Each season our trees work hard to convert sunlight into carbohydrates in the leaves and then at the relevant time to send this 'food' into the developing fruit to give us our crop.
Food movement is all about the points of demand. The signals that determine food movement are driven by hormones.
Hormones determine the greatest 'sink' for food at different growth stages and these sinks will change in strength.
At times most of the food is directed to vegetative growth, at other times it is delivered to the fruit, and at times we see a stronger sink from the roots and storage tissue.
The hormone signals in the plant are quite complex and it is more the ratio of one hormone to another, rather than the absolute amount of hormone that determines the transfer of food to each site.
The main hormone that directs food is IAA. When IAA moves down from the vegetation, sugar and carbohydrates move in the opposite direction (up to the shoots). When IAA moves from the fruit, food (sugar and carbohydrates) move in the opposite direction (into the fruit).
Harvest
Leading up to harvest, if we have the crop managed properly, we hope that most of the food has been moving to the fruit. This is where the production comes from.
If there is a big fruit load, we hope that the fruit is sending IAA out and receiving food from the leaves in return.
We know that too much late vegetative growth can lessen the trees ability to size its fruit. This is because the vegetative growth competes with the fruit for food.
When we harvest the fruit we suddenly lose the largest sink. The trees are still producing food and the food has to go somewhere.
Where will the food go now? It will go to the point that is presenting the next biggest sink. Most likely it will be shared amongst the vegetation, the roots and the storage tissue in the tree.
Where do we want it to go? Well we don’t want it to move to the vegetative growth because that will be falling on the ground soon. We want it to move to the roots and storage tissue.
The storage tissue includes the buds that will be important next season so it helps if they are furnished with food.
Movement to the roots keeps the roots alive for longer and keeps their ability to supply water and nutrients to the trees as they approach dormancy.
What does this mean in practice?
There is valuable food stored in the leaves. After harvest we want it to move back into the tree and not into new vegetation.
If possible, we want the vegetative growth to slow down leading up to harvest so that all the energy is not put into feeding the new vegetation.
Post harvest fertiliser (cont next month)
See this article in Tree Fruit April 2018
