Response of Malus x domestica Borkh to metamitron and high night temperature: Effects on physiology and fruit abscission

Periods of high nighttime temperature may induce carbohydrate (CH) shortage by increased dark respiration. Metamitron is a thinning agent that inhibits photosynthesis and enhances fruit abscission due to a reduction in CH production. To clarify how both interact in apple tree physiologic mechanisms and on fruit abscission, five field trials were carried out in Lleida, Girona and Sint-Truiden (2017 + 2018), using orchards of ‘Golden’ apple trees. At the stage of 12–14 mm fruit diameter, four treatments were established: (A) CTR – control, trees under natural environmental conditions; (B) HNT – high nighttime temperature, trees exposed to artificially increased nighttime temperature during 5 nights after the day of spraying, without metamitron application; (C) MET - 247.5 ppm of metamitron application and (D) MET + HNT - trees submitted to the combined exposure to metamitron application (MET) and to artificially increased nighttime temperature (HNT). HNT did not affect metamitron absorption, net photosynthesis (P_n) and stomatal conductance however, promoted significant reductions in leaf CH content mainly before sunrise, especially in sucrose (18–45%) and in sorbitol (19–26%). Metamitron significantly reduced P_n to about 50% of CTR, which resulted in decreases in leaf sucrose and sorbitol, reaching minimum values 5 days after spraying, between 21 and 57% and 19–26%, respectively. Fruit growth rate of both treatments was retarded by 30%, 2 days after either metamitron application or HNT. Both treatments originated a similar reduction in the number of fruits and size improvement. The combined exposure (MET+HNT) promoted similar P_n reductions as MET, but showed the greatest sucrose (44–60%) and sorbitol (73–84%) decreases which resulted in the strongest thinning efficacy. Lipid peroxidation was not affected by the treatments however, antioxidant enzyme activity showed moderate changes with activity increases mainly under MET and MET + HNT, accompanied by a rise in glutathione content and reduction in ascorbate. This work shows that the overlap of photosynthesis inhibition (reducing CH production) by means of metamitron spraying, and likely greater respiration (increased CH consumption), by HNT imposition, translates less CH production than the growing fruits demand (negative CH balance) leading to a metamitron thinning effect enhancement. Periods of high nighttime temperature must be considered when deciding the best metamitron rate to achieve an optimal crop load result.