The application of black food colorants - a possible alternative to chemical thinning agents in apple?

In order to match market and consumer demands with regard to fruit size and quality, fruit thinning is not only one of the most important but also one of the most challenging measures in apple production. However, the list of registered chemical thinning agents is getting shorter, mechanical thinners remove mainly peripheral fruit of expectably high quality and labour costs for manual thinning are relatively high. Therefore, the search for alternative thinning agents or methods remains an important task in fruit science. Transient reduction of light availability, hence reduced carbohydrate availability, for the developing fruit by covering trees with shade nets already proved to be a possible, yet expensive way to reduce fruit set in apples. We modified this approach in a two-year experiment by transiently blackening flowers and leaves of three-year-old ‘Kanzi’, ‘Cameo’ and ‘Topaz’ apple trees on rootstock M.9, respectively, with the water soluble food colorant E151 BrilliantSchwarz. The effect on fruit set and fruit size was compared with the efficacy of common chemical flower and fruitlet thinners such as Ammoniumthiosulphate (ATS), Ethephon, Benzyladenine (BA), Naphtylacetic Acid (NAA) and a mixture of the gibberellins GA4 and GA7. Thinning agents were applied either once or twice at recommended flower/fruitlet growth stages. At balloon stage, the total numbers of flower clusters on 5 trees for each treatment in 2008 and 7 trees per treatment in 2009 were counted. Fruit numbers per tree and maximum fruit diameter were determined at weekly intervals from application time until the end of June. Fruit set was calculated as number of fruit per 100 flower clusters. In comparison to control trees, ATS application resulted in the highest reduction of fruit set (67.2%, 47%), followed by ATS+BA (66.3%, 40.3%) and BA+GA4+7 (40.3%, 30.2%) for ‘Kanzi’ and ‘Cameo’, respectively. While NAA reduced fruit set by 31.2% in ‘Cameo’ it was less effective in ‘Kanzi’. Blackening with BrilliantSchwarz reduced fruit set by 28% in ‘Kanzi’ and 7.6% in ‘Cameo’. Interestingly, besides Ethephon, BrilliantSchwarz was the only treatment where fruit size of ‘Kanzi’ was significantly increased, whereas fruit size of ‘Cameo’ was only increased by ATS. Return bloom in 2009 on ‘Kanzi’ treated with ATS was increased by 75.2% over the control trees followed by ATS alone and BrilliantSchwarz with 33.1 and 26.6%, respectively. For ‘Cameo’ significantly higher return bloom was observed in ATS, ATS+BA and BA+GA4/7 treatments. The 2009 experiments also included the applications of the fungicide Armicarb and combinations of BrilliantSchwarz with ATS or Kaolin. Leaf photosynthesis rates were significantly reduced for the longest period in the combination of BrilliantSchwarz and ATS. Most treatments, except ATS+BA, reduced significantly ethylene evolution and auxin export from flowers or fruitlets. It can be hypothesized that fruit thinning activity following application of black food colorant alone or in combination with other compounds is rather caused by reduced auxin flow due to limited carbohydrate availability than by the formation of stress ethylene.