SUMMARY
Current views on membrane construction are presented as a basis for discussing certain modifications of structure that may render membranes leaky, allowing solutes to diffuse from them. Biophysical experiments on dehydrated membranes indicate that, when their water content is reduced below 20%, they no longer adopt the lipid bilayer conformation. When dry seeds, spores or lichens are placed in water, soluble cell constituents leak from them for a few seconds or minutes until membrane integrity is re-established; the same may apply also to the liberation of solutes from pollen grains. Moreover, there are indications that ‘dry’ in this context means having less than about 20% water.

Some plants are sensitive to chilling, their tissues becoming leaky when exposed to temperatures between 0 and 10°C. This chilling-induced leakage, attributed to a phase change in all the phospholipids of which the membrane is composed occurs at a slightly lower temperature than the change in the activation energy of membrane-bound enzymes.

At senescence, leaf tissues become leaky and may eventually dry out. So much phospholipid disappears from senescing cotyledons of cucumber that there is no longer sufficient to fabricate complete and intact membranes around the cells. Treating cucumber cotyledons with iodoacetate also causes a loss of phospholipids, allowing solutes to leak out of the cells.

It is possible that exposing seeds or mature plant tissues to oxygen at pressures of 1 or more atm renders them leaky because of lipid peroxidation.

Phospholipids and plant membrane permeability

Publisher Summary This chapter discusses the biochemical adaptations to anoxia. The ability of plants to survive anaerobic conditions depends on morphological and anatomical features. It also depends on physiological and biochemical factors. Leaves seldom experience anaerobic conditions; however, flooding frequently produces anaerobic conditions for roots. Thus, structural features such as aerenchyma may provide the roots with a limited supply of oxygen from the aerial parts of the plant. At the biochemical level, a pronounced Pasteur Effect may ensure an adequate supply of ATP during short periods of anaerobiosis. However, in the longer term, the accumulation of the products of glycolysis may injure the plant. To survive long periods of anaerobiosis, plants may have evolved at least three biochemical strategies. Plants have developed mechanisms to maintain a high energy charge and allow limited growth. This mechanism appears to be found in few cases such as rice. Plants maintain the glucose supply and the adjustment of the carbon metabolism to avoid the accumulation of toxic products.

Anaerobic metabolism and the production of organic acids

Publisher Summary This chapter discusses the regulation of glycolysis and the pentose phosphate pathway. Glycolysis and the pentose phosphate pathway are the two main pathways of carbohydrate degradation in plants. Sucrose and starch are the principal sources of substrates for glycolysis in plants. The enzymes involved in the breakdown of sucrose and starch in plants do not appear to have regulatory properties. The localization of metabolic pathways in subcellular compartments is an aspect of control that has significance for the regulation of carbohydrate metabolism in plants. The chapter presents compartmentation of the enzymes glycolysis. Glycolysis occurs in plastids and in the cytoplasm of plant tissues; the reactions in the respective compartments are catalyzed by separate isoenzymes. The relative amounts of plastid and cytoplasmic isoenzymes differ for the various glycolytic reactions and depend on the type of tissue and its stage of development.

The regulation of glycolysis and the pentose phosphate pathway

In a tuber which is in thermal equilibrium with its surroundings, metabolic heat production raises the temperature slightly above the environment — the specific heat of the tissue is about 3.6 J (g°C)−1 (Mann, reported in Burton, 1966) and a typical heat production of, say, 50 mJ g−1 h−1 (Section 14.1.4) could thus raise the temperature 0.014°C h−1 were it not transferred to the surrounding air. This transfer involves a temperature gradient of about 0.2°C from the centre to the periphery, and about 0.1°C from just below the skin to the surrounding air (Burton, 1969). These are the approximate equilibrium values.

The physics and physiology of storage

SUMMARY: The influence of dissolved oxygen concentration on the metabolism and respiration of growing Klebsiella aerogenes NCTC 8017 was studied by means of a continuous-flow culture technique. Different dissolved oxygen tensions (equivalent partial pressures) were obtained by varying the partial pressure of oxygen in the gas phase. The respiration rate (oxygen uptake rate per unit mass organism) was independent of dissolved oxygen tensions above 10-15 mm. Hg when the culture was said to be in the ‘excess oxygen state’. With dissolved oxygen tensions below about 10 mm. Hg, the ‘limited oxygen state’ developed. In the transition to this state, complex oscillations in the dissolved oxygen tension occurred. The oscillations reflected alternate stimulation and inhibition of the respiration rate. A decrease in dissolved oxygen tension to below 5 mm. Hg immediately increased the respiration rate; an increase in dissolved oxygen tension from 5 to 10 mm. Hg immediately decreased the respiration rate. These effects formed part of the mechanism of the oscillations which could be sustained indefinitely. The oscillations in respiration rate occurred irrespective of change from pH 6·0 to 7·4 or whether growth was limited by glucose or ammonium supply. The pattern of oscillations in respiration rate varied with growth rate over the range 0·2-0·5 hr−1. It is suggested that decreasing the oxygen tension to about 5 mm. Hg acted like an uncoupler of oxidative phosphorylation. In the excess oxygen state with excess glucose and growth limited by ammonium supply, the glucose carbon was largely accounted for as CO2, pyruvate and organisms, irrespective of the pH value. Pyruvate was not accumulated when growth was glucose-limited. In the limited oxygen state, the glucose carbon was largely accounted for as: organisms, CO2, 2:3-butanediol, ethanol, acetic acid, formic acid, lactic acid. The proportions of these fermentation products varied with oxygen supply, pH value and whether growth was ammonium-limited or glucose-limited.

