The spring trial of strawberry cultivation is on its way. The first strawberries have been harvested. Now that the sun is going to shine more strongly, the optimal climate setting appears to be quite a puzzle. Peter Geelen, involved in the project: "Early in the morning, the leaf temperature already rose above the greenhouse air temperature. This difference can go up to 5 °C. The VPD then rises to 2 kPa. That is a shame because the CO2 level in the greenhouse air is 800-750 ppm and the RV is also on level with 75-80%. All ingredients are available to make optimal use of the light."
Water stress and closing stomata
"However, the crop experiences water stress because the supply of water is less than the evaporation. This causes the stomata to close and the VPD (vapor pressure difference between the leaf and the greenhouse air) to rise. The question is how do we keep the stomata open under these circumstances? For this we can look at the energy balance of the plant. Because the leaf temperature is higher than the greenhouse temperature, the supply of energy for these leaves only consists of radiation. The discharge of energy takes place via radiation evaporation and convection. The hot leaf releases energy (convection) via contact with the colder greenhouse air, so that the greenhouse air is heated by the leaf or the leaf is cooled by the greenhouse air. The other way to discharge energy is the radiation evaporation. This is independent of the air humidity and the greenhouse temperature. By increasing the convection at the discharge side, we can reduce the evaporation so that the balance between discharge and supply of water is improved and the stomata can open further. To increase this discharge through convective cooling can be achieved by creating more air movement with the present vertical fans."
Screen away light
"Another way to even out the water balance is to reduce the energy supply by screening away light. In the trial this has proven to be very effective in getting the VPD back towards 1 kPa, which indicates that the stomata have opened further again. It would be better, however, to be able to let more light in. The question then remains what causes the water stress so early in the morning? Since the humidity of the air is high, a high convection evaporation cannot be the cause. So only the supply side of water in the plant remains."
Low root temperature
"Uptake and/or transport of water in the strawberry plant cannot keep up with the increase in evaporation in the morning. A factor that may be important here is the (too) low root temperature in the morning as in the strawberry cultivation the nighttime temperature decreases to between 8 and 15 °C. When the uptake of water is lower than the evaporation, the crop extracts this water from itself, so also from the fruits. About this matter, the research raises more questions about the cause of soft fruit. This research is now facing the challenge of optimum use of screens, heating, misting and fans to optimize light utilization as the irradiance increases more." For more background information, see weblectures part 12 'Evaporation according to the energy and water balance of the plant' and part 13 'Effect of stomata' or paragraph 2.7 of the book 'The basic principles of Next Generation Growing’.
For more information:
Kas als Energiebron
www.kasalsenergiebron.nl
info@kasalsenergiebron.nl