Effects of fruit thinning agents on apple tree canopy photosynthesis and dark respiration

Effects on photosynthesis of the fruit thinning agents naphthaleneaceticacid (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, usingthe alternate-bearing sensitive apple cv. 'Elstar'. This work wasbased on the hypothesis that plant stress in the form of large reductions inleaf photosynthesis are a pre-requisite for successful fruit thinning. A newtechnology was employed for continuous recording of tree canopyphotosynthesis, dark respiration and carbon balance of apple trees. This wasbased on six canopy chambers, which enclosed apple trees under naturalconditions in the field, with on-line measurements and continuous analysis ofCO₂ exchange and automated data acquisition. All employed thinningagents reduced whole tree canopy photosynthesis consistently by3–34% on the five days following their application, withphotosynthesis still declining thereafter in the case of the NAA and'Amid-thin' application. The reduction after application of either'Rhodofix' or 'Ethrel', declined within five days, suchthat most of the original photosynthetic potential was restored, indicatingacceptable phytotoxicity of these three plant growth regulators at theconcentrations used. The effects on dark respiration differed markedly. NAA and'Ethrel' increased dark respirationover-proportionally by up to 106%, whereas 'Amid-thin' and'Rhodofix' decreased it by up to 46%inthe first night after application, thereby drastically affecting the carbonbalance of the tree in opposite ways. These results are integrated into ahypothesis linking basipetal auxin transport, phloem loading, translocation anddeficiency of photoassimilates.