The following points highlight the top thirteen factors influencing the respiratory rate. The factors are: 1. Type and Age of Plant Tissue 2. State of Respiring Protoplasm 3. Temperature 4. Hydration of Tissues 5. Light 6. Oxygen 7. Carbon Dioxide 8. Inorganic Salts 9. Mechanical Stimulation 10. Wounding as a Respiration Stimulator 11. Cyanide-resistant Respiration and Others.
Factors Influencing Respiratory Rate:
- Type and Age of Plant Tissue
- State of Respiring Protoplasm
- Temperature
- Hydration of Tissues
- Light
- Oxygen
- Carbon Dioxide
- Inorganic Salts
- Mechanical Stimulation
- Wounding as a Respiration Stimulator
- Cyanide-resistant Respiration
- Oxidases other than Cytochrome Oxidases
- Pentose Phosphate Pathway
Role # 1. Type and Age of Plant Tissue:
The respiratory rate depends upon the age and the nature of the tissues, since it is concerned with energetic activities. Young tissues, like the actively growing regions of the plant, have higher respiratory rate than the old or matured tissues.
Depending upon the physiological status of the tissues a difference in the respiratory activity is also seen. In general tissues with high metabolic turnover or energetic activities respire more than the resting tissues.
Difference and the change of the substrate composition with the change in age cause variability in the respiratory rate of different types of tissues. In the overall growth of a seedling, respiratory rate is high during its initial growth, and this gradually declines. It is lowest in the mature seedling. If we compare the respiratory rates of developing leaves, it is highest during early leaf growth and falls to the steady state when the leaf attains maturity.
In a mature tissue there is breakdown of the integrated system of energy transfer and further increase in age is associated with an increase in senescence. During senescence proteins breakdown releasing some of the substrate for respiration. This normally results in an increase in respiratory rate with more production of CO2 for a brief period. This marks the collapse of the cellular organization and cells die.
The respiratory rate also varies with the type of tissues depending mostly on their metabolic status and relative availability of non-metabolic or structural components and their accessibility to oxygen (Table 18-1).
Tissues with high dead mass or non-metabolic components like woody stems, have low respiratory rate. On the other hand in bulky organs like potatoes and carrots the respiratory rate depends more on the rate of oxygen diffusion in the tissue. Dormant seeds represent a state of autoxidation and decay without any sequential metabolism as these seeds carry on very low gas exchange.
The state of the respiring protoplasm, concentration of respirable sugar, the concentration and activity of the necessary enzymes are some of the additional internal factors which affect respiration.
For most plant species temperature, acidity, salt concentration and the amount of moisture, carbon dioxide and oxygen are some of the additional important factors which affect respiration. Sometimes it becomes difficult to distinguish between the effect of two or more factors since all of them have almost similar response.
For example, the enzymic activity depends upon temperature, acidity, presence of certain activating and depressing substances which are often of a simple inorganic nature.
Role # 2. State of Respiring Protoplasm:
The rate of respiration depends upon the developmental stage of protoplasm. In the actively growing regions of the plant, cells are relatively rich in protoplasm with high enzymatic activity. As these regions grow, numerous synthetic processes are taking place which require energy and this is supplied by respiration.
Thus, the rate of respiration is very high in these tissues. In the mature plant organ where growth has ceased, protoplasm abounds in inert matter and metabolic processes gradually decrease. Here the rate of respiration is also low. The inert matter also increases with the age of plant cells. Evidently the rate of respiration is connected with growth.
Role # 3. Temperature:
Temperature significantly affects the rate of respiration as it does other enzymatic processes. Lower temperature limits of respiration of a planty lie below 10°C conditioned by the freezing of the tissues.
In the hibernating green parts of the plants, e.g., in the buds of the deciduous trees, needles of conifers, it is much lower, about 20-25°C below 0°C. With certain limits, with an increase in temperature respiratory rate increases rapidly and up to 40°C follows the Vant Hoffs rule.
It has been calculated that within limits, with every 10°C rise in temperature, the enzymatic reactions get doubled which can be expressed as:
Q10 = rate at (1 + 10)°C / rate at t°C
The Q10 value of respiration in the temperature range 0 to 20° is usually between 2 and 3. Q10 value decreases with a further rise in temperature and mostly beyond 35°C there is reported to be a great breakdown in respiratory mechanism possibly because of the destruction of enzyme by heat. At temperatures above 30°C time factor begins to operate resulting in the fall of respiratory rate.
More is the period for which a plant is exposed at high temperature, greater is the fall in the respiratory rate. Thus, with temperatures in excess of 33°C, the process of respiration which was initially very intense, soon begins to decrease and after few hours becomes weaker than at temperatures of about 20-25°C and even lower.
Temperature above 20°C changes protoplasm and results in decrease of respiration. The various explanations offered for this decreased respiration are: oxygen may become limiting to the cells at high temperature because of its reduced solubility and diffusion; CO2 may accumulate in cells in such concentrations at high temperatures as to check the rate of respiration and the supply of oxidizable food may be insufficient to maintain high rates of respiration. The optimum temperature at which the rate of respiration remains steady at a high level cannot be the same for every plant.
