Effects on photosynthesis of the fruit thinning agents naphthaleneacetic acid (NAA) and three commercial plant growth regulator formulations, naphthaleneacetic acid ('Rhodofix') and naphthaleneacetamide ('Amidthin') and 2-chloroethylphosphonic acid ('Ethrel') were evaluated with respect to the stress they impose on the fruit tree, using the alternate-bearing sensitive apple cv. 'Elstar'. This work was based on the hypothesis that plant stress in the form of large reductions in leaf photosynthesis are a pre-requisite for successful fruit thinning. A new technology was employed for continuous recording of tree canopy photosynthesis, dark respiration and carbon balance of apple trees. This was based on six canopy chambers, which enclosed apple trees under natural conditions in the field, with on-line measurements and continuous analysis of CO, exchange and automated data acquisition. All employed thinning agents reduced whole tree canopy photosynthesis consistently by 3-34% on the five days following their application, with photosynthesis still declining thereafter in the case of the NAA and 'Amidthin' application. The reduction after application of either 'Rhodofix' or 'Ethrel', declined within five days, such that most of the original photosynthetic potential was restored, indicating acceptable phytotoxicity of these three plant growth regulators at the concentrations used. The effects on dark respiration differed markedly. NAA and 'Ethrel' increased dark respiration over-proportionally by up to 106%, whereas 'Amid-thin' and 'Rhodofix' decreased it by up to 46% in the first night after application, thereby drastically affecting the carbon balance of the tree in opposite ways. These results are integrated into a hypothesis linking basipetal auxin transport, phloem loading, translocation and deficiency of photoassimilates.
