Achieving Shear Growth: Pruning to Maximize Fruit Size
When we prune or top off plants, we are removing the fastest-growing shoots. This is done to remove apical dominance, the phenomenon where the main, central stem of the plant grows more strongly than the side stems.
Removing apical dominance forces the remaining growth points to produce and grow faster. Apical dominance is controlled by the production of a plant growth hormone called an auxin that is developed in the plant’s leading shoot. Auxins, in higher concentrations, inhibit and slow the growth of side branches and lateral shoots, giving plants a familiar triangular, or Christmas tree, shape.
Removal of this dominant shoot promotes better growth throughout the rest of the plant, resulting in new lateral shoots that are denser and more vigorous. The lower branches and architecture of the plant become stouter instead of being restricted by the apical dominance of the main shoot.
The result is a plant that has a more full-bodied vascular system. The channels and pathways leading into other parts of the plant develop bigger than they normally would have become, which is why the excessive pruning of annual fruiting plants is so successful. Forcing the plants’ vascular systems and structures to grow bigger signals fruits to develop bigger.
Pruning has three main benefits. The first two—controlling the direction of growth and encouraging vigorous new growth of side shoots—are well-known and understood. A third and less understood benefit to removing apical-dominant growth is increased yields. Removing apical dominance just before the fruiting buds begin to develop can enhance yields.
During growth, young fruit buds demand much of the plant’s resources. It is thought that removing apical dominance stimulates the development of cell division in branches leading into fruiting trusses or pedicles. These trusses develop larger-functioning xylems and phloems that may allow for the greater importation of carbons from the leaves, potentially resulting in greater cell division in fruits, which may translate into a greater expansion potential for the fruits.
A fruit that has more cells ultimately has more expansion potential, regardless of the size of the plant. If you are busy growing leaves, you’re never going to have the necessary cellular division to drive fruit yields to their maximum potential.
When to Prune
For most fruiting annuals, it is important that all shoot growth be halted at the beginning stage of flowering and fruiting growth. This will help ensure there is no competition for immature young fruitlets. Also, helping the fruit or its truss grow and have its cells divide for a longer period time is critical. Typically, cells within fruiting bodies will divide for a limited time period. Fruit development occurs in four phases:
- fruit set
- a short period of rapid cell division
- a longer cell expansion phase
- ripening or maturation stage
Research has proven it is possible to stimulate extended periods of cell division. This has been achieved by applying ethylene during the early flowering stage, immediately after pollination. The exact reason why ethylene drives this increased rate of cell division is unknown, but one of the theories is that it may signal a genetic trigger telling the plant to boost cell production to safeguard the seeds of the next generation with a larger, protective envelop (the fruit).
The rate and duration of cell division varies among fruits and also among tissues within a fruit. However, the cell division phase gradually shifts fruit into the cell expansion phase. Cell division seems to be more critical than cell expansion in determining final fruit size as generally, the more cells a fruit has, the larger its final size will be.
During peak fruit expansion, there is an intense flow of photoassimilates from mature leaves into the rapidly enlarging fruits. Sugars generated by photosynthesis, along with amino acids, calcium and other nutrients, move via the enlarged phloem into the ever-enlarging fruit.
The relative strength of the plant’s transportation structure is a major factor for fruit enlargement. Enhanced transport is dictated by cell divisions that have stout vascular connections. During plant growth, xylem and phloem enlargement occurs in an orderly and patterned manner of superior vascular connections.
In a nutshell, to a plant, if you build a bigger house, the seeds inside will be afforded greater protection. For a grower, this means maximum yields and greater fruit size. One other benefit that comes along with building a bigger house is that it often takes longer to ripen and mature a fruit with a robust architecture.
This effect further enhances fruit size and yield, as slowing or delaying the death of the parent plant hinders aging and lengthens the growth period. If you want the largest yields possible, it is imperative to increase the length of time cells divide in the fruit after pollination.
Written by Russ Landry | President
Russell Landry is the former vice-president of the Great Pumpkin Commonwealth and its many competitive weigh-off sites held worldwide. He is now the current president of the Giant Vegetable Growers of Ontario. Russ publishes the GVGO Growers’ Vine newsletter.