摘要:

Abstract.A mechanistic model of photosynthesis is developed based on the characteristics of ribulose 1,5‐bisphosphate (RuBP) carboxylase and the assimilation of CO2as an ordered reaction with RuBP binding before CO2. An equation is derived which considers the effects of light (for regeneration of RuBP) and CO2. Taking values for the maximum turnover of RuBP carboxylase at substrate saturation, the maximum carboxylation efficiency (maximum increase in rate per increase in CO2concentration) and the minimum quantum requirement for the C3pathway, photosynthesis in the absence of photorespiration is simulated. In the model, at varying concentrations of CO2, the efficiency of light utilization approaches a maximum value as photon flux density decreases. Similarly, with a given maximum carboxyation capacity, at varying photon flux densities the carboxylation efficiency approaches a constant maximum value (equal toVmax/KmCO2) as CO2is decreased. However, a decrease in the state of activation of RuBP carboxylase under low light results in a lower carboxylation efficiency. Limits on the rate of photosynthesis, as the maximum capacity for regeneration of RuBP is reduced relative to carboxylation potential, or as the maximum capacity of the carboxylase varies, are considere

ISSN:0140-7791    

DOI:10.1111/j.1365-3040.1988.tb01904.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

Abstract.A model is developed for photosynthesis and photorespiration in C3plants, using an equation for the multisubslrate ordered reaction of ribulose 1,5‐bisphosphalc carboxylase‐oxygenase (Farazdaghi&Edwards, 1988). The model examines net CO2fixation with O2inhibition, and mutual inhibition when equilibrium exists between carboxylation and oxygenation (at the CO2compensation point). It is based on the stoichiometry of energy requirements and O2, and CO2exchange in the cycles, the quantum efficiency for RuBP generation, the maximum capacity for RuBP generation, the carboxylation efficiency with respect to [CO2], and the oxygenation efficiency with respect to [O2]. With increasing concentrations of CO2above the CO2compensation point, decreasing quantum flux density, or decreasing O2, simulations show that the rate of photorespiration progressively decreases. The two components of O2inhibition of photosynthesis change disproportionately with increasing CO2concentration. According to the model, the energy utilized during photosynthesis at the CO2compensation point is about half that under atmospheric conditi

ISSN:0140-7791    

DOI:10.1111/j.1365-3040.1988.tb01905.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

Abstract.Portulacaria afra(L.) Jacq. is a perennial facultative CAM species showing a seasonal shift from C3to CAM photosynthesis. The shift to CAM during the summer occurs despite continued irrigation of the plants. The authors examined the hypothesis that the seasonal shift to CAM occurred because of low transient water potentials. They measured changes in whole leaf water, osmotic and pressure potentials over the course of the shift. They also studied changes in enzyme activity to ascertain if PEP carboxylase and PEP carboxykinase were induced during the seasonal shift to CAM. Water potentials were high, from ‐0.1 to ‐0.5 MPa, predawn and midday, when the C3pathway of photosynthesis was utilized. Osmotic potentials were constant, from ‐0.7 to ‐ 0.8 MPa, indicating very little change in turgor.P. afrashifted to CAM indicated by large diurnal acid fluctuations (300 400 meq m−2) despite C3‐like predawn water potentials. Midday water potentials usually decreased 0.2‐0.7 MPa, while the osmotic potential remained unchanged or decreased slightly. Thus, a midday loss of turgor was associated with the use of the CAM pathway. The results support the hypothesis that the induction of CAM occurred due to low transient water potentials and may be partially mediated through the loss of turgor. The shift to CAM is only a partial induction with PEP carboxykinase showing high activity all year round while PEP carboxylase increases three‐to five‐fold over C3levels. Relatively high levels of CAM enzyme activity enables the utilization of the CAM pathway in the winter and spring in response to high daytime temperatures and increased evaporative demand. These results would lead to an increase in water use efficiency during such periods when compared to other indu

ISSN:0140-7791    

DOI:10.1111/j.1365-3040.1988.tb01906.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

Abstract.Nuclei fromZea maysandPhaseolus vulgarisroot meristematic and differentiated cells were treated according to a recently developed simple procedure for isolation of nuclear lamina of Ehrlich Ascite Tumor (EAT) cells. As revealed by electron microscopy, the residual structures obtained represented empty nuclear shells, resembling those previously isolated from animal cells. Moreover, the composition of the residual nuclear structures from plant cells was found to be very similar to that described previously for the nuclear lamina purified by the adopted procedure. As demonstrated by SDS‐polyacrylamide gel electrophoresis, the plant nuclear shells contained a small number of proteins in the 65‐45 kD range. Two proteins — 62 and 50 kD—were most characteristic for beans, while a 55‐kD protein was abundant in maize. When blotted on nitrocellulose paper, some of the proteins of plant nuclear shells were immunoreactive with sera containing antibodies against the proteins of EAT nuclear envelopes. The degree of phosphorylation of the proteins of plant nuclear shells was found to be higher in meristematic than in differentiated maize root cells, correlating with the mitotic activity of the starting

ISSN:0140-7791    

DOI:10.1111/j.1365-3040.1988.tb01907.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

