bluuebunny/crossref_metadata_2025 huggingface dataset:
Mechanical freeze‐thaw damage and frost hardening in leaves and isolated thylakoids from spinach. I. Mechanical freeze‐thaw damage in an artificial stroma medium
Authors: HINCHA D. K. , SCHMITT J. M.
Published: April 2006

Abstract Freeze‐thaw damage to thylakoids in spinach leaves has been simulated in vitro, using a complex, defined artificial stroma medium. The resulting mechanical damage was quantified by measuring the loss of the marker protein plastocyanin from the thylakoid lumen, which is released as a result of membrane rupture. Loss of plastocyanin was already apparent at 0°C and became more severe at subzero temperatures. The time course of plastocyanin loss during freezing was biphasic: after an initial rapid loss, plastocyanin release was linearly dependent on incubation time. In short‐term experiments a linear dependence on freezing temperature was observed. Solute diffusion into the thylakoids, leading to influx of water and eventually membrane rupture, has been observed in vitro as well as after freezing of leaves.
Differences between the effects of partial and whole plant chilling on carbon translocation of a C4 grass
Authors: POTVIN C.
Published: April 2006

Abstract Experiments were conducted with Echinochloa crus‐galli to partition the effects of chilling the leaf vs. chilling the whole plant on subsequent 11C translocation. The results clearly demonstrated that whole plant chilling was very detrimental whereas chilling only the leaf had no effect on subsequent translocation nor on 11C uptake. The inhibition of translocation was due to a reduced rate and percentage of export while 11C fixation rate was not significantly altered. When the leaf of a chilled plant was maintained at 22 °C, there was no impairment of the transport system nor of photosynthesis. The decrease in export with whole plant chilling may have been due to carbon movement into storage carbohydrates, resulting in a low sucrose gradient.
The heat‐shock response in higher plants: a biochemical model
Authors: BURKE J. J. , ORZECH USDA‐ARS K. A.
Published: April 2006

Abstract A compilation of existing data on higher plant responses to heat‐shock temperatures has been utilized to produce a biochemically based model of integrated cellular responses to elevated temperatures. This model describes a potential mechanism for the triggering of several biochemical responses to a thermally induced leakage of extracellular or vacuolar ions into the cytoplasm. It seems possible that many of the observed heat‐shock responses are involved in the protection of (a) enzymes from inactivation and (b) nucleic acids from cleavage induced by the presence of elevated levels of specific metals.
Water‐use efficiency in rice (Oryza sativa L.) in relation to resistance to salinity
Authors: FLOWERS T. J. , SALAMA F. M. , YEO A. R.
Published: April 2006

Abstract The effects of sodium chloride on water‐use efficiency of a number of varieties of rice have been investigated. Sensitivity to salinity in rice arises in large part from excessive rates of sodium ion transport to the shoot consequent upon high rates of transpirational water loss. Varietal differences in water‐use efficiency were found: these were greater when measured for whole shoots over a period of one week, than when made as instantaneous measurements on individual leaves. Salinity had rather little effect on water use efficiency but overall resistance of seedlings to salt was, in general, greater the greater the water‐use efficiency. The reasons for the differences between varieties are discussed in terms of differences in growth pattern: among the seven varieties investigated, water‐use efficiency and salt‐resistance were lower in dwarfed as opposed to non‐dwarfed varieties.
The effect of divalent cations on Na+ tolerance in Charophytes. II: Chara corallina
Authors: TUFARIELLO J. A. M. , HOFFMANN R. , BISSON M. A.
Published: April 2006

Abstract The freshwater Charophyte Chora corallina dies when subjected to 70 molm−3 NaCl if the Ca2+ concentration is 0.1 mol m −3. This stress is accompanied by a depolarization of the cell to a membrane potential more positive than EK, a net influx of Na+ into the vacuole, and a net loss of K+ from the vacuole. Raising the Ca2+ concentration to 7 mol m −3 in the presence of elevated Na+ restores the Na+ to Ca2+ ratio to 10: 1 as in the control solution, and results in enhanced survival even though turgor is not regulated. Mg2+ is not a good substitute for Ca2+. It is suggested that the main reason that C. corallina fails to occupy saline habitats is its failure to regulate turgor, not sensitivity to Na +, since the latter is similar to that seen in C. buckellii, which is found in saline habitats.
The roles of low temperature and light in accumulation of a 31‐kDa polypeptide in the light‐harvesting apparatus of maize leaves
Authors: COVELLO P. S. , HAYDEN D. B. , BAKER N. R.
Published: April 2006

