bluuebunny/crossref_metadata_2025 huggingface dataset:
A compact lighting system for use with portable leaf chambers to measure photosynthesis
Authors: YOUNG A. T. , HARRISON G. E. , LAWLOR D. W.
Published: April 2006

Abstract. A compact, portable lighting system, developed for the measurement of photosynthesis in the field with portable chambers and applicable to the laboratory, is described. The system consists of a miniature 50‐W, 12‐V tungsten‐halogen lamp and a light guide which is constructed with randomized fibre bundles and mounted above the window of a portable assimilation chamber. The light guide distributes light uniformly across the leaf surface; the photon irradiance at the leaf surface is monitored by measuring the irradiant emitted from one fibre‐optic bundle using a sensor connected to the body of the light guide. Up to 1600 μmol quanta photosynthetic active radiation m−2s−1 may be achieved at the leaf surface; irradiance may be varied by wire mesh screens of different densities. Leaf temperature follows air temperature outside the chamber to within ±2 °C over the range 10–30 °C and within the range of photon irradiances from 0 to 1600/μmol quanta m −2s. The power supply for the lamp is a 12‐V, 24‐A h lead‐acid battery and the photon irradiance at the leaf surface gradually decreases by c. 3% over 2.5 h of measurement. With this system, response of photosynthetic rate to irradiance and to CO2 partial pressure at constant irradiance may be measured in the field, independent of natural variations in solar irradiance.
Cell specialization within the parenchymatous bundle sheath of barley
Authors: WILLIAMS M. L. , FARRAR J. F. , POLLOCK C. J.
Published: April 2006

Abstract. Structural and physiological aspects of the parenchymatous bundle sheath (PBS) were studied in cultivars of Hordeum distichum L. 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 × 10−5mm3 compared to 1.23 × 10−5mm3 for 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 relation to these findings.
Extraction and analysis of sap from individual wheat leaf cells: the effect of sampling speed on the osmotic pressure of extracted sap
Authors: MALONE M. , LEIGH R. A. , TOMOS A. D.
Published: April 2006

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 and Tradescantia. 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 in Tradescantia cells 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 sampling.
Cold‐acclimation of Hibiscus rosa‐sinensis L. and Hibiscus syriacus L. in natural and controlled environments
Authors: TEETS T. M. , HUMMEL R. L. , GUY C. L.
Published: April 2006

Abstract Seasonal cold‐acclimation patterns and the effects of photoperiod and temperature on cold‐hardiness of Hibiscus rosa‐sinensis L. and Hibiscus syriacus L. were determined. Field‐grown H. rosasinensis consistently failed to survive freezing at ‐ 2°C. Two genotypes of field‐ and container‐grown H. syriacus initiated cold‐acclimation in mid September, in response to decreasing daylength, and continued to an ultimate midwinter hardiness level of ‐ 27°C in early February. Controlled environment experiments using combinations of short days (SD) and cool day/night temperatures were unable to induce even minimal cold acclimation of H. rosasinensis. In controlled environments, H. syriacus attained a moderate amount of cold tolerance at warm temperatures and long days (LD). Low night temperature combined with LD, warm day produced the same degree of cold‐acclimation as the SD treatments. While not essential, SD enhanced H. syriacus cold‐acclimation in controlled environments. A ‐ 5°C frost treatment of intact plants did not enhance cold‐hardiness of H. syriacus.
The ‘end‐of‐day’ phytochrome control of internode elongation in mustard: kinetics, interaction with the previous fluence rate, and ecological implications
Authors: CASAL J. J. , SMITH H.
Published: April 2006

Abstract The ‘end‐of‐day’ phytochrome control of internode growth was characterized in Sinapis alba, seedlings previously grown under continuous white light for 13 d. The transition from white light to darkness caused a reduction in internode extension rate with a lag of less than 10 min. Following this, extension rate remained almost constant for at least 48 h. i.e. ‘re‐etiolation’ was not noticed. The phytochorme controlling the growth processes was stable in the Pfr form. The growth rate of plants receiving a red light pulse, and the growth promotion caused by a far‐red light pulse, increased with increasing fluence rate of the previous white light treatment. In far‐red treated plants a first growth rate acceleration peaked at 20–30 min after the end of white light, followed by a transient deceleration which led to a growth rate minimum at 40–60 min, followed by a final growth rate recovery yielding a more‐or‐less steady elevated rate. Pulses establishing different Pfr/P modified the extent, but not the early kinetics, of the growth response. The relative promotion of growth caused by low Pfr/P was limited by darkness as follows: (a), The growth promotion caused by far‐red directed to the internode alone was transient. (b), The promotion caused by a reduction of Pfr/P in the whole shoot persisted in darkness for at least 48 h and also persisted if, after a 3–9 h dark period, the plants were returned to continuous white light. In darkness, however, the magnitude of this growth rate promotion decreased with time, particularly when the previous white light fluence rate was low, or the pulse preceding darkness provided the lowest Pfr/P. (c), When compared over the same period in darkness, growth rate was higher in those seedlings in which Pfr/P was reduced during the continuous white light pretreatment than in those ones in which the Pfr/P was only reduced immediately before darkness. It is proposed that in the natural environment, red/far‐red signals could be more effective when provided during daytime than at the end of the photoperiod, as both the background growth rate and the relative promotion caused by low Pfr/P are reduced by darkness.
Some effects of nitrate‐treated soil upon the sensitivity of buried redroot pigweed (Amaranthus retroflexusL.) seeds to ethylene, temperature, light and carbon dioxide
Authors: EGLEY G. H.
Published: April 2006

