Photorespiration: Meaning, Measurement and Metabolism

In this article we will discuss about:- 1. Definition of Photorespiration 2. Measurement of Photorespiration 3. Metabolism 4. Inhibition of Photorespiration and the Increase in Yield.

Definition of Photorespiration:

Photorespiration is a process or a group of processes by which certain plants release CO₂ in the light. The phenomenon can be demonstrated readily if a photorespiring plant is illuminated in a closed environment. Initially, the rate of photosynthesis will be more than the rate of photorespiration, and the plant will deplete the CO₂ concentration within that environment.

As the CO₂ is used up, the rate of photosynthesis will gradually decreases and at a particular point the rate of photosynthetic CO₂ uptake will be equal to the rate of photorespiratory CO₂ output and no further net CO₂ exchange is observed.

The CO₂ concentration at which this occurs is called the CO₂ compensation point. If the plant does not photorespire, it will be able to remove all the CO₂ from its environment and will have zero compensation point.

Plants are divided into two groups on the basis of their compensation points: those with high (50 ppm CO₂) and low compensation points (5 ppm CO₂), respectively. To what extent the low compensation point is due to reduced photorespiration or efficient refixation of photorespired CO₂ is still a matter of debate.

Measurement of Photorespiration:

It is extremely difficult to measure the true rate of photorespiration, since under normal circumstances, photosynthesis and photorespiration occur simultaneously in the same tissue as follow:

One of the simplest ways of measuring photorespiration is inhibition of photosynthesis and measuring O₂ uptake or CO₂ output. Here a plant or a leaf is put in the light into CO₂– free air stream. The aim is to inhibit photosynthesis by the non- availability of CO₂. Photorespiration can be measured at the rate of CO₂ output.

However, this is not an accurate method since some of the CO₂ released by photorespiration is re-fixed through photosynthesis and consequently the rate of photorespiration is underestimated. The degree of underestimation can be minimized by using fast air flow rate to sweep away as much of the CO₂ as possible, before it is re-fixed. Even then the error cannot be eliminated altogether.

The alternative method involves ¹⁸O₂ uptake. Here again photorespiration is underestimated because some of the O₂ released by photosynthesis is taken up by photorespiration in addition to the supplied ¹⁸O₂.

Recently infrared gas analyser is used to study the rate of CO₂ evolution. As seen in figure 15-1 the leaf is enclosed in a chamber and the gas surrounding the leaf is circulated by a pump through an infrared gas analyser. The latter component mainly comprises a light source which gives off infrared rays which fall on a photoelectric detector.

CO₂ released from the leaf, absorbing infrared rays when cross the infrared beam (lR-beam) some of the rays are intercepted and the detection response changes. Thus it is possible to measure the amount of CO₂ released initially and then after specific periods of darkness or illumination.

Similarly mass spectrometer is also employed to determine the amount of CO₂ released. Here the leaves of a photorespiring plants are sealed in a chamber. The one end of this chamber is connected to a source of mixture of isotopes in a gaseous form e.g. 18O₂ and the other end is attached to a mass spectrometer. The latter instrument is employed for the exact determination of the mass of an isotope.

To date, all the available methods for measuring photorespiration are inaccurate.

Metabolism of Photorespiration:

i. Biosynthesis of Glycolate:

In most of the photosynthetic organisms like algae and higher plants glycolate synthesis occurs. That glycolate is an unwanted metabolite is evidenced by the fact that many of the algae excrete it into the surrounding water. However, higher plants cannot excrete it in this way, but instead oxidize it through photorespiration.

Chloroplast is the site of CO₂ fixation and transketolase is the enzyme which catalyzes the transfer of two carbon fragments between certain sugar phosphates during the operation of the cycle. This two carbon fragment Dihydroxyethylthiamine pyrophosphate (DHETPP) can be oxidized non-enzymatically into glycolate and thiamine pyrophosphate (TPP) by ferricyanide, H₂O₂ and other oxidizing agents.

According to the scheme of Coombs and Whittingham (1966) in light ferredoxin is reduced by the light reaction of photosynthesis. In low CO₂ and high O₂ little PGA is available and reduced ferridoxin is oxidized by atmospheric O₂ to H₂O₂ and the latter oxidizes DHETPP to glycolate.

Under high CO₂ concentration much of the PGA is available which is converted to glyceraldehyde 3-P. Evidently low CO₂ and high O₂ levels stimulate peroxide formation and consequently photorespiration. On the other hand, high CO₂ and low O₂ levels stimulate 3 PGA reduction and therefore photosynthesis.