Abstract Previously, accumulation of a 31‐kDa polypeptide had been observed in the light‐harvesting apparatus of thylakoids of maize leaves exposed to 5°C and high light (Hayden et al., 1986). The accumulation and disappearance of this 31‐kDa polypeptide in thylakoids of maize leaves are examined as a function of photon flux density and temperature. The accumulation of large amounts of the polypeptide at 5°C was light‐dependent during a 6‐h chill period, with 50% of maximal accumulation occurring at a photon flux density of 60 μmol m−2 s−1.Some polypeptide accumulation did occur in leaves kept in the dark at 5°C for 6 h, i.e. ca. 18% of that accumulating at a photon flux density of 1500 μmol m−2 s−1. The temperature optimum for polypeptide accumulation was ca. 9°C with greater than 50% of maximal accumulation being achieved between 5 and 11°C. The breakdown of maximally accumulated polypeptide on returning leaves to 25°C was complete after 1 h, had a half‐time of ca. 20 min and was independent of light. Breakdown of the polypeptide was also observed when thylakoids isolated from chilled leaves were incubated at 25°C. Reductions of thylakoid incubation temperature between 13 and 5°C markedly reduced the rate of polypeptide disappearance. The accumulation of the polypeptide is discussed in relation to temperature and light effects on the rate of the polypeptide synthesis and of peptidase activities. The results are also discussed in the context of accumulation of the polypeptide in maize leaves in the field and consideration is given to the possible physiological significance.
Nitrogen‐stimulated potassium influx into maize roots: differential response of components resistant and sensitive to ambient ammonium1
Authors: VALE F. R. , JACKSON W. A. , VOLK R. J.
Published: April 2006

Abstract Potassium (86Rb) influx from 200 mmol m −3 KCl into dark grown, decapitated maize seedlings 6 d old) was stimulated by nitrate pretreatment. The stimulus was clearly evident by 6h exposure to nitrate and required 12–24 h for maximal expression. Decay of the nitrate‐stimulated potassium influx was more than 50% complete within 3 h after transfer to nitrogen‐free solutions. The stimulation of potassium influx was entirely accounted for by an increase in the influx component that was resistant to inhibition by presence of 200 mmol m−3 ambient ammonium. In contrast, the component of potassium influx that was sensitive to inhibition by ambient ammonium was unaffected by nitrate pretreatment. Exposure to the glutamine synthetase inhibitor L‐methionine‐dl‐sulphoximine (MSX) during nitrate pretreatment stimulated the resistant component but the sensitive component was nearly eliminated. Pretreatment with ammonium increased the resistant component of potassium influx within 3 h, i.e. before it was increased by nitrate pretreatment, but the sensitive component was concomitantly restricted. The latter recovered partially during extended pretreatment with ammonium. The data indicate that the resistant component responded positively to increases in tissue ammonium concentrations whereas the sensitive component was unaffected by tissue ammonium except at concentrations in excess of 10μmol g−1. Ammonium influx was also stimulated by nitrate pretreatment and to a greater extent than potassium influx. Presence of MSX with nitrate during pretreatment resulted in a further stimulation in ammonium influx. The parallel increases in root ammonium concentrations with the two pretreatments imply that part of the increase in ammonium influx was a consequnce of increased counter‐transport with endogenous ammonium.
Anomalous kinetics of sucrose uptake in maize scutellum slices1
Authors: HUMPHREYS T. E.
Published: April 2006

Abstract The kinetics of sucrose uptake into maize scutellum slices showed that the uptake mechanism had a saturable component with a Km of l.5mol m−3 sucrose. Nevertheless, uptake rate was constant (zero order) over extended periods of time until the bathing solution was nearly depleted of sucrose. It is concluded that these anomalous uptake kinetics reflect sucrose influx across the plasmalemma because of the following results: (a) Efflux of sucrose into buffer was negligible compared with uptake rate, (b) When slices were incubated in fructose, sucrose was synthesized and there was a net release of sucrose to the bathing solution until a steady‐state was reached when influx and efflux were equal in magnitude. After the steady‐state was reached, efflux of sucrose from the slices was nearly the same in magnitude as the estimated rate of uptake that would have occurred from bathing solutions initially containing the steady‐state sucrose concentration, (c) Exchange of sucrose between bathing solution and slices was negligible compared with uptake rate, (d) Pretreatment of slices with uranyl nitrate abolished sucrose uptake, but uptake rate was re‐established in these slices after treatment with HCl (pH 2). Uptake rate was set by the initial sucrose concentration of the bathing solution, and was not influenced by the level of endogenous sucrose or by the rate at which the sucrose concentration of the bathing solution declined. Abrupt increases in sucrose concentration during the uptake period increased the rate of uptake only if the concentration was increased above that at the start of the uptake period. Following abrupt decreases in sucrose concentration, there was a lag of about 30 min before uptake rate decreased greatly. If slices were washed and replaced in a fresh sucrose solution during the uptake period, a new uptake rate was set to correspond to the new initial sucrose concentration. It is suggested that the sucrose carrier has a transport site with a relatively low Km (much below 1.5mol m−3) and that the measured Km (1.5mol m−3) is that of a site that binds sucrose and thereby controls the rate of uptake. The low Km suggested for the transport site would explain the zero order kinetics but a model of the uptake mechanism that includes the control site cannot, as yet, be constructed from the data.
Compartmentation and fluxes of carbon in leaf blades and leaf sheaths ofPoa annuaL. andPoa x jemtlandica(Almq.) Richt
Authors: BORLAND A. M. , FARRAR J. F.
Published: April 2006

AbstractThe allocation of photosynthetically fixed carbon in the leaf blades and sheaths ofPoa annua(a ruderal grass) andPoa x jemtlandica(a sub‐arctic grass) was followed over a light‐dark cycle. Labelling with14Carbon and gas exchange measurements provide data for an eight‐compartment model describing the partitioning of carbon between spatially and chemically separated pools and their rates of turnover. Soluble sugars and fructans were turned over rapidly in the leaf blades of both species. The flux of carbon through pools of storage carbohydrates was higher in the leaves ofP. x jemtlandicathan inP. annua.The exchange of carbon between pools was slower in the sheath than the blade. Carbohydrates stored in the sheath appeared to have no significant role in metabolism over the light‐dark cycle studied here.
Blue light‐regulation of cell division in Adiantum protonemata: kinetic properties of the photosystem
Authors: IINO M. , NAKAGAWA Y. , WADA M.
Published: April 2006

