In this article we will discuss about the problem of nutrition in photosynthesis.
Every year plant food production of the world is nearly 1.4 x 109 tons of dry matter. This estimate includes cereals, starch, roots, sugar and oil seeds but did not take into account vegetables and fruits. In other words, plant production thus covers only about 1.3% of the total photosynthetic production of the land area. Comparatively sea contributes nearly 1% to world food production.
By conservative estimate one man needs about 1 million kcal of food per year. This is equivalent to 250 kg of carbohydrates. From the energy standpoint world harvest is sufficient to supply about 5.6 billion people. It is important to increase the soil productivity on the world-wide basis.
We need to achieve high food production targets by increased use of fertilizers, improved agricultural methods, use of biocides, planned irrigation facilities. All these factors will optimize photosynthesis.
It is imperative to take up a physiologically adjusted and varied supply of plant species and, to test available potential intensively in the fields for growing ability, productivity, protein composition, quality, storage conditions, etc.
Photosynthesis, its Redirection towards Production of fuel:
Figure 13-40 shows two diagrams which sum up schematically how the natural process of photosynthesis operates and how it is being redirected towards the production of fuel. Note a single chloroplast with its heart having ‘reaction centre’ consisting of a single molecule.
When a proton of light impacts a reaction centre, it sets in motion a series of events that within five minutes or so, lead to the synthesis of carbohydrates (left diagram). On the left side of this figure is given the diagram showing what the scientists are now engaged in redirecting this process towards generation of hydrogen gas. To produce hydrogen, enzyme hydrogenase is introduced. This enzyme is extracted from photosynthetic bacteria or from algae.
Even though the precise working of hydrogenase is still not clear, it induces the electrons and hydrogen ions extracted from water to link up and form hydrogen gas. Yet another promising line being followed is to re-create the bare essentials of the photosynthetic apparatus with synthetic chemicals that mimic the abilities of chlorophyll and dehydrogenase to generate hydrogen.
A chemist, Hertha and Gerhard Sprintschnik at the University of North Carolina (USA) have successfully fabricated an apparatus for the production of hydrogen photochemically. Here the synthetic compounds take the place of chlorophyll and dehydrogenase.
The apparatus is fitted with mirrors to direct sunlight into a glass vessel which holds the glass slides covered with a chemical compound containing the light-sensitive element ruthenium.
The compound absorbs light, becomes activated and splits water into oxygen and hydrogen, bubbles of which rise to the surface. It may be added that this apparatus is a forerunner of a large solar-powered hydrogen generator.
Dr. Malvin Calvin tried to create an energy- producing synthetic membrane that could duplicate plant’s photosynthetic process. This Nobel laureate researched on some Euphorbia species in which latex contains as much as 30-40 per cent hydrocarbons and are similar to crude oil in several oils.
Thehydrocarbons from this plant have a molecular weight in the order of 20,000. Once water is removed from this hydrocarbon it feels like oil. It is estimated that a hectare of metre, tall plants like those of Euphorbia species could produce the equivalent of 50 barrels of crude oil annually.
Further, the latex carries stored solar energy and with the greatest sunshine provides the best prospects for hydrocarbon plantations. There is a good possibility that some of the plants growing in rocky and substandard soil are plants which combine hardiness, hydrocarbons and are efficient to capture maximum sunlight.
Ptotosynthesis and Food Problem:
It is surmised that by the end of 2000 A.D. the population increase in the world will demand doubling of cereal grain and quadrupling grain legume production. In order to achieve this goal the production of cereal grain and legumes has to increase by three and six per cent, respectively. So far the increase in crop productivity occurred through conventional approaches of plant breeders and agronomists.
In recent years plant physiology, microbiology and biochemistry are increasingly being employed to gain further increase in crop productivity. Consequently basic processes concerned with carbon fixation or photosynthesis are gaining increased attention.
Biochemical, biophysical and physiological constraints on photosynthesis in relation to crop productivity are being increasingly studied. In the following we shall discuss some of these aspects briefly:
Rate of Photosynthesis:
This is a complex character which can be estimated either by measuring the total photosynthetic area or the rate of photosynthesis per unit area. In fact some workers have established a correlation between the photosynthetic area and the rate of photosynthesis per unit surface. This is best exemplified in wheat where diploid species have small leaf area and high rate of photosynthesis than the tetraploid and hexaploid species having large area.
Further, expansion of leaf area was also very significant than the rate of leaf photosynthesis. However, high yielding ability is associated with higher photosynthetic rate as in soybean. Much information has appeared on differences as regards rate of photosynthesis in different varieties of rice, maize, wheat, pea and some beans. In rice and maize the varietal differences range from 50 to 200 per cent, respectively.
The selection of varieties provides a suitable approach for increasing production. Available literature also shows that in a suitable environment, rate of photosynthesis is higher in C4 plants, when compared with C3 plants.
Some attempts have also been made to make a cross between these two categories of plants. So far no success has been achieved in this direction. Singh et al., (1980) have discussed the photosynthetic efficiency and its prospects of genetic modifications. According to these workers the rate of net photosynthesis is limited by the rate of CO2 uptake and O2 evolution due to photorespiration in wheat.
