RESPONSES OF GREENHOUSE TOMATOES TO SUMMER CO₂ ENRICHMENT

To evaluate the feasibility of summer CO₂ enrichment for greenhouse tomato production in South-western Ontario, the largest and most concentrated greenhouse vegetable production area in North America, its effects on leaf photosynthesis, fruit yield and quality of long tomato crops planted in January were investigated in six greenhouse compartments over three summers from 2005 to 2007. Due to strong ventilation in the summer seasons, higher than ambient concentration of CO₂ was only achieved on sunny days before 10–11 a.m. when outside global solar radiation was less than 400–500 W m^-2 (depending on ventilation setting) or after 6–7 p.m. or on overcast days when outside global solar radiation was less than 200 W m^-2. In summer 2005, the warmest on record in South-western Ontario, the leaf photosynthesis of the CO_2-enriched plants partially acclimated to high levels of CO₂. However, the leaf photosynthesis of the CO_2-enriched plants did not show any acclimation in summers of 2006 and 2007, a normal and a cooler than usual summer. The photo-assimilate gains with high concentration of CO₂ under high light intensity was much larger than under low light intensity, and the response of leaf photosynthesis to CO₂ did not change during the course of the day from early morning to late afternoon. Therefore, CO₂ enrichment will be more effective under strong solar radiation and the enrichment in the afternoon would be as effective as in the morning, if same concentration of CO₂ could be achieved with same amount of CO₂ supplied. Summer CO₂ enrichment to a level above the ambient did not affect fruit quality. It reduced fruit yield in August 2005, did not change fruit yield in summer 2006, and slightly increased fruit yield in summer 2007. High fruit load increased fruit yield per unit of area but it did not improve the crop response to CO₂ enrichment. Therefore, high temperature stress and the inability to achieve higher than ambient concentration CO₂ under high solar radiation due to strong ventilation, might have limited the crop response to summer CO₂ enrichment.