Relationship with chlorophyll

The influence of sunlight on vegetable growth, and the results of etiolation are, of course, well known to botanical students. There is no room for doubt that the production and evolution of the odour-bearing constituents of a plant are in direct relationship with the chlorophyll... 

Figure 12.9 Relationship between chlorophyll-a ( ig L 1) and particulate thiol (nM) in upper and lower regions of Galveston Bay (UGB and LGB). Stations with open circles (river and seawater end-members) were not used in the regression. (Modified from Tang et al., 2000).

Should it be definitively proven that the plastids are not formed anew in the egg cells, then their relationship to the organism that contains them would more or less remind us of a symbiosis. It is possible that the green plants indeed owe their origin to the union of a colourless organism with one evenly stained with chlorophyll . 

Publications of Mees, Homer and Tomlinson in the 1960s on general herbicidal properties indicated that the phytotoxic action is connected with chlorophyll and light. The authors presumed, on the basis of the relationships between the reducibility of the single compounds and the phytotoxic action, that a reduction to a stable free radical occurs in the plant and that this free radical is responsible for phytotoxic action. The redox potentials of -0.446 and 0.346 mV of paraquat and diquat, respectively, are ensured by the reduction potential of light reaction I of photosynthesis (Calderbank, 1968). 

Shetlar has derived non-linear relationships of the Stem-Volmer type suitable for systems in which quenching occurs by more than one mechanism. Rate constants for heavy atom fluorescence quenching of polynuclear aromatic hydrocarbons by 1-iodopropane in benzene have been found to decrease exponentially with the energy difference between the fluorescing state and the nearest lower triplet state (Dreeskamp et al.). Bromocyclopropane has been recommended as a heavy-atom quencher of excited singlet states since it is more photostable than simple alkyl bromides (Flemming, Quina, and Hammond). Laser studies with chlorophyll a have provided evidence for the interesting radiationless intermolecular process Tx + Sx -> T2 + S0 (Menzel). 

The pathway from Mg-protoporphyrin IX to the chlorophylls is still not known in complete detail. For that reason we must be content with mentioning the names of a few probable intermediates (Fig. 132). The biogenetic relationship between chlorophylls a and b is also still a matter of dispute. Chlorophyll a may be converted into b, but it is also possible that the biosynthetic pathways separate from each other earlier, at the state of protochlorophyllide a. For physiologists, an important point about these last stages of the route to the chlorophylls is that the biosynthesis of the chlorophylls is light-dependent in higher plants. The light-controlled reaction is the conversion of protochlorophyllide a into chlorophyllide a. 

In this way, the near-linear chlorophyll-phosphorus relationship in lakes depends upon the outcome of a large number of interactive processes occurring in each one of the component systems in the model. One of the most intriguing aspects of those components is that the chlorophyll models do not need to take account of the species composition of the phytoplankton in which chlorophyll is a constituent. The development of blooms of potentially toxic cyanobacteria is associated with eutrophication and phosphorus concentration, yet it is not apparent that the yield of cyanobacterial biomass requires any more mass-specific contribution from phosphorus. The explanation for this paradox is not well understood, but it is extremely important to understand that it is a matter of dynamics. The bloom-forming cyanobacteria are among the slowest-growing and most light-sensitive members of the phytoplankton. ... 

Hemin is shown on the right in Figure 22-7. It is shown beside the model of chlorophyll A to emphasize the astonishing similarity. The portions within dotted lines identify the differences. Except for the central metal atom, tl.e differences are all on the periphery of these cumbersome molecules. We cannot help wondering how nature managed to standardize on this molecular skeleton for molecules with such different functions. We cannot avoid a feeling of impatience as we await the clarification of the possible relationship, a clarification that will surely be provided by scientists of the next generation. 

The analysis of carotenoid identity, conformation, and binding in vivo should allow further progress to be made in understanding of the functions of these pigments in the photosynthetic machinery. One of the obvious steps toward improvement could be the use of continuously tuneable laser systems in order to obtain more detailed resonance Raman excitation profiles (Sashima et al 2000). This technique will be suitable for the investigation of in vivo systems with more complex carotenoid composition. In addition, this method may be applied for the determination of the energy of forbidden Sj or 2 Ag transition. This is an important parameter, since it allows an assessment of the energy transfer relationship between the carotenoids and chlorophylls within the antenna complex. 

The conversions conducted in both steps are currently based on empirical relationships that are more or less robust. For example, the relationship between the chlorophyll and carbon content in an average phytoplankton cell is dependent on factors that influence cell metabolism, including nutrient arailability, temperature, and light. The temperature dependence of photosynthesis is associated with an enzyme-mediated step in the Calvin cycle (Figure 7.6a). 

The similarity between the structures of the corrinoids and the porphyrins becomes evident from comparison of cobyrinic acid (75) (the simplest of the corronoids so far isolated) with uroporphyrinogen III (70). The possibility of a biosynthetic relationship between these structures was suggested by Shemin, who reported the incorporation of [14C]ALA into vitamin Bn and confirmed by the subsequent demonstration that PBG was also incorporated. The ubiquitous precursorial role of uroporphyrinogen III in heme, chlorophyll and corrinoid biosynthesis proposed by Porra (65BBA(107)176) was, however, not substantiated by experimental evidence until much later, when under carefully controlled conditions cells of Propionibacterium shermanii were shown to incorporate radioactivity from [14C]uroporphyrinogen III into vitamin Bn (72JA8269). 

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