Assimilates accumulation

In this chapter we review recent developments, concentrating mostly on the effects of water stress because of its great importance, but trying where possible to identify general problems. As noted above, accumulation of ABA in leaves is common under stress, and we review its effects on gas exchange. If the sole direct effect of ABA were to reduce stomatal aperture, then assimilation rate would also decrease because of a lower intercellular partial pressure of C02,pi. Some experiments have suggested that pi is little affected by ABA and that the capacity for CO2 fixation has actually decreased. The situation has become complicated with the observation (Terashima et al., 1988) that application of ABA can cause non-uniform... 

To become intimately familiar with a process takes time. For a process engineer this may take two weeks or more, depending on the complexity of the system and the engineer s previous experience. This time is not reduced substantially by the presence of large computers. It is a period for assimilating and categorizing a large amount of accumulated information. 

Sometimes pollution is acceptable in small amounts. When Columbus landed in the New World there were an estimated 15,000 Indians living in the area that is now Ohio (1970 population 10,652,017). Did these Indians pollute the environment They made open tires, something that is now banned in all municipal areas, and they urinated and defecated at large. Still, since they were so few in number, the cleansing actions of the air, water and biosphere could rapidly assimilate then-wastes and no accumulating harm was done. In the strict sense of the word pollution, they did dirty the streams and anyone drinking the water shortly afterwards could contract the diseases whose infective agents were present in their wastes. However, with a little care this was unlikely, and their pollution was not considered bad. 

Bosma W, Kamminga G, De Kok LJ. 1990. Hydrogen sulfide-induced accumulation of sulfhydryl compounds in leaves of plants under field and laboratory exposure. In Rennenberg H et al., eds. Sulfur nutrition and sulfur assimilation in higher plants Fundamental environmental and agricultural aspects. The Hague, Netherlands SPB Academic Publishing, 173-175. 

Schindler, C. and H. K. Lichtenthaler. 1996. Photosynthetic C02-assimilation, chlorophyll fluorescence and zeaxanthin accumulation in field grown maple trees in the course of a sunny and a cloudy day. J. Plant. Physiol. 148 399-412. 

Hinesly, T.D., R.L. Jones, E.L. Ziegler, and J.J. Tyler. 1977. Effects of annual and accumulative applications of sewage sludge on assimilation of zinc and cadmium by com (Zea mays L.). Environ. Sci. Technol. 11 182-188. 

In humans the clearance rate of Hb is higher than that of HpHb (L4, Lll). Murray et al. (M6) found this also to hold for rabbits and, by studying the elimination in nephrectomized animals, they also proved that the difference was not due to urinary loss of Hb. Analysis of the organs proved that the HpHb complex and Hb were assimilated mainly in the liver and were catabolized with an early reappearance of the iron as transferrin iron within 30 minutes. The free Hb accumulated also in the tubular cells of the kidneys. No data have been published suggesting that the spleen is of any appreciable importance in this respect. No typical exponential clearance of the HpHb complex from plasma was observed (L10, Lll) in the first few experiments. Lathem and Worley (L4) found that HpHb disappeared at a simple exponential rate in 5... 

The bioavailability of selenium to a benthic deposit-feeding bivalve, Macoma balthica from particulate and dissolved phases was determined from AE data. The selenium concentration in the animals collected from San Francisco Bay was very close to that predicted by a model based on the laboratory AE studies of radiolabelled selenium from both particulate and solute sources. Uptake was found to be largely derived from particulate material [93]. The selenium occurs as selenite in the dissolved phase, and is taken up linearly with concentration. However, the particle-associated selenium as organoselenium and even elemental selenium is accumulated at much higher levels. The efficiency of uptake from the sediment of particulate radiolabelled selenium was 22%. This contrasts with an absorption efficiency of ca. 86% of organoselenium when this was fed as diatoms - the major food source of the clam. The experiments demonstrated the importance of particles in the uptake of pollutants and their transfer through the food web to molluscs, but the mode of assimilation was not discussed. 

Under certain conditions in fresh and wastewaters, essentially all the carbon is mineralized, and little or none accumulates in the biomass. This is unexplained because mineralization generates energy, and the metabolic pathways leading to the formation of C02 are assumed to involve biochemical sequences that result in carbon assimilation. The following are some examples reported by several workers [40,47,57,87] ... 

The metabolism of sulphur plays an important role in metal response, since most chelators, including metallothioneins and phytochelatins, contain this element. APSl encodes an ATP sulfurylase and Brassica juncea has been engineered to constitutively express this gene (Pilon-Smits et al., 1999). The plants showed increased S assimilation, higher glutathione (GSH) levels and were more tolerant to Se, accumulating 2-fold higher Se levels in the shoots. 

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