The regulation of turgor pressure during sucrose mobilisation and salt accumulation by excised storage-root tissue of red beet.

The changes in turgor pressure that accompany the mobilisation of sucrose and accumulation of salts by excised disks of storage-root tissue of red beet (Beta vulgaris L.) have been investigated. Disks were washed in solutions containing mannitol until all of their sucrose had disappeared and then were transferred to solutions containing 5 mol·m(-3) KCl+5 mol·m(-3) NaCl in addition to the mannitol. Changes in solute contents, osmotic pressure and turgor pressure (measured with a pressure probe) were followed. As sucrose disappeared from the tissue, reducing sugars were accumulated. For disks in 200 mol·m(-3) mannitol, the final reducing-sugar concentration equalled the initial sucrose concentration so there was no change in osmotic pressure or turgor pressure. At lower mannitol concentrations, there was a decrease in tissue osmotic pressure which was caused by a turgor-driven leakage of solutes. At concentrations of mannitol greater than 200 mol·m(-3), osmotic pressure and turgor pressure increased because reducing-sugar accumulation exceeded the initial sucrose concentration. When salts were provided they were absorbed by the tissue and reducing-sugar concentrations fell. This indicated that salts were replacing sugars in the vacuole and releasing them for metabolism. The changes in salf and sugar concentrations were not equal because there was an increase in osmotic pressure and turgor pressure. The amount of salt absorbed was not affected by the external mannitol concentration, indicating that turgor pressure did not affect this process. The implications of the results for the control of turgor pressure during the mobilisation of vacuolar sucrose are discussed.