AbstractFresh dormant redroot pigweed (Amaranthus retroflexusL.) seeds were buried 5 cm deep in the field at Stoneville, MS in November 1981. Potassium nitrate (200 kg ha1) or nothing was applied to the soil in the fall of 1981 and the late winter of 1982. Seeds were recovered at intervals under darkness during the following 2 years and tested for responses to ethylene, temperature, light and carbon dioxide. During the first overwintering, nitrate enhanced loss of primary dormancy and increases seed sensitivity to temperature, light and ethylene. The loss of dormancy reached a maximum at 25 to 30 weeks (early summer) after burial. Examination of the recovered seeds indicated that about 80% of the non‐treated seeds and 98% of the nitrate‐treated seeds germinatedin situduring the period of maximum loss of dormancy. Thus, after one overwintering period, about 20% of the original buried seed population remained dormant in nontreted soil and 2% remained dormant in the nitratetreated soil. After the second overwintering, the percentages of dormant seeds remaining in nontreated or treated soil were both only 1–2%. Nitrate reduced dormancy and enhanced germination in early summer following the first overwintering. Regardless of treatment, the remaining 1 2% of seeds in soil after the second year were of low sensitivity to the germination stimuli (ethylene, temperature, light) and constituted the long‐lived portion of the original seed population.
Chloroplast movement
Authors: HAUPT W. , SCHEUERLEIN R.
Published: April 2006

Abstract. Chloroplasts redistribute and/or reorientate in the cell as a response to the light direction, resulting in patterns typical for light of low or high fluence rate, respectively. Usually, the main photoreceptor pigment is a blue‐UV‐absorbing pigment (‘cryptochrome’), but in a few exceptional cases, the reversible red/far‐red system phytochrome is involved. Detection of light direction is based on light refraction and/or on dichroic orientation of photoreceptor molecules. Membrane effects, intracellular calcium redistribution and calcium‐calmodulin interaction are discussed as likely steps in signal transduction. In the response mechanism the actin‐myosin system is involved. However, several details of perception, transduction and response are still unsolved and open for discussion. Particularly interesting are the cases of multiple photoreceptor systems, i.e. those where separate transduction chains are started which coact or interact with each other. This raises the question as to the evolution of multiple photoreceptor systems under the assumption that light‐oriented chloroplast movements serve to optimize photosynthesis.
Chromatic adaptation and the events involved in phycobilisome biosynthesis
Authors: GROSSMAN A. R.
Published: April 2006

Abstract. The major light‐harvesting complex in cyanobacteria and red algae is the phycobilisome, a macromolecular complex that is attached to the surface of the photosynthetic membranes. The phycobilisome is composed of a number of different chromophoric polypeptides called phycobiliproteins and nonchromophoric polypeptides called linker proteins. Several environmental parameters modulate the synthesis, assembly and degradation of phycobilisome components. In many cyanobacteria, the composition of the phycobilisome can change to accommodate the prevalent wavelengths of light in the environment. This phenomenon is called complementary chromatic adaptation. Organisms that exhibit complementary chromatic adaptation must perceive the wavelengths of light in the environment and transduce the light signals into a sequence of biochemical events that result in altering the activities of genes encoding specific phycobiliprotein and linker polypeptides. Other environmental parameters such as light intensity and nutrient status can also have marked effects on both the number and composition of the phycobilisomes. The major concern of this article is the molecular events involved in chromatic adaptation. Most of the information concerning this process has been gained from studies involving the filamentous cyanobacterium Fremyella diplosiphon. However, also briefly considered are some of the complexities involved in phycobilisome biosynthesis and degradation; they include post‐translational modification of phycobilisome polypeptides, the coordinate expression of chromophore and apobiliprotein, the specific degradation of phycobilisomes when cyanobacteria are deprived of macronutrients such as nitrogen, sulphur and phosphorus, and the assembly of the individual phycobilisome components into substructures of the light harvesting complex.
Sensing soil oxygen
Authors: DREW M. C.
Published: April 2006

Abstract. Under natural conditions where gaseous exchange between soil and atmosphere is restricted by excess water, the concentration of O2 in the rooting zone can become very low while reduced ions and organic compounds that are potentially phytoxic may accumulate. Mechanisms by which shoots and roots detect, and adjust to, this O2‐deficient environment are reviewed. Injury to roots and their inability to function because of insufficient O2 is communicated to the shoot in a variety of ways, so that it adjusts physiologically. Roots may acclimate metabolically to a gradual fall in O2 supply, so that they either improve their tolerance of anoxia, or partially avoid O2‐deficiency by structural changes that aid internal transfer of O2 to the roots from the shoot. Molecular mechanisms regulating such metabolic changes, including environmental cues, are discussed.
Cytokinins and tRNAs: a hypothesis on their competitive interaction via specific receptor proteins
Authors: ROMANOV G. A.
Published: April 2006

