Leaf stomata and tomato plants
Plants are like humans in as much that they have pores.
In plants the pores are called stomata and are mainly on the underside of leaves.
The stomata are used to release water vapour. As water is lost out of the leaves, water is drawn in through the roots.

Because water molecules tend to stick together, this acts as a chain from the roots to the leaves from where the water evaporates. This process is called transpiration.

Leaf stomata also open to allow carbon dioxide into the leaves which is essential for photosynthesis – the process by which plants make their own food (sugars). Commercial greenhouses add extra carbon dioxide to the air so that plants will grow faster.

Stomata also release oxygen which is very important for us!

The stomata open and close depending on weather conditions.
When it is sunny and bright, plants open their stomata to release water vapour in order to draw up water and nutrients through their roots and allow more carbon dioxide into the leaves.

Of course if leaves release too much moisture the plant wilts. If they keep their stomata closed they won’t be able to absorb carbon dioxide and that will slow growth – it’s a continual compromise.

A plant can easily become stressed if the water supply at its roots is erratic as this will affect transpiration which includes nutrient take up. Blossom End Rot can also be caused by an erratic water supply – a good reason to have a reservoir for the grow bag!

Stress can cause so many other issues including, blossom drop and smaller ripe fruit than expected.

Guard Cells – The Doors of the Stomata
Each stoma (singular) in the leaf has a guard cell on either side that opens and closes. They look a bit like bananas and sausages!

Leaf Stomata and Tomato Plants

The guard cells open and close by pressure – as water (including sugars etc.) is pumped into the guard cells they open the stomata (3), as pressure is reduced they close (lower photo).

This is good, because on a hot day if a plant runs low on water, the guard cells close (less pressure) and prevent more water from escaping from the leaves. Eventually, if the plant is not watered it wilts to the point of no return as the pressure within it is lost. The guard cells will be closed at this stage trying to conserve as much moisture as possible.

Top photo guard cells open – bottom photo they are closed.

How This Relates To My Plants

Foliar Spraying
Because most stomata (pores) are found on the underside of the leaves, it is best to foliar spray the underside of the leaves. Moisture and nutrient absorption into a plant’s system is also quickest through the underside of the leaves because of the many stomata and the cuticle (outer waxy layer) is much thinner than on the upper surface.

Spraying the upper leaf surface takes longer for nutrients to enter a plant’s system because of the thick waxy layer and fewer stomata. However, it has been shown that nutrients are absorbed to the same amount over time (top and bottom leaf surfaces), just not at the same rate.

The problem with spraying the upper leaf surface is the danger of fungal spores landing on the leaves. Blight spores are totally harmless on a dry leaf – that’s worth remembering!

Interruption of Water Flow
Spraying leaves too often (several times a day) is not a good idea because every time a leaf is wet, the release of water vapour from the stomata slows because moisture (humidity) outside the leaf is greater than the moisture inside – making it harder for leaves to release water vapour.

This also interrupts the flow of nutrients from the roots. Excessive foliar spraying could lead to deficiency of the immobile elements such as calcium and therefore cause Blossom End Rot. A foliar spray once every three or four days is the most often I would spray.

Foliar spraying/feeding is best used as a supplement – when leaves show a nutrient deficiency. Or at a time in a plant’s growth when more of a particular nutrient is needed and a deficiency is more likely to occur.

A build-up of nutrients on the leaf surface can also cause leaf damage and make further foliar spraying less effective.

To Recap:

  • Spray the underside of leaves for quickest results
  • Wet leaves are vulnerable to blight
  • Spraying too often may interrupt nutrient supply from roots and cause BER
  • Use foliar spraying as a supplemental feed
  • Too much foliar feeding can damage leaf surfaces

See also: Transpiration and tomato plants