Abstract
If low night temperatures can be combined with high day temperatures, providing optimal growth conditions for plants, a significant energy saving can be achieved in greenhouses. Lowering the night temperature from 18˚C to 10˚C - 11˚C for 8 h had no negative effect on the CO2 exchange rate (CER) during the following light period in tomato. This was found both in plants grown in artificial light only or in combination with daylight. Allowing the temperature to increase from 20˚C to about 40˚C, in parallel with an increasing solar photon flux density (PFD) from 0 up to about 800 µmol·m −2 ·s −1 in the greenhouse during summer, progressively increased CER when the CO2 concentration was maintained at 900 µmol·mol −1 . At 400 µmol·mol −1 CO2, maximum CER was reached at about 600 µmol·m −2 ·s −1 PFD combined with a temperature of 32˚C, and leveled out with a further increase in PFD and temperature. Maximum CER at high CO2 concentration was around 100% higher than at low CO2 level. Under early autumn conditions, CER increased up to about 500 µmol·m −2 ·s −1 PFD/32˚C at low CO2 and up to about 600 µmol·m −2 ·s −1 PFD/35˚C at high CO2. An elevated CO2 level doubled the CER in this experiment as well. Measurements of chlorophyll fluorescence showed no effect of low night temperature, high day temperature or CO2 concentration on the quantum yield of photosynthesis, indicating that no treatment negatively affected the efficiency of the photosynthetic apparatus. The results showed that low night temperatures may be combined with very high day temperatures without any loss of daily photosynthesis particularly in a CO2 enriched atmosphere. If this can be combined with normal plant development and no negative effects on the yield, significant energy savings can be achieved in greenhouses.
Highlights
Within a certain range, plant growth generally depends more on the mean temperature than on the diurnal temperature variation [1,2]
The effect of decreasing the night temperature from 18.1 to 15.0 ̊C or 10.8 ̊C in Experiment 1A had no effect on carbon dioxide exchange rate (CER) during the light period when the plants were grown under 200 μmol·m−2·s−1 photon flux density (PFD) (Figure 1)
The results clearly show that daily photosynthesis was not negatively affected by night temperatures of down to about 10 ̊C
Summary
Plant growth generally depends more on the mean temperature than on the diurnal temperature variation [1,2]. Temperature integration, where high day temperatures are compensated by lower night temperatures, has the potential to reduce energy consumption in greenhouses [3]. This implies that the temperature is allowed to increase with increasing irradiance levels up to an acceptable maximum temperature when ventilation takes place. This is done in combination with a drop in night temperature in order to achieve an acceptable mean temperature, a drop that reduces the heating demand. As it is much easier in practice to maintain a high CO2 concentration at higher temperatures than at lower temperatures, the high temperatures might be preferred due to higher
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