Abstract. A hypothesis concerning the molecular mechanism of cytokinin action in plants is presented. Free cytokinins and some cytokinin‐containing tRNAs are supposed to compete in the binding to a postulated receptor protein. As a result, the activity of some isoacceptor tRNAs would depend on cytokinin concentration in a cell. This explains a stimulatory effect of cytokinins on the translation of a definite set of cellular mRNAs. Various aspects of the hypothesis are briefly discussed.
L‐acetylcarnitine, a substrate for chloroplast fatty acid synthesis
Authors: MASTERSON C. , WOOD C. , THOMAS D. R.
Published: April 2006

Abstract. H14CO3 was not incorporated into fatty acids by isolated pea leaf chloroplasts, which, therefore, do not possess a self‐contained pathway for the synthesis of fatty acids from early intermediates of the Calvin cycle. Citrate, pyruvate, acetate and L‐acetylcarnitine were all shown to act as sources of acetyl groups for fatty acid synthesis by pea leaf chloroplasts. L‐acetylcarnitine was the best substrate, being incorporated into fatty acids at rates that were at least five‐fold higher than those achieved with the other substrates. Citrate was incorporated into fatty acids at the lowest rate, followed by pyruvate, with acetate being incorporated at the second highest rate of all. When the isolated chloroplasts were ruptured, an inhibition of L‐acetylcarnitine incorporation into fatty acids was noted, whilst acetate incorporation remained unaffected. L‐acetylcarnitine also increased the ratio of monoenoic: saturated fatty acids synthesized, compared with a 1:1 ratio observed when citrate, pyruvate and acetate were supplied as substrates. It is suggested that L‐carnitine and carnitine acyltransferases play a central role in plant acyl CoA metabolism by facilitating the transfer of activated acyl groups across membranes (acyl CoA barriers).
Comparative effects of selenite and selenate on nitrate assimilation in barley seedlings
Authors: ASLAM M. , HARBIT K. B. , HUFFAKER R. C.
Published: April 2006

Abstract. The effect of SeO3 and SeO4 on NO3 assimilation in 8‐d‐old barley (Hordeum vulgare L.) seedlings was studied over a 24‐h period. Selenite at 0.1 mol. m− in the uptake solutions severely inhibited the induction of NO3 uptake and active nitrate reductases. Selenate, at 1.0 mol m−3 in the nutrient solution, had little effect on induction of activities of these systems until after 12 h; however, when the seedlings were pretreated with 1.0 mol m−3 SeO4 for 24 h, subsequent NO3 uptake from SeO4‐free solutions was inhibited about 60%. Sulphate partially alleviated the inhibitory effect of SeO3 when supplied together in the ambient solutions, but had no effect in seedlings pretreated with SeO3. By contrast, SO4 partially alleviated the inhibitory effect of SeO4 even in seedlings pretreated with SeO4. Since uptake of NO3 by intact seedlings was also inhibited by SeO3, the percentage of the absorbed NO3 that was reduced was not affected. By contrast, SeO4, which affected NO3 uptake much less, inhibited the percentage reduced of that absorbed. However, when supplied to detached leaves, both SeO3 and SeO4 inhibited the in vivo reduction of NO3 as well as the induction of nitrate reductase and nitrite reductase activities. Selenite was more inhibitory than SeO4; approximately a five to 10 times higher concentration of SeO4 than SeO3 was required to achieve similar inhibition. In detached leaves, the inhibitory effect of both SeO3 and SeO4 on in vivo NO3 reduction as well as on the induction of nitrate reductase activity was partially alleviated by SO4. The inhibitory effects of Se salts on the induction of nitrite reductase were, however, completely alleviated by SO4. The results show that in barley seedlings SeO3 is more toxic than SeO4. The reduction of SeO4 to SeO3 may be a rate limiting step in causing Se toxicity.
Do detached root‐cap cells influence bacteria associated with maize roots?
Authors: GOCHNAUER M. B. , SEALEY L. J. , McCULLY M. E.
Published: April 2006

Abstract. A model rhizosphere has been used which consisted of detached root‐cap cells of maize in their surrounding root‐cap mucilage on the surface of Noble agar. These cells were co‐cultured for periods up to 32 d with eight different bacterial isolates from soil‐grown roots and surrounding soil and two laboratory cultures. Cap cells were unaffected by the bacteria. There were five different type‐specific responses of the bacteria in proximity to the cap cells. There were, strong growth inhibition (Rhizobium sp. and Escherichia coli), strong stimulation (Pseudomonas fluorescens, laboratory strain), mixed weak inhibition or stimulation (Pseudomonas fluorescens, field isolate), early inhibition followed by strong stimulation then spore formation (Bacillus spp.), no effect (Streptomyces sp. and Cytophaga sp.). It is concluded that detached root‐cap cells are actively involved in the establishment of characteristic rhizosphere bacterial microflora.
Rapid concentration changes of phosphoenolpyruvate carboxylase mRNA in detached leaves of Mesembryanthemum crystallinum L. in response to wilting and rehydration
Authors: SCHMITT J. M.
Published: April 2006

