Use temperature and light to steer respiration

The plant makes sugars from CO₂ and water through photosynthesis. Exactly the opposite happens during respiration: The sugars are broken down with the help of oxygen. This is necessary to release energy that is stored in the sugars. At the same time CO₂ and water are released. All processes in the plant that use oxygen are called respiration and there are three types: Photorespiration; growth respiration; and maintenance respiration. 

Photorespiration

During the photosynthetic process the CO₂ required for photosynthesis first binds with the enzyme rubisco before the next steps can occur. Photorespiration is a photosynthetic reaction in which oxygen instead of CO₂ binds to this enzyme. This situation occurs if not enough CO₂ is available for photosynthesis. One reason this happens is when the stomata are closed and if the greenhouse air contains 300 to 500 times as much oxygen as CO₂. In certain situations, photorespiration can rise sharply, accounting for up to 30% of photosynthesis. As a grower, you want to limit this as much as possible; after all you want to maximum photosynthesis. This can be done by optimising the climate so that the stomata stay open and by increasing the CO₂ level in the greenhouse. This way you change the ratio between the amount of oxygen and CO₂. There are two ways to increase the level: By adding CO₂ and keeping the vents closed. The latter can be kept closed for longer if a heat reflecting and/or diffuse coating is used.

Growth respiration

The second form of respiration is growth respiration, which is very useful because it is directly linked to the formation of new plant material. The plant converts the sugars, which formed during photosynthesis, into useful substances such as fats, proteins, fibres and the like. This conversion costs energy, which is provided by the breakdown of some of the sugars. On average, growth respiration accounts for 30% of photosynthesis. A grower has no control over this form of respiration.  

Maintenance respiration

However, a grower does have control over the third form: Maintenance respiration. This is needed to maintain the plant. Many processes cost energy and plant parts wear or break down and need to be replaced. That too costs energy. The speed of photosynthesis – under normal greenhouse circumstances – shows little sensitivity to temperature. But maintenance respiration is sensitive. It doubles when the temperature rises by 10ºC. That means: As the temperature rises, photosynthesis remains the same while the maintenance respiration goes up. The net result is a decrease in production. However, there is a side note: This is only true if the light level remains the same as the temperature rises. If the light level also rises the situation is different. Then photosynthesis also rises sharply and the increasing temperature is useful for the quick accumulation of the sugars that have formed.   

Grow with light levels in mind 

Light and temperature have to rise equally and this is the principle behind: ‘Grow based on light’. If there is more light you can allow the temperature to rise. Conversely on dark days the temperature needs to fall. Otherwise the total respiration can rise and may account for 100% of the photosynthesis. On such a day nothing at all is produced. 

The principles are not complicated, but even so steering based on maintenance respiration is not easy in practice. It requires a combination of heating, ventilation, screening and using the appropriate coatings on the glass. Growers are usually very adept at this thanks to years of experience. It could be improved further but we first need more insight. It is very probable that the maintenance respiration changes throughout the day so if this could be managed it may be possible to make gains. However, there is still insufficient information about these changes to control it well.