Fruit Splitting (Cracking) – What causes it and can we do anything about it?

Overview

Like much of biology, it’s a multifactorial phenomenon which has no simple one-step answer.  Research is on-going, but detailed mechanistic understanding and appropriate management strategies are still incomplete.  Almost all fruits and nuts can be variously affected, and for some may represent a significant economic loss for growers.  As well as damaged fruit being unmarketable, they also serve as entry points for fungal infections.  Frequency and extent of splitting varies widely across different species, and even within species, different cultivars can show great variation.  Outcomes are driven by plant genetics and environmental and management conditions and how these factors interact.

 

Causes - general.

Unlike soft and flexible animal cells, plant tissues including fruit cells are encased within cell walls and other stiff materials to maintain form and function.  For fruit to grow and expand, there has to be a controlled loosening of these firm structures in concert with cellular expansion through increased turgor pressure.  If this coordination is out of phase, then cells may be ruptured where loosening gets ahead of expansion, or there may be no increased space for cells to expand into if behind; proper cell function may then be compromised.  This synchronisation is controlled at the genetic level where families of proteins are synthesised and released, and environmental factors such as temperature and management conditions such as soil moisture may impact.  The desired outcome is obviously for an orderly unfolding of these events such that fruit mature with sugars, acids and other substances developing properly, astringency is removed, fruits soften, etc.

 

Causes - specific.

For a given plant, probably the most important factor is the state of fruit hydration throughout development.  This is influenced by foliar absorption such as when it rains or through the roots.  Most fruits increase in size in the early stages by cellular division and in later stages by cell expansion.  If moisture status is too low in the early phase, there may be insufficient cells to accommodate cellular expansion later.  Usually, rainfall or over-watering around harvest time increases the percentage of split fruit.  The relative importance of root and aerial delivery of moisture seems to vary for different fruits.  However, the observation that splitting may be just as prevalent in greenhouse-grown plants where moisture levels can be controlled precisely illustrates that moisture is not the sole answer to the problem.  As with grapes, reducing water in the approach to harvest has one other plus, namely of increasing fruit solute concentrations and flavour.

 

What can we do?

 

Obviously if you are only at the planning stage of growing a particular crop of 10,000 trees and your chosen species has a propensity for significant splitting, then you need to select the most resistant cultivar available in balance with other desirable properties.  Climate is the next concern, and usually drier conditions around harvest time are preferable.  For example, rain near harvest with grapes, blueberries and cherries can cause massive splitting.  So, the further out of range your site is from the plant's naturally adapted climate, the more problems may ensue.

If your plant is already in the ground, then genes and climate are decided and you have to rely on management.  To repeat the general statement above, foolproof and universal remedial strategies have not yet been devised because we still don't have a complete understanding of mechanisms.

Barry

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