Abstract. Mesembryanthemum crystallinum switches the mode of carbon assimilation from C3 to Crassulacean acid metabolism (CAM) when stressed with sodium chloride. The induction of PEPCase (phosphoenolpyruvate carboxylase), a key enzyme of CAM, has been investigated in detached leaves. Detached C3‐leaves wilt rapidly, losing about 40% of their water during 8 h in the light. Under these conditions, mRNA for PEPCase is induced up to 13‐fold, PEPCase enzyme is induced up to twofold and mRNA for the small subunit of ribulose bisphosphate carboxylase (rbcS) is decreased down to below 20%. The changes are not due to wounding, as could be shown by immersing the leaves in water. Increase of PEPCase transcripts under wilting conditions can be seen after only 2 h. Conversely, when leaves from prestressed plants are detached and allowed to imbibe water, the level of PEPCase mRNA is decreased by 80% in an 8‐h period, whereas rbcS transcripts are increased to 150%. Infiltration with 500 mol m−3 salt solutions is less effective than wilting in inducing PEPCase transcripts. Different salts (sodium, potassium, chloride and sulphate ions) give similar responses, both in detached leaves and in whole plants. The data show that signals from the root are not required for both the increase in PEPCase transcript levels during water stress and their decrease during relief from stress.
The role of pH and ion transport in oligosaccharide‐induced proteinase inhibitor accumulation in tomato plants
Authors: DOHERTY H. M. , BOWLES D. J.
Published: April 2006

Abstract. The induction of proteinase inhibitor (PrI) activity in young tomato plants by wounding and oligosaccharides has been shown to be prevented by pretreatment of the plants with phenolic acids such as aspirin. Aspirin applied over a wide range of concentrations is able to inhibit PrI induction by pectic fragments. The possibility that other weak acids may also inhibit PrI induction was investigated. Isobutyric acid and trimethylacetic acid were shown to be less effective as inhibitors than aspirin, and weak bases were without effect. However, it was demonstrated that various agents known to influence ATPase activity and intracellular pH were able to inhibit PrI induction, and in particular the striking effect of low concentrations of fusicoccin on PrI induction was noted.
Physiology of the interaction of angiosperm parasites and their higher plant hosts
Authors: PRESS M. C. , GRAVES J. D. , STEWART G. R.
Published: April 2006

Abstract.Interactions between parasitic angiosperms and their hosts occur at the level of seed germination, haustorial development and resource transfer. Chemicals released from the host function as cues for host recognition, and trigger germination as well as haustorial initiation. Transpiration is a key process regulating solute transfer from host to parasite, and some parasitie plants have unusual stomatal characteristics. Although solute transfer is apoplastic, the haustorium appears to play a role in regulating solute composition. Host responses to infection are reviewed, and it is concluded that competition for water and solutes are unlikely to play a major role in determining reductions in host productivity: metabolic incompatability is suggested to be the major cause of this.
Salinity response of a freshwater charophyte, Chara vulgaris
Authors: WINTER U. , KIRST G. O.
Published: April 2006

Abstract. Chara vulgaris L. growing in an oligohaline lake was adapted to laboratory conditions and subjected to long‐term salinity treatments ranging from 0 to 350 mol m 3 NaCl added to the lake water (40–680 mosmol kg 1). Osmotic potential and concentration of the main osmotically active solutes (K+, Na+, Mg2+, Cl and sucrose) in the vacuolar sap of the central internodal cells were estimated. C. vulgaris did regulate turgor but incompletely. Turgor decreased from 335 mosmol kg 1 under control conditions to 52–111 mosmol kg 1 at 350 mol m 3 NaCl. The enhancement of πi was achieved by increase in both ions and sucrose. Sterile and fertile plants differed in their response to osmotic stress. In sterile plants, the ions accounted for about 87% of the vacuolar osmotic potential. The increase of πi under osmotic stress was exclusively due to an accumulation of Na+ and Cl‐. In fertile plants, sucrose accounted for about 35% of πi and ions for about 51% Under osmotic stress, sucrose content increased together with the ionic content of Na+ and Cl‐.
The influence of high levels of brief or prolonged supplementary far‐red illumination during growth on the photosynthetic characteristics, composition and morphology of Pisum sativum chloroplasts
Authors: CHOW W. S. , GOODCHILD D. J. , MILLER C. , ANDERSON J. M.
Published: April 2006

Abstract. Peas were grown in controlled environments (12h white fluorescent light. ∼47 μmol photons m‐2 s 1/12 dark, 25 °C), using (1) 15‐min far‐red illumination at the end of each photoperiod (brief FR) to simulate the increase in the far‐red/red ratio near the end of the day, and (2) high levels of supplementary far‐red light (red:far‐red ratio=0.04) during the entire photoperiod (long‐term FR) to simulate extreme shade conditions under a plant canopy. Brief FR illumination led to marked morphological effects attributable to phytochrome regulation, namely, an increase in internodal length, but a decrease in leaflet area, chloroplast size and chlorophyll content per chloroplast compared with the control. Significantly, brief FR illumination had little or no effect on the amounts of the major chloroplast components (ribulose 1.5‐biphosphate carboxylase, adenosine triphosphate synthase, cytochrome b/f complex and Photosystem II) relative to chlorophyll or Photosystem I, and the leaf photosynthetic capacities per unit chlorophyll were similar. In contrast, supplementing high levels of far‐red light during the entire photoperiod not only led to the phytochrome effects above, but there was also a marked increase in leaf photosynthetic capacity per unit chlorophyll. due to increased amounts of the major chloroplast components relative to chlorophyll or Photosystem I. We hypothesize that supplementary far‐red light, absorbed by Photosystem I, induced an increase in the major chloroplast components by a photosynthetic feedback mechanism. In fully greened leaves, we propose that the two photosystems themselves, rather than phytochrome, may be the predominent sensors of light quantity in triggering modulations of the stoichiometries of chloroplast components, which in turn lead to varying photosynthetic capacities.
Gas exchange measurements required to predict the newly fixed carbon pool size in a source leaf of corn (Zea mays L.)
Authors: KOSHKIN E. , TRETIAKOV N. N.
Published: April 2006

