Under natural conditions plants are autotrophic and their carbon source is the environmental CO₂ . In tissue culture the carbon sources are the carbohydrates in the medium on the one hand and the CO₂ in the headspace on the other hand (mixotrophy). Compared to greenhouse grown plants or seedlings, in most cases low rates of photosynthesis have been recorded for plants in culture. 

 

Evolution of photosynthesis during acclimatization Evolution of sugar and starch reserves during acclimatization

 

Full autotrophy under TC conditions requires extra CO₂ as well as the possibility that it enters the container and high PAR (250 µmol.m^-2.s^-1). However, autotrophic plants are not necessarily better than mixotrophic ones. 

When the sugar concentration in the medium is lowered, the plants will approach autotrophy. On the other hand, high sugar concentrations during stage 3b  result in extra starch and/or sugar reserves in the chloroplasts, which are metabolized during the first days of the weaning process. 

• Effects of sugar concentration in the medium on photosynthesis

• Effects of sugar concentration in the medium on carbohydrate reserves