CO2 Limitation:
CO2 concentration is a definite limiting factor for attaining optimal level of photosynthesis. With an increase in CO2 concentration, 2 to 3 times over the normal ambient value the crop yield also increases. In grain legumes and cereals such an increase ranges from 50 to 15 per cent respectively over the control.
Even though the results are excellent yet impractical under prevailing agronomic conditions. CO2 increase has been shown to delay senescence by a week or so and the latter character can be successfully exploited for enhanced output. In wheat with delay in senescence, yield was likely to increase by 3 per cent per day.
Efficiency of Conversion of Solar Energy:
It is a daily experience that much of the solar energy is wasted in several ways by the time it reaches canopy level and reduction of CO2 to carbohydrates takes place. Little more than 60 per cent of sunlight is absorbed by photosynthetically active pigments.
Further, reflection by the leaf etc. also adds to the general loss of sunlight (Fig. 13-41). In India, maximum light is available during April to June. However, during these months water is a limiting factor to capture solar energy and converting it to biochemical energy.
Photorespiration:
Lot of energy is wasted in achieving light driven process. The interaction between photosynthesis and photorespiration is mediated by RuBP carboxylase. Initially in photosynthesis RuBP is carboxylated while in photorespiration it is oxygenated. Photorespiration can be reduced by decreasing the phosphoglycolate synthesis by inhibiting RuBP oxygenase; or by checking the conversion of phosphoglycolate to CO2 or by refixing the photorespiratory CO2. It may be added that this suggestion may not be a practical solution to inhibit photorespiration since it may decrease RuBP and photosynthesis.
Thus, in practice if photorespiration is to be checked, then oxygenase activity of RuBP carboxylase must be attacked either by changing pH or temperature. Fortunately in C4 plants there is no loss due to photorespiration because of the specialized anatomy, presence of unique enzymes and cellular compartmentation of these enzymes.
There is a remote hope that perhaps through genetic manipulation C3 species could be converted into C, species. The study of C3 into C4 crosses has brought out some interesting points, for instance, the inheritance of leaf anatomy and biochemical apparatus is complex.
Further simple mutagenesis may not be an important factor in converting a C3 species into C4 species. It may be added that some plants e.g. Panicummillioides have Kranz anatomy and low photorespiration but has height CO2 compensation concentration.
It also exhibits CO2 inhibition of RuBP carboxylase activity. In the remote past a C3 species might have undergone mutation and due to interspecific hybridization between C3 and C4 species such species arose. One conclusion which can be drawn is that in nature species with C4 and C3 combination of characters could be produced which will improve the photosynthetic efficiency.
Harvest Index:
In evaluating crop production, biological and economic yields are commonly employed terms. The former refers to production of total dry matter whereas the latter takes into account the economically important part of the biological yield.
Harvest index refers to the ratio of economic yield to biological yield. In a large number of cereals, improvement in grain yield is attributed to the enhanced harvest index. Studies on the relationship between photosynthesis and harvest index. Studies on the relationship between photosynthesis and harvest index may cause maximization of seed yield in grain crops.
Total dry matter yield is a consequence of crop canopy efficiency in intercepting and utilizing the solar radiation variable during the growing season. Leaves are the main plant organs which intercept solar energy. For maximum crop growth rates, sufficient leaves must be available in the canopy to intercept most of the solar radiation incident on the crop canopy.
When this takes place the level of crop photosynthetic efficiency or CGR is determined by the photosynthetic efficiency of leaves or the NAR. The efficiency of net assimilation rate can be affected by the extent of solar radiation, the efficiency of leaves to photosynthesize, the LEAF AREA INDEX (LAI), how evenly the solar radiation level is divided among leaf surfaces, and the extent of plant respiration.
It may be noticed that crop plants do not maintain a critical LAI over the whole growing season. In the annual plants, leaf area accumulation begins from seedling stage, and radiation interception by the canopy is almost zero.
But the LAI increases and ultimately intercepts most of the solar radiation. After total ground cover is attained, total dry matter production is a factor of how long the crop can maintain an active, green leafy canopy.
Some strategies have been proposed to maximize solar radiation utilization and hence crop yields and these are:
i. Early planting for rapid LA development. This requires development of varieties which are frost resistant and can withstand cold temperatures.
ii. Use of seeding rate which will develop an optimal LAI at the maximum leaf area development.
iii. Planting at a given time when total ground cover could be obtained at the maximum solar energy levels.
iv. Undertake uniform planting over the land to avoid early interplant competition and enhance the rate of solar radiation interception.
v. Use fertilizers to enhance the rate of growth and photosynthetic efficiency of leaf surfaces.
vi. Extending the leaf area duration through PGRs (e.g. cytokinins) for maximum radiation interception.
Use of Unconventional Photosynthates:
Process of photosynthesis fixes carbon in the form of sugars, starch and cellulose. Of these maximum carbon fixation is in the form of cellulose which cannot be used as a food by men. In several countries research is going on to convert cellulose into some type of food.
For instance, cellulose could be converted into glucose, glucose-fructose syrup etc. through enzymes. However, acid hydrolysis is not a well-accepted process. On the contrary, alkali treatment of cellulose has been widely utilized for ruminant feeding. Some attempts have also been made to bring about enzymatic hydrolysis of starch through amylases.
We need to know more about the precise basic processes operative during photosynthesis. As yet not much information is available with regard to several processes including light mediated reactions.