Abstract. Steady‐state photosynthesis (Pn), post‐illumination CO2 release rates (R), sucrose‐phosphate synthase (SPS) activities, and levels of starch, sucrose and hexoses were measured in the source leaf of corn (Zea mays L.) during a 16‐h photoperiod at 800 μmol m 2 s 1. Pn and SPS activity remained constant. Carbohydrate pools increased at a linear rate, except the first and last hour of the photoperiod. Both the CO2 evolution rate at the moment of light removal (Rmax) and SPS activity decreased by one half after the onset of darkness (0 60 min). Sucrose diminished during this period by 40%, whereas the starch remained constant. Thereafter, starch mobilization began, followed by a gradual decline in leaf respiration. The average dark export rate was calculated to be 60% less than that during the day. Maintenance respiration (Rm) of an attached leaf after 48 h darkness was determined. Plants were illuminated for different intervals (e.g. 5, 10 or 20 min), each followed by dark periods sufficient for respiration to decline to Rm. The ratio of C assimilated in light to that released in dark was 6:1. After the 48‐h dark period, the minimal period of illumination (Tmin) required to restore Pn and Rmax to the original level was determined. A mathematical analysis of the kinetics involved in the recovery of Pn and Rmax provided an estimate of turnover time (0.22h) and pool size 9.15 mmol m 2) for the newly fixed carbon.
Lipids, carbohydrates and amino acids exuded from the axenic roots of rape seedlings exposed to water‐deficit stress
Authors: SVENNINGSSON H. , SUNDIN P. , LILJENBERG C.
Published: April 2006

Abstract. Rape (Brassica napus [L.]) plants were cultivated for 25 d with axenic roots in a growth system with quartz sand and circulating nutrient solution. After a water stress period of 24h. fresh nutrient solution was added and root exudates were collected 3d later. The stress did not cause significant differences in the weights between the stressed and the control plants. The stressed plants tended to exude more soluble organic carbon than the control plants. This was not reflected in the amounts of low molecular weight carbohydrates exuded. A significantly lower proportion of the soluble organic carbon exuded by the stressed plants was made up of amino acids in comparison with the control plants, 7% and 28% respectively. Exuded hydrophobic substances could be recovered from the sand particles. The stressed plants exuded more sterols than the control plants, and an increased number of polar lipid types. The exudates of the stressed and control plants also differed regarding their fatty acid composition.
Temperature sensitivity of in vitro germination and seedling development of Dactylorhiza majalis (Orchidaceae) with and without a mycorrhizal fungus
Authors: RASMUSSEN H. , ANDERSEN T. F. , JOHANSEN B.
Published: April 2006

Abstract. Symbiotic germination and development in vitro of Dactylorhiza majalis seeds with a strain of Rhizoctonia is very temperature dependent. Above an optimum at 23–25 °C there is a marked decline in germination percentage. Seeds that did germinate at higher temperatures had only little or no development of mycorrhiza, and developed few or no rhizoids compared with seedlings raised at optimal or lower temperatures. Six‐week‐old seedlings grown for additional 4 weeks on a range of temperatures had an optimal length increase at 23–24.5 °C mean temperature. At superoptimal temperatures (26 °C), the seedlings contained smaller starch reserves than those at lower temperatures and increased about as much in length as seedlings grown at 13 °C but much less than those grown at optimum. Temperature also influenced the differentiation of the leafy shoot, seedlings growing to a larger size before shoot initiation in the temperature range of optimal growth. Because of the small span between optimal and too‐high temperatures, a careful assessment of temperature optimum will be necessary in any orchid/fungus relationship before judging the success of symbiosis. At optimal temperature, symbiotic germination gave a germination percentage about twice that using a good asymbiotic method. The increase in seedling length was about 45% per week in symbiotic culture compared with less than 30% in the asymbiotic culture.
C, N and P nutrition of Lemanea mamillosa Kütz. (Batrachospermales, Rhodophyta) in the Dighty Burn, Angus, U.K.
Authors: MacFARLANE J. J. , RAVEN J. A.
Published: April 2006

