摘要:

Abstract.It has previously been proposed that the fundamental function of phytochrome in the natural environment is the perception of the relative proportions of red and far‐red light, i.e. the red: far‐red ratio. This paper re‐evaluates this hypothesis, for vegetative green plants, in the light of recent findings. Essentially, three issues are considered: (a) the modulation of the response to red: far‐red by fluence rate: (b) the anticipation of competition for light by perception of changes in red: far‐red that precede actual shading: and (c) characteristics of phytochrome that may be important in the mechanism of photoperception (i.e. the accumulation of photoconversion intermediates, and the stability of Pfr). We conclude: (a) the red: far‐red ratio provides a reliable signal of plant density, even before shading by neighbours occurs: (b) plants are able to perceive and respond to these signals, and that possible ambiguities due to low red: far‐red at low solar angles may be avoided by modulation of the perception process by fluence‐rate dependent mechanisms; (c) although direct experimental evidence does not yet exist, circumstantial evidence suggests that the perception of red: far‐red may confer positive adaptive advantage; and (d) plants of certain species perceive and respond to fluence rate changes, mediated perhaps by a blue‐light absorbing photoreceptor or by phytochrome, but that these responses do not necessarily lead to shade avoidance reactions and their ecological relevance is n

ISSN:0140-7791    

DOI:10.1111/j.1365-3040.1989.tb01966.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

Abstract.The model comprises three submodels, which together give an integrated picture of nitrogen pools and fluxes in grassland under grazing or cutting. The first submodel represents the interaction of the grazing animal with the sward through intake and the production of excreta: the second is concerned with the growth of the vegetative grass crop and its response to light, temperature and nitrogen; these two submodels are interfaced with a submodel of soil carbon and nitrogen pools and processes, including dead shoot and root material, dead and live soil organic matter, and three pools representing mineral nitrogen. No account is taken of water, which is assumed to be non‐limiting, or the possible effects of soil pH and soil aeration. The model is used to simulate a range of management strategies as applied to stocking density and fertilizer application, examining both steady‐state and non‐steady‐state conditions. The model highlights the long time scales associated with grassland systems, the role of the grazing animal in modifying carbon and nitrogen flows, and the importance of soil conditions to grassland productivity and fertilizer response. The productivity of grazed swards may be greater or less than that of cut swards depending on stocking density and fertilizer application, although nitrogen recovery (as calculated here) is always lower in grazed swards. The model is able to stimulate mineralization and immobilization, and place these in the context of plant processes and the grazing

ISSN:0140-7791    

DOI:10.1111/j.1365-3040.1989.tb01967.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

Abstract.A novel technique to record the variability of stomatal aperture over the leaf surface is described. This combines observations of leaf surfaces using low‐temperature scanning electron microscopy (LTSEM), with digital image analysis to produce the most accurate aperture measurements obtained to date. Leaf samples are rapidly immobilized by cryo‐fixation in liquid nitrogen and stored in a purpose‐built cryo‐storage system. Specimens can be collected in the field, remote from the cryopreparation system, and stored for up to several weeks before being examined on the LTSEM. The advantages of this method are that the time frame of the measurements is accurately known, and is identical for all stomatal apertures in a sample, and the precision of the measurements is not limited by the resolving power of the microscope. Measurements of stomatal aperture were obtained from leaves of field grownAvena fatuausing the above procedure. Leaf surface conductance (gsur) was determined by porometry immediately before cryo‐fixation of the same region of the leaf. Measurements of aperture size showed a high degree of variability within each specimen, with coefficients of variation similar to those found in previous studies. Stomatal conductance (gs) was calculated from stomatal dimensions using formulae derived elsewhere. A linear regression between the computed values of gsand porometric estimates of gsurshowed good agreement with the regression line passing through the origin with a slope of 1.0 (R2=0.96). Applications of the experimental system are

ISSN:0140-7791    

DOI:10.1111/j.1365-3040.1989.tb01968.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

