Essay on “Photosynthesis” (1600 Words)

Photosynthesis is the most important single process involved in the productive potential of any ecosystem and for a long time it used to be thought that photosynthetic CO₂ fixation in plants occurs exclusively through the reductive pentose phosphate cycle (the Calvin pathway) and also that the rate of respiration of green plants in light was equal to that in the dark.

Within the last few decades the inadequacy of both these views has been demonstrated following the recognition of an important alternative pathway of CO₂ fixation in certain tropical grasses and dicotyledonous plants and also from the fact that light respiration (photorespiration in many temperate plants may be quite different both in rate and in the pathway of carbon metabolism from the more normal and well-known dark respiration (see Black, 1973; Laetsch, 1974).

Extensive researches on various aspects of carbon metabolism in tropical and temperate plants, done during the last few decades, have brought to light hitherto unknown features of vital signif­icance not only in plant physiology, anatomy and morphology, but also plant productivity, ecology, and competition.

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It is now known that higher green plants can be broadly classified into three major groups on the basis of their net CO₂ uptake mechanism, viz., (a) those which primarily utilize the reductive pentose phosphate pathway, such as most temperate plants; (b) those which primarily utilize the C₄-dicarboxylic acid cycle, e.g, sugar cane and certain tropical plants, especially tropical grasses; and (c) those which exhibit the Crassulacean acid metabolism, e.g., some succulents.

Leaves of a given plant may exhibit at least one of these pathways. A plant exhibiting a particular one of these pathways generally also exhibits a characteristical­ly distinctive leaf anatomy.

The pentose plants or C₃ plants (category A) have a diffuse distribution of organelles in their leaf mesophyll or palisade cells and generally lack distinct bundle sheaths around their leaf vascular bundles.

The leaves of C₄ plants (category B) possess a distinct layer of bundle sheath cells and these prominent cells have a high concentration of organelles. Those of CAM plants (category C) have fleshy leaves that have a spongy appearance in cross-section, with mesophyll cells having large vacuoles and the organelles being evenly distributed in the thin cytoplasm. These plants are devoid of a definite layer of palisade cells.

When examined in a light or electron microscope, the bundle sheath cells of neither A nor C plants show any extensive development of chloroplasts, mitochondria, per­oxisomes or other sub cellular organelles. On the other hand, C₄ plants show a high concentration of these organelles in their bundle sheath cells and often have dimorphic chloroplasts, one kind being located in mesophyll cells and the other in bundle sheath cells.

In pentose plants the chief carboxylating enzyme in light is the ribulose diphosphate carboxylase; in C₄ plants it is phospho enol pyruvate carboxy­lase; and in CAM plants, both these enzymes occur more or less in equal proportions.

The C₄ plants are much more efficient photosynthetically at optimum day temperatures of 30-45°C, and at relatively higher light intensi­ties, and they lack photorespiration.

This is why most tropical plants, espe­cially grasses, are in general more highly productive than most temperate plants. The CO₂ compensation concentration is much lesser (0-10 ppm) in C₄ plants. The CAM plants seem to be intermediates between C₃ and C₄ plants with respect to carbon fixation and productive potential.

The mostly tropical C₄ plants are widespread in xerophytic habitats. Though concentrated primarily in the Gramineae and Caryophyllales, it is now known that the following families also have at least some C₄ species: Amaranthaceae, Aizoaceae, Chenopodiaceae, Compositae, Cyperaceae, Euphorbiaceae, Portulacaceae, Nyctaginaceae, and Zygophyllaceae.

According to Laetsch (1974), the CAM plants seem adapted to con­stantly arid environments whereas C₄ plants have apparently evolved in regions of intermittent aridity Both C₄ plants and CAM plants are well- represented in the flora of saline and brackish habitats and both these groups of plants have obviously evolved and adapted in response to selective pres­sures prevalent in xeric and saline habitats.

It may well be that the evolution of structural and functional adaptations to these environments was pre- adaptive with regard to the evolution of efficient methods for the assimila­tion and retention of carbon (Laetsch, 1974). In fact, the most remarkable characteristic of C₄ plants is their greater maximum capacity for net CO₂ uptake and a more efficient utilization of the available water as compared to the pentose plants.

The transpiration rates of many C₄ grasses are much lower than their C₄ counterparts, and the evolution of a highly efficient carbon-trapping system in the vicinity of the vascular bundle sheath results in rapid accumulation of dry matter, and relatively low water loss. The ability to grow rapidly seems to be responsible for the observed capacity of C₄ plants to compete successfully with mesophytes (Laetsch, 1974).

