Impact of in vitro CO2 enrichment and sugar deprivation on acclimatory responses of Phalaenopsis plantlets to ex vitro conditions

We assessed the effect of growth at either 400 μmol mol−1 (ambient) or 1000 μmol mol−1 (elevated) CO2 and 0 g L−1 (deprivation) or 30 g L−1 (supplementation) sugar on morphological traits, photosynthetic attributes and intrinsic elements of the CAM pathway using the CAM orchid Phalaenopsis 'Amaglade'. The growth of shoot (retarded) and root (induced) was differently affected by CO2 enrichment and mixotrophic regime (+sugar). The Fv/Fm ratio was 14% more in CO2-enriched treatment than at ambient level during in vitro growth. At elevated level of CO2 and sugar treatment, the content of Chl(a + b), Chl a/b and Chl/Car was enhanced while carotenoid content remained unaltered. During in vitro growth, gas-exchange analysis indicated that increased uptake of CO2 accorded with the increased rate of transpiration and unchanged stomatal conductance at elevated level of CO2 under both photo- and mixotrophic growth condition. At elevated level of CO2 and sugar deprivation, activities of Rubisco (26.4%) and PEPC (74.5%) was up-regulated. Among metabolites, the content of sucrose and starch was always higher under CO2 enrichment during both in vitro and ex vitro growth. Our results indicate that plantlets grown under CO2 enrichment developed completely viable photosynthetic apparatus ready to be efficiently transferred to ex vitro condition that has far-reaching implications in micropropagation of Phalaenopsis.