Abstract Blue‐light‐induced cell division in single‐celled Adiantum protonemata was studied by using two or three pulses (pulse duration: 30 s) separated by various periods and by using relatively long irradiation (e.g. 30 min). The results showed: (1) that the response is saturated by a single pulse, (2) that after the application of a saturating pulse, the protonemata gradually become responsive to another pulse, showing time‐dependent saturation to the second pulse, and (3) that although reciprocity holds in the pulse‐induced response, it becomes invalid as the exposure duration extends in the range of minutes. These results were analysed in view of a reaction model in which a molecular component is considered to exist in two forms A and B. The response to a single pulse is considered to result from a light‐dependent conversion of the component (A→B), and the restoration response measured by two pulses, from its dark reversion (B→A). The analyses yielded (1) the value of the constant which relates the fluence rate to the rate constant of the light‐induced reaction, and (2) the rate constant of the dark reaction. The model was extended to formulate the responses to long irradiations as a function of the integrated concentration of B over time. The responses predicted by the formula by using the parameter values estimated from the pulse responses were able to explain the responses measured for long irradiation.
Water economy and photosynthesis of the CAM plant Senecio medley‐woodii during increasing drought
Authors: RUESS B. R. , FERRARI S. , ELLER B. M.
Published: April 2006

Abstract CO2 gas exchange, transpiration and water uptake of the succulent Senecio medley‐woodii were monitored simultaneously during a 10 day period of increasing drought. The measurements were performed with a combination of a CO2 gas exchange chamber and a potometer system. Further, leaf water relations and CO2 gas exchange of a branched potted plant were measured during 15 days of water shortage. The enhancement of CO2 dark fixation at the beginning of drought modifies the leaf water relations according to the increased malate accumulation during the dark period. The enhancement of water uptake from dusk to dawn corresponds to the increase of Ψleaf during the same period. Therefore at the beginning of drought a short time improvement of plant water status through the increased CO2 dark fixation and malate accumulation can be maintained.
Carbon balance during CAM: an assessment of respiratory CO2 recycling in the epiphytic bromeliads Aechmea nudicaulis and Aechmea fendleri
Authors: GRIFFITHS H.
Published: April 2006

Abstract The regulation of crassulacean acid metabolism (CAM) under controlled environmental conditions has been investigated for two tropical epiphytes, relating plant water and carbon balance to growth form and habitat preference under natural conditions. Aechmea fendleri is restricted to wet, upper montane regions of Trinidad, while A. nudicaulis has a wider distribution extending into more arid regions of the island. Morphological characteristics of these plants are related to habitat preference in terms of leaf succulence (0.44 and 0.94 kg m−2 for the two species respectively) and a distinct layer of water storage parenchyma in A. nudicaulis In contrast, the thinner leaves of A. fendleri contain little water‐storage parenchyma and less chlorenchyma per unit area, but the plants have a more open leaf rosette. The two species differ in expression of CAM, since the proportion of respiratory CO2 recycled as part of CAM had been found to be much lower in A. fendleri This study compared the efficiency of water use and role of respiratory CO2 recycling under two PAR regimes (300 and 120 μnol m−2 s−1) and three night temperatures (12, 18 and 25 °C). Dark CO2 uptake rates for both species were comparable to plants in the field (maximum of 2.3 ± 0.2 μmol m−2s−1± SD, n= 3). Total net CO2 uptake at night increased on leaf area basis with temperature for both species under higher PAR, although under the low PAR regime CO2 uptake was maximal at 18 °C. Water‐use efficiency (WUE) increased at 18 °C and 25 °C during dark CO2 uptake (Phase I) and also during late afternoon photosynthesis (Phase IV) in both species. For A. fendleri, dawn to dusk changes in titrable acidity (ΔH +) were similar under high and low PAR, although ΔH+ was correlated to night temperature and PAR in A. nudicaulis. The proportion of ΔH+ derived from respiratory CO2 also varied with experimental conditions. Thus percentage recycling was lower in A. fendleri under high PAR (0–10%), but was only reduced at 18 °C under low PAR. Recycling by A. nudicaulis ranged from 32–42% under high PAR, but was also reduced to 6% under low PAR at 18 °C; at 12 °C and 25 °C, recycling was 37% and 52% respectively. Previous studies have suggested a relationship between the proportion of recycling and degree of water stress. This study indicated that CAM as a CO2 concentrating mechanism regulates both water‐use efficiency and plant carbon balance in these epiphytes, in response to PAR and night temperature. However, the precise relationship between respiratory processes and the balance between external and internal sources of CO2 is as yet unresolved.
Ideal nutrient productivities and nutrient proportions in plant growth
Authors: ÅOREN G. I.
Published: April 2006

AbstractI propose that one single formulation can be applied to relate growth and content of several nutrients, including the most important macronutrients, of most plant species. The plant growth rate is proportional to the nutrient content minus a given minimal concentration of the nutrient in minimum. The proportionality factor, the nutrient productivity, and the minimum concentration are species specific properties. The nutrient productivity formulation is shown to apply for very different plant species and for different nutrients.
Early bending kinetics in response to geostimulation or unilateral IAA application in Lolium nodal segments
Authors: CLIFFORD P. E.
Published: April 2006