Abstract.Plantago majorssp.majorandP. lanceolatawere grown in solution culture with exponential nutrient addition rates. Compared withP. lanceolata, P. major majorshowed a higher shoot weight ratio (SWR, fraction of plant dry weight in the shoot) and a higher net assimilation rate (NAR, expressed on a leaf dry weight basis) at equal plant (PNC) and shoot (SNC) nitrogen concentration, respectively. No difference was observed in shoot nitrogen ratio (SNR, fraction of plant nitrogen in the shoot) against PNC between the two species. The effect of these differences in matter partitioning and NAR on plant growth was examined by using a growth model. The model assumed (1) that the SWR and SNR are a linear function of PNC and (2) that the NAR is a rectangular hyperbolic function of SNC. Curvilinear relationships were observed between relative growth rate (RGR) and PNC.P. major majorhad a higher RGR at equal PNC and, thus, a higher nitrogen productivity (NP) thanP. lanceolata.Steady‐stale exponential growth was simulated for different nitrogen availability in the environment.P. major majorhad a higher RGR over the whole range of nitrogen availability but the difference attenuated with decreasing availability of nitrogen. The simulation also showedP. lanccolatahaving higher plasticity in the shoot/root ratio, which resulted from its higher variability in PN

ISSN:0140-7791    

DOI:10.1111/j.1365-3040.1988.tb01908.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

Abstract.Drought effects on the final leaf area of individual leaves were hypothesized to depend on the leaf developmental stage at which drought occurred. To evaluate this hypothesis, final leaf area and cell number were measured for soybean (Glycine max(L.) Merr.) leaves that were at different stages of development when single or cyclical drought treatment was imposed. Leaf emergence rate from the meristem, as depicted by changes in the plastochron index, was not as sensitive as leaf expansion to cyclical droughts. For leaf expansion, small leaves, once they emerged from the meristem, suffered larger decreases in growth than leaves undergoing rapid leaf area expansion. Decreases in final leaf area as a result of a cyclical drought were correlated with decreases in final cell number. Decreases resulting from a single 8‐d drought were dependent on the age of the leaf at the time of drought, because small leaves were found to have proportionately larger decreases in final cell number and area than larger leaves. These results indicated that age‐dependent leaf responses to drought are based on the relative activity of cell division and expansion at the time stress was impo

ISSN:0140-7791    

DOI:10.1111/j.1365-3040.1988.tb01909.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

Abstract.Seed germination characteristics were investigated in the most commonCistusspecies in Greece, namelyC. incanusssp.creticusandC. salvifolius.In addition to the soft seed subpopulation, both species produce a large fraction of hardcoated, water‐impermeable seeds which can be softened and, thus, promoted to germinate by mechanical scarification and thermal pretreatment. Temperature and light control of seed germination are unimportant. In the ecological context of the Mediterranean ecosystems, the eventually advantageous, opportunistic strategy of germination is based on: (a) seed heterospermy (which allows the smaller, softcoated fraction to germinate promptly each year while the majority of the seeds, the hard ones, accumulate in the soil); (b) the seed population heterogeneity in relation to coat hardness (so that any heat conditions produced by fire induce the softening and germination of a certain seed fraction); (c) the notably low germination rate (which suppresses premature germination); (d) the wide, Mediterranean‐type (relatively cool), temperature range of germination (while higher temperatures simply inhibit but do not induce any dormancy); and (c) the apparent lack of photo‐sensitivity (enabling germination under every light regime). In non‐fire years, the temporal distribution of field germination and seedling appearance might be partly determined by the seed dispersal strategy of the individualCistusspecies. Nevertheless, the post‐fire regeneration response is manifested in the form of a huge wave of germination (of practically all seeds softened by the fire heat), shortly after the onset of the rai

ISSN:0140-7791    

DOI:10.1111/j.1365-3040.1988.tb01910.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

Abstract.Positron emission tomography (PET) has been utilized to obtain dynamic images of long distance nutrient translocation in plants. Positron emitting18F, produced by a Van de Graaff accelerator using the reaction18O(p,n)18F, was fed in solution to excised stems ofGlycine maxpositioned vertically in a large‐aperture PET detector system. Images of tracer activity were recorded with a time resolution of 0.5 min and a spatial resolution of 4 mm. Maximum tracer activities at stem sites were obtained within 3 min of the pulse feed. A model is presented enabling evaluation of regional values for tracer flow, tracer binding, flow speed and flow volume. Analysis of data for one stem position yielded a flow volume of 2.1mm3min−1and a flow speed of 36cm min−1. Comparison with the distribution of14C‐inulin, which was simultaneously fed to the cut stems, indicates the18F is suitable for use as an apoplastic tracer; 92% of the tracer activity accumulated in the leaves. The fraction of18F that remained bound was most concentrated at stem nodal regions, an observation consistent with the existence of transfer cells at these sites. Advantages and limitations of PET applied to plant physiological investigations are di

ISSN:0140-7791    

DOI:10.1111/j.1365-3040.1988.tb01911.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

Abstract.Wide variations in total aluminum (AL) concentration reported to have a toxic effect on plants arc attributed to imprecise definition of the chemical environment at the root interface. Because AL can complex with natural ligands and form time‐dependent metastable species in water, the requirements of a fixed AL speciation in the nutrient medium can only be met by simultaneously holding constant the pH, the AL/OH ratio and the renewal time of the solution. It is also important that the root be constantly perfused by a fresh solution so that the root interface is as chemically close to the bulk solution as possible. The rhizostat presented here uses a process controller to constantly replenish four tanks containing different combinations of nutrients and AL. The solution is pumped from each tank to a tray where it is continuously delivered to seedlings growing on artificial medium. This rhizostat provides an accurate control for AL speciation. Preliminary experiments on white spruce seedlings show that levels of 10 micromolal or less total AL are sufficient to cause toxic effects at pH of 4.5 and without the formation of AL complexe

ISSN:0140-7791    

DOI:10.1111/j.1365-3040.1988.tb01912.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY