The following points highlight the ten factors affecting the the rate of aerobic respiration. The factors are: (1) Protoplasmic Conditions (2) Temperature (3) Supply of Oxidisable Food (4) Oxygen Concentration of the Atmosphere (5) Carbon Dioxide Concentration of the Atmosphere (6) Supply of Water (7) Light (8) Inorganic Salts (9) Injury and the Effects of Mechanical Stimulation and (10) Effect of Various Chemical Substances.
Factor # 1. Protoplasmic Conditions:
Young actively growing meristematic tissues have always higher rates of respiration than older and more mature parts. The proportion of protoplasm, both relative and absolute, is always greater in the young cells compared to maturer, vacuolated cells. There seems to be a direct relationship between the amount of protoplasm and the respiration rates—the greater the protoplasm, the higher is the respiration rate. Hydration of protoplasm and the quantity of respiratory enzymes of the mitochondria (all the respiratory Krebs cycle enzymes are known to occur, in mitochondria) are important protoplasmic factors which contribute to the effects obtained.
Factor # 2. Temperature:
Within certain limits (0-45°C.) increase in temperature leads to an increase in the initial respiration rate. Above 30°C., the respiration rate slows down and the decrease above the optimum may possibly be due to the progressive inactivation of respiratory enzymes. Other causes which might contribute to this lowering of respiration rate may include: (a) O2 may not get access to the cell fast enough, (b) CO2 may accumulate in toxic concentration, (c) the supply of oxidisable respiratory substrate may be inadequate to keep pace with the high rates of respiration, etc. At 0°C., respiration rate greatly diminishes and soon becomes imperceptible though there are some records of measurable respiration even at — 20°C. The decline of respiration at temperatures below zero can be attributed to the formation of ice and consequent dehydration of protoplasm.
The temperature coefficient (Q10) of respiration within the temperature range of 0-35°C appears to be about 2.0 to 2.5.
Factor # 3. Supply of Oxidisable Food:
Increase in soluble food content readily available for utilisation as respiratory substrate, generally leads to an increase in the rate of respiration up to a certain point when some other factor becomes limiting.
Factor # 4. Oxygen Concentration of the Atmosphere:
In considering first the effect of oxygen concentration in gas mixtures at N.T.P., the whole range of oxygen concentrations falls in two clear cut divisions, a lower range, in which anaerobic respiration occurs and an upper range, in which the entire volume of CO2 evolved results from aerobic respiration.
The lower range extends from zero to the extinction point (generally about 4-5% oxygen) which, as we know, is that concentration of oxygen just high enough to completely suppress the anaerobic component. The upper range extends from the extinction point to 100% Oxygen.
In general, the rate of respiration is decreased by oxygen concentration smaller than that of air (21%) which drops off very sharply at concentration of oxygen less than 5-10%.
The oxygen poisoning, i.e., the significant fall in respiration rate, was observed in many tissues in pure O2, even at N.T.P. This inhibiting effect was also observed with green peas when they were exposed to pure oxygen exerting a pressure of 5 atm.— the respiration rate fell rapidly. The oxygen poisoning effect was reversible, if the exposure to high oxygen pressure was not too prolonged.
The relation of oxygen concentration to respiration has a particularly important implication in the growth of the roots. Roots must respire vigorously if they are to grow and take up minerals from the soil by active absorption. The vigorous root respiration is only possible if the space surrounding the roots and root hairs has ample supply of oxygen. When the roots are poorly aerated as in water-logged or heavy soils, growth of the plant may be significantly restricted.
Factor # 5. Carbon Dioxide Concentration of the Atmosphere:
In general, the higher the concentration of CO2 of the atmosphere, the lower is the rate of respiration. This fact is made use of in storage of fruit. Air containing 10% CO2 (in atmosphere it is only 0.03%) retards respiratory breakdown and therefore reduces sugar consumption and thus prolongs the life of the fruit.
The oxygen content of the air, however, must be maintained as high as normal to prevent anaerobic respiration. In some plant tissues, however, respiration rate actually increases when exposed to relatively high concentration of CO2. It is thought now that the effect of CO2-concentration on the respiration rate is influenced not only by its concentration in the medium but also depends upon the kind of tissue and the period of exposure.
Factor # 6. Supply of Water:
Increase in the percentage of moisture leads to a general increase in respiration rate. This increase is slow at first but very rapid later. This is very clearly seen in the tissues of many xerophytes. As the water content of such plants is increased, often there is no great immediate effect upon the rate of respiration, until a certain water content (which varies according to the tissue) is attained after which respiration rate shows rapid increase. On the other hand, minor variations in water content of well-hydrated plant cells do not appear to have very great influence upon the rate of respiration.
