Improving Red Colouration of Apple Fruit | Möglichkeiten zur Verbesserung der Rotfärbung bei Äpfeln

Measures for (a) improving the intensity of red colouration, and (b) increasing the proportion of red peel on otherwise shaded parts of the fruit are reviewed with the aim of assessing their potential to meet consumer and market demands, produce healthier fruit and improve economic returns to the producer of bi-coloured apple and other fruits in the light of often unknown or under-estimated adverse side-effects of existing and new compounds mostly in the US in 2014; hence the underlying regulatory mechanisms of colour formation and inhibition in apple fruit are explained.Foliar fertilisers, used to enhance fruit colouration, so far had a limited effect principally in the order of 3–5 % better coloured fruit for most monophosphates—only ‘Seniphos’ (with 4 % CaO) consistently increased the proportion of well-coloured fruit by up to 8 %. Kaolin, a white particle film, appeared an unsuitable measure for improving fruit colouration at 50°N when sprayed onto the fruit, while it can significantly reduce sunburn at lower latitudes.Undesired side-effects such as lack of colouration can originate from misuse such as late application of excessive doses of 6-BA formulations, as otherwise properly used for thinning, or the gibberellin antagonist ProhexadionCalcium for shoot growth regulation. The mode of action is explained of colour promoting agents like methionin/phenylalanine (‘Sunred’), methyl jasmonate (‘Blush’), ethylene (‘Motivate’), aminoethoxvinylglycine (AVG, ‘Retain’) and abscisic acid (ABA, ‘ProTone’) although presently without registration in Europe.Light reflectors such as woven PE/PVP, aluminium-coated plastics, industrial-grade paper, straw, lime, kaolin, titanium oxide and lime-based compounds as mulch in the alleyways at least 4 weeks prior to anticipated harvest have been successfully used to enhance fruit colouration. They can particularly enhance the desired red colouration on the otherwise shaded green, (inner and lower) sides of the fruit. Colour improvements in the order of 8–10 % make reflective mulches particularly Extenday type PVP/PE/aluminium foils economically viable, particularly if used more than once a year. Light reflectors are more effective in years of poor colouration and in late ripening varieties, as intended; countryside concerns have to be considered.Cultivation measures at 50°N such as winter and summer as well as root pruning are the first steps to improve fruit (red) colouration by improving light penetration into the tree canopy and to the fruits, followed by light reflectors, ‘Seniphos’ like monophosphates, other fruit colour enhancing chemicals and evaporative cooling using overhead sprinklers at times of low humidity and little radiation (evening) and choice of lighter hail net (white or grey), hillsides (rather than valleys) and change to a variety with better colour. In any case, the greater content of anthocyanins in red apples provides an additional health benefit for the consumer without extra charge.