Abstract In Lolium multiflorum nodal segments, bending responses both to geostimulation and unilateral indole‐3‐acetic acid (IAA) application exhibited much variability in their lag times and speeds of early bending. Despite this variability, mean response curves to gravity and auxin stimulus were markedly similar with each having a phase of immediate, negative bending followed by phases of slow, positive bending and eventually more rapid, positive bending within 40 min of initial treatment. Comparison of lag times for response to geostimulation and unilateral IAA application, whether derived from the mean of individual replicates, or from mean curve data, showed that at least 4 min is available in this geotropic system for establishment of asymmetric auxin levels that could lead to differential growth. The hypothesis that variability in georesponse in Lolium nodal segments is linked to variable sensitivity of geosensitive tissue to auxin was tested using matching longitudinally‐halved nodal segments and evidence was obtained in support of the hypothesis from lag time but not from early bending speed data. The implications of the findings for an involvement of endogenous IAA in shoot geotropism together with the necessity to understand better the complex behaviour of bending response in individual replicates are discussed.
An artificial osmotic cell: a model system for simulating osmotic processes and for studying phenomena of negative pressure in plants
Authors: STEUDLE E. , HEYDT H.
Published: April 2006

Abstract An artificial osmotic cell has been constructed using reverse osmosis membranes. The cell consisted of a thin film of an osmotic solution (thickness: 100 to 200 μm) containing a non‐permeating solute and was bounded between the membrane and the front plate of a pressure transducer which continuously recorded cell turgor. The membrane was supported by metal grids to withstand positive and negative pressures (P). At maximum, negative pressures of up to –0.7 MPa (absolute) could be created within the film on short‐term and pressures of up to –0.3 MPa could be maintained without cavitation for several hours. As with living plant cells, the application of osmotic solutions of a non‐permeating solute resulted in monophasic relaxations of turgor pressure from which the hydraulic conductivity of the membrane (Lp) and the elastic modulus of the cell (ɛ) could be estimated. The application of solutions with permeating solutes resulted in biphasic pressure relaxation curves (as for living cells) from which the permeability (Ps) and reflection (σs) coefficients could be evaluated for the given membrane. Lp, Ps, and σs were independent of P and did not change upon transition from the positive to the negative range of pressure. It is concluded that the artificial cell could be used to simulate certain transport properties of living cells and to study phenomena of negative pressure as they occur in the xylem and, perhaps, also in living cells of higher plants.
Different forms of phototropic response induced by microbeam irradiation of discrete regions of an organ
Authors: TAYLOR A. , HART J. W. , BERG A. R.
Published: April 2006

AbstractContinuous unilateral microbeam (1 mm) irradiation of coleoptiles ofAvena salivaL. produces growth responses in non‐irradiated regions of the organ. Irradiation of the apex is followed by differential growth which commences in the apical zone, but then moves in a basipetal direction and results in positive curvature. Irradiation of the base also results in differential growth which commences at the apex and moves basipetally. However, the growth differential in this case results innegativecurvature. The results of basal irradiation suggest the likely occurrence of acropetal transmission of the phototropic signal. The rate of movement of this signal is faster than the documented rate of auxin transport.
Control of hypocotyl gravitropism by photochrome in a dicotyledonous seedling (Sesamum indicumL.)
Authors: WOITZIK F. , MOHR H.
Published: April 2006

AbstractThe present study was prompted by the question as to whether the strong effect of red and far‐red light treatments on blue‐light‐mediated phototropism in the sesame (Sesamum indicumL.) hypocotyl (Woitzik & Mohr, 1988) should be attributed in part to changes initialed by light in the gravitropic counter‐response. Light treatments, operating through phytochrome, do indeed strongly affect the gravitropic response. However, the direction of the light effect is the same in gravitropism, as in phototropism. Thus, the gravitropic counter‐response leads to an underestimate, rather than an overestimate, of the importance of phytochrome action on phototropic responsiveness. The effect of red and far‐red light, operatingviaphytochrome, on the gravitropic response of the sesame hypocotyl could be studied in the present paper without any interference due to phototropism or light control of longitudinal growth. It was found that the effects of red and far‐red pretreatments (given prior to the onset of the stimulus) as well as the action of simultaneously applied red or far‐red light (simultaneous to the phototropic or gravitropic stimulus) are very similar in both phototropism and gravitropism. In particular, the seedling is capable of superimposing information about the actual light conditions during bending on the ‘memory’ it has about the light conditions prior to the onset of phototropism or gravitropic stimulation, This striking similarity between the phototropic and gravitropic responses possibly indicates that phytochrome affects the signal‐response‐chain at a relatively late stage, after the phototropic and the gravitropic signal‐response chains have merged. From a teleonomic point of view the action of red and far‐red light on phototropic, as well as gravitropic, responsiveness can be conceived as part of a shade escape strategy.
Stable isotope carbon study: long‐term partitioning during progressive drought stress in Brassica napus var. oleifera
Authors: DELEENS E. , MARCOTTE L. , SCHWEBEL‐DUGUE N. , VARTANIAN N.
Published: April 2006