Abstract.Poplar shoots (Populus euramericanaL.) obtained from cuttings were exposed for 6 or 8 weeks to NH3concentrations of 50 and 100 μgm−3or filtered air in fumigation chambers. After this exposure the rates of NH3uptake, transpiration, CO2assimilation and respiration of leaves were measured using a leaf chamber. During the long‐term exposure also modulated chlorophyll fluorescence measurements were carried out to obtain information about the photosynthetic performance of individual leaves. Both fluorescence and leaf chamber measurements showed a higher photosynthetic activity of leaves exposed to 100 μg NH3m−3. These leaves showed also a larger leaf conductance and a larger uptake rate of NH3than leaves exposed to 50 μg m−3NH3or filtered air. The long‐term NH3exposure did not induce an internal resistance against NH3transport in the leaf, nor did it affect the leaf cuticle. So, not only at a short time exposure, but also at a long‐term exposure NH3uptake into leaves can be calculated from data on the boundary layer and stomatal resistance for H2O and ambient NH3‐concentration. Furthermore, the NH3exposure had no effect on the relation between CO2‐assimilation and stomatal conductance, indicating that NH3in concentrations up to 100 μg m−3has no direct effect on stomatal behaviour; for example, by affecting the guard or contiguous

ISSN:0140-7791    

DOI:10.1111/j.1365-3040.1989.tb01969.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

Abstract.Structural and physiological aspects of the parenchymatous bundle sheath (PBS) were studied in cultivars ofHordeum distichumL. The PBS of intermediate, lateral and midrib veins consisted of a single layer of cells closely appressed to the mestome sheath. These cells were large, vacuolate and approximately cylindrical in shape, extending parallel to the vein. Mean PBS cell volume was 4 × 10−5mm3compared to 1.23 × 10−5mm3for mesophyll cells. Transverse sections revealed three cell types within the PBS, cells with small chloroplasts (S‐type), cells with large chloroplasts (L‐type) and structural cells. The majority of cells were S‐type, containing chloroplasts of approximately a third of the volume of mesophyll chloroplasts; they were able to reduce tetranitro blue‐tetrazolium and synthesize starch. Structural cells interrupted the phloem and xylem are of the sheath in lateral veins and the midrib, whilst between one and four PBS cells within the phloem are of each vein type contained chloroplasts similar in volume and starch content to those of the mesophyll. Only these L‐type cells contained noticeable starch grains at the end of an 8‐h dark period, a further 4 h darkness being required for complete mobilization of starch. Starch deposition within S‐type and structural cells was detectable after 4 h illumination but was only appreciable in leaves excised from the plant and illuminated for 9–12 h. The role of S‐type PBS cells in assimilate transport is discussed in rel

ISSN:0140-7791    

DOI:10.1111/j.1365-3040.1989.tb01970.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

Abstract.A modification to the pressure probe is described which allows very rapid extraction of sap samples from single higher plant cells. The performance of this rapid‐sampling probe was assessed and compared with the unmodified probe for cells of both wheat andTradescantia.Under some conditions, the unmodified probe operated too slowly to avoid dilution of cell sap during the extraction process. This led to values for apparent sample osmotic pressures that were below the turgor pressures for the same cells. The problem was particularly acute in young wheatleaf epidermal cells which are small, elongate and have high turgor pressure. These exhibited rapid water influx when their turgor was depressed during the sampling of their contents (half‐time for pressure recovery in wheat cells was less than 1 s while inTradescantiacells it was 3–5 s). Dilution during sampling was apparently negligible when the rapid sampling probe was used. The study was complemented by a simple model of the way cells dilute during sampling. Quantitative predictions of the model were consistent with our observed findings. The model is used to assess the major factors which determine a cell's susceptibility to dilution during sam

ISSN:0140-7791    

DOI:10.1111/j.1365-3040.1989.tb01971.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

Abstract.Two‐year‐old potted sweet chestnut seedlings were grown at 350 ppm CO2and 700 ppm, day and night in constantly ventilated tunnels during two full growing seasons, near Paris, France (48° N, 2° E). Enrichment with CO2caused an unusual shoot growth response. After the end of July, stem elongation ceased in 62% of the CO2enriched plants as compared with 37% in the control. The leaves of CO2‐enriched seedlings showed early senescence, indicated by premature yellowing and a decrease in chlorophyll content. This was associated with nutrient dilution brought about by the rapid growth of these trees. The increase in total dry weight of the CO2‐enriched seedlings was essentially the result of increase in the root dry weight (69%). Shoot weight decreased by 22% relative to the control. Total leaf area per enriched plant was 25% smaller than the control. This unusual pattern of growth and carbon allocation of the CO2treated Chestnut trees emphasizes the concept of a response specificity within trees to an increase of atmosp

