The effect of CO₂ on ethylene evolution and elongation rate in roots of sunflower (Helianthus annuus) seedlings

Both carbon dioxide and ethylene can affect the rate of root elongation. Carbon dioxide can also promote ethylene biosynthesis by enhancing the activity of 1‐aminocylopropane‐1‐carboxylic acid (ACC) oxidase. Since the amount of CO 2 in the soil air, and in the atmosphere surrounding roots held in enclosed containers, is known to vary widely, we investigated the effects of varying CO 2 concentrations on ethylene production by excised and intact sunflower roots ( Helianthus annuus L. cv. Dahlgren 131). Seedlings were germinated in an aeroponic system in which the roots hung freely in a chamber and were misted with nutrient solution. This allowed for treatment, manipulation and harvest of undamaged and minimally disturbed roots. While exposure of excised roots to 0.5% CO 2 could produce a small increase in ethylene production (compared to roots in ambient CO 2 ), CO 2 concentrations of 2% and above always inhibited ethylene evolution. This inhibition of ethylene production by CO 2 was attributed to a reduction in the availability of ACC: however, elevated CO 2 had no effect on ACC oxidase activity. ACC levels in excised roots were depressed by CO 2 at a concentration of 2% (as compared to ambient CO 2 ), but n ‐malonyl‐ACC (MACC) levels were not affected. Treating intact roots with 2% CO 2 inhibited elongation by over 50%. Maximum inhibition of elongation occurred 1 h after the CO 2 treatment began, but elongation rates returned to untreated values by 6 h. Supplying these same intact roots with 2% CO 2 did not alter ethylene evolution. Thus, in excised sunflower roots 2% CO 2 treatment reduces ethylene evolution by lowering the availability of ACC. Intact seedlings respond differently in that 2% CO 2 does not affect ethylene production in roots. These intact roots also temporarily exhibit a significantly reduced rate of elongation in response to 2% CO 2 .