11th Class Biology Respiration In Plants Factors Affecting Rate of Respiration

Many external and internal factors affecting the rate of respiration are as follows :

(1) External factors

(i) Temperature : With every \[10{}^\circ C\] rise of temperature from \[0{}^\circ C\] to \[30{}^\circ C\]the rate of respiration increases 2 to 2.5 times (i.e., temperature coefficient \[({{Q}_{10}}{}^\circ )\] is = 2 to 2.5), following Vant Hoff’s Law. Maximum rate of respiration takes place at \[{{30}^{o}}C,\]there is an initial rise, soon followed by a decline. Higher the temperature above this limit, more is the initial rise but more is the decline and earlier is the decline in the rate of respiration. Probably this is due to denaturation of enzymes at high temperature.

Below \[0{}^\circ C\]the rate of respiration is greatly reduced although in some plants respiration takes place even at \[-20{}^\circ C.\] Dormant seeds kept at \[{{50}^{o}}C\]survive.

(ii) Supply of oxidisable food : Increase in soluble food content readily available for utilization as respiratory substrate, generally leads to an increase in the rate of respiration upto a certain point when some other factor becomes limiting. 

(iii) Oxygen concentration of the atmosphere : The amount of oxygen in the environment of plants is increased or reduced upto quite low values the rate of respiration is not effected. On decreasing the amount of oxygen to 1.9% in the environment aerobic respiration become negligible (extinction point of aerobic respiration) but anaerobic respiration takes place.

(iv) Oxygen poisoning : The significant fall in respiration rate was observed in many tissues in pure \[{{O}_{2}},\]even at N.T.P. This inhibiting effect was also observed in green peas when they are exposed to pure oxygen exerting a pressure of 5 atm- the respiration rate fall rapidly. The oxygen poisoning effect was reversible, if the exposure to high oxygen pressure was not too prolonged.

(v) Water : With increase in the amount of water the rate of respiration increases. In dry seeds, which have \[8-12%\] of water the rate of respiration is very low but as the seeds imbibe water the respiration increases. As water is necessary for activity of enzymes.

(vi) Light : Respiration takes place in night also which shows that light is not essential for respiration. But light effects the rate of respiration indirectly by increasing the rate of photosynthesis due to which concentration of respiratory substrates is increased. More the respiratory substrate more is the rate of respiration.

(vii) Carbon dioxide\[(C{{O}_{2}})\]:  If the amount of CO₂ in the air is more than the usual rate of respiration is decreased. Germination of seeds is reduced and rate of growth falls down. Heath, (1950) has shown that the stomata are closed at higher cone. of \[C{{O}_{2}},\] due to which oxygen does not penetrate the leaf and rate of respiration is lowered.

(viii) Inorganic salts : The chlorides of alkali cations of Na  and K, as also the divalent cations of Li, and Ca and Mg, generally increase the rate of respiration as measured by the amount of \[C{{O}_{2}}\]evolved. Monovalent chlorides of K and Na increases the rate of respiration, while divalent chlorides of Li, Ca and Mg causes less increase in respiration.

(ix) Injury and effects of mechanical stimulation : Wounding or injury almost invariably results in an increase in the rate of respiration.

(x) Effect of various chemical substances : Certain enzymatic inhibitors like cyanides, azides, carbon monoxide, iodoacetate, malonate etc. reduce the rate of respiration even if they are present in very low concentration.

However, various chemical substances such as chloroform, ether, acetone, morphine, etc., brings about an increase in respiratory activity.

(xi) Pollutants : High concentration of gaseous air pollutants like \[S{{O}_{2}},N{{O}_{X}}\] and \[{{O}_{3}}\] inhibit respiration by damaging cell membrane. These gaseous pollutant causes increase in pH which in turn affects the electron transport system thus inhibiting respiration.

Heavy metal pollutant like lead (Pb) and cadmium (Cd) inhibit respiration by inactivating respiratory enzymes.

(2) Internal Factors

(i) Protoplasm : The meristematic cells (dividing cells of root and shoot apex) have more protoplasm than mature cells. Hence, the meristematic cells have higher rate of respiration than the mature cells. Respiration rate high at growing regions like floral and vegetative buds, germinating seedlings, young leaves, stem and root apices.

(ii) Respiratory substrate : With the increase of in the amount of respiratory substrate, the rate of respiration increases.