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ISHS Acta Horticulturae 903: IX International Symposium on Integrating Canopy, Rootstock and Environmental Physiology in Orchard Systems

PREDICTING CHEMICAL THINNER RESPONSE WITH A CARBOHYDRATE MODEL

Authors:   T.L. Robinson, A.N. Lakso
Keywords:   Malus × domestica chemical thinning, naphthaleneacetic acid, benzlyadenine, carbohydrate supply, carbohydrate demand, temperature, light intensity
DOI:   10.17660/ActaHortic.2011.903.103
Abstract:
Chemical thinning remains one of the more unpredictable parts of apple production with large variations from year to year and within years. To quantify the variability within years and between years, we conducted spray timing trials each year from 2002-2008 on mature vertical axis ‘Royal Gala’/M.9 trees. We applied a tank mix of either 7.5 mg•L-1 of naphthaleneacetic Acid (NAA) plus 600 mg•L-1 of carbaryl or 75 mg•L-1 of 6-benzlyadenine (BA) plus 600 mg•L-1 of carbaryl at 3 or 4 day intervals beginning at petal fall until 21 days after petal fall. We estimated the combined effects of temperature and sunlight on thinning efficacy carbohydrate balance (supply-demand) model developed by Alan Lakso. The model uses daily maximum and minimum temperatures and sunlight level to estimate whole tree carbohydrate supply and demand by the various organs of the tree. To evaluate the usefulness of the carbohydrate model in explaining the variability in thinning response, we compared the model estimates of carbohydrate balance and field results of thinning trials to look for periods of particularly good or poor supply:demand balance. Model simulations from several years showed that there are often periods of particularly negative carbohydrate balance which were associated with severe thinning. There were also years in which there was essentially a balance in carbohydrate demand and supply which gave a traditional pattern of chemical thinning response from petal fall to 20 mm fruit size unaffected by carbohydrate deficits. This traditional pattern shows that at petal fall there is the least thinning while the greatest thinning occurs at 10-15 mm fruit size which is followed by reduced thinning at greater fruit sizes. Periods of significant carbohydrate deficit, as estimated by our model, appear to dramatically change the underlying pattern which gives rise to the year to year variability in thinning response. The carbohydrate model was used in 2008 to help predict and explain chemical thinning response in New York state.
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