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- Author or Editor: THEOPHANES SOLOMOS x
Abstract
Determination of diffusivity of gases in bulky plant tissues is of both theoretical and practical interest. For instance, a precise knowledge of O2 diffusion is needed for studying the nature of “oxidases” that may be involved in fruit respiration and also for predicting minimum O2 levels that can be safely used in controlled atmosphere (CA) storage. Further, a precise knowledge of the internal concentration of ethylene may be useful in determining the maturity of apples before harvest (15). Principles and techniques used for determining resistance to gas diffusion in bulky plant organs, and some practical applications for CA storage of apples are presented here.
We have investigated the effects of different O2 concentrations on the onset of the climacteric rise in C2H4 evolution and subsequent ripening of `Gala' apples. The data show that the effect of low O2 on the timing of the onset of the C2H4 climacteric is saturable in that for low O2 to retard the onset of the rise in C2H4 evolution its concentration must be decreased below 8%. Further, the results suggest that the retarding effects of low O2 on the onset of ripening cannot be ascribed to the inhibitory effects of low O2 on the action of the low levels of C2H4 evolution by preclimacteric apples. The results concerning the rate of C2H4 evolution and accumulation of the ACC-oxidase protein show that application of low O2 after the initiation of the climacteric rise in C2H4 is not as effective in delaying senescence as when it is imposed early, i.e., at the preclimacteric stage.
Additions of adjuvants which inhibit the biosynthesis or action of C2H4 show that the climacteric rise in respiration during senescence of cut carnations is a facet of ethylene action and not senescence as such. The rate of CO2 output of carnation flowers was diminished in a dose-dependent mode by low O2. The data indicate that the diminution of respiration by low O2 may not be attributed to the restriction of either of the mitochondrial terminal oxidases. The steady-state concentration of ATP was similar in both air and 2% O2-treated flowers. 2% O2 eliminated for 32 days any rise in C2H4 evolution. In addition the longevity of the flowers kept under 2% O2 was longer than those which were treated with STS. The results are taken to indicate that hypoxia affects developmental events leading to the induction of C2H4 and/or the synthesis of transducer of C2H4 action.
In the past four years the effects of levels of O2 from 1 % to 100%1 on ripening of Gala apples were studied. It was observed that oxygen concentrations larger than 8% did not delay the onset of the climacteric rise in ethylene evolution and respiration, and had no effect on any parameters of ripening, such as texture, acidity and soluble solids. The timing of the onset in the rise of ethylene evolution differed with the year. Low O2 environments of 1-2% did not induce any rise in ethanol concentration. One hundred percent O2 was highly detrimental in that it induced visible symptoms akin to low O2 injury and enhanced the accumulation of ethanol. Hypoxic environments induced a novel 61 kd polypeptide whose quantity was inversely related to the levels of O2. The data also indicate that the effect of low O2 environments on respiration is a function of the physiological stage of the fruits.
Previous research has shown that subjecting bananas to low O2 treatment during the climacteric rise decreases the rate of sugar accumulation but the fruits eventually ripen. In the present study we applied low O2 in fruits whose ripening had been initiated by exogenous C2H4 and in preclimacteric ones. In preclimacteric fruits low O2 suppressed the climacteric rise during the duration of the experiment (20 days). It completely inhibited the increase in sugars, invertase and sucrose phosphate synthase (SPS) activities while there was a sharp increase in sucrose synthase (SS). In control fruits the increase in sugar content coincides with a sharp increase in invertase, and SPS and a decline in SS. Hypoxia inhibited the increase in invertase and SPS while it induced an increase in SS. Nevertheless, the activities of invertase and SPS in the climacteric hypoxic fruits was higher than in hypoxic preclimacteric ones. The results, thus, indicate that the imposition of low O2 at the preclimacteric stage is much more efficient in delaying banana ripening than when it is applied after the initiation of ripening.
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 well known that storage of potato tubers at chilling temperatures results in an increase in the content of simple sugars, mainly sucrose, glucose, and fructose. We have observed that in Russet Burbank potatoes sucrose is the first sugar to accumulate, followed by a rise in glucose and fructose. The ratio of the increment of glucose to fructose is one, indicating that sucrose is the sole source for the reducing sugars. Our data show that there is a close temporal relationship between the accumulation of reducing sugars and rise in the activity of acid invertase (AI). Purification of combining Con A-Sepharose 4B affinity followed by DEAE-Sephacel ion exchange columns resulted in 144-fold purification of AI. SDS-PAGE indicating that the above procedure resulted in the concentration of 58 kDa polypeptide. IEF electrophoresis revealed the presence of five isoforms of AI with the following pI: 5.16, 5.25, 5.37, 5.55, 5.85. Two-D gel electrophoresis (native-to-denature) showed that a polypeptide of 49.48 kDa was associated with all isoforms of AI. There were also two addition polypeptides with 37.94 and 20.07 kDa which could be breakdown products of the 58 kDa protein or subunits of AI.
Freshly harvested carnation flowers (Dianthus caryophyllus L. cv. White's Sim) were kept for 4 days either in air or low O2 before they were transferred to N2. Low O2 in the range of 1.2–2.7% resulted, depending on the concentration, in a decrease in respiration and 3–5 fold increase in alcohol dehydrogenase (ADH) activity, without a concomitant increase in ethanol production. Anoxia initially, within 4 hours, depressed by about 40% the rate of CO2 evolution in air, but had no effect on low O2-treated flowers. Anoxia induced in all treatments an increase in the activity of ADH, but the levels of ADH were 1.5 fold higher in the low O2-treated flowers than those kept in air. This difference increased to 10-fold after 6 days because by then, the air treated flowers were almost dead. Prior exposure to hypoxia enhanced the anoxic life of flowers by 3–4 days. Anoxia also induced an increase in ethanol production in both air and low O2-treated flowers. The peak value of ethanol evolution was about 1.5-fold higher in the low O2 than in air-treated flowers. The data are discussed in terms of the effect of hypoxia on carnation flower metabolism and longevity.
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.
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.