The biggest limiting factor in the tree’s ability to produce large crops of high quality fruit is the shade the tree casts upon itself.
It is not enough to know how and how much sunlight fruit trees must intercept. It is also important to think about managing shade because it is the balance between sunlight distribution and how uniform that sunlight is throughout the canopy that matters for yield, fruit size, fruit maturity and fruit quality.
The position of fruit in the canopy plays a big role in its maturity and post-harvest storage.
Sunlight hits the trees in different ways depending on the position in the canopy, causing fruit quality to vary within the tree, especially in trees with large canopies.
Get fruit out of the shade
To properly manage the fruitful part of the canopy you must reduce the interior structure of the trees and get the fruit out of the shade.
The higher the proportion of the fruit is exposed to adequate sunlight, the more uniform will be fruit ripening, and fruit quality at harvest and post-harvest.
Shade reduces fruit weight at harvest, skin colour, soluble solids concentration (sugars), flower bud numbers and fruit set in future years.
Shade has a positive effect on bitter pit of apples and skin russet.
Flowering & flower bud formation
For good flowering the wood needs to be exposed to 30 per cent or more full sunlight on a diurnal basis. On large trees with dense canopies there are commonly fewer flowers or no flowers on the shaded parts of the tree.
It takes a certain amount of energy to drive the process of flower bud formation and initiation—the sunlight must hit that part of the tree, not elsewhere on the tree, for flower buds to develop.
Photosynthates for quality fruit
To grow quality fruit the fruit and its nearby leaves must be exposed to about 50 per cent or more full sun, so that there is enough sunlight to produce the photosynthates for the fruit to grow to its marketable sizes.
The bigger you want to grow the fruit, the more photosynthates that fruit need to have. That is why you see differences in the size of the fruit from different parts of the tree: the larger fruit tends to be on the parts of the tree that get adequate sunlight.
Light through leaves
Very little light passes through a leaf to the leaves below. A leaf absorbs at least 90 per cent of the sunlight that hits it.
Only 10 per cent or less shines through to the leaves below, and the quality of that light is changed, so that it is not useful for photosynthesis.
When leaves are stacked on top of each other there can be a lot of dark areas in the tree where fruit has poor colour and size.
Fruit in sun vs shade
Fruit that receives adequate sunlight lose more water during the day than shaded fruit, but this also causes sun-exposed fruit to gain more sugar, flavonoids and nutrients, and promotes greater fruit growth overnight than shaded fruit.
Researchers in the USA compared pears harvested from trees with large canopies which grew inside the shaded areas, with pears that grew in areas that were more exposed to the sun.
They found that harvest maturity varied depending upon the fruit’s position in the tree. Fruit that was more exposed to the sun was riper and tastier than shaded fruit. There were also fewer pears and the pears were smaller when they grow in shade than pears which received adequate sunlight.
In the shade there was quite a loss of diffuse sunlight which affected the development of the fruit. The difference was considerable and a huge variation was found in the development of the fruit in the same tree.
Chemical analysis also indicated that position in the tree has a major impact on storability and fruit quality.
(cont next issue)
See this article in Tree Fruit Dec 2021
