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It is well known that high temperatures can cause a reduction of fruit set resulting in lower yields and for this reason it is important to look for varieties that do well in the heat. In many varieties day/night temperatures higher than 34/200C or a period of four hours at 400C will cause blossom drop. There are several reasons why this problem occurs:
The rate of photosynthesis influences fruit set. Observations and experiments have shown that different segments of tomatoes and even different varieties within a segment can react differently to heat. Some varieties adapt to heat stress and may have only a small reduction in photosynthesis whereas others with no heat tolerance can have a high reduction in photosynthesis of even 65% or more.
At very high temperatures the reduced photosynthesis means that there are less assimilates (the carbohydrates produced in photosynthesis) going to vegetative growth and usually even less to generative growth/flower production.
Pollen viability and the amount of pollen produced can be greatly reduced by high temperatures. The lack of viable pollen is a major cause of poor fruit set. There can also be a lack of pollen dehiscence (release of the pollen) caused by an abnormal endothecium (an envelope containing the pollen) that does not open properly.
In addition, ovules in the ovary (the part that develops into the fruit) can also be damaged from the heat.
There is very often style elongation which results in the stigma being thrust out of the flower while the anthers with the pollen are still inside. This makes normal flower pollination very difficult and often results in flower drop.
Assisting Pollination in Heat
When the temperatures are very high it is helpful if they can be reduced. Greenhouses that have cooling pads should be activated in the heat of the day in order to maintain optimum temperatures.
A plastic roof can be whitewashed or shade cloth can be spread on the roof and if there are thermal curtains they can be used during the main hours of heat. There should always be enough light of 600-800 micro-Einstein’s so that photosynthesis can continue normally.
In a greenhouse or nethouse it is recommended to not remove too many leaves. Leaves help cool the plants and protect the fruit from the sun. In addition, shading can help reduce heat stress.
Although the conditions for pollination are not very good in the heat the following methods also help obtain better results. These methods are best applied in the morning when the temperature and relative humidity are more suitable.
- Crop wires can be shaken to help free the pollen.
- Air blasting with an air blower also helps to release the pollen grains.
- Electric vibrators that shakes the flowers releasing the pollen grains that can fall onto the stigma.
- Bumblebees are not very sensitive to extreme weather and can help give better results. They should be used even in high temperature conditions.
Hormones are often used in extreme temperature conditions. They help improve fruit set and give parthenocarpic fruit. In high heat conditions it may be necessary to spray hormones twice a week and then the concentrations are often lowered to prevent excessive hormone amounts from causing damage. Follow the manufacturer’s recommendations.
 
Left is a normal flower. Right is a flower with elongated style and the stigma outside the flower.
Diagram showing how flower fertilization takes place.
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