Temperature affects the rate of respiration indirectly. For instance when the temperature of a potato tuber is lowered from a few degrees above to about 0°C, the respiration rate increases. This is because low temperature causes a shift in the starch-sugar equilibrium towards the sugar side.
Role # 4. Hydration of Tissues:
The percentage of bound water in the seeds is 16% and their rate of respiration is low. With an increase in the moisture content up to 16% the respiration also increases slightly but beyond this, rise in respiration is very rapid.
The increase in respiration due to increase in water content may either be because of water which makes the respiring cells turgid; the hydrolysis of carbohydrates to soluble sugars; accelerating the action of respiratory enzymes; acting as a medium in which oxygen diffuses into the respiring protoplasm or the water concentration of the cell membranes would reduce the permeability to oxygen and CO2, the former acting as a deficient factor and the latter as a retarding factor.
Tissues saturated with water have low rate of respiration. In the wilted leaves the rate of respiration rises because starch is converted into sugar which serves as a respiratory material.
Role # 5. Light:
Light is not essential for respiration and indirectly affects the respiratory rate. It increases respiration by raising the temperature of the green plant and also increases the amount of photosynthetic material required for respiration. In some green tissues, it has an inhibitory effect. Red light has a greater retarding effect than blue light. In some instances, red light may increase respiration.
Role # 6. Oxygen:
The presence of oxygen is essential for the Krebs cycle. Oxygen is the terminal acceptor of electron in the electron transport system. In general at low O2 concentrations both aerobic and anaerobic respirations occur in the plant.
In general as the oxygen concentration increases from zero, the rate of aerobic respiration increases. In most of the plants the rate falls with an increase in oxygen concentration. In germinating rice grain, the increase in the rate of aerobic respiration is linear over a range of oxygen concentration.
This is perhaps, due to the fact that oxygen consumption is limited by an oxygen diffusion barrier present in the outer covering of the rice grain (production rate in air = 1.0).
The influence of O2 on respiration depends upon the way in which it is measured. When pure N2 is used and there is general lack of oxygen, no O2 is absorbed. Under these circumstances, the rates of sugar loss and output of CO2 are not reduced to zero. Surprisingly, they increase sometimes. This is explained by the fact that oxygen can actually retard the utilization of sugars in respiration.
The effect is on glycolysis and is known as Pasteur effect. In apple fruits and in flooded roots where there is deficiency of O2, Pasteur effect is highly significant to maintain food reserves. In apples and other fruits the process is of prime importance during storage when loss of sugars extensively is checked during over ripening.
It is ideal to check aerobic respiration by decreasing O2 to save loss of sugars. However, care is exercised to check stimulation of anaerobic processes. Thus CO2 is additionally added to the surroundings, the temperature is lowered and these factors check further over ripening. Additionally, CO2 also inhibits the action of ethylene.
Role # 7. Carbon Dioxide:
Increasing the CO2 concentration of air depresses respiration. Normally, inhibition of respiration is only there if the CO2 concentration increases too high over the normal. The anaerobic respiration in the germinating pea seeds is inhibited by about 50% carbon dioxide in the air. This is helpful in maintaining the dormant state of the seeds. Relative changes in the carbon dioxide concentration does affect stomatal closure and opening.
Stomata get closed under the environment with high concentrations of carbon dioxide and this may thus cause inhibition of respiration. In leaf respiration, the effect of CO2 concentration is indirect one that is through the closure of stomata which limit a gaseous exchange. This may result in increasing the internal concentration of CO2 considerably and in this way limit respiration.
Role # 8. Inorganic Salts:
The rate of respiration increases when a plant or a tissue is transferred from water to a salt solution. The amount by which the respiration is increased over normal has been called salt respiration. This process has been linked with absorption of salts and ions, which have high requirement of energy supplied by respiration.
Role # 9. Mechanical Stimulation:
Leaf respiration increases by handling, stroking or bending of leaves. Response to handling decreases if it is repeated over a period of time. It has been observed that shearing stress, gives more of stimulation to respiration and almost very little effect by compression or tension. It has also been reported that sound waves stimulate respiration but no conclusive evidence has so far been given.
Role # 10. Wounding as a Respiration Stimulator:
Wounding of a plant organ stimulates respiration in that organ. It initiates meristematic activity in the region of the wound resulting in the development of “wound callus”. A great increase in sugar contents in the half cut potato is reported. Perhaps the increase in respiration is due to an increased availability of respiratory substrate in wounded tissues.
Further evidences for the stimulation of respiration in response to wounding are associated with the rapid oxidation of phenolic compounds, the increase in the normal process of glycolysis and oxidative catabolism in wounded tissue and the reversion of certain cells to meristematic state, followed by callus formation to heal the wound which will have high rate of respiration to those of resting or mature tissues.
Role # 11. Cyanide-Resistant Respiration:
Certain negative ions that combine with the iron in cytochrome oxidase tend to inhibit aerobic respiration strongly. These ions are cyanide and azide. In addition carbon monoxide also forms a strong complex with this ion causing poisoning of respiration by preventing electron transport. In some plants and some organs such inhibitors have very little effect on respiration.
