Photosynthesis and transpiration of tomato and CO2 fluxes in a greenhouse under changing environmental conditions in winter

Responses of tomato leaves in a greenhouse to light and CO2 were examined at the transient stage at the end of winter, when both photoperiod and irradiance gradually increase. Additionally, CO2 fluxes were calculated for a greenhouse without supplementary lighting and without CO2 enrichment based on CO2 sinks (plant photosynthesis) and CO2 sources (plant and substrate respiration). In January, tomato leaves in the greenhouse showed low photosynthesis with a maximum assimilation of 6-8 mu mol CO2 m(-2) s(-1), a quantum yield of 0.06 mu mol CO2 mu mol(-1) photosynthetic active radiation (PAR) and a low light compensation point of 26 mu mol PAR m(-2) s(-1), a combination which classifies them as shade leaves. In February, tomato leaves increased their light compensation point to 39 mu mol PAR m(-2) s(-1) and quantum yield to 0.08, the former indicating the adaptation to increased irradiance and photoperiod. These tomato leaves increased their transpiration from 0.4 to 0.9 in January to similar to 2 mmol H2O m(-1) s(-1) in February. Both photosynthesis and transpiration were primarily limited by light but neither by stomatal conductivity nor by CO2. In January, light response of photosynthesis, dark respiration and transpiration were negligibly affected by increasing CO2 concentrations from 600 to 900 ppm CO2 under low light conditions, indicating no benefit of CO2 enrichment unless light intensity increased. In February, tomato leaves were photoinhibited at inherent greenhouse CO2 concentrations on the first sunny day; this photoinhibition was further enhanced by an increased CO2 concentration of 1000 ppm. CO2 fluxes in the greenhouse appeared strongly dependent on solar radiation. After exceeding the light compensation point in the morning, greenhouse CO2 concentrations decreased by 58 or by 110 ppm CO2 h 21 on a sunny day in January or February and by 23 ppm on overcast days in both months. Calculated per overall tomato canopy, plant photosynthesis contributed 42-50% to the morning CO2 depletion in the greenhouse. Dark respiration of tomato leaves was; 2 mu mol CO2 m(-2) s(-1) in January and; 3 mu mol CO2 m(-2) s(-1) in February. This dark respiration resulted in rises of 15 and 17 ppm CO2 h(-1) at night in the greenhouse compartment and was identified as primary source of CO2. Respiration of the substrate used to grow the plants, which produced 7.3 ppm CO2 h(-1), was identified as secondary source of CO2. The combined plant and substrate respiration resulted in peaks of up to 900 ppm CO2 in the greenhouse before dawn.