Increasing Photosynthetic Efficiency

Photosynthesis typically only captures the energy from about 1% of incident light, leaving ample room for improvement of biomass yields, culture density, and growth rates. Several strategies for increasing photosynthetic efficiency have been employed in algae, as strains with modified photosynthetic apparatuses are quick to make and can be rapidly screened and characterized, at least compared to terrestrial plants. 

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This set of reactions is very detrimental to the efficiency of photosynthesis. Oxygen is added to carbon, CO2 is lost, energy is consumed, and ribulose bisphosphate is destroyed. For a plant to be able to increase the discrimination of Rubisco for CO2 would obviously be advantageous, but that hasn t happened, either naturally or through the efforts of scientists. An increased concentration of CO2 in the atmosphere may lead to increased photosynthesis and decreased photorespiration, but high CO2 concentrations would also contribute to global warming (and the increased photosynthetic carbon fixation would not likely reduce the amount of CO2 in any event). 

The acquisition of carbon is strongly modulated by the surface area of photosynthesizing leaves hence, understanding leaf area development is germane to efforts to increase yield. In many crops, biomass is linearly related to the amount of light intercepted (Monteith, 1977). This is certainly the case for Jerusalem artichoke, where total productivity is strongly correlated with the amount of solar radiation intercepted (Denoroy, 1996 Meijer et al., 1993). Leaf area, leaf duration, and photosynthetic efficiency of the crop canopy determine how much light is intercepted and subsequently utilized (Table 10.9). 

The use of water as the electron donor significantly increases the efficiency of photosynthetic ATP synthesis because the generation of one molecule of oxygen is accompanied not only by the release of four electrons (e ), but also by the release of four protons on one side of the membrane. Thus, an additional proton is released for each proton equivalent produced by the initial electron-transfer process, so twice as many protons are available to drive ATP synthesis. Oxygen generation will be considered in Chapter 19. 

An increase in the photosynthetic efficiences for C02 fixation and H2 production through accessory (antenna) pigment reduction ... 

A measure of photosynthetic efficiency and productivity can be obtained from the light-saturation curve of photosynthesis. This type of analysis is necessary and sufficient for the measurement of the vital signs of photosynthesis [Melis et al. 1999], In such measurements, the rate of O2 evolution, when plotted as a function of irradiance, first increases linearly and then levels off as the saturating irradiance (Is) is approached [Neale et al. 1993]. The slope of the... 

The data show that SSHB increased the root dry matter in the same way it did in the other experiments. Simultaneously, we observed an increase in leaf length and leaf area which corresponded with an increase in invertase activity. The higher enzyme activity appeared to be the key to the whole problem. It is well known that acid invertase splits the disaccharide sucrose so that the two monosaccharides formed from it can become building stones for additional leaf substances. That is, glucose is recycled through the Embden-Meyerhof-Parnas pathway. Likewise, fructose is readily available for conversion into fructose 1,6-diphosphate. Subsequently, more leaf area arises which is able to produce more assimilates. The consequence is an increased yield. Thus, photosynthetic efficiency increases considerably. 

In wheat, the ear weights of both tillers and the main stem were increased, in comparison with untreated controls by brassinolide (BR) sprays (I). In corn, BR treatment had significant influence on the length of the unfertile tip portion of the ear and on the number of vacant kernels. The yield increased compared with untreated controls (2). Little is known of the effect of BR on the ripening of rice plants, and the mode of action of BR on the ripening of rice has not been studied. To increase crop yield, it is necessary to improve the photosynthetic efficiency and promote the translocation of photosynthetic products in the panicles. Therefore, we assessed the ripening of rice, the translocation of photosynthetic products, and plant hormone levels in panicles. 

Predictions of changes in productivity are difficult in part because a major cause of probable climate change is increasing atmospheric carbon dioxide, which is also the raw material for photosynthesis. Enriching the atmosphere with CO2 could potentially speed rates of photosynthesis and, thereby, increase agricultural production. Indeed, laboratory experiments in which CO2 concentrations were increased from 300 to 600 ppm, increased photosynthetic rates by 20% in maize and 60% in wheat (Akita and Moss, 1973). Laboratory experiments also show that enrichment with CO2 increases the growth of root shoots and increases tire efficiency of water use. 

The use of water as the electron donor significantly increases the efficiency of photosynthetic ATP synthesis because the generation of one molecule of oxygen is accompanied not only by the release of four electrons (e ), but... 

The increased formation of solanine in potatoes exposed to light is a familiar fact and occurs over a wide range of wavelengths (20). Its glycoside constitution might appear to offer an explanation of the effect, but the most effective radiation appears to lie in the ultraviolet rather than in the photosynthetically efficient ranges. Conner (21) found 171% increase over controls with wavelengths around 300 m i and relatively little with visible radiation. 

Biological response Unlike all the other enrichment experiments, IRONEX I did not stimulate a sustained phytoplankton bloom. The biological response, consisting of increases in photosynthetic efficiency, primary productivity, and chlorophyll a, was largely confined to an extant... 

Figure 15 Curve-fit of the modei of Jassby and Piatt to the photosynthesis response on increasing photon flux density as measured for the green microaiga Chlamydomonas reinhardtii. Data derived from measurements by Vejrazka C Janssen M, Benvenuti G, etal Photosynthetic efficiency and oxygen evolution of Chiamydomonas reinhardtii under continuous and flashing light, Appi Microbioi Biotechnoi 97(4) 523-1532,2013.

The enzyme Rubisco is a target for genetic manipulation because it catalyses a rate limiting step in photosynthesis- Moreover Rubisco is inefficient in that not only does it have a low catalytic rate but also catalyses a wasteful reaction with oxygen. Consequently researchers hope that by modification of genes for Rubisco the enzyme s characteristics may be improved, leading to a significant increase in photosynthetic efficiency. 

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