The respiration in such systems or plants continues and is termed cyanide-resistant respiration. There are many fungal and algal species which show such type of respiration. In these systems/plant mitochondria possess an alternate branch in the electron transport pathway (Fig. 18- 2). This alternate route permits transport of electrons to oxygen. Terminal oxidase and other components of this route are still unknown.
However, very little or almost nil oxidative phosphorylation is coupled to it. Thus this route produces heat but not ATP. The heat is important and useful to some systems like pollination in arum lily.
From Figs. 18-2, 18-3 (A,B) it will be observed that branching of route takes place from the cytochrome oxidase pathway only after the first oxidative phosphorylation ‘site’, i.e., after the point of ATP production. Thus, in situations where electron transport through the cyanide-resistant pathway is fast, there will be heat and ATP formation at quick rates. However, the amount of substrates utilized will be enormous.
Figure 18-3 (A, B) show schematic representation of the electron transport chain and proton ‘pumping’ sites in the inner membrane of the plant mitochondrion (A), the path of electrons from NADH or succinate to molecular oxygen is indicated by solid arrow. Note that energy conserved in the proton gradient is consumed to drive ATP synthesis through F sub 0 ATPase coupling factor present in the membrane.
Alternate electron transport pathways in plant mitochondrion is shown in B Fig. 18.4. Electrons entering the chain through the alternate dehydrogenases will pass through two phosphorylating sites. Plant mitochondrion have an ‘external’ dehydrogenase that faces the inter-membrane space and is capable of oxidizing cytoplasmic NADPH. Here only two ATP can be formed from the transfer of each pair of electrons to oxygen.
Role # 12. Oxidases other than Cytochrome Oxidases:
Oxidase may be defined as an enzyme which catalyzes the transfer of electrons from substrate to molecular oxygen releasing water and H2O2 as end products. In addition to cytochrome oxidase which is a mitochondrial enzyme there are other oxidases which take part in oxygen consumption by plant cells.
These are described below:
1. The alternate oxidases:
In arum lily, there are two alternate electron transport pathways from substrate to oxygen. Nothing is known regarding the ‘switch’ which causes shift from oxygen consumption mediated through cytochrome oxidase and that mediated through alternate oxidase. Further, much remains to be discovered about its general occurrence in other plant organs or systems.
2. Glycolic acid oxidase:
In the green tissues of several species glycolic acid oxidase is present which is flavin-containing oxidase. This enzyme reduces oxygen to hydrogen peroxide, and plays an important role in photorespiration. Hydrogen peroxide is toxic when accumulated in a tissue. Hence, catalase decomposes it to water and oxygen.
3. Flavin oxidase:
This enzyme has a role in the oxidative degradation of fatty acids in some seedlings. It is present in glyoxysomes. FAD is its coenzyme. The hydrogen peroxide produced in the reaction is decomposed to water and oxygen through catalase.
4. Phenolase:
This is a copper containing oxidase. The activity of this enzyme can be made out when the injured surface of an apple, brinjal, peach or some other fruits turn brown or black. Phenol is the substrate for phenolase. Phenols and phenolases are present in different cell compartments.
5. Ascorbic acid oxidase:
This enzyme is copper-containing and is reported from divergent plant tissues. Nearly all the living cells contain the substrate vitamin C or ascorbic acid. There is a general
suggestion that asorbic acid oxidase may act as a terminal oxidase and compete with cytochrome oxidase.
6. IAA oxidase:
This enzyme brings about the oxidation of indole-3 acetic acid and the oxidation products do not act as plant growth substances. This enzyme appears to act as a regulatory system for the control of IAA concentration within the plant cells. IAA oxidase belongs to a class of enzymes known as peroxidases.
Role # 13. Pentose Phosphate Pathway:
Since 1950 it is increasingly appreciated that in addition to glycolysis and Krebs cycle, energy could also be obtained through the oxidation of sugars via pentose phosphate pathway. It is also called hexose monophosphate shunt.
Several of the compounds of the PPP are also components of the Calvin cycle. The chief difference between the two lies in the fact that during PPP sugar phosphates are oxidized rather than synthesized as during Calvin cycle in the chloroplast. In this manner the reactions are similar to those of glycolysis. In addition both PPP and glycolysis take place outside the organelles in the cytosol.
In fact the two pathways are interwoven and have some reactants in common. The chief difference is that in PPP it is the NADP which accepts the electron from the sugar phosphates while in glycolysis NAD is the common acceptor.
It will be observed that the first reaction involves glucose 6-phosphate which is made available through different reactions and is immediately oxidized by glucose 6-phosphate dehydrogenase. It is an instance of the oxidation of an aldehyde to an acid. Rest of the steps in the reaction are clearly made out in the same figure.
Under dark conditions PPP also takes place in the chloroplast. In the light G 6-P dehydrogenase seems to be inactivated. Thus, PPP stops and Calvin cycle operates. The chief importance of this pathway lies in its glucose breakdown, supply of NADPH for synthetic reactions, making available ribose 5-phosphate, and also erythrose 4-phosphate.