On Which Side of the Tree Are Reflective Mulches More Efficient to Improve Fruit Colour?

Abstract

The objective of the present study was to investigate how far and from which side reflective mulches effectively reflect light into the tree crown to improve the colour of apple fruit under hail net in the autumn when light is limiting at 50° N latitude. Therefore, polypropylene mulch (LumilysTM) was spread 2.6 m wide in the alleyways and (recycled) aluminum foil in the herbicide strip under the trees (0.6 m either side of the tree) six weeks prior to anticipated harvest. Previous work had shown the light penetration and distribution within the tree canopy using polypropylene mulch (Lumilys TM) in the alleyways, but without showing the effect on fruit colouration and without using other mulches or strategies. Hence, ninety 9‑year-old cv. ‘Braeburn, Hillwell‘ apple trees on M 9 rootstock at CKA Klein-Altendorf, University of Bonn were employed for the experiment in 2018, a year with generally good fruit colouration. Trees were trained to slender spindles and were planted in N‑S rows to optimize light utilization. A bio stimulant (StimplexTM) was included as a chemical colour enhancer, while untreated trees of the same rows served as control. 180 apple fruits were marked and repetitively assessed four-times prior to harvest resulting in 3600 colour measurements on attached fruit with the following results 1)Fruit on the East side of the tree developed less red colouration (69°hue) than those on the West side, identifying the need for colour improvement measures such as reflective mulches, chemicals or other on the East side2)Low hanging fruit under 1 m height in the tree canopy developed insufficient colour (70.4°hue), which was improved by Stimplex to 66.4°hue with a one week earlier harvest3)In this otherwise shaded area, reflective mulches showed the greatest effect: Apple fruit reached good red colouration with polypropylene (LumilysTM) in the alleyways of every row (59.6°hue), every other row (57°hue) or with aluminum foil directly under the tree (53.3°hue), where lower hue values represent better red colour4)By contrast, the positive effect on colouration of the reflective mulches appeared less pronounced for fruit in the light-exposed Westside of the tree canopy, irrespective of their material and application (polypropylene in the alleyways or aluminum directly under the trees).5)Spreading the reflective mulch LumilysTM every second row viz alleyway improved fruit colouration, to the same extent, as spreading the same material in the traditional way in every row viz alleyway or spreading (recycled) aluminum foil directly under the trees.Regardless of the material and spreading approach, the reflective mulches showed the best effect on fruit colouring in the more shaded parts of the tree canopy.