摘要:

Abstract.The sieve‐tube elements of long‐lived arborescent monocotyledons which lack secondary thickening remain functional for many decades despite lacking a nucleus. A minimal requirement for transport by mass flow powered by the Munch mechanism is maintenance of semi‐permeability of the plasmalemma of the sieve tube elements; loading and re‐loading could be deputed to nucleate cells symplastically linked to the sieve elements. An additional requirement in the long‐term relates to replacement of components damaged by mechanical, chemical or radiation intrusions. Minimizing the damage from radiation and chemical agents can be related to a number of commonly observed features of sieve tubes. Damage from O2and radicals derived there from is minimized in these essentially aerobic cells by (1) the absence of intercellular gas spaces in the phloem combined with the lower O2solubility and diffusivity in concentrated disaccharide (or sugar alcohol) solutions, (2) the absence of photosynthetic machinery which could generate singlet oxygen, and (3) the presence of at least some components of scavenging mechanisms (glutathione, peroxidase, abscorbate). Non‐enzymic glycosylation (exacerbated by O2) of proteins is minimized by the low concentrations of reducing sugars in sieve‐tubes. Ultraviolet damage is minimized by UV‐absorbing materials between the plant surface and the sieve tubes, including the selerenchymatous cap of fibres on the vascular bundles. The extent to which repair involves symplastic import of polypeptides from nucleate companion cells, thus breaching the −800‐Da limit on symplastic transport, is unclear, but it could occur in fully differentiated companion cell‐sieve element associations without necessarily

ISSN:0140-7791    

DOI:10.1111/j.1365-3040.1991.tb01330.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

Abstract.The photosynthetic characteristics ofElodea nuttalliigrown in wastewater in continuous flow reactors in a greenhouse were investigated. The diurnal changes in dissolved inorganic carbon (DIC), dissolved oxygen (DO) and pH were monitored. Photosynthesis removed both CO2(aq) and HCO3−from the reactors. A stoichiometry of 1.19:1 was observed between HCO3−removal during photosynthesis and OH−production during photosynthesis, consistent with theories regarding direct bicarbonate utilization. In laboratory experiments, the light compensation points (гPPFD) were similar (31–35μmol m−2s−1) to reported values for other macrophytes; however, the light saturation level was high (1100μmol m−2s−1) and similar to values reported for aerial portions Of heterophyllous macrophytes. The kinetics of photosynthetic oxygen evolution (Km(CO2) = 96mmol m−3; Vmax= 133mmol g−1Chl h−1) and the CO2compensation point (г= 44cm3m−3) suggested an adaptive, low photorespiratory state in response to low carbon concentrations. Photosynthetic Vmaxvalues were slightly, but significantly higher (P0.001) at pH 8.0 compared to pH 4.5. While CO2utilization at pH 8 could account for most of the observed phototsynthetic rates, an HCO3−component was present, suggesting two separate transport systems for HCO3−and CO2(aq) inE. nuttallii.The activity of RUBISCO (160.3 mmol g−1Chl h−1was one of the highest reported values for aquatic macrophytes. Compared to RUBISCO, we observed lower activities of the β‐carboxylating enzymes phophoenolpyruvate carboyxlase (PEPcase), 24.1 mmol g−1Chl h−1; phosphorenolpyruvate carboxykinase (PEPCKase), 14 mmol g−1Chl h−1. This suggests that the potential light‐independent fixation of carbon inE. nuttalliiwas much less than RUBISCO‐dependent fixation. The RUBISCO/PEPcase ratio was 6.6, indicating thatE. nuttalliiwas similar toMyriophyllumsp. in possessing a physiological adaptation to low CO2levels which is hypothesized to include carbonic anhydrase (CA) and an active transport system for HCO3−. CA levels

ISSN:0140-7791    

DOI:10.1111/j.1365-3040.1991.tb01331.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

Abstract.Elodea nuttalliiwas grown under greenhouse conditions in domestic wastewater in an aquatic treatment system under conditions conducive to photorespiration. Initial research on the photosynthetic characteristics ofE. nuttalliisuggest that the submergent macrophyte possessed a carbon concentrating mechanism. Isotopic disequilibria H14CO3‐uptake studies (5‐80s) were used to assess the bicarbonate active‐transport capabilities inE. nuttalliileaves. Using a range of substrate concentrations (50‐50200mmol m−3), the accumulation of label (mmol g−1Chl) over time due to transport was found initially to exceed accumulation due to fixation until steady state rates were observed. Internal steady state pools of dissolved inorganic carbon (DIC) ranged from 40 to 80 mol m−3. The concentration factor (CF: the ratio of internal cyroplasmic (DIC] to external medium [DIC]) decreased from 800 to 114 as external bicarbonate concentrations were increased. Inhibition of transport by uncouplers (2,4‐dinitrophenol (DNP), carbonyl cyanide‐m‐chlorophenylhydrazone (CCCP)); ATPase inhibitors (dicylcohexocarbodiamide (DCCD), phloridzin, arsenate); electron transport inhibitors (DCMU, Antimycin A), and carbonic anhydrase inhibitors (ethoxyzolamide, acetazolamide) suggest that bicarbonate transport required (1) a proton motive force, (2) a functional ATPase, (3) a chloroplast carbon sink, and possibly (4) a CA‐like moiety associated with the transport protein. While plasmalemmasomes were not observed, the plasmalemma was vesiculated and acid and alkaline banding was observed when leaves were incubated under light in the presence of bicarbonate. These data are consistent with the operation of a bicarbonate‐cation symport which concentrates substrate against a concentration gradient at the expense of metabolic energy. The presence of an active transport system for bicarbonate ensures that internal carbon concentrations are high when carbon dioxide, is scarce and bicarbonate is the only carbon species available in aquatic treatment systems during photorespiratory conditions. Therefore,E. nuttalliiis particularly well suited f