Abstract. Long‐term carbon partitioning was analysed by stable carbon isotope labelling of CO2 during the adaptive response of Brassica napus to progressive drought stress. This method allowed us to distinguish between the pre‐existing carbon, which had accumulated before the change in CO2 isotope composition (on 20 d) and the recent photosynthetic input occurring during the following period. Three successive adaptive phases characterized the plant response to drought. In the first period of water shortage (20–30 d), growth was progressively slowed down: the recent C input (28.3 mg per plant) was mainly allocated to mature leaves (14 mg) and roots (8.9 mg); the pre‐existing C chains were partly lost by respiration (12.1 mg) and partly translocated to the apex (2.3 mg). The increase in dry matter (13.0 mg) was mainly due to the recent C input (19.1 mg) in shoots but the roots appeared also as an important sink despite their small dry matter increase (3.2 mg). Root dry matter turnover corresponded to the elimination of pre‐existing C chains (6.0 mg) and their renewal with the recent C input (9.2 mg). Root sink strength was related also to the development of drought‐induced short tuberized roots (0.1 mg from pre‐existing C and 0.3 mg from recent C). In the second period of water stress (from 30 to 40 d), the whole plant biomass remained constant in spite of efficient allocations from pre‐existing and recent C sources to two main sinks: shoot apex (3.1 and 5.4 mg, respectively) and short tuberized roots (1.0 and 0.2 mg, respectively). The hypocotyl acted as a transient storage organ for the recent C input (2.0 mg). In the third period of recovery upon rehydration (from 46 to 51 d), the short tuberized roots gave rise to a new root system using C chains from pre‐existing and recent sources (1.7 and 2.8 mg, respectively). Such concomitant sink‐source behaviour of tuberized roots underlines the adaptive value of drought rhizogenesis.
Salt stress in cultured rice cells: effects of proline and abscisic acid
Authors: KAVI KISHOR P. B.
Published: April 2006

Abstract. The presence of 1 and 10 mol m−3 proline in media containing 100 and 200 mol m−3 of NaCl, had little effect on the growth of salt‐adapted callus of rice. However, in such callus proline accumulation was stimulated by 10 mol m−3 proline in the presence of 100 mol m−3 NaCl. On the other hand, with 100 mol m−3 NaCl, both 1 and 10 mol m−3 proline significantly increased both the growth and proline content of salt‐unadapted callus. On replacing NaCl with KCl (100 and 200 mol m−3), growth of saltadapted as well as unadapted callus was inhibited, but the presence of 10 mol m−3 proline had an ameliorating effect. Abscisic acid (ABA) supressed the growth of both salt‐adapted and unadapted callus of rice in the absence of salt stress. ABA inhibited the growth of callus adapted to and grown in 100 and 200 mol m−3 of NaCl or when it was replaced by equimolar concentrations of KCl. Growth of 100 mol m−3 NaCl adapted cells was inhibited when they were transferred to a medium containing 200 mol m−3 of NaCl, but in the presence of ABA it was stimulated. ABA increased the growth of unadapted cells when subjected to different salts. Also, ABA accelerated the adaptation of cells exposed to salt but not to water deficits imposed by nonionic solutes.
Stomatal control of transpiration from a developing sugarcane canopy
Authors: MEINZER F. C. , GRANTZ D. A.
Published: April 2006

Abstract. Stomatal conductance of single leaves and transpiration from an entire sugarcane (Saccharum spp. hybrid) canopy were measured simultaneously using independent techniques. Stomatal and environmental controls of transpiration were assessed at three stages of canopy development, corresponding to leaf area indices (L) of 2.2, 3.6 and 5.6. Leaf and canopy boundary layers impeded transport of transpired water vapour away from the canopy, causing humidity around the leaves to find its own value through local equilibration rather than a value determined by the humidity of the bulk air mass above the canopy. This tended to uncouple transpiration from direct stomatal control, so that transpiration predicted from measurement of stomatal conductance and leaf‐to‐air vapour pressure differences was increasingly overestimated as the reference point for ambient vapour pressure measurement was moved farther from the leaf and into the bulk air. The partitioning of control between net radiation and stomata was expressed as a dimensionless decoupling coefficent ranging from zero to 1.0. When the stomatal aperture was near its maximum this coefficient was approximately 0.9, indicating that small reductions in stomatal aperture would have had little effect on canopy transpiration. Maximum rates of transpiration were, however, limited by large adjustments in maximum stomatal conductance during canopy development. The product of maximum stomatal conductance and L. a potential total canopy conductance in the absence of boundary layer effects, remained constant as L increased. Similarly, maximum canopy conductance, derived from independent micrometeorological measurements, also remained constant over this period. Calculations indicated that combined leaf and canopy boundary layer conductance decreased with increasing L such that the ratio of boundary layer conductance to maximum stomatal conductance remained nearly constant at approximately 0.5. These observations indicated that stomata adjusted to maintain both transpiration and the degree of stomatal control of transpiration constant as canopy development proceeded.
Shoot temperatures and thermal tolerances for succulent species ofHaworthiaandLithops
Authors: NOBEL P. S.
Published: April 2006

Abstract.Temperatures of small succulent plants, such as species in the generaHaworthiaandLithops, are highly influenced by temperatures of the surrounding soil. Indeed, the minimum and the maximum temperatures of the upper leaf epidermis ofHaworthia retusa. H. turgida. Lithops leslei, andL. turbiniformiswere generally within 1°C of the accompanying soil surface temperatures. An energybudget model closely predicted such soil‐to‐plant temperature differences as well as the effect of the greater convective exchange for the protrudingHaworthiaspecies compared with theLithopsspecies, which were flush with the soil surface. Although a lower shortwave absorptance would reduce maximum shoot temperatures, the shortwave absorptances of all four species were similar to those of the soil in their respective native habitats in South Africa. Tolerances of the four species to low and to high temperatures at three different day/night air temperatures (15°C/5°C, 30°C/20°C, and 45°C/35°C) were analysed using cellular accumulation of a vital stain, neutral red. Chlorenchyma cells were slightly more tolerant of extreme temperatures than were cells of the water‐storage parenchyma. In this regard,H. retusasurvived low and high temperatures that killed the water‐storage parenchyma but not the chlorenchyma. Acclimation to low temperatures and to high temperatures, which was exhibited by all four species, led to estimated tolerances to 1 h at −16°C and 68°C. Although the low temperature tolerance is not particularly noteworthy, very few vascular plants are reportedly able to tolerate such high temperatures.
Variation in stomatal characteristics over the lower surface of Commelina communis leaves
Authors: SMITH S. , WEYERS J. D. B. , BERRY W. G.
Published: April 2006

Abstract. The silicone rubber impression technique was used to measure stomatal apertures in 9 mm2 sampling areas covering the entire lower surface of leaves of Commelina communis L. The data were analysed using a computer program which produced ‘iso‐aperture' contours illustrating local differences in mean stomatal aperture. Little consistency was seen in the iso‐aperture patterns among sampling times, although the stomata were always relatively closed at the leaf tip and base. When stomata in the middle of the lamina were open, those near to the leaf margin tended to be relatively closed. In places, gradients of mean stomatal aperture were as high as 1 μm mm−1. Measurements along a transect across the lower epidermis revealed no correlation of stomatal aperture with the presence of major veins in the mesophyll tissue. Variation in guard‐cell size and stomatal frequency on the lower leaf surface was also analysed. The guard cells were smallest and the stomata more frequent near to the leaf margins. The significance of the results is discussed in relation to measurements of leaf conductance and models of stomatal function.
Theory and practice of a portable photosynthesis instrument
Authors: LEUNING R. , CSIRO P. SANDS
Published: April 2006

Abstract. Theory and practice of non‐steady‐state portable photosynthesis instruments (LI‐6000 and 6200, LI‐COR Inc., Nebraska, U.S.A.) are presented. Mass balance equations for the time dependence of H2O and CO2 mol fractions within the leaf chamber were used to describe instrument function. Measurements for each run were fitted to an exponential function to estimate average rates of CO2 assimilation and transpiration during the measurement period. Stomatal conductances and intercellular CO2 mol fractions were also computed. Linear data analysis used in the LI‐6200 produced similar results for assimilation rates, stomatal conductances and intercellular CO2 concentrations compared to a more rigorous nonlinear analysis, provided humidity within the chamber was kept constant during the measurement period. Instrument performance for CO2 fluxes was confirmed by injecting pure CO2 at steady rates from a microsyringe into the chamber. Miniature evaporimeters were designed to check H2O flux measurements. Significant discrepancies were observed between LI‐6200 estimates of H2O fluxes and direct measurement and errors were attributed to adsorption desorption of water vapour on chamber walls or to leaks. The leaf chamber should be stored at humidities and temperatures similar to those during measurement conditions for maximum reliability of results.
On the significance of C3—C4 intermediate photosynthesis to the evolution of C4 photosynthesis
Authors: MONSON R. K. , MOORE B. d.
Published: April 2006

Abstract Evidence is drawn from previous studies to argue that C3—C4 intermediate plants are evolutionary intermediates, evolving from fully‐expressed C3 plants towards fully‐expressed C4 plants. On the basis of this conclusion, C3—C4 intermediates are examined to elucidate possible patterns that have been followed during the evolution of C4 photosynthesis. An hypothesis is proposed that the initial step in C4‐evolution was the development of bundle‐sheath metabolism that reduced apparent photorespiration by an efficient recycling of CO2 using RuBP carboxylase. The CO2‐recycling mechanism appears to involve the differential compartmentation of glycine decarboxylase between mesophyll and bundle‐sheath cells, such that most of the activity is in the bundlesheath cells. Subsequently, elevated phosphoenolpyruvate (PEP) carboxylase activities are proposed to have evolved as a means of enhancing the recycling of photorespired CO2. As the activity of PEP carboxylase increased to higher values, other enzymes in the C4‐pathway are proposed to have increased in activity to facilitate the processing of the products of C4‐assimilation and provide PEP substrate to PEP carboxylase with greater efficiency. Initially, such a ‘C4‐cycle’ would not have been differentially compartmentalized between mesophyll and bundlesheath cells as is typical of fully‐expressed C4 plants. Such metabolism would have limited benefit in terms of concentrating CO2 at RuBP carboxylase and, therefore, also be of little benefit for improving water‐ and nitrogen‐use efficiencies. However, the development of such a limited C4‐cycle would have represented a preadaptation capable of evolving into the leaf biochemistry typical of fully‐expressed C4 plants. Thus, during the initial stages of C4‐evolution it is proposed that improvements in photorespiratory CO2‐loss and their influence on increasing the rate of net CO2 assimilation per unit leaf area represented the evolutionary ‘driving‐force’. Improved resourceuse efficiency resulting from an efficient CO2‐concentrating mechanism is proposed as the driving force during the later stages.
The determination of sensitivity parameters for auxin‐induced H+‐efflux from Avena coleoptile segments
Authors: FITZSIMONS P. J.
Published: April 2006

