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BOOKS REVIEWS     
Book reviewed in this article:
Archeology: Computer Analysis of Chronological Seriation . FRANK HOLE and MARY SHAW
Archeology: Archéologie et calculateurs: problèmes sémi-ologiques et mathématiques . CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE  相似文献   
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This study was undertaken to determine which of the two NO3? fluxes (influx or efflux) across plasma membranes of root cells is the target of those amino acids which have been shown to inhibit net NO3? uptake (Muller & Touraine 1992, Journal of Experimental Botany 43 , 617–623). Parallel experiments were performed to mea-sure either the time course of 15NO3? release from roots of soybean seedlings previously labelled with this isotope into non-labelled solution, or the time course of 15N accumulation from labelled 15NO3? solution in non-labelled seedlings. Focusing on the fate of 15NO3? in the cytoplasmic compartment, a model is developed to describe the time courses of the accumulation and release of tracer across the plasma membranes of root cells. Both time courses can be described by the sum of an exponential plus a linear term. In our material, the linear part of the accumulation time course is obscured by the NO3? fluxes exiting the cytoplasm, and the curve thus appears to be quasilinear over several minutes. However, we show that the use of the net tracer accumulation rate during this time period as an estimate of NO3? influx does not provide accurate estimates of influx and efflux. By contrast, 15NO3? efflux analysis permits calculation of the unidirectional fluxes across plasma membranes of root cells and the kinetic parameters of the cytoplasmic NO3? pool. Under our experimental conditions, efflux accounted for 30 to 50% of influx, and the cytoplasmic NO3? content was found to be in the 70–400nmol g?1 fw range. Using this methodology, the effect of amino acid accumulation on unidirectional fluxes of nitrate was then examined. Pretreatments of the seedlings with an amino acid which has been shown to inhibit net NO3? uptake led to concomitant decreases in net accumulation rates of 15NO3? and of reduced 15N in roots and total 15N in cotyledons. NO3? influx was markedly inhibited by these treatments, while NO3? efflux remained essentially unaffected, or even decreased. It is concluded that the target of the regulation of NO3? uptake by phloemtranslocated amino acids is the influx system.  相似文献   
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Dark CO2-fixation in guard cells of Vicia faba was much moresensitive to ammonium than in mesophyll cells. Addition of ammonium(5.0 mol m–3; pH0 7.6) caused up to a 7-fold increasein dark CO2-fixation rates in guard cell protoplasts (GCP),whereas in leaf slices, mesophyll cells, and mesophyll protoplaststhe increase was only about 1.4-fold. In both cell or tissuetypes, total CO2-fixation rates were higher in the light (2–12-foldhigher in GCP and 28-fold in mesophyll); these rates were onlyslightly changed by ammonium treatment. However, separationof 14C-labelled products after fixation of CO2 in the lightby GCP revealed a large ammonium-induced shift in carbon flowfrom starch and sugars to typical products of C4-metabolism(mainly malate and aspartate). In contrast, in mesophyll cellsamino acid and malate labelling was only moderately increasedby ammonium at the expense of sucrose. The data suggest thatin vivo ammonium might facilitate stomatal opening and/or delaystomatal closing through an increased production of organicacids. Key words: PEP-carboxylation, guard cell protoplasts, ammonium, fusicoccin  相似文献   
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Western Atlantic coral reefs were differentially affected bya mass bleaching (discoloration) event in 1987. We periodicallyassessed the "appearance" of zooxanthellate organisms betweenDecember 1987 and June 1988 at nine conspicuously affected sitesin the Bahamas, Florida, St. Croix, and Venezuela, using a standardizedpoint-count technique. Three to four months after the localinitiation of the event, the "bleached" state was still presentin one to three of the most abundant reef coral taxa and ina few of the less common species (n = 5 sites). "Recovery" occurredsomewhat faster at shallower depths, at least in the Bahamasand Florida. Scleractinian corals which were "prolonged bleachers"had foliaceous or massive, rather than branching, morphologies."Bleached" points disappeared from the point counts after $6to $8 months. Long-term field data on spatial and temporal variability inthe dynamics of zooxanthellate organisms would help us to understandthe ecological consequences of bleaching. More generally, weneed to distinguish anthropogenic changes in the structure andfunctioning of reef ecosystems from those which occur naturally.Point-count techniques are well suited for collaborative studiesinvolving rapid quantification of coloration states and healthin reef corals.  相似文献   
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