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

Studies were made of metabolism in highly vacuolated and slightlyvacuolated Zea mays root tissue both during and after plasmolysis. Plasmolysis resulted in decreased respiration and carbon dioxideevolution from glucose and an increased sucrose synthesis. Inhibitionof respiration during plasmolysis in both the highly vacuolatedand slightly vacuolated tissue was not relieved by supply ofglucose, 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 negativeosmotic potential. It is suggested that low TCA cycle activityin vivo must be due either to inhibition of mitochondrial activityor to reduced flow of carbon through the glycolytic pathway. A low potential for TCA cycle activity after deplasmolysis issuggested, as addition of pyruvate stimulated carbon dioxideevolution but not oxygen uptake, which was severely decreased.This was presumably due to severe mitochondrial damage as shownby their activity in vitro. However, it is not clear whetherrespiration in vivo is rate limited by rapid leakage of metabolicintermediate (reported earlier) or by lysis of mitochondria. Deplasmolysis did not damage mitochondria from slightly vacuolatedtissue, a result which was consistent with respiratory measurementsmade in vivo. The data show that mitochondria in vacuolated tissue are damagedduring and after deplasmolysis and not before. It is suggestedthat lysis of mitochondria occurs in vivo as a result of a sharpincrease in the osmotic potential of the cell fluids.