Respiration Efficiency and Heterosis in Sugarbeet 1

with which carbohydrate energy is transferred to ATP would explain heterosis. Mitochondrial respiration (ADP:O ratio) and storage respiration rate (CO₂ evolution) are generally considered to be measuring the same oxidative phosphorylation process, even though other reactions may also be involved. A respiration efficient genotype would, therefore, have a high ADP:O ratio and a low CO₂ evolution rate per unit of growth. Mitochondrial respiration efficiency (ADP:O ratio) was measured in a set eight sugarbeet (Beta vulgaris L.) hybrids and their respective parents at four different stages of growth. Storage respiration (CO₂, evolution) of sugarbeets stored 30 days at 5 ° was measured on an eight parent, diallel cross and a two-by-six males ✕ females cross. All hybrids exhibited significant heterosis for root and total sucrose yield. The ADP:O ratios of the hybrids were higher (more efficient) than those of the inbred parents at each stage of growth. The ability of both inbreds and hybrids to transfer carbohydrate energy to ATP (ADP:O ratio) changed significantly with growth demand. The highest ADP:O ratios were obtained when the plants were growing most rapidly (2.07 and 2.26 for inbreds and hybrids, respectively), and the lowest during storage at 5 °C (1.54 and 1.68 for inbreds and hybrids, respectively). Hybrids had significantly lower storage respiration rates (0.456 μmol CO₂ kg⁻¹ s⁻¹) than the inbred parents (0.614 μmol CO₂ kg⁻¹ s⁻¹). The two males (L33 and L53) in the males ✕ females cross had similar root and sucrose yields, and similar storage respiration rates. In hybrid combination, the L53 hybrids produced significantly larger root and sucrose yields, and a faster cell division rate and a lower storage respiration rate than the L33 hybrids. These data indicate that a major factor contributing to root and sucrose yield heterosis in sugarbeet is the ability of the hybrids to transfer more of the carbohydrate energy into ATP for growth, maintenance, and membrane structure, compared with the inbred parent.