This scheme is open to criticism since H₂O₂ formation saturates at 8 of O₂ in the broken chloroplasts and intact leaves, whereas O₂ stimulation of photorespiration is not saturated at 100% O₂.

Based on the discovery by Ogren and Bowes (1971) of photorespiratoryglycolate synthesis a recent hypothesis is given. According to this a derivative of glycolate, namely 2- phosphoglycolate was produced as a result of the apparent malfunctioning of the enzyme RuBP carboxylase.

In its normal state, this enzyme fixes CO₂ for the Calvin cycle by adding a molecule of CO₂ to the RuBP to form the two, three carbon molecules of phosphoglyceric acid (PGA) and the latter is metabolized into sugar.

According to these workers, oxygen could function as an alternative substrate for this enzyme, in place of CO₂. This CO₂ and O₂ apparently compete for RuBP at the same or the adjacent sites on the carboxylase, since O₂ is the competitive inhibitor with respect to oxygenase reaction.

Thus the Warburg effect or the balance between photosynthesis and photorespiration is based on the dual activity of RuBP carboxylase. The competition between CO₂ and O₂ for RuBP carboxylase determines the relative rate of photosynthesis and photorespiration.

ii. Glycolate Oxidation and CO₂ Evolution:

No further metabolism of glycolate takes place within the chloroplast. Glycolate from the chloroplast enters the small microbodies called peroxisomes which are packets of enzymes, at least one of which generates hydrogen peroxide as a byproduct.

Catalase is also present to destroy this peroxide which might otherwise be toxic. Here glycolate is also oxidized to glyoxylate releasing H₂O₂ as well. Glyoxylate is then converted to glycine by an amino transferase reaction. The donor of the amino group in this reaction may possibly be glutamate.

Within the peroxisomes, the microbody reaction may occur for the conversion of two mole of glycine into one of those of serine releasing one molecule each of CO₂ and ammonia. In this process, two ATP are produced.

The CO₂ given off at this stage is the one evolved in the process of respiration. Within the peroxisomes, glycerate from the chloroplast is converted into hydroxypyruvate and serine, respectively.

iii. Conversion of Serine to Carbohydrate:

14C-serine when added is shown to be incorporated into carbohydrates but only in the presence of light, suggesting that the last stage in the photorespiratory pathway involves, conversion of serine to carbohydrate and is an energy requiring process.

Some of the stages in this reaction sequence could occur in peroxisomes whereby serine is converted into hydroxypyruvate and subsequently D-glycerate is known to be converted into 3-phosphoglycerate, a compound intermediate of the Calvin cycle. If glycerate (PG) enters the chloroplast it is converted into carbohydrate using ATP and NADPH.

iv. Energetics of Photorespiration:

In true respiration large amount of energy is available in the form of ATP and NADPH. But in photorespiration, ATP is generated during conversion of two molecules of glycolate to carbohydrate. These ATP molecules are made available by conversion of glycine to serine and CO₂.

However, the subsequent conversion of serine to carbohydrate uses two molecules of ATP and two of NADPH. If all the carbon of the original two glycolate molecules be recovered, then CO₂ released during serine formation must be refixed by normal photosynthesis and this requires two farther molecules of NADPH and three of ATP.

In summary, to restore the level of carbohydrate, plants must use a total of four nucleotides and three ATP. To fix four carbon atoms by normal photosynthesis nearly double the amount of energy is required. Table 15-1 shows comparison of photorespiring and non-photorespiring plants.

Inhibition of Photorespiration and the Increase in Yield:

By growing plants under conditions where photorespiration is inhibited, 50-100% increase in growth rate is demonstrated. In fact synthesis of phosphoglycolate is inhibited by reducing the O₂/CO₂ ratio in the air surrounding the plant. This is accomplished by reducing the oxygen concentration or else by increasing the CO₂ concentration.