Abstract A method is described for the measurement of auxin‐induced H+‐efflux from small populations of Avena coleoptile segments. The method allows the simultaneous investigation of the kinetics of rapid auxin responses over a wide concentration range. IAA promoted linear rates of H+‐efflux and the change in amplitude of response occurred mainly over a low, narrow concentration range (10–50 μmol m‐3). The sigmoidal curve of best‐fit to each set of dose‐response data was determined using non‐linear regression techniques, allowing the objective determination of characteristic tissue sensitivity parameters (RMIN, RMAX, KD and p). The sensitivity parameters for the auxin‐type herbicide fluroxypyr are also presented as well as IAA parameters determined in the presence of abscisic acid and the ‘antiauxin’ PCIB. The interpretation of the parameter values and the potential use of sensitivity parameter analysis for the evaluation of theories concerning plant hormone action and interaction is discussed.
The effect of photoperiod length on respiration in leaves of Saxifraga cernua L., an arctic herb
Authors: McNULTY A. K. , CUMMINS W. R.
Published: April 2006

Abstract Rates of oxygen uptake were measured in leaves of Saxifraga cernua which had been exposed to an 18‐h photoperiod. These rates were compared to those in plants which had been exposed to continuous light. Rates of total dark respiration and alternative pathway respiration measured at the end of the photoperiod gradually decreased over the initial 3 d of exposure to an 18‐h photoperiod. Thereafter, respiratory rates were constant. Rates of total dark respiration and alternative pathway respiration decreased during the 6h dark period. Rates of normal and alternative pathway respiration are equally affected during the dark period. The respiratory rates had reached a new minimum level 3 d after the initiation of a dark period. These results suggest that respiration rates in arctic plants are high because of the long photoperiod in the arctic. The kinetics of photoperiod induced changes in respiration are slow enough to suggest the involvement of the biological clock in setting respiration rates. Indeed, total dark respiration and alternative pathway respiration show a definite circadian rhythm. Free‐running experiments show that normal respiration changes much less (has a smaller amplitude of variation) than alternative pathway respiration and that alternative pathway respiration accounts for most of the rhythmicity of respiration.
Photosynthetic activity of chloroembryos of a few selected tropical plants
Authors: ANTOSZEWSKI R. , DABYDEEN S. , LALLA A.
Published: April 2006

Abstract The role of chlorophyll in the mature embryos of several tropical plants (Phthirusa pyrifolia [H.B.K.] Eichl. [Loranthaceae]. Murraya koenigia Kurz. [Rutaceae], Murraya paniculata Jack. [Rutaceae], Syzygium cuminii [L.] Skeels [Myrtaceae]) was investigated. Extracted chloroembryos of all species do photosynthesize when illuminated. Whole mature fruits of M. koeningii, M. paniculata and Syzygium cuminii exhibited some photosynthetic activity, but pericarps of none of the fruits photosynthesized when exposed to light. Thus the photosynthetic activity of fruits may be ascribed to CO2 uptake by chloroembryos embedded in the fruits. A specific aspect of plant physiology, namely the re‐utilization of respired CO2 in the process of photosynthesis is emphasized. It is postulated that within embedded chloroembryos, conditions such as high CO2 concentration, high light intensity, and low oxygen concentration are favourable for conducting intensive photosynthesis. Photosynthesis within enclosed organs has an additional advantage in that is does not expose the plant to any risk of water loss usually associated with photosynthesis.
Adaptations by macroalgae to low carbon availability. II. Ultrastructural specializations, related to the function of a photosynthetic buffer system in the Fucaceae
Authors: AXELSSON L. , CARLBERG S. , RYBERG H.
Published: April 2006

Abstract Among the brown algae, species of the Fucaceae (Pelvetia, Fucus and Ascophyllum) were found to have a ‘photosynthetic buffering’ system, allowing the algae to carry out oxygen production without a concomitant uptake of inorganic carbon. This system was not found in other brown algae examined (e.g. Halidrys, Laminaria and Desmarestia) nor in 16 examined species of red and green algae. Pelvetia, Fucus and Ascophyllum belong to the littoral algae which are periodically emersed. In the Fucaceae, the meristodermal cells were found to have a special organization of organelles. Towards the outer cell wall there was a prominent layer of mitochondria while the chloroplasts were concentrated towards the inner and side walls. Between the mitochondria and the chloroplasts there was a large number of physodes. This arrangement of organelles was not found in the other brown algae examined nor in red or green algae. The significance of this organization of the mitochondria is discussed in connection with the function of the ‘photosynthetic buffering’ system.
Growth and nitrate uptake properties of plants grown at different relative rates of nitrogen supply. I. Growth ofPisumandLemnain relation to nitrogen
Authors: OSCARSON P. , INGEMARSSON B. , LARSSON C.‐M.
Published: April 2006

