EFFECT OF HIGH-PRESSURE MIST AND DAYTIME CONTINUOUS CO2 ENRICHMENT ON LEAF DIFFUSIVE CONDUCTANCE, CO2 FIXATION AND PRODUCTION OF ROSA HYBRIDA PLANTS GROWN ON ROCKWOOL

Rose plants (Rosa hybrida cultivar 'Sonia') were grown on rockwool in three plastic greenhouses: the first one was used as a control greenhouse, the second one had a relative humidity (RH) level of 75 to 100 % provided by high-pressure mist (HPM), and in the third greenhouse, HPM was associated with daytime continuous CO₂ enrichment (HPM+CO₂). CO₂ was injected from 09.00 to 16.00. The daytime CO₂ concentration was kept at around 900±100 ppm. CO2 injection could be maintained with reasonably low gas waste by limiting aperture rates of openings to less than 5 %. HPM was used to reduce air temperatures by evaporative cooling. Plants were irrigated at rates proportional to incident radiation. The effect of HPM and HPM+CO₂ was investigated from mid-summer to fall, under natural conditions. These were characterized by high incident photosynthetic photon flux density, high temperature and low RH. HPM+CO₂ helped to substantially improve net photosynthesis when compared to the control or to the HPM treatment. Leaf diffusive conductance and leaf water potential remained unaffected by HPM+CO₂, when compared to HPM. HPM+CO₂ improved the growth, measured as an increase in dry matter, the number of bud breaks and the number of marketable flowers m^-2 by 57 %, 23 % and 19 %, respectively. Moreover, HPM+CO₂ improved stem diameter, this may indicate that long-term benefits can be expected from this technique. As a conclusion, it appears that combining daytime continuous CO₂ enrichment with HPM offers very good opportunities for improving greenhouse rose production even during summer in hot climate conditions.