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The Influence of Dissolved Oxygen Concentration on the Respiration and Glucose Metabolism of Klebsiella aerogenes during Growth

Summary
The changes of glucose-6-phosphate, fructose-6-phosphate, fructose diphosphate, dihydroxy-acetonephosphate, 3-phosphoglycerate, 2-phosphoglycerate, phosphoenolpyruvate, adenosine triphosphate, adenosine diphosphate, adenosine 5′-monophosphate and oxidized co-enzyme I were determined in shelled green peas held in air and in nitrogen. In anoxia 3-phosphoglycerate, 2-phosphoglycerate, phosphoenolpyruvate, adenosine triphosphate and oxidized co-enzyme I, each decreased markedly to new steady levels after ahout 15 minutes; except in one experiment, adenosine diphosphate increased markedly for about 15 minutes. Similarly, the development of the Pasteur effect in shelled green peas is rapid, i.e. less than 30 minutes (Wager, 1961). In contrast, glucose-6-phosphate decreased for some 24 hours of anoxia, while fructose diphosphate and dihydroxyacetonephosphate first increased, then decreased and finally might rise again.

From our results, the metabolic fate of adenosine diphosphate appears to be of primary importance in controlling the rate of glycolysis. Thus, with transfer to anoxia, the most consistent ehange in the glycolytic intermediates is the rapid decrease in the contents of 3-phosphoglycerate and of phosphoenolpyruvate. One explanation of these changes is that the pyruvate kinase reaction is accelerated, due possibly to a local increase of adenosine diphosphate. With a faster pyruvate kinase reaction, the additional adenosine triphosphate formed might hasten glycolysis and this faster rate might then be maintained.

The fact that, in anoxia, smaller amounts of adenosine triphosphate support a higher rate of glycolysis than in air has suggested that in air this nucleotide is not readily available to the sugar-phosphorylating enzymes. Similarly, the present results could be explained if the glycolytic enzymes were mainly confined to an organized structure showing a difference in accessibility for adenosine di- and triphosphates.

On the other hand, our results may be related to a feed-back mechanism not yet specified. How important such mechanisms might be is shown by the complex effeets of adenosine nucleotides, of inorganic phosphate and of other compounds on the activity of phospho-fruetokinase in vitro.

The changes m anoxia, stated above, of glueose-6-phospliate, of fructose diphosphate and of dihydroxyacetone phosphate are considered.

A difference in the metabolic fate of adenosine triphosphate in aerobie and anaerobie conditions is described. The difference is readily explained in terms of a glyeolytic structure but could also be due to an acceleration of the hexokinase and phosphofructokinase reaetions in anaerohiosis either by activation or by a decrease in the inhibition of these enzymic stages.

While the existence of a glycolytic structure has not been proved beyond doubt, evidence that such a strueture may exist is accumulating.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-8137.1968.tb05471.x

Studies in the respiratory and carbohydrate metabolism of plant tissues

THE Pasteur effect was early attributed to the influence on the rate of glycolysis of changes in the balance of inorganic phosphate and of adenosine diphosphate and triphosphate following a transition from air to nitrogen1–3. The changes in content of several phosphate compounds during various experimental treatments of yeast suggested that ‘mitochondria’ adenosine triphosphate might not be freely available for phosphorylating fructose-6-phosphate4,5. An increase in fructose diphosphate was observed in muscle under anaerobic conditions6, and this was attributed to the activation of phosphofructokinase caused by the increased contents of adenosine monophosphate and inorganic phosphate and to a decreased content of adenosine triphosphate7. Moreover, an increase in the permeability of the mitochondrial membrane to adenosino diphosphate under anaerobic conditions has been postulated8. Finally, an acceleration in the turn-over rate of phosphoenolpyruvate followed by an increase of fructose diphosphate and of pyruvate were observed during the large increase in output of carbon dioxide in bananas ripening in air; an increase in the rate of glycolysis was postulated and this increase was attributed mainly to the localization of ‘glycolytic’ adenosine triphosphate (initially produced in the pyruvate kinase reaction) near to the sugar phosphorylating and glycolytic enzymes9.

M. A. A. Khan

Papers

Studies in the respiratory and carbohydrate metabolism of plant tissues

Mechanism of the pasteur effect.