Abstract. Field measurements of the growth rate of the red freshwater macroalga Lemanca mamillosa Kutz, in the Dighty Burn, together with measurements of water velocity, [CO2], [NO3], [NH3+ NH4+] and [phosphate], have been made between February and July. This period covers the growth of the erect gametophyte and later of the carposporophyte inside the gametophyte. Hydrodynamic studies in the laboratory on benzoic acid models of the gametophyte suggest an average in situ unstirred layer some 12 μm thick. For growth of the gametophyte, this estimated boundary layer thickness, together with the measured inorganic C transport pathway within the plant, suggest that growth is not significantly restricted by CO2 transport from the bulk phase to the plastids. δ13C measurements on source CO2 and on plant organic C bear this out. Habitat choice (low temperatures: CO2 enrichment from ground‐water input: rapid water flow), plant morphology and anatomy (turbulence‐generating ‘knobbles’ on the nodes; plastids close to the outside of the plant), and plant biochemistry (high CO2 affinity of the RUBISCO carboxylase; quite high carbonic anhydrase activity) are responsible for this lack of limitation by inorganic C transport in the growing gametophyte which lacks HCO3 transport and a CO2 concentrating mechanism. Transport through the boundary layer does not significantly restrict acquisition by the plant of N (probably as NH4+, despite the preponderance of NO3 in the environment) or of P (as orthophosphate) in the field. The membrane transporters, which have high substrate affinities (K½'s about 2 mmol NH4+ m‐3 and < 2 mmol inorganic phosphate m−3), probably impose the major limitation. The development of the carposporophyte later in the season, and an increase in the thickness of the cortex of the gametophyte, result in an increased (less negative) δ13C, suggesting a significant diffusion limitation to CO2 transport. This conclusion is reinforced by consideration of the opposing effect on Δδ13 C of the decreased demand for products of phosphoenolpyruvate carboxylase activity as the N/C ratio decreases late in the growing season.
Natural deuterium and oxygen‐18 enrichment in leaf water of cotton plants grown under wet and dry conditions: evidence for water compartmentation and its dynamics
Authors: YAKIR D. , DeNIRO M. J. , GAT J. R.
Published: April 2006

Abstract. Significant differences in leaf water oxygen and hydrogen isotopic composition were observed between cotton plants grown under wet and dry conditions. The magnitude of the differences could be fully explained by the conventional model that describes the isotopic composition of an evaporating water pool under steady state conditions. The results indicate that leaf water isotopic composition is strongly influenced by transpiration rate via its effects on relative humidity adjacent to the leaf surface and on the isotopic composition of the air moisture. Our application of the model, however, provides evidence that leaf water must consist of a mixture of several isotopically distinct pools. These pools are suggested to reside in the symplast, in the cell walls and intercellular spaces and in the veins. A model is proposed suggesting that only the water residing in the cell walls and the intercellular spaces (the transpiration pool) interacts directly with the external environment. The large symplastic pool responds to the external environment to a limited extent via its relatively slow exchange with water in the transpiration pool. It is likely that the isotopic composition of water in the symplastic pool is strongly buffered against shortterm environmental variations, a possibility that would have important implications for the isotopic conditions under which organic matter biosynthesis occurs.
Chlorophyll fluorescence measured using the Fraunhofer line‐depth principle and relationship to photosynthetic rate in the field
Authors: CARTER G. A. , THEISEN A. F. , MITCHELL R. J.
Published: April 2006

Abstract.A field study was conducted to determine the relationship of solar‐excited chlorophyllafluorescence to net CO2assimilation rate in attached leaves. The Fraunhofer line‐depth principle was used to measure fluorescence at 656.3 nm wavelength while leaves remained exposed to full sunlight and normal atmospheric pressures of CO2and O2. Fluorescence induction kinetics were observed when leaves were exposed to sunlight after 10 min in darkness. Subsequently, fluorescence varied inversely with assimilation rate. In the C4Zea mays, fluorescence decreased from 2.5 to 0.8 mW m‐2nm‐1as CO2assimilation rate increased from 1 to 8 μmol m‐2s‐1(r2= 0.520). In the C3Liquidambar styracifluaandPinus taeda, fluorescence decreased from 6 to 2 mW m‐2nm‐1as assimilation rate increased from 2 to 5 or 0 to 2 μmol m‐2s‐1(r2= 0.44 and 0.45. respectively). The Fraunhofer line‐depth principle enables the simultaneous measurement of solar‐excited fluorescence and CO2assimilation rate in individual leaves, but also at larger scales. Thus, it may contribute significantly to field studies of the relationship of fluorescence to photosynthesis.
Wall extensibility and gravitropic curvature of sunflower hypocotyls: correlation between timing of curvature and changes in extensibility
Authors: BAGSHAW S. L. , CLELAND R. E.
Published: April 2006

Abstract. Gravitropic curvature results from unequal growth rates on the upper and lower sides of horizontal stems. These unequal growth rates cold be due to differences in wall extensibility between the two sides. To test this, the time course of curvature of horizontal sunflower (Helianthus anmus L.) hypocotyls was determined and compared with the time courses of changes in Instron‐measured wall extensibility (PEx) of the upper and lower epidermal layers. As gravicurvature developed, so did the difference in PEx between the upper and lower epidermis. The enhanced growth rate on the lower side during the period of maximum increase in curvature was matched by PEx values greater than those of the vertical control, while the inhibited growth rate on the upper side was accompanied by PEx values below that of the control. The close correlation between changes in growth rates and alterations in PEx demonstrates that changes in wall extensibility play a major role in controlling gravicurvature.
Effects of metals on enzyme activity in plants
Authors: VAN ASSCHE F. , CLIJSTERS H.
Published: April 2006