ISSN:0140-7791    

DOI:10.1111/j.1365-3040.1989.tb01972.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

Abstract.The effect of soil drought on water‐use efficiency (WUE) and water relations was examined for pottedArtemisia tridentataNutt. andPinus ponderosaLaws., a dominant Great Basin desert shrub and a Sierran montane tree, respectively. Before the onset of drought,A. tridentatahad slightly higher photosynthetic rates thanP. ponderosaandA. tridentatamaintained positive photosynthetic rates at substantially lower water potentials (Ψ). Complete stomatal closure and cessation of photosynthesis occurred at a Ψ of ca. −2.5 MPa forP. ponderosaand less than −5.0 MPa forA. tridentata.Repeated drought cycles caused a small increase in bulk modulus of elasticity forA. tridentataand neither species exhibited significant osmotic adjustment. WUE was similar at Ψ≥−1.0 MPa but as Ψ decreasedP. ponderosaconsistently maintained higher WUE thanA. tridentata.The primary factor contributing to higher WUE forP. ponderosawas the rapid decrease in stomatal conductance with decreasing Ψ. Comparatively low WUE forA. tridentata, a drought tolerant species, suggests that efficient use of water is a conservative ecophysiological ‘strategy’ that can be detrimental in a competitive water‐limited environment. The combination of profligate use of water and a high degree of drought tolerance may be a more successful combination of physiological characteristics in

ISSN:0140-7791    

DOI:10.1111/j.1365-3040.1989.tb01973.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

Abstract.Marked increases in growth and nitrogen content were found withGunnera tinctoriaMolina (Mirbel) plants infected (+Nostoc) with the cyanobacteriumNostoc punctiformeL., in comparison to uninfected (—Nostoc) plants and this was attributed to N2‐fixation by the phycobiont. Whilst host and symbiont can be grown separately, preliminary data indicates that the host plant is reliant on the cyanobacterium to meet its nitrogen requirements because it has little capacity to assimilate nitrate. Although the maximum light‐saturated rate of photosynthesis was higher in the +Nostocplants, there was no reduction in photosynthetic efficiency under lightlimiting conditions, despite marked differences in plant nitrogen status. Differences in photosynthetic rate were implicated as the major reason for the differences in plant productivity. Stomatal conductance was insensitive to changes in plant nitrogen status and did not parallel the variation in photosynthetic rates. The ecological significance of the largely invariant stomatal response and the consequences of differences in water and nitrogen‐use efficiencies between + and —Nostocplants is

ISSN:0140-7791    

DOI:10.1111/j.1365-3040.1989.tb01974.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

Abstract.A portable apparatus has been constructed to measure simultaneously the quantum yield of CO2assimilation, light absorption, chlorophyll fluorescence emission and water vapour exchange of attached intact leaves in the field. The core of the instrument is a light‐integrating spherical leaf chamber which includes ports for a light source, photosynthetically active radiation sensor, fluorescence probes and gas inlet and outlet manifolds. Measurement of the quantum flux inside the empty chamber and with a leaf present allows determination of leaf absorptance. An open gas exchange system is employed using an infra‐red analyser to measure leaf CO2exchange. Using a DC white light source the quantum yield of CO2assimilation based on absorbed light (φabs) may be determined rapidly in either ambient air or artificial gas mixtures. Inclusion of capacitance humidity probes into the gas inlet and outlet ports allows simultaneous determination of water vapour exchange and subsequent estimation of stomatal conductance to CO2and intercellular CO2concentration. Measurement of fluorescence emission by the sample leaf exposed to white light is achieved by a modulated fluorescence detection system. In addition to determination of the minimal, maximal and variable fluorescence levels, a further analysis allows the photochemical and non‐photochemical components of fluorescence quenching, to be estimated. The theory and design of this apparatus is described in detail. The use of the apparatus in the field is demonstrated through a study of the photosynthetic performance of a maize and bean crop during the growing season and by analysis of the photosynthetic performance of crops subjected to nitrogen‐stress and a herbicide t

ISSN:0140-7791    

DOI:10.1111/j.1365-3040.1989.tb01975.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY