Sugar uptake and storage by sugarcane suspension cells at different temperatures and high sugar concentrations

Experiments with sugarcane cell suspensions were performed for two purposes: (a) To reveal the temperature dependence of sugar transport systems for interpretation of sugarcane growth at suboptimal temperatures; and (b) to test whether different temperatures or high sugar concentrations elevate the intracellular sucrose level by shifting the sucrose cycle of synthesis and degradation. It was found that the hexose uptake systems (for glucose and fructose) of suspension cells were strongly inhibited at low temperature (e.g. 15 degrees), whereas high temperatures were much less harmful. The optimum temperature was 45 degrees. Hexose uptake was modulated by temperature and by development during the batch culture growth period. Neither the presence of high concentrations of hexoses, sucrose or osmoticum (mannitol) nor the growth at low or high temperature did increase the intracellular sucrose concentration. The best effect was obtained at high sucrose concentration in the medium, when the cells were grown in chemostat culture, but still no sucrose accumulation relative to the concentration in the medium occurred. Analysis of enzymes involved in sucrose metabolism showed that the correlation between internal sucrose and sucrose phosphate synthase minus acid invertase, a correlation that had been observed in sugarcane plants, did not exist in sugarcane suspension cells. Also, neutral invertase activity did not give any correlation to sucrose. it is speculated that the low sucrose level in suspension cells and the absence of correlation to acid invertase is basically due to the low degree of vacuolization and the substantially faster sucrose transport at the tonoplast of suspension cells compared with internodal storage parenchyma cells.