Abstract.Uptake of phytotoxic amounts of metal by higher plants or algae can result in inhibition of several enzymes, and in increase in activity (= induction) of others. Two mechanisms of enzyme inhibition predominate: (1) binding of the metal to sulphydryl groups, involved in the catalytic actionor structural integrity of enzymes, and (2) deficiency of an essential metal in metalloproteins or metal‐protein complexes, eventually combined with substitution of the toxic metal for the deficient element. Metal accumulation in the cellular compartment of the enzyme is a prerequisite for enzyme inhibitionin vivo.The induction of some enzymes is considered to play a significant role in the stress metabolism, induced by metal phytotoxicity. Peroxidase induction is likely to be related to oxidative reactions at the biomembrane; several enzymes of the intermediary metabolism might be stimulated to compensate for metal‐sensitive photosynthetic reactions. The induction of enzymes and metal‐specific changes in isoperoxidase pattern can be used as diagnostic criteria to evaluate the phytotoxicity of soils, contaminated by several metals. Lines for future research on metal phytotoxicity are proposed, involving the study of inhibition and induction of enzymes at the different cell membranes (especially the plasmamembrane)in vivo.
Rapid, bilateral changes in growth rate and curvature during gravitropism of cucumber hypocotyls: implications for mechanism of growth control
Authors: COSGROVE D. J.
Published: April 2006

Abstract. The growth response of etiolated cucumber (Cucumis sativus L.) hypocotyls to gravitropic stimulation was examined by means of time‐lapse photography and high‐resolution analysis of surface expansion and curvature. In comparison with video analysis, the technique described here has five‐ to 20‐fold better resolution; moreover, the mathematical fitting method (cubic splines) allows direct estimation of local and integrated curvature. After switching seedlings from a vertical to horizontal position, both upper and lower surfaces of the stem reacted after a lag of about 11 min with a two‐ to three‐fold increase in surface expansion rate on the lower side and a cessation of expansion, or slight compression, on the upper surface. This growth asymmetry was initiated simultancously along the length of the hypocotyl, on both upper and lower surfaces, and did not migrate basipetally from the apex. Later stages in the gravitropic response involved a complex reversal of the growth asymmetry, with the net result being a basipetal migration of the curved region. This secondary growth reversal may reflect oscillatory and or self‐regulatory behaviour of growing cells. With some qualifications, the kinetics and pattern of growth response are consistent with a mechanism involving hormone redistribution, although they do not prove such a mechanism. The growth kinetics require a growth mechanism which can be stimulated by two‐to three‐fold or completely inhibited within a few minutes.
Gravitropism of cucumber hypocotyls: biophysical mechanism of altered growth
Authors: COSGROVE D. J.
Published: April 2006

Abstract. The biophysical basis for the changes in cell elongation rate during gravitropism was examined in aetiolated cucumber (Cucumis sativus L.) hypocotyls. Bulk osmotic pressures on the two sides of the stem and in the epidermal cells were not altered during the early time course of gravitropism. By the pressureprobe technique, a small increase in turgor (0.3 bar, 30 kPa) was detected on the upper (inhibited) side, whereas there was a negligible decrease in turgor on the lower (stimulated) side. These small changes in turgor and water potential appeared to be indirect, passive consequences of the altered growth and the small resistance for water movement from the xylem, and indicated that the change in growth was principally due to changes in wall properties. The results indicate that the hydraulic conductance of the watertransport pathway was large (.25 h 1 bar 1) and the water potential difference supporting cell expansion was no greater than 0.3 bar (30 kPa). From pressureblock experiments, it appeared that upon gravitropic stimulation (1) the yield threshold of the lower half of the stem did not decrease and (2) the wall on the upper side of the stem was not made more rigid by a cross‐linking process. Mechanical measurements of the stress/strain properties of the walls showed that the initial development of gravitropism did not involve an alteration of the mechanical behaviour of the isolated walls. Thus, gravitropism in cucumber hypocotyls occurs principally by an alteration of the wall relaxation process, without a necessary change in wall mechanical properties.
Cloning and characterization of a wound‐specific hydroxyproline‐rich glycoprotein in Phaseolus vulgaris
Authors: SAUER N. , CORBIN D. R. , KELLER B. , LAMB C. J.
Published: April 2006

Abstract. We have isolated a cDNA clone whose corresponding transcript encodes an apoprotein of a hydroxyproline‐rich glycoprotein. Both infection of plants with fungal spores and treatment of suspension‐cultured bean cells with a fungal elicitor preparation cause a rapid decrease of the level of this mRNA. Wounding of bean tissue, however, leads to a very rapid, transient induction of this transcript. In contrast, all bean hydroxyproline‐rich glycoproteins described previously are induced both after infection with fungal spores and after mechanical damage. The protein encoded by this new clone contains a proline‐rich domain of about 30 kilodaltons. Most of these proline residues are part of the characteristic peptides Ser‐Pro4, Scr‐Pro5 and Ser‐Pro6, which are often tandemly repeated. These tandem‐repeats are not found in any of the other bean hydroxyproline‐rich glycoproteins, where almost all of the Ser‐Pro4 units are integrated into higher order repetitive units of 16 amino acids. The gene is present at a single or low copy number in the haploid genome and exhibits an unusual codon usage bias for the amino acids proline and serine.
Some spectral properties of several soil types: implications for photomorphogenesis*
Authors: MANDOLI D. F. , FORD G. A. , WALDRON L. J. , NEMSON J. A. , BRIGGS W. R.
Published: April 2006