Some of the procedures adopted are described below:

i. Increasing Productivity at Low Oxygen Concentration:

It is now well documented that low oxygen conditions inhibit photorespiration and bring about a corresponding increase in the rate of photosynthesis in several plants. Zelitch (1971) indicated that reducing the oxygen concentration from its normal 21 to 20% level, increased the rate of photosynthesis in a variety of plants by factors ranging from 35 to 100%.

ii. Increasing Productivity at high CO₂ Concentration:

The effect as mentioned above can be achieved by increasing the CO₂ concentration. The process is cheap and easy. In the atmopsphere, CO₂ concentration is 0.03% and consequently very little CO₂ is required additionally to alter O₂/CO₂ ratio in order to inhibit photorespiration. The greatest disadvantage lies in the necessity to employ air tight enclosures where plants are grown. However, the technique has greatest limitations if desired to apply for large scale agricultural uses.

iii. Increasing Productivity with Chemical Inhibitors:

There is great need to discover an enzyme inhibitor which could be sprayed on to the plants to suppress phosphoglycolate synthesis successfully and thereby check photorespiration. In fact two major inhibitors—HPMS (Hydroxyl 2-pyridine methane sulfonic acid) which inhibits the oxidation of the glycolic acid into glyoxylate and glycolate, and inhibitor of glycolate synthesis are used.

Recently Zelitch and Day have shown that a mutant from slow growing variety of tobacco had lower rate of net photosynthesis in normal air and faster photorespiration compared with its fast growing wild type. Evidently, the genetic control through a pleiotropic effect was capable of regulating photorespiration within the species.

The efforts of Bjorkman to convert a C₃ species into a C₄ species by interspecific hybridization in Atriplex were not successful. In fact, the hybrid of the weed had a decreased net photosynthesis. This is understandable since conversion of C₃ and C₄ plants would involve large changes in leaf morphology, chloroplast type and enzyme activities.

In the early stages of evolution, CO₂ concentration was very high in the atmosphere perhaps because of inhibition of glycolate synthesis and absence of photorespiration. In all likelihood, it is comparatively recently on a geological scale that CO₂ level fell down sufficiently to allow photorespiration. However, in evolution, some plants have evolved where photorespiration was inhibited, possibly by possessing CO₂ pump mechanism which increased the CO₂ concentration in the region of their RuBP-carboxylase to such a level as to inhibit phosphoglycolate production.

All the C₄ plants have a comparable anatomy in which photosynthetic tissue is confined to two concentric layers of cells, each being only one cell deep and surrounding each of the vascular bundles. The inner one is called bundle sheath and the outer one as mesophyll. By preparing extract from each of these layers, it is observed that two layers contain different photosynthetic enzymes.

Nearly, all of the leaf’s RuBP-carboxylase is in the bundle sheath cells whereas the mesophyll cells contain PEP- carboxylase. The latter fixes CO₂ by reacting with PEP and forming oxaloacetate. In general C₄ plants are divisible as follows:

Those which have high concentration of malic dehydrogenase in mesophyll cells and produce malic acid from oxaloacetate and the latter is transferred to bundle sheath and converted to pyruvate. Second, those which have alanine—aspartic transaminase which produce asparate from oxaloacetate. In bundle sheath aspartic acid is again transferred to oxaloacetate.

Further, it was shown that the chloroplast of bundle sheath contains enzymes which liberate CO₂ from C₄ acid leaving behind C₃ acid pyruvate. The CO₂ released is refixed by the RuBP-carboxylase in the bundle sheath chloroplast, whereas in remainder of plant photosynthesis occurs via normal Calvin cycle.

The pyruvate formed is translocated back to the mesophyll where it is converted to PEP by enzyme PEP synthetase using ATP and inorganic phosphate, so as to be ready to pick up another molecule of CO₂. Gold-worthy and Day (1970) put forward a suggestion that the C₃ pathway is acting as a pump generating high CO₂ concentration in bundle sheath chloroplast, and in this way, inhibits glycolate production.

However, one point still remains unexplained. For instance: ‘if C₄ plants do not normally produce glycolate where is the need for them to have peroxisomes and why was glycolate detected in them by Zelitch?’

One of the possible explanations could be that under normal circumstances glycolate is not produced; however, it can be synthesized under adverse conditions e.g. if stomata are closed due to adverse conditions or the CO₂ concentrations in the air space of the leaf fall due to photosynthesis.

Sooner or later a point is reached when the pump starved of CO₂ input, fails to maintain the CO₂ concentration in the bundle sheath chloroplast at a level adequate to prevent glycolate synthesis. Glycolate production and its consequent photorespiration would then occur and peroxidase would be required.

The regulation of photorespiration is a challenging scientific area for future studies. During the current energy crisis and food shortage, the regulation of photorespiration provides an opportunity to supply a large measure of solution to these pressing problems.

Science will succeed where nature has failed as far as C₃ plants are concerned. Photorespiration is suppressible, if not unable. The medicine is free and is becoming available in abundance. It is CO₂!