Although the collective assemblage of morphological and physiological characters of C₄ plants are quite distinct from those of the C₃ plants, never­theless, it has been possible to produce hybrids between the C₄ species Atriplex rosea and the C₃ A. patula. The F₂ and F₃ generations of these hybrids manifested various morphological and functional attributes interme­diate between the two parents. Possibly, the CAM plants may be considered to have originated by such hybridization occurring in nature during their evolutionary history.

It summarizes the currently accepted views on metabolic compartmentation of biochemical reactions between the chloroplasts of bundle sheath cells and mesophyll cells of a C₄ plant.

In a comprehensive mathematical analysis of net primary productivity of grasslands and forest ecosystems of Gujarat (India), Pandeya (1974) concluded that the values of total net primary production in the grazing lands per m²/year are considerably higher than the respective values for grazing lands in temperate countries; in other words, the tropical grasslands are much more productive than temperate.

The physiological explanation for this disparity is thought to lie in the fact that the tropical grasses possess the C₄ dicarboxylic acid pathway of photosynthetic CO₂ fixation (the Hatch- Slack Cycle), lack photorespiration, and are photosynthetically highly effi­cient, whereas the temperate grasses have the C₃ pathway (Calvin Cycle), possess photorespiration, and are less efficient photosynthetically.

However, another similar study, by Caldwell (1974), on the comparative product­ivities of a C₄ plant (Atriplex confertifolia) and a C₃ plant (Ceratoides lanata) has led to the inference that the higher observed productivity of the Atriplex -dominated community may not necessarily depend on the C₄ path­way but may rather be due to the tendency of Atriplex to support higher quantities of foliage through a major part of the year and also to maintain a low but positive CO₂ balance for a greater proportion of the year. Compara­tive values for carbon fluxes for the two kinds of cool desert shrub community respectively dominated by Atriplex and Ceratoides are given.

However, it should be clearly understood that the C₃ and C₄ plants do not necessarily have distinct and dissimilar methods of carbon assimilation (Moore, 1974). Even though the initial reaction of CO₂ in C₄ plants is with phosphoenolpyruvate to form oxaloacetate, this is followed by a second fixation process, involving release of CO₂ and carbon acids within the bun­dle sheath cells where it is refixed in the chloroplasts by the C₃ pathway.

The chief significance of the C₄ pathway lies in its usual operation at very low CO₂ concentrations: the C. reactions seem merely to concentrate the ambient CO₂ in the mesophyll cells before passing it on to the bundle sheaths. The C₄ process of CO₂ fixation may not constitute an alternative to the Calvin Cycle but may be just an additional device which can be of greater utility under certain environmental conditions.

Although it is generally believed that the C₄ mechanism is the basis of higher primary production rate in tropical plants, not all workers accept this. These dissenters think that at least some of the very high forage yields reported from some tropical grasses, e.g., Pennisetum may be due to the longer growth season rather than the higher short-term production efficiency of these species (see Moore, 1974).

Laetsch (1974) mentions that the C₄ process is commonest in such plants as are subjected to intermittent aridity; this is unlike C₄ assulacean acid metabolism plants which mostly experience permanently dry conditions. The main adaptive significance of the C₄ path­way of carbon fixation may well be the maintenance of growth in water- deficient conditions when open stomata may result in water stress (Moore, 1974).

Another finding of some ecological significance is the existence of plant species intermediate for C₃ and C₄ photosynthesis (Kennedy and Laetsch, 1974). Most species investigated were found to be either entirely C^ or entirely C₄ kind even though some species within the same genus could be C₃ and others C₄. Kennedy and Laetsch have shown that Mollugo verticillata (belonging to family Aizoaceae) possesses features intermediate between C₃ and C₄ plants in respect of leaf anatomy, cell ultrastructure, photorespiration and primary photosynthetic products.

There have been two chief patterns of approaches to ecology; viz., (1) the Ecosystem approach tends to study the interrelationships of the species in a community with regard to their roles in energy flow and nutrient cycles. The chief emphasis is on whole and beneficial relationships; (2) the Individ­ualistic or Organismic approach emphasizes that certain species play a key role in giving a distinctive character to a community, and that multispecies group mutualisms are important in this context (Richardson, 1980).

Indeed, Richardson sees mutualisms as integrating mechanisms for whole communities, and several field studies have established that mutualism is widespread and important to many population and community characteristics (Fleming and Heithans, 1981).