ISSN:0140-7791    

DOI:10.1111/j.1365-3040.1991.tb01332.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

Abstract.Elodea nuttalliiwas grown in primary effluent from domestic wastewater under conditions in which CO2may be limiting. However, high photosynthetic inorganic carbon uptake rates have been reported forElodeaunder these conditions. In order to determine if leaf cell structure showed modifications which would support proposed models of a plasma membrane (PM) bicarbonate transport system and observed high photo‐synthetic rates, leaf cell ultrastructure was analysed using quantitative techniques (stereology). A Fold Index (FI) calculated for the PM showed that infolding increased surface area to 2.15 times that of an idealized cell of the same shape and size. Association Indices (Sa) showed a significant association of the mitochondria with the PM. These observations support models for an ATPase‐driven HCO3‐cation cotransport system inElodeacells. High chloroplast thylakoid surface density values (Sv) were similar to C4:monocots and indicated high light‐gathering potential. The granal/stromal (g/s), granal/cristae (g/c), and stromal/cristae (s/c) membrane ratios were similar to those of C3plant cells. Thus,Elodeachloroplasts and cells exhibited some structural features similar to both C3and C4plants. Membrane‐bound inclusion bodies, which occupied 1% of the cell volume, were also observed in the leaf cells. Cell walls and nucleoid regions were absent in these bodies and X‐ray (EDAX) analysis failed to detect any element (above the resolution limit of atomic no. 12) in substantial quantities. Inclusion bodies were observed in both the cytoplasm and the periplast and remain unidentified. A model is proposed forElodeausing an ATPase proton pump in the plasma membrane which extrudes protons into the periplast space between the cell wall and plasma membrane. This proton gradient is coupled to a bicarbon

ISSN:0140-7791    

DOI:10.1111/j.1365-3040.1991.tb01333.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

Abstract.With the aid of specifically designed potometer experiments, it is shown that, after ozone fumigation, twigs transpiring in gas exchange chambers show poor water balance in decreasing humidity. The quotient of water uptake to water loss never falls below 0.9 in healthy material because of the control capacity of the stomata. In twigs from a tree fumigated with ozone irregular and delayed stomatal closure results in values of<0.5 or even lower, depending on the degree of damage. As a result, in dry air, the transpiration rates of fumigated twigs often fall far below those of the control material, even if they were higher than the latter in humid air. In analogous experiments, the difference in behaviour between twigs of densely (‘healthy’) and sparsely needled (‘damaged’) trees from the natural stand is comparable to the difference between controls and ozone‐fumigated trees in most respects. In soil that is more or less dried out and after the best possible saturation of the twigs during the night, the transpiration rates of fumigated trees increase fairly strongly in the humid chamber air at dawn, but finally decrease more or less suddenly to lower values than in the controls. The results are placed in the context of the basic research on plant water relations and compared with histological changes in the stomatal apparatus after a period of fumigation as described earlier. Therefore, long‐term effects of pollution can be explained as a specific distrubance of hydr

ISSN:0140-7791    

DOI:10.1111/j.1365-3040.1991.tb01334.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

Abstract.After a period of one week at 11m s−1in a wind tunnel, the leaf surface ofPicea sitchensisandPinus sylvestrishad undergone structural modification. Low‐temperature scanning electron microscopy was used to examine these changes.Piceashowed flattening and smearing of wax crystals, as well as cracks in some of the wax structures filling the stomatal antechambers. InPinus, most damage was on the cells surrounding the stomatal antechamber or on needle ridges. Artificially abraded surfaces were of similar appearance. Minimum epidermal conductance to water vapour (geMIN) was determined gravimetrically. InPiceagcMINwas more than doubled by wind treatment and increased eightfold by rubbing. Similar but less extreme increases occurred inPinus.Neither species showed recovery ofgeMINafter 1 week at low windsp

ISSN:0140-7791    

DOI:10.1111/j.1365-3040.1991.tb01335.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

