Effects of fungicide and insecticide mixtures on apple tree canopy photosynthesis, dark respiration and carbon economy

Fungicide/insecticide mixtures were applied at times and doses commonly used in commercial orchard practice. Their effects on photosynthesis and dark respiration were evaluated in two seasons with respect to the potential stress they impose on an apple tree using cv. 'Elstar'. The mixtures included the fungicides mancozeb, flusilazol and dithianon, and the insecticides oxydemeton-methyl or pirimicarb. A new technology was employed to continuously examine photosynthesis, dark respiration and carbon balance of apple trees based on six canopy chambers, which enclosed apple trees under natural conditions in the field, with on-line measurements and continuous analysis Of CO2 exchange and automated data acquisition. The fungicides mancozeb and flusilazol combined with the insecticide oxydemeton-methyl reduced whole tree canopy CO2 assimilation mostly at midday and, using hourly means, by an averaged 7.4% on the day of its application. This reduction in whole canopy photosynthesis declined with time, restoring most of the original photosynthetic potential within 3-5% in 3 days, hence, indicating acceptable phytotoxicity. This fungicide/insecticide mixture overproportionally, in relation to the changes in photosynthesis, increased dark respiration by up to 72% in the night after application, thereby drastically affecting the tree's carbon balance in an adverse way. In contrast, the fungicide dithianon combined with the insecticide pirimicarb decreased dark respiration by 15-21% with reductions in canopy photosynthesis in the order of 6-9%. Because the decrease in dark respiration exceeded that in photosynthesis, the apple tree overall gained carbon in a balance. Overall, effects on photosynthesis were smaller than on dark respiration. The effects of the pesticide combinations on photosynthesis are attributed to the CO2-independent Hill reaction in photosynthesis and to uncoupling the photosynthetic electron flow from phosphorylation, thereby inhibiting energy, viz. ATP formation or its transfer, rendering dissociation of ATP into ADP and P-i. (C) 2004 Elsevier Ltd. All rights reserved.