Walnut (Juglans regia L.) plantlets were incubated during micropropagation in standard vessels (quasi confmed vessels) or in aerated vessels flushed with 360 or 20 000 cm^(C02) under irradiances of 70 (LI) and 250 (HI) pmol m"^ s'*. Plantlet morphology was strongly affected by the environment; leaf surface was increased, but shoot elongation and number of stems were reduced after increasing the irradiance of culture. Gross photosynthesis (Pq) capacity measured by using the •®02 isotope and mass-spectrometry techniques was increased by increasing photosynthetic photon flux (PPF) and CO2 concentration. Plantlets exhibited a potential for photorespiratory activity and Mehler-type reaction and a high rate of mitochondrial respiration in all vessel types and irradiances. When a long-term HI was applied, gas exchange rates (Pq and O2 uptake) were reduced in most of the vessel and PPF conditions, except in quasi confmed vessels. Under all the growth conditions, net photosynthetic rate (P^) was zero or slightly positive and the dry matter accumulation was very similar. Changes in O2 exchange, growth rate or enzyme activities linked to carbon fixation that were induced by changes in PFD and CO2 concentration showed that the photosynthetic characteristics of plantlets were typical for hetero-mixotrophic tissues.