Involvement of ethylene in the morphological and developmental response of rice to elevated atmospheric CO2 concentrations

We tested the hypothesis that increased carbohydrate flux under elevated CO2 regulates accelerated development using rice ( Oryza sativa L. cv. Jarrah). Plants were grown either in flooded soil or solution culture at either 360 or 700 muL CO2 L-1. Total dry mass, shoot elongation rates (SER), tiller appearance rates ( TAR) and ethylene release from intact rice seedlings were measured from 5 to 42 days after planting (DAP). At maturity, shoot and sheath length, tiller number and grain mass were also measured. Elevated CO2 had a profound effect on growth, morphology and development and the effects were more pronounced during the early growth phase. Total above ground biomass increased at elevated CO2 and this was accounted for by enhanced tiller number. Grain yield was increased by 56% under elevated CO2 mainly due to increased tiller number and hence panicle number. TAR and SER were enhanced at elevated CO2 but SER increased only untill 25 DAP. Elevated CO2 stimulated a 2-3-fold increase in endogenous and ACC-mediated ethylene release but the ACC concentration in the leaves was little affected showing that rates of ACC synthesis matched its oxidation. Inhibition of ethylene action by 1-aminocyclopropane (1-MCP) had a more pronounced inhibitory effect on ethylene release in plants that were grown at 700 as compared to 360 muL CO2 L-1. Feeding sucrose to intact plants enhanced ethylene synthesis and these results are consistent with the hypothesis that increased accumulation of sucrose at elevated CO2 may enhance expression of genes in the ethylene biosynthetic pathway. We conclude that increase in ethylene release may be central in promoting accelerated development under elevated CO2 and this coincides with the release of auxiliary buds and accelerated rates of tiller appearance hence increased grain yield at elevated CO2.