Factor # 7. Light:
As far as we know light has no direct effect on respiration except in bringing about an increase in temperature which certainly, as we know well, influences respiration. The indirect effect of light on respiration is, of course, immense because only in light the primary respiratory substrates are synthesised.
That light has no direct effect on respiratory activity has been shown in some mutant strains of Chlorella which, although green, cannot photosynthesise. These algae, however, grow and respire when supplied with a suitable source of carbon. The respiration rate of these forms was entirely unaffected by illumination. With blue-green alga, Anabaena, however, respiration rate was found to depend on light, and the effect was also influenced by O2-concentration.
The term photorespiration has attracted a lot of attention during the last few years. It is used to indicate increased respiratory activity in light, regardless of the pathways of respiration, by which CO2 is released and oxygen consumed.
Some plants, e.g., tobacco, evolve CO2 when brightly illuminated in CO2-free air whereas others, e.g., maize, do not. This light respiration is stimulated by high oxygen concentration of the medium. The different responses by plants to temperature also suggest that this photorespiration is different from normal mitochondrial respiration. There are strong evidences also for progressive increase in photorespiration with increasing light intensities.
In tropical HSK-plants (e.g., sugar cane leaves), photorespiration is very difficult to detect as these species are extremely efficient in photosynthesis. Compared to these plants, the temperate species, which fix CO2 primarily by following the classical cycle, have very high respiratory rates.
Factor # 8. Inorganic Salts:
The chlorides of alkali cations of Na and K, as also the divalent cations of Li, Ca and Mg, generally increase the rate of respiration as measured by the amount of CO2 evolved, although there is considerable difference in the effects of monovalent and divalent cations. With monovalent chlorides of K and Na, the high respiration rates may be maintained for some 7—10 days whereas with divalent chlorides of Li, Ca and Mg, the increased rate observed generally falls off after about a day. Similar results are obtained with NH4Cl but in all cases this effect seems to be transitory.
The transitory increased respiration rates on addition of chlorides, particularly of potassium and sodium may be calculated as total respiration in salts minus the ground- respiration (ordinary respiration), unconnected with salt uptake. The ground respiration is essentially cyanide resistant whereas the extra respiration due to salt addition was found to be cyanide sensitive—10-4M cyanide completely abolished the increased respiration.
This enhanced respiration has been termed salt respiration or anion respiration which, according to Lundegardh, was directly related to the total amount of anion absorbed by plant cells rather than to the absorption of cations.
Factor # 9. Injury and the Effects of Mechanical Stimulation:
Wounding or injury almost invariably results in an increase in the rate of respiration. Broken and shrivelled seeds and kernels have always higher respiration intensities than clean, intact seeds of the same type and moisture content. It is possible that the increased respiration rate associated with injury may partly he correlated to an increase in the sugar content, actually observed in the cells close to the injury, which-may be as high as 70% compared to intact parts. This increased rate of respiration following injury is not, however, maintained for more than 48—72 hours.
A considerably higher respiration rate is observed when leaves are first touched or handled and this effect could also be produced by stroking or bending the leaf by a mechanical arrangement. The possibility that mechanical stimulation actually affects the oxidation process itself is supported by the observation that mechanical stimulation has no effect on respiration rate in an atmosphere of only nitrogen.
The effect of two successive stimulation with a short interval between them is interesting. Both the first and second stimulations increase the respiration rate but although the respiration rate reached as a result of second stimulation may be actually higher than the peak reached by the first stimulation, the net increase in respiration rate after the second stimulation is always less than that resulting from the first stimulation, if the time between the two stimulations is less than three days. It is evident that the sensitivity of a tissue to respiratory mechanical stimulation is lowered by a previous stimulation.
Factor # 10. Effect of Various Chemical Substances:
Various chemical substances such as chloroform, ether, acetone, morphine, etc., temporarily increase respiration rates if given in small doses but if the dose is large, a rapid decrease in respiration rate is observed.
Ripening fruits produce ethylene and this is accompanied by an increase in respiration rate; other volatile products responsible for the flavour (aroma), e.g., methyl, ethyl, amyl esters of formic, acetic, caproic and caprylic acids also associated with increased respiration rate, reach a maximum during ripening of fruits. Artificial ripening of fruits may be induced by treating unripe fruits with ethylene gas (1: 1000) and this is always accompanied by an increase rate of respiration.