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- Author or Editor: Theophanes Solomos x
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.
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.
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.
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.
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 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.
Farnesene and its conjugated triene oxidation products in apple peel are positively correlated with, and thought to be involved in, the storage disorder superficial scald. Levels of these compounds are often estimated by dipping fruit in hexane and measuring the absorbance of the crude extracts at 232 nm (farnesene) and 269 or 281 minus 290 nm (trienes). We have devised a C18 HPLC method with UV detection at 232 and 269 nm that allows the simultaneous quantitation of 80 ng of farnesene and trienes. Using this method we have confirmed the recent report that one conjugated trien-6-ol comprises 90% of the stable oxidation products of farnesene. It was also found that crude hexane extracts of apple peel can give spuriously high values for farnesene and/or trienes when levels of these compounds are low and other UV-absorbing components are present. A group of compounds unrelated to farnesene, with an absorbance maximum at ≈259 nm, were noted in the peel of cv. Gala apples, which produced little farnesene or trienol. This may explain the report that fruit with a high ratio of A258nm/A281nm in peel extracts have a low incidence of scald. The new HPLC method will be applied in subsequent studies of postharvest factors involved in regulation of farnesene synthesis and oxidation.
Previous observations have shown that the diffusivity of water vapors is much larger than the value that is predicted theoretically from the magnitude of the diffusion coefficient of CO2, C2H4, or both. This has been ascribed to the ability of water to diffuse through the cuticle and to the transport of water via the capillaries of cellulase micorfibrels to the surface of the lenticels, where it evaporates. We measured the diffusivity of CO2 in `Gala' and `Granny Smith' apples. The former are more permeable to CO2 than the latter cultivar, in particular after prolonged storage at 2°C. The diffusivity of H2O was 10- to 20-fold larger than that of CO2. Furthermore, the ratio of D(H2O)/D(CO2) was similar for both cultivars. Infiltration of dyes and gas flow through apples submerged in water show that in `Gala' apples, the number of open lenticels is larger than in `Granny Smith'. Thus, the data indicate that lenticels are the main avenue of gas exchange in apples.
Continuous postharvest treatment of carnation flowers (Dianthus caryophyllus L. cv. Elliot's White) with 50 or 100 mM aminotriazole significantly extended useful vase life relative to flowers held in distilled H2O. No morphological changes symptomatic of floral senescence appeared in treated flowers until 12 to 15 days after harvest. The longevity of aminotriazole-treated flowers was extended to ≈18 days. The respiratory rate of aminotriazole-treated carnations was suppressed, and they exhibited no respiratory climacteric throughout the period of observation. The responsiveness of aminotriazole-treated flowers to exogenous ethylene appeared temporally regulated. Flowers treated with 50 mM aminotriazole for 2 days senesced in response to application of 10 μl exogenous ethylene/liter, whereas flowers treated for 24 days exhibited no morphological response to ethylene treatment. Chemical name used: 3-1H-amino-1,2,4-triazole-1-yl (aminotriazole).