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).