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.
Fruit in sun vs shade (cont from last issue)
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.
Another example is with cherry trees where researchers found that as shoots of young trees grow, each leaf grows at a particular angle from the previous leaf, moving into the best position to create minimal shading for the leaves below. The tree maintains this high-efficiency pattern as new shoots develop from the same growing points.
However, by year three or four, shade starts to increase and leaves on the inside of the canopy cannot photosynthesise adequately to produce the sugars and flavonoids for good fruit size, firm fruit and good taste at harvest. Tree training and summer pruning are then often used to remedy the shade problem. (Figure 1).
Because sunlight can only travel for short distances in tree canopies before it disappears, it is important to know that each fruit needs a certain amount of leaf area to develop into marketable size, fruit quality and taste. It is a matter of arranging that leaf area in the form of a certain number of leaves per fruit.
To complicate things, leaves are not all created equally when it comes to contributing sugars to the fruit. Shoot leaves and spur leaves of apple and pear with and without fruit, all play a role at different times during fruit growth.
Managing those types of leaves helps to keep the tree in balance in terms of crop load and leaf area.
Keeping those leaves exposed to adequate sunlight will ensure uniformity in the photosynthates delivered to the fruit. It gives you the ability to grow a high quality piece of fruit with good sugar, good size, good firmness and good taste throughout the tree.(Figure 2).
To obtain high production of fruit of uniform harvest maturity, you have to have an orchard system that has a high-volume canopy to absorb about 70 per cent of incoming sunlight. For this the orchard must have the correct density of trees, have a narrow canopy with minimal structural wood and uniform vigour. (Figure 3).
Light interception and shade (cont next issue)
See this issue in Tree Fruit Jan 2022
