Sugar metabolism in buds during flower bud formation: a comparison of two Japanese pear [Pyrus pyrifolia (Burm.) Nak.] cultivars possessing different flowering habits

In order to elucidate the role of carbohydrate in flower bud formation, the seasonal changes of carbohydrate concentrations (fructose, glucose, sorbitol, sucrose and starch) and the activities of enzymes catabolizing sorbitol (NAD-dependent sorbitol dehydrogenase (NAD-SDH), NADP-dependent sorbitol dehydrogenase (NADP-SDH) and sorbitol oxidase (SOX)) and sucrose (sucrose synthase (SS) and acid invertase (AI)) were investigated during flower bud formation using the terminal bud of the spur of ‘Kosui’ and ‘Chojuro’ Japanese pears (Pyrus pyrifolia (Burm.) Nak.). Bud size measured after leaves had fallen was larger in ‘Chojuro’ than in ‘Kosui’. The buds containing one primordium were significantly smaller than those containing two or three primordia for both cultivars. Concentrations of the carbohydrates in the bud were not significantly different between the cultivars. On the other hand, sucrose metabolizing enzymes showed higher activities in ‘Chojuro’ than in ‘Kosui’ throughout the measurement period. Similarly, the activities of sorbitol metabolizing enzymes were higher in ‘Chojuro’ than in ‘Kosui’ though the differences were temporal. Additionally, bud growth rate correlated positively with the activities of NAD-SDH and AI (soluble) during the period of flower bud development. These results suggest that sugar metabolic activity was involved in bud growth, whereas the role of sugar concentration in buds remained undetermined. We postulate that greater activities of sugar catabolizing enzymes may enhance the capacity of buds to attract assimilates, thereby accelerating bud growth and increasing the number of primordia in the bud.