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It has been demonstrated previously that 1.5% 02 suppresses the rise in respiration and appreciably diminishes the accumulation of sugars associated with the transfer of potatoes to chilling temperatures. The temporal relationships between respiration and the capacities of the terminal mitochondrial oxidases, and between sucrose metabolism and invertase activity were studied in tubers kept in air and under 1.5% at 1° C. Chilling temperatures induced de novo synthesis of the alternative oxidase. Initially there was a close temporal relationship between rise in respiration and capacity of the alternative oxidase. With time the rate of respiration declined while the capacity of the AO continued to increase. Low oxygen inhibited by 94-97% the rise in the AO. Paralled with the rise in glucose and fructose there was an increase in invertase activity which increased by twofold after 20 days at 1°. Four isoforms of invertase were identified with PI values of 5.8, 5.6, 5.4 and 5.17 from chilled tubers. Low O2 suppressed the rise in invertase.
It is known that pure O2 enhances in sweet potato roots the respiratory increment produced by C2H4 (Theologis and Laties, 1982, Plant Physiol.). Our experimental results indicates that the decrease in respiration with decreasing O2 concentration is due to the restriction of an “oxidase” whose apparent Km for O2 is 5-6 fold higher than that of cytochrome oxidase. The magnitude of the apparent Km for O2 is affected by the diffusion of O2 and experiment conditions. The effect of O2 on respiratory rise produced by C2H4 is due to the curtailment of C2H4 action rather than to respiration as such. The apparent Km for O2 of this system is larger than that of respiration.
The mechanism of C2H4 action on plant respiration is not well understood. In the present work we treated peeled sweet potato roots (Ipomea batatas cv. MD715) with 10 ppm C2H4 in air and 3% O2 Analytical data showed a close relationship between respiration and activity of phosphofructokinase while the activity of pyrophosphate fructose-6-phosphate phosphotransferase remained constant under all experimental treatments. At the respiratory peak there was an increase in both pyruvate and fructose-2,6-diphosphate. The change in the levels of pyruvate, followed closely that of the respiration drift, while those of fructose-2,6-diphosphate did not correlated so closely. The data indicate that the stimulation of respiration by C2H4 in sweet potato roots is closely associated with an enhancement of glycolysis. The levels of ATP also increased with the rise in respiration and reflected the magnitude of the respiratory increment.