Polysorbates are potential agents for the chemical thinning of apple fruitlets, but their mechanism of action has not yet been elucidated. One possible mechanism may involve increased cuticular transpiration, as their chemical properties could alter cuticle structure and its permeability to water. This master’s thesis investigated the effects of the polysorbates Tween 20 and Tween 80 on cuticular transpiration and on the histological structure and ultrastructure of the leaf cuticle of 'Brookfield Gala' apple (Malus domestica Borkh.). Leaves were treated with Tween 20 or Tween 80, while untreated leaves served as controls. Cuticular transpiration was determined gravimetrically and by gas-exchange measurements, whereas cuticle structure was analysed using light and transmission electron microscopy. A system for the semiquantitative characterisation of cuticle structure was also established. Treatment with polysorbates did not result in a consistent increase in cuticular transpiration. Differences among treatments were observed primarily in the ultrastructural organisation of the external cuticular layer and the morphology of epicuticular waxes. The proportion of chaotic lamellar structure was increased from 10.9% in the control to 32.1% with Tween 20 and 58.7% with Tween 80, while the proportion of amorphous epicuticular wax was decreased from 20.6% in the control to 7.4% with Tween 20 and 1.4% with Tween 80. The results suggest that, at the concentration applied, both polysorbates may affect the reorganisation of the lipid components of the cuticle; however, the observed ultrastructural changes were not reflected in a clear alteration of its barrier function.
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