Quantitative trait loci for stomatal density and size in lowland rice

Genotypic variation in stomatal density and size has been reported but little is known of the genetic mechanisms behind these leaf traits. Using 101 recombinant inbred lines derived from a cross between a tropical japonica, IR69093-41-3-2-2 and an indica variety, IR72, we conducted a field study to determine stomatal density and size and identify quantitative trait loci (QTL) controlling these traits under lowland conditions. Ten QTLs for stomatal density and four QTLs for size were identified across growth stages and leaf surfaces (adaxial and abaxial). The contribution of each QTL to total phenotypic variation ranged from 9.3 to 15.2% for stomatal density and 9.7 to 14.3% for size. The allele from IR72 increased stomatal density and that from IR69093-41-3-2-2 increased size. The expression of the QTLs for stomatal density and size differed by growth stage indicating that these traits might be genetically controlled depending on growth stage or that each QTL had a different function by growth stage. Significant negative genetic correlations between stomatal density and size at both vegetative (r = -0.308**) and heading (r = -0.484**) stages were observed but no common QTL for these traits was detected across growth stages and leaf surfaces. These results indicate that the QTLs for density and size may neither be genetically linked nor pleiotropically controlled and findings can be used as basis for selection at the leaf level on the balance of carbon and water uptake. Further study is needed to fully understand the mechanism underlying the observed genetic association and to elucidate the function of the QTLs involved.