Sugar metabolism in spur bud during flower bud formation:: A comparison between exposed and shaded buds of Japanese pear [Pyrus pyrifolia (Burm.) Nak.] 'Kosui'

To elucidate the role of carbohydrates and their metabolism during flower bud formation, the effects of shading the terminal buds of spurs of the Japanese pear 'Kosui' for 3 weeks were investigated. The carbohydrates were: fructose, glucose, sorbitol, sucrose and starch, whereas the related enzymes were: NAD-dependent sorbitol dehydrogenase (NAD-SDH), NADP-dependent sorbitol dehydrogenase (NADP-SDH), sucrose synthase (SS) and acid invertase (Al). Early treatments between 24 May to 28 July decreased both the concentrations of sorbitol, glucose and fructose in the buds and the bud fresh weight as compared with the non-treated, exposed buds. However, late treatments between 28 July to 2 September did not. Activities of NAD-SDH, NADP-SDH, soluble Al and SS increased in the shaded buds, although the differences between the treatments were temporary and sometimes insignificant. In contrast, cell wall-bound Al was temporarily decreased by the shading treatment. Contents of fructose, glucose and sorbitol in buds were positively correlated to the growth rate of the bud. On the other hand, sorbitol concentration was correlated negatively to the activities of total SDH (NAD-SDH + NADP-SDH) and soluble Al, and glucose with NADP-SDH. It is postulated that the sugar concentration may be the limiting factor for bud growth when the carbohydrate supply is insufficient. Shading increased the activities of sugar catabolizing enzymes in the bud that enhanced the capacity of buds to attract photosynthates (sink strength). It could be possible that the decrease of sugar concentration may act as a signal in the regulation of sugar catabolizing enzymes.