The Abscission Layer, Pruning Tomatoes & Other Erroneous Practices
Online Advice To Prune & De-leaf Tomatoes Can Increase Disease & Decrease Yield. Don’t Just Take My Professional Grower Advice, Check Out The Research on Pruning & De-Leafing.
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The internet is full of advice to remove leaves and prune tomatoes and other veg plants But few mention the abscission layer. There are instances where advice on removing leaves can be beneficial but most of it is very poor advice based on folklore. And using secateurs or snips to remove plant growth increases the potential for bacterial, viral and fungal plant diseases. This article explains the Abscission Layer.
Don’t just take my advice as someone that owned a tomato growing business and grew a wide range of salad and veg crops over two decades. Look at the cited research from around the world. Though I often suggest not paying too much attention to American and other overseas advice of a generic nature (our climates, pests and diseases are very different), but on this occasion we are looking at research on specific plant physiology and related areas. It is universal.
The Purpose Of Leaves
Plants grow leaves for a reason. Plant leaves take carbon dioxide and moisture to photosynthesise. Sunlight is what drives this process and it produces the “energy”, ie plant sugars, that power the plant. It grows the flowers and fruit.
An explanation of photosynthesis
Without leaves there can be no flowers out fruit.
Do Old Leaves Contribute To Plant Growth?
Yes.
I’m often told that the upper leaves contribute 90% of the photosynthesis and the old bottom leaves on tomatoes are useless and should be removed. Certainly the upper leaves contribute a great deal. They are more numerous so we should expect that. But without the older leaves we lose around 10% of the energy that could be produced. Now imagine your car being only 90% efficient. Or your health being such that you only had 90% of the energy you need! That’s not a great position to be in is it?
So keeping the bottom, older leaves, while they are functional, makes a lot of sense.
Eventually they will senesce, i.e. go yellow and die back. But that is no reason to remove them early. And one reason for not removing them is that they contain valuable plant nutrients. When they die back naturally the plant removes the plant nutrients and uses them for new growth. But if we chop off the leaves we rob the plant of its nutrients. And that means less growth and flavour in our fruit.
How About Removing Leaves to Increase Air Flow?
Poor airflow leads to stagnant conditions and that can lead to disease. But there are other solutions to this situation. One is to increase the natural ventilation in the greenhouse or tunnel .. don’t forget polytunnels are far more airtight than greenhouse and not always suitable for tomato growing UNLESS you can provide good ventilation. Polytunnels with one door at one end and no other ventilation is just asking for disease issues.
However, removing a few leaves from the bottom of the plant isn’t going to solve poor air flow much. It is far better to solve the fundamental problem than to tinker with the plant.
Encarsia formosa predated whitefly on tomato leaf. Biological control example
I’m also often reminded that plant leaves that touch the soil need removing as they will get wet and get disease. I grew 10,000 tomatoes plants a year and that isn’t my experience. Of course, if you have poor ventilation and humid, stagnant conditions that is a different story. But the solution is to improve ventilation. Then you can “harvest” the 10% of energy those leaves provide. In nature plant leaves touch the ground .. just look at a lawn or a field of wheat, barley, oats etc. Touching the ground is very common. That isn’t the issue. The real issue are the conditions in which they touch the ground. If we don’t adequately ventilate our greenhouse and polytunnels then we are setting ourselves up to get diseases. Especially fungal disease such as botrytis.
Natural Leaf Removal – What Is The Abscission Layer?
Plants lose leaves all the time. The most obvious way is the one we see each autumn when deciduous trees lose their leaves. Our American cousins call this time the Fall. It’s quite descriptive of the natural process that takes place.
