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Shading as an effective means for crop load management and fruit quality enhancement in apple trees

Aliev, T., Solomakhin, A., Blanke, M., Kunz, A., Klad, A.
Acta horticulturae 2012 no.956
alternate bearing, apples, carbon, color, firmness, flowering, flowers, fruit drop, fruit quality, fruit set, fruit trees, photosynthesis, photosynthetically active radiation, shade, starvation, sugar content, sustainable technology, taste
Since carbon starvation due to photosynthesis inhibition may induce fruit abscission, four-year-old apple trees 'Golden Delicious', 'Elstar' and 'Gala' on M9 at Bonn were shaded to achieve desired crop load viz., thin. Whole trees were covered with shade cloth (80% shade/PAR reduction, 90% UV reduction) for either 3, 6 or 9 days either at the end of bloom (EB) or 23 days after full bloom (DAFB); uncovered adjacent trees served as control. While shading for 3 days (23 DAFB) was optimal for fruit quality (fruit size, colour and sugar content) with only a slight decrease in yield, prolonged shading (for >6 days) also led to fruit quality improvement, but also to a considerable decrease in yield, caused by a higher rate of June drop; fruit trees shaded earlier at the end of bloom shоwed weaker June drop and more fruit set with insufficient thinning efficacy. Later and prolonged shading (more then 6 days) increased fruit sugar content (SSC) by 1.0-2.3% (and taste) by improving tree source: sink relationships, and fruit mass by 41% in 'Gala' and 13% in 'Elstar', with better (75-100% red surface) colouration of 85-96% than ca. 65% in the un-shaded control; the same shading treatment also induced the desired stronger return bloom viz., less alternate bearing, with the least flowers in the un-shaded control. Thus, this study has shown that shading for 3-6 days at 23 DAFB due to its effects on source:sink relationships and tree carbon starvation may be an environmentally-friendly technology to obtain fruit of the desired quality in terms of firmness, size, colour and sugar content and prevent biennial bearing of fruit trees.