Sunlight within the canopy controls distribution of mineral elements and carbohydrates.
Shade and mineral nutrition (cont from last month)
Well-exposed portions of the tree will have leaves with high levels of leaf nitrogen, while shaded portions have lower levels.
When leaves are shaded, nitrogen is exported out of the shaded trees to leaves which are better exposed. Thus, leaves at the top and edge of dense canopies will have higher nitrogen levels and shoots will tend to be more vigorous than leaves in open canopies.
Zinc deficiencies may be diagnosed by poor light. Leaves and shoots with low zinc, which are formed under low light conditions, may not show deficiency symptoms of small size, narrow shapes, and rosettes, although tissue levels are low.
Good light distribution within the canopy will result in more uniform distribution of nitrogen and other elements, along with better distribution of vigour.
Ideally, to maximize light interception and have adequate light distribution within the canopy, tree canopies should have a large surface area to volume ratio. Many high density training systems take advantage of this ratio. Thus, canopies of the most efficient training systems are often as narrow as 0.60 to 1.00 metre, and may extend from 2.50 to 3.50 metres in height.
Skin colour and shade
Too much shade is detrimental for skin colour in some fruit.
The colour pigment is formed from carbohydrates manufactured via photosynthesis. But sunlight must also hit the fruit directly to allow the chemical reactions, which create the red anthocyanin pigments, to proceed.
Depending upon the specific type of fruit and variety, generally 50 to 70 per cent full sunlight is necessary to develop a full, maximum red or pink skin. You don’t want too much shade and you don’t want too much sun—you want something in between.
See this article in Tree Fruit March 2022