Another example can be seen in the veg garden. Crops such as Brussels experience natural senescence of lower leaves as the plant grows. Left to its own devices the leaf starts to go yellow as the plant scavenges it to remove valuable nutrients. Once it has recovered these nutrients it can recycle them elsewhere in the plant. At this stage the leaf is no longer of any value to the plant and it treats it in the same way a deciduous tree sheds leaves in autumn. At the base of the leaf stem, where it attaches to the stem, there is an area called the abscission layer. It is a few cells thick and is where the plant “amputates” the leaf stalk. It is a natural layer and quickly closes to stop pathogens entering (unlike when secateurs are used on plant and leaves an open wound!).
Plants grow abscission layers in various places. Tomatoes also have them at the knuckle behind the fruit. Its purpose is to allow the fruit to fall from the plant (or be picked) without leaving a gaping wound that would allow pathogens in.
Now For The Science Bit
Above is the layman’s guide to de-leafing, abscission layers, pruning leaves etc. It is based on the knowledge I have acquired over decades as a professional grower and horticulture lecturer. But don’t just take my word on this .. always check your sources .. read the science bit below and see the research sources that confirm my experience.
The Abscission Layer in Tomatoes and Its Importance in Vegetable Cultivation
The abscission layer is a specialised zone of cells that forms at the base of plant organs (such as leaves, flowers, or fruits) to facilitate their controlled detachment from the plant. This process, called abscission, is crucial for plant health, resource allocation, and reproduction. Below is a detailed explanation of its structure, function, and importance in growing tomatoes and other vegetables.
Structure and Formation of the Abscission Layer
The abscission layer consists of small, densely cytoplasmic cells that differentiate early in organ development.
In tomatoes, it forms at the midpoint of the pedicel (fruit stalk) and includes 6–8 cell layers that extend across the tissue.
These cells remain undifferentiated and retain meristem-like activity, allowing them to respond to hormonal signals.
Before detachment, enzymes like polygalacturonase (PG) and cellulase break down the cell walls in this layer, weakening the tissue.
Role of Hormones in Abscission
Abscission is regulated by a balance of plant hormones:
Auxin (high in young organs) suppresses abscission by maintaining cell adhesion.
Ethylene (produced during stress or ripening) promotes abscission by activating cell wall-degrading enzymes.
When auxin levels drop (e.g., in ageing leaves or unfertilised flowers), ethylene sensitivity increases, triggering separation.
Importance in Tomato and Vegetable Cultivation
Fruit Yield and Harvest Efficiency
In tomatoes, the abscission layer determines whether fruits detach cleanly (important for mechanical harvesting) or retain stems (undesirable for processing).
Mutants like “jointless” tomatoes lack this layer, causing fruits to break off at the calyx instead, which is useful for industrial processing.
Preventing Premature Fruit Drop
Environmental stresses (drought, storms) can trigger early abscission, leading to yield loss.
In apples and tomatoes, typhoons or strong winds can cause fruit drop if the abscission layer weakens prematurely.
Flower and Fruit Thinning
Some crops (e.g., apples) undergo “June drop”, where excess fruitlets abscise to balance nutrient allocation.
Farmers use chemical thinners (e.g., ethylene inducers) to mimic this process and improve fruit size.
Disease and Pest Management
Shedding infected leaves (e.g., from aphid galls) can reduce pest spread.
The abscission layer also forms a protective barrier (lignified cells) to prevent pathogen entry after detachment.
Practical Applications for Professional Growers
Tomatoes: Selecting cultivars with strong abscission zones (e.g., non-jointless types) ensures fruits remain attached until ripe.
Leafy Greens: Removing yellowing leaves manually can prevent disease spread if natural abscission is slow.
Orchards: Applying anti-ethylene sprays (e.g., 1-MCP) can delay abscission and extend harvest windows.
Conclusion
The abscission layer is vital for controlled shedding of plant organs, influencing yield, harvest quality, and stress resilience. In tomatoes and other crops, understanding its regulation helps optimise fruit retention, thinning practices, and disease management. By manipulating hormonal signals or selecting appropriate varieties, growers can improve productivity and reduce losses.
