mineral concentration were crossed to develop segregating progeny in which to molecularly map genes affecting rice nutritional value.