Abstract. Light transmission through several sands, soils and size classes of glass beads was investigated. The glass beads served as a highly reflective model system for the sands. Light penetration was significantly better through sands than through either sandy or silty loams. For sandy soils, there was a shift in the spectrum leading to a change in the red‐to‐far‐red photon ratio with increase in depth: this ratio dropped by more than 30% in the first few millimetres, and in some cases increased again with further increases in depth. The depths at which these changes occurred in the soil profile were specific for a given soil type. Similar spectral changes were obtained when sample depth for a sandy soil was held constant, and particle (ped) size of that soil was decreased. No such changes, either with sample depth or ped size, were observed with loams. Per cent transmittance in the blue (442 nm) was more than three orders of magnitude lower than in the red (632.8 nm) for sandy soils. When dry sandy soils were water‐saturated, their transmittance increased by up to three orders of magnitude, and the red far‐red ratios in the transmitted light increased in all cases. By contrast, water‐saturated loams were essentially opaque. The possible significance of these spectral properties of soils in seed germination and seedling photomorphogenesis is discussed.
Zeta potential measurements of cell wall preparations from Regnellidium diphyllum and Nymphoides peltata
Authors: O'SHEA P. , WALTERS J. , RIDGE I. , WAINRIGHT M. , TRINCI A. P. J.
Published: April 2006

Abstract. Cell wall particles were prepared from the semi‐aquatic plants Regnellidium diphyllum and Nymphoides peltata with minimum disruption to the integrity of the cell wall. The behaviour of freshly‐prepared and frozen‐thawed particles in a D.C. electric field was monitored with a microscope attached to video recording apparatus. From the respective particles mobility in a well‐defined electric field. it was possible to determine their electrostatic potential and consequently estimate the corresponding surface charge density. Experiments were performed in media of different pH and cation concentration (ie, K+ Ca2+). A significant electronegative potential was found in cell wall preparations of both plants. Freezing and thawing further reduced the electrostatic potential for both plant species in all the media utilized for electrophoresis. A reduction of pH or an increase of the cation concentration was found to neutralize the electrostatic potential in a sigmoidal fashion. Ca2+ was more than 10 times more effective than K+ at neutralizing the apparent electrostatic potential of the cell wall preparations. Regnellidium was found to have a lower electrostatic potential than Nymphoides, although both responded in a similar manner to the various treatments. The possible relevance of the cell wall electrostatic potential, pH and [Ca2+] and particularly their inter‐relationship is discussed for the two species of plants in terms of their differing growth responses to the ionic environment of the plant.
Survival and death of Chara internodal cells in electrolyte solutions and calcium release from the cell wall
Authors: KIYOSAWA K. , ADACHI T.
Published: April 2006

Abstract. Survival and death of Chara internodal cells were investigated in one of the alkali metal salts KCl, some of the alkali earth metal salts CaCl2, Ca(NO3)2, MgCl2, Mg(NO3)2, SrCl2, Sr(NO3)2, BaCl2 and Ba(NO3)2, potassium phosphate pH buffer solution (pH 7.0), Tris‐maleate pH buffer solution (pH 7.0), HEPES (N‐2‐hydroxyethylpiperazine‐N′‐2‐ethanesulphonic acid)‐KOH (pH 7.0) pH buffer solution, calcium buffer solutions, and deionized water. Most of the internodal cells died within a day or a few days in KCl, MgCl2, Mg(NO3)2, BaCl2 and Ba(NO3)2 solutions of higher concentrations, calcium buffer solutions of pCa 6.0, 10.0 mol m‐3 potassium phosphate pH buffer solution and 10.0 mol m‐3 Trismaleate pH buffer solution. However, all of the internodal cells survived more than 10 d in deionized water, 80.0 mol m‐3 CaCl2, 80.0 mol m‐3 Ca(NO3)2, 80.0 mol m‐3 SrCl2, 80.0 mol m‐3 Sr(NO3)2 calcium buffer solutions of pCa 4.0 and pCa 5.0, and 10.0 mol m‐3 HEPES‐KOH (pH 7.0) pH buffer solution. Addition of Ca2+ or Sr2+ to K+, Mg2+ and Ba2+ salt solutions increased the survival rates of the internodal cells. Calcium release from the internodal cell wall was measured in deionized water, KCl, NaCl, MgCl2, CaCl2, SrCl2 and BaCl2 solutions. Except in deionized water and CaCl2 solution, most of the calcium binding to the cell wall was released within one or a few hours in respective electrolyte solutions. Thus, survival and death of the internodal cells in the electrolyte solutions tested were interpreted in terms of the calcium release from the cell wall and the cell membrane, and intrinsic ability of Sr2+ to maintain the cell membrane normal.