Respiration and Mitochondrial Activity in Zea mays Roots As Affected by Osmotic Stress

Studies were made of metabolism in highly vacuolated and slightly vacuolated Zea mays root tissue both during and after plasmolysis. Plasmolysis resulted in decreased respiration and carbon dioxide evolution from glucose and an increased sucrose synthesis. Inhibition of respiration during plasmolysis in both the highly vacuolated and slightly vacuolated tissue was not relieved by supply of glucose, organic acids, or uncouplers of oxidative phosphorylation. Mitochondria isolated from plasmolysed tissue were tightly coupled, but activity in vitro was inhibited by exposure to a high negative osmotic potential. It is suggested that low TCA cycle activity in vivo must be due either to inhibition of mitochondrial activity or to reduced flow of carbon through the glycolytic pathway. A low potential for TCA cycle activity after deplasmolysis is suggested, as addition of pyruvate stimulated carbon dioxide evolution but not oxygen uptake, which was severely decreased. This was presumably due to severe mitochondrial damage as shown by their activity in vitro. However, it is not clear whether respiration in vivo is rate limited by rapid leakage of metabolic intermediate (reported earlier) or by lysis of mitochondria. Deplasmolysis did not damage mitochondria from slightly vacuolated tissue, a result which was consistent with respiratory measurements made in vivo. The data show that mitochondria in vacuolated tissue are damaged during and after deplasmolysis and not before. It is suggested that lysis of mitochondria occurs in vivo as a result of a sharp increase in the osmotic potential of the cell fluids.