Non-structural carbohydrate status and CO2 exchange rate of apple fruitlets at the time of abscission influenced by shade, NAA or BA

Four days of 90% shading at the time of fruit setting (1-5 days after petal fall, DAPF) of mature 'Jonagold'/M.9 apple trees was combined with the app lication of thinning agents to establish the possible interaction of both m ain treatments on fruit retention, and the effects of the main treatments o n fruitlets CO2 exchange and their carbohydrate concentration. Shading alon e reduced yield by up to 35%. When shading was followed by the application of 15 ppm 1-naphthaleneacetic acid (NAA) or 50 ppm 6-benzyladenine (BA), th e interaction between shading and chemical thinners was on the border of si gnificance. Following the assumption that abscission was induced by shading before the application of chemical thinning agents, the same process of di minished dark respiration rate (R-d) was found on fruitlets sampled 3 days after the end of shading and 7 days after NAA and BA spraying, i.e. in the fruitlets that were considered prone to drop. A week after the end of shadi ng the abscission of fruitlets due to shading was completed and on the rema ining fruitlets that were not prone to abscission any more, larger negative net CO2 assimilation rates (A) and enhanced R-d were recorded. Fruitlets p rone to abscission sampled 2 days after the end of shading showed a lower c oncentration of glucose and ascorbic acid and a higher concentration of sta rch. Eight days after the end of shading, the fruitlets from shaded trees t hat were not prone to abscission any more contained less fructose while the fruitlets from NAA and BA splayed trees had a higher concentration of fruc tose a week after application. The results do not support the hypothesis th at a good assimilate supply is needed for the retention of apple fruitlets, because the fruitlets retained an trees expressed even stronger CO2 loss a nd because the concentration of non-structural carbohydrates did not decrea se in fruitlets prone to drop with the exception of glucose. (C) 2001 Elsev ier Science B.V. All rights reserved.