Autotrophic and heterotrophic carbon assimilation of in vitro grown potato (Solanum tuberosum L.) plants

The effects of sugar concentration in the medium on the autotrophic/heterotrophic carbon metabolism were studied in tissue cultured potato plants. The weight of plants grown in the light on media containing 3 % sucrose was significantly higher than that of plants grown on 8 % sucrose media. Similarly, the weight increased when plants were transferred from 8 % to 3 % sucrose and decreased in the reverse case. In contrast, under dark conditions, a higher weight was observed in plants grown on higher sucrose concentration. The uptake of ¹⁴C-sucrose from the medium was higher when the sugar's concentration was 8 % as compared to 3 %, indicating a higher proportion of heterotrophic carbon in the former. These results were further supported by the natural abundance of ¹³C/¹²C ratios in the plant tissue. Analyses of the ¹³C/¹²C in the plants, sugars and the air CO₂, indicated that 90 % of the tissue carbon was of heterotrophic origin in light-grown plants on 8 % sucrose. Only 50 % of the tissue carbon was of heterotrophic origin when the plants were grown on 2 % glucose medium. These results provide evidence that a high sugar concentration in the medium significantly reduced the contribution of autotrophic CO₂ assimilation to the accumulation of dry weight. The possible mechanisms inhibiting CO₂ assimilation in potato plants grown in vitro, under conditions of high sugar concentrations (or low osmotic potential) are discussed.