Alterations in growth and crassulacean acid metabolism (CAM) activity of in vitro cultured cactus

Unlike C-3 plants, cacti possess a crassulacean acid metabolism (CAM) physi ology that can alter the pattern of carbon uptake and affect plant growth u nder artificial environmental conditions, especially in tissue culture. In vitro-derived plantlets of Coryphantha minima grew 7-fold larger than plant s cultured under similar ex vitro conditions. Growth regulators incorporate d into the culture media during shoot proliferation stage of micropropagati on had a strong influence on this increased growth. Other important factors that contributed to increased growth under in vitro conditions were high r elative humidity and sugar in the culture medium. An analysis of gas exchan ge and daily fluctuations of malic acid levels revealed an increase in net photosynthetic rate, in terms of carbon assimilation, by in vitro plants co mpared with that of ex vitro plants. This stimulated photosynthesis in the presence of an external carbon source was unexpected but apparently true fo r cacti exhibiting CAM physiology. Unlike CAM plants grown in ex vitro cond itions, net CO2 uptake by in vitro-cultured cacti occurred continuously in the light as well as the dark. Once regenerated, cacti were transferred to ex vitro conditions where the normal CAM pathway resumed with a concomitant reduction in growth and CO2 uptake. These results showed that growth of ca cti can be considerably accelerated by in vitro culture.