Rice genotypes that respond strongly to elevated CO2 also respond strongly to low planting density

Screening for cultivars that respond strongly to elevated atmospheric CO2 concentration ([CO2]) is essential for increasing the productivity of C3 crops in the future high-[CO2] world. However, the cost of research facilities capable of increasing [CO2], such as free air-CO2 enrichment (FACE) rings, limits the number of accessions that can be tested. Since low planting density and elevated [CO2] can both increase individual crop growth, I therefore hypothesized that cultivars capable of responding strongly to high [CO2] would also respond strongly to low planting density. To test this hypothesis, I grew four rice cultivars, previously determined to respond differently to elevated [CO2] in a 2-year FACE experiment, under normal and half-normal planting densities in a paddy field. The plant biomass was significantly increased (by 36–64%) by low planting density, with a significant cultivar × density interaction. Panicle number and grain weight showed similar responses. The change in grain weight caused by low planting density was closely, significantly (P < 0.05), and positively correlated with the response to elevated [CO2] in the previous research. These results support the hypothesis that low planting density may emulate the effects of high [CO2] and therefore facilitate pre-screening of cultivars.