Work conducted in the USA, using equipment capable of measuring the light interception of the various types of leaves and the gas exchange associated with a whole apple tree, has enabled scientists to work out the photosynthetic potential and the tree’s assimilate needs to sustain a crop load at various times during the season (Lakso et al., 1998).
- This works shows that the demand for assimilates increases rapidly following bloom, peaking at about 4-6 weeks after full bloom.
- Thereafter, it remains fairly stable through until harvest.
- Comparing this demand with the tree’s potential to supply showed that the carbon production by the apple tree leaf canopy cannot support all of the fruits, which potentially could set, for more than a week or more after bloom.
- The work shows that, during the season there are two periods of potential limitation of carbon supply; one about 2-4 weeks after blooming and the other in the last few weeks prior to harvest.
This identification of a critical period for assimilate supply is supported by other recently published research.
- Researchers in Michigan showed that applications, 15 to 30 days after full bloom of chemicals known to limit photosynthesis temporarily (e.g. terbacil) caused fruitlet abscission (Flore et al., 1998).
Research at East Malling has shown that shading of apple trees also reduces photosynthesis and may cause reductions in fruit size, as well as reducing flowering and fruit set (Jackson and Palmer, 1977).
- However the shading needed to significantly reduce fruit size was 63% or more.
- Severe shade (only 10% of full sunlight) reduced fruit size by 30%.
- Further work by the same team (Jackson et al., 1977) showed that the shading reduced fruit size by reducing both cell numbers and individual sizes within fruits.
- Shading also reduced fruit colour, russeting, cracking, and the dry matter and starch per unit weight.
- Research in Japan has shown that glucose, fructose and sucrose contents of apple fruits are all decreased by shading.
It is argued that the high productivity of compact trees, such as those produced on dwarfing rootstocks, is attributable to a high level of light interception within the tree canopy; this in turn improves photosynthetic capacity (Robinson and Lakso, 1991).
- Training systems also influence light interception and hence photosynthesis and fruit size.
- German research has shown that trees trained to a Y trellis system bore more fruits and larger fruits than trees grown on spindles (Chen et al., 1998).
- Fruits from the Y trellis system were also higher in dry matter, total soluble solids, starch, glucose, fructose, sorbitol and total non-structural carbohydrates.