AbstractThe relations between growth and internal nitrogen concentrations were investigated in nonnodulatedPisum sativumL. cv. Marma andLemna gibbaL. grown at relative rates of nitrate‐N additions (RA) varying from 0.03 to 0.27 d1(Pisum) and 0.05 to 0.40 d1(Lemna). At RA≤0.21 d1(Pisum) and ≤0.30 d1(Lemna), the relative growth rate (RGR) correlated well with RAwhereas higher RAwas not met by any further increawse in growth rate. The tissue nitrogen concentrations at growth‐limiting RAincreased linearly with RGR. The slope of these lines indicate a maximum nitrogen productivity (amount of biomass formed per unit nitrogen and time) of 14.4 g DW g1Nd1forPisumand 15.9 g DW g1N d1forLemna.Extrapolation of the plots to RGR=0 yielded intercepts of 10–15 mg N g−1DW forPisumtissue, whereas forLemnathe intercepts were closer to the origin than forPisum.These intercepts formally define a fraction of the total plant nitrogen that appears not to be active in production of new biomass, her termed ‘non‐growth nitrogen’. The partitioning of nitrogen as well as biomass to the roots increased at low RA, and is discussed in relation to activity of shoots and roots, respectively.
Carbon isotope discrimination and the ratio of carbon gained to water lost in barley cultivars
Authors: HUBICK K. , FARQUHAR G.
Published: April 2006

Abstract A negative correlation between water‐use efficiency (W), defined as the ratio of moles of carbon in the plant to moles of water transpired, and carbon isotope discrimination (Δ) was established for barley in pot experiments using 12 cultivars. The correlation was strong in two independent experiments in four different controlled environment where ambient temperature and vapour pressure deficit were varied and plants were either well‐watered or given limited amounts of water. Variation among cultivars was found in both Δ and W and rankings of both parameters, according to cultivar, were similar in different environments. Limiting water usually increased water‐use efficiency of plants. Total dry matter can be substituted for moles of carbon when calculating water‐use efficiency but the correlation between W and Δ were calculated using the carbon content of dry matter. There were differences varied significantly among cultivars. Despite these differences, correlations were also large between whole plant W and Δ of any of the plant parts. The amount of dry matter partitioned into reproductive growth varied genetically, as did the effect of stress on the partitioning. Growth, W and Δ of barley were compared with theory derived from gas exchange properties and with other literature. The effect on W of variation in vapour pressure deficit in these experiments was removed by multiplying W by vapour pressure deficit to derive the parameter, k(Pa mol C/mol H2O). This allowed comparisons among experiments with different vapour pressure deficits. The mean k for these barley cultivars was similar to that calculated by others for grasses. However, variation was found, and, in contrast with previous work which treats k as a species constant, we conclude that there is promise in selecting for increased k.
Measurement of the photorespiratory activity of the submerged aquatic plantMyriophyllum spicatumL.
Authors: OWTTRIM G. W. , COLMAN B.
Published: April 2006

AbstractThe CO2compensation concentrations (points) of leaves of the submerged vascular aquatic plantMyriophyllum spicatumL. were determined in a closed aqueous system at pH 7.0 by a gas chromatographic technique and over the range 10–30deg;C were found to range from 36 to 46 cm3m−3in medium equilibrated with 21% O2(0.03 kgm−3), and 25 to 35 cm3m−3in medium equilibrated with 2% O2(0.03 kgm−3). The rates of true (TPS) and apparent (APS) photosynthesis of leaves were measured in medium equilibrated with 21% O2and buffered at pH 7.0, at subsaturating concentrations (12.8–18.8 mmol m−3) of dissolved inorganic carbor. (DIC) containing H14CO3, by determining the initial rates of uptake by the leaves of DIC and14C‐activity from the medium. The rate of photorespiration, the difference between TPS and APS, was 7.0–13.3% of TPS over the range of 10–25°C and rose to 29% of TPS at 35°C. The magnitude of the compensation point of this plant is therefore similar to, but is much less O2‐sensitive than, those of C3plants, and the photorespiratory rate, at DIC concentrations near the CO2compensation point, is very low compared to that of C3plants.
Effects of prolonged freezing stress on the photosynthetic apparatus of moderately hardy leaves as assayed by chlorophyll fluorescence kinetics
Authors: SCHULTEIS C. , SANTARIUS K. A.
Published: April 2006

Abstract Moderately frost‐hardy leaves of the wintergreen broadleaf woody shrubs Pyracantha coccinea and Ligustrum ovalifolium and the winter annual herb Spinacia oleracea were subjected to extended freezing stress up to 15 d at temperatures 2–8°C above the mean lethal temperature (LT50). After thawing, the fast kinetics of in vivo chlorophyll fluorescence of photosystem II (PSII) and the potential of linear photosynthetic electron transport of isolated thylakoid membranes was measured at room temperature. The lower the minimum freezing temperature and the longer the time of exposure, the greater was the suppression of the fluorescence signals of the leaves and decrease of the electron transport capacity of the thylakoid membranes. The pattern of inactivation of PSII‐mediated electron flow, i.e. inhibition of photoreaction to photochemistry and/or electron donation to the photochemical reaction, during long‐term freezing at temperatures somewhat above the LT50 of the leaves was similar to that observed earlier after relatively brief exposure of leaves and isolated thylakoid membranes to more severe freezing stress. As injury occurred during freezing in complete darkness, it is likely that prolonged winter stress under natural environmental conditions causes changes in the photosynthetic apparatus of moderately hardy leaves which are not due to photoinhibition.