Apparent photosynthesis

Acute Inhibition of Apparent Photosynthesis by Phytotoxic Air Pollutants... 

Sublethal plant exposures to a number of phytotoxic air pollutants can cause the reversible suppression of one of life s most basic processes— photosynthesis. The possibility of plant growth suppression by atmospheric pollution is a concern of many people. We need to know if subnecrotic pollutant exposures that may occur in ambient air can repress photosynthesis rates sufficiently to cause significant retardation of plant growth. Some insight into the capability for short-term exposures to HF, CI2, O3, S029 NO2 and NO— applied singly and as dual pollutant mixtures— to suppress apparent photosynthesis rates of several important crop species is presented here. 

Given these restrictions, oat, barley and alfalfa canopies required treatments with more than 1 pphm HF, about 5 pphm O3 or CI2, 20 pphm SO2, and 40-60 pphm NO2 or NO before apparent photosynthesis rates were measurably depressed by the end of 2 hr of exposure. Above these apparent threshold values the 2-hr depressions induced were linearly related to pollutant concentrations applied up to those that caused visible foliar injury to the tissues. Foliar necrosis occurred to some plant tissues within the canopies exposed to approximately 15 pphm HF,... 

Table II includes supporting data for greater-than-additive inhibition of alfalfa apparent photosynthetic rates induced by SO2+NO2 mixtures. The enhanced effects were most marked at the lower concentrations applied, becoming less pronounced as pollutant levels were raised. At 50 pphm of each gas no synergism was evident. At this SO2 exposure concentration, sulfur dioxide appeared to regulate the observed plant responses. Significant amounts of inhibition resulted from the lowest bipollutant concentrations used (15 pphm of each gas) these concentrations were well below those required for the individual pollutants to measurably suppress apparent photosynthesis rates. At these exposure levels where no tissue necrosis occurred, the plants recovered completely within 2 hr after fumigation. The manner by which this inhibiting interaction occurred is not well understood. This pollutant combination is also known to act in a synergistic fashion to cause visible injury to plants, and further study of this mixture may be warranted.

Of the phytotoxic air pollutants and mixtures tested, O3 or combinations of SO2+NO2 are most likely to occur in ambient atmospheres in sufficiently high concentrations to acutely depress apparent photosynthesis. Ambient HP concentrations of the magnitudes which inhibited CO2 uptake rates in an acute, reversible manner would be rare. Studies into longer-term exposures (several days or weeks) to HP concentrations in the low ppb range have suggested that reduced photosynthesis under these conditions correlated with the amount of necrosis that developed (, ). 

Pollutants on Apparent Photosynthesis and Water Use by Citrus Trees, Environ. Sci. Technol. (1967) 1, 644. 

Watanabe T, Kondo N (1976) Isolation of a proteinaceous inhibitor of ethylene biosynthesis from marine algae. Agric Biol Chem 40 1877-1878 Webster PL (1967) Cell cycle kinetics in meristems Effects of colchicine and lAA. PhD Thesis, Case-Western Res Univ, Cleveland, Ohio Webster PL, Langenauer HD (1973) Experimental control of the activity of the quiescent centre in excised root tips of Zea mays. Planta 112 91-100 White KL, Hill AC, Bennett JH (1974) Synergistic inhibition of apparent photosynthesis rate of alfalfa by combinations of sulfur dioxide and nitrogen dioxide. Environ Sci Technol 8 574-576... 

Fig. 8. Freezing tolerance of soaked thalli of Vmbilicaria vellea indicated by maximum restoration of gross photosysnthesis in the postculture (% of normal). The thalli were cooled and rewarmed rapidly (—and gradually (—) in September ( A) and in December (O A). Gross photosynthetic rates were calculated by summing up the rates of apparent photosynthesis and 50% of the dark respiration. (From Kappen and Lange, 1972.)...

If a plant is illuminated in a closed container the carbon dioxide concentration inside the container eventually reaches a steady level which is known as the carbon dioxide compensation point. The final level represents the balance attained between carbon dioxide fixing and carbon dioxide releasing reactions. That carbon dioxide releasing reactions do occur is readily demonstrated if leaves which have previously been photosynthesising in labelled carbon dioxide are transferred to unlabelled carbon dioxide—labelled carbon dioxide can be detected in the atmosphere surrounding the leaves. This sort of experiment underlines the fact that when photosynthesis is measured as CO2 uptake or oxygen evolution the value obtained represents apparent photosynthesis—the balance between reactions resulting in... 

Of the various factors that cause redox disequilibria, the most effective are biologic activity (photosynthesis) and the metastable persistence of covalent complexes of light elements (C, H, O, N, S), whose bonds are particularly stable and difficult to break (Wolery, 1983). For the sake of completeness, we can also note that the apparent redox disequilibrium is sometimes actually attributable to analytical error or uncertainty (i.e., difficult determination of partial molalities of species, often extremely diluted) or even to error in speciation calculations (when using, for instance, the redox couple Fe /Fe, one must account for the fact that both Fe and Fe are partly bonded to anionic ligands so that their free ion partial molalities do not coincide with the bulk molality of the species). 

Iron is required by essentially all living organisms, participating in respiration, photosynthesis, and DNA synthesis. An apparent exception is certain members of the bacterial genus Lactobacillus (1). In general, the reactions mediated by iron proteins are electron transfer or activation of dioxygen or nitrogen. However, the very properties of iron which render it so... 

Finally, a promising trend in the study of the photoelectrochemical behavior of objects, whose nature is close to that of semiconductors, is related to photobiology, in particular to processes of light conversion in natural photosynthesizing objects. Certain elementary stages of photosynthesis, particularly photoelectrochemical ones, can, apparently, be simulated in some cases within the framework of the concepts of photoelectrochemistry of semiconductors. 

Racker and Schroeder (85) questioned the importance of the alkaline FDPase in photosynthesis because of its lack of activity at neutral pH, its apparent cytoplasmic localization, and the presence of a second enzyme or enzymes which appeared to be associated with the chloroplasts and which hydrolyzed both FDP and SDP. Later work, however, has clearly established the function of this enzyme in the photosynthetic carbon cycle. Smillie has shown that the alkaline FDPase is associated with photosynthetic tissues in higher plants and Euglena (101, 102). The enzyme was also shown to be localized in the chloroplasts and to be absent in nonphotosynthetic tissue or bleached algae. It was the only FDPase detected in the photosynthetic bacterium Chromatium grown under autotrophic conditions (102). Preiss et al. (103) have pointed... 

Distinct differences in cells with regard to the presence or absence of target structures or metabolic processes also offer opportunities for selectivity. Herbicides such as phenylureas, simazine, and so on, block the Hill reaction in chloroplasts, thereby killing plants without harm to animals. This is not always the case because paraquat, which blocks photosynthetic reactions in plants, is a pulmonary toxicant in mammals, due apparently to analogous free-radical reactions (see Figure 18.4) involving enzymes different from those involved in photosynthesis. 

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