Abstract.Nitrogen‐fixing peanut root nodules have oleosomes (lipid bodies) in the infected cells. The oleosomes have been characterized and compared with their counterparts in the seed tissue. Eighty per cent of the nodule oleosomes were found to be of smaller size (0.16–1.0 μm diameter [dia.]) while in the seeds the larger size (2.0–5.5 μm dia.) dominated. The larger oleosomes were exclusively found in the uninfected three layers of cortical cells adjacent to the infected zone. Morphometric analyses have revealed significantly higher numbers of oleosomes covering about 8% of the infected cell area in immature (white) nodules, whereas the mature/old (pink) nodules had lesser numbers occupying about 4–5% of the cell area. The decrease in the amount of oleosomes possibly reflects their utilization in mature/old nodules which effectively fix nitrogen. The oleosomes were distinctly stained by p‐phenylenediamine (pPD) at both light and electron microscopic levels. An electron‐dense rim was observed around the nodule oleosomes; where lipolytic activity was also demonstrated using cytochemical methods. The rim was absent in the seed oleosomes. The defatted oleosomes were found to be surrounded by a ‘half unit membrane’ and a non‐extractable rim of possible pro‐teinaceous substance. Gas chromatographic analyses of the lipid from the isolated oleosomes indicated the presence of higher amounts of saturated fatty acids in the nodule oleosomes than the ones in the seed. The study indicates that the nodule oleosomes differ from seed oleosomes, with respect to the presence of (1) an electron‐dense rim showing (2) lipolytic activity and (3) higher amounts of saturated fatty acids. Nodule oleosomes seem to be transient storage organelles to be metabolized, while in the seed they are mean

ISSN:0140-7791    

DOI:10.1111/j.1365-3040.1991.tb01336.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

Abstract.The physiological characteristics and photo‐system composition of the photosynthetic apparatus ofSilene dioica, a woodland plant, grown in sun and natural shade are examined. As expected, shade leaves exhibited lower chlorophyll a/b ratios, light saturated rates of CO2assimilation (Asat), dark respiration (Rd,) and light compensation points (Г), with both sun and shade leaves having similar absorptances and quantum yields of CO2assimilation (φ). Shade leaves were able to utilize far‐red light for electron transport and carbon assimilation and reach the compensation point. Sun leaves in far‐red light had a rate of carbon assimilation equivalent to their dark respiration rate. Chlorophyll fluorescence kinetics from leaves at 77 K together with analyses of thylakoid contents of photosystems (PS) I and II and the light‐harvesting cholorphyll a/b protein complex associated with PSII (LHCII) demonstrated that the antenna size of PSII was similar in thylakoids of sun and shade leaves, but shade leaves containedca.20% more PSII andca.12% less PSI complexes. The increased PSII/PSI ratio in shade leaves accounted for their ability to achieve the compensation point in far‐red light. An important feature of photosynethic shade adaptation inS. dioicais an increase in the PSII/PSI ratio and not an increase in the antenna size of PSII. The adaptive response of sun leaves when placed in a shade environment was rapid and had a half‐

ISSN:0140-7791    

DOI:10.1111/j.1365-3040.1991.tb01337.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

Abstract.Glutaraldehyde fixation was used to determine the solute concentrations in the various cell types present in tissue cultures of squash (Cucurbita pepo). Small pieces of callus were plasmolyzed in a graded series of mannitol solutions and fixed in 20 kg m−3glutaraldehyde adjusted to be isosmotic with the particular plasmolysing solution. The callus samples were further processed using standard electron microscopy techniques. Using this procedure, mature sieve elements that form in squash callus have an osmotic potentional of ‐2.4MPa. The osmotic potential of the callus sieve elements was comparable to values reported for the sieve tube members of the phloem in intact plants. This ability of callus sieve elements to develop high internal hydrostatic pressures demonstrates that they are capable of phloem loading. However, the osmotic potentials of the surrounding parenchymatous cells and companion cells were only –1.15 and –1.5 MPa, respectively. In contrast to the companion cells of the phloem in intact plant tissues, the osmotic potential of the callus companion cells indicated that they were not directly involved in phloem loading. Several immature sieve elements containing distinct nuclei and vacuoles were observed in the callus granules. These immature sieve elements were plasmolyzed in weaker mannitol solutions (below 0.6kmol m−3) than the enucleate sieve elements (1.01 kmol m−3mannitol). The low solute concentrations in immature sieve elements indicated that the ability to load sugars occurs concomitantly with the maturation of the sieve element

ISSN:0140-7791    

DOI:10.1111/j.1365-3040.1991.tb01338.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

Abstract.In peas (Pisum sativumL.) homozygous forsym5, nodulation has an unusual temperature dependence. Thesesym5 mutants nodulate poorly at a root temperature of 20°C but nodulate better at 12°C. By lowering the root temperature of thesym 5mutants from a lightroom temperature of 20/15°C to a constant 12°C, 8d after planting, the number of nodules can be further increased. A cool period (12°C) as short as 6h, early in the infection process, is sufficient to significantly increase nodulation of plants otherwise growing at 20/15°C. This temperature‐sensitivity of nodulation is not due to a temperature induced change of asym5‐related, 66‐kD peptide but may involve accumulation of a gas in the

ISSN:0140-7791    

DOI:10.1111/j.1365-3040.1991.tb01339.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY