bluuebunny/crossref_metadata_2025 huggingface dataset:
The effect of water stress on photosynthetic carbon metabolism in four species grown under field conditions
Authors: QUICK W. P. , CHAVES M. M. , WENDLER R. , DAVID M. , RODRIGUES M. L. , PASSAHARINHO J. A. , PEREIRA J. S. , ADCOCK M.D. , LEEGOOD R. C. , STITT M.
Published: April 2006

Abstract. The effect of gradually‐developing water‐stress has been studied in Lupinus albus L., Helianthus annuus L., Vitis vinifera cv. Rosaki and Eucalyptus globulus Labill. Water was withheld and diurnal rhythms were investigated 4–8d later, when the predawn water deficit was more negative than in watered plants, and the stomata closed almost completely early during the photoperiod. The contribution of ‘stomatal’ and ‘non‐stomatal’ components to the decrease of photosynthetic rate was investigated by (1) comparing the changes of the rate of photosynthesis in air with the changes of stomatal conductance and (2) measuring photosynthetic capacity in saturating irradiance and 15% CO2. Three species (lupin, eucalyptus and sunflower) showed larger changes of stomatal conductance than photosynthesis in air, and showed little or no decrease of photosynthetic capacity in saturating CO2. Photosynthesis in air also recovered fully overnight after watering the plants in the evening. In grapevines, stomatal conductance and photosynthesis in air changed in parallel, there was a marked decrease of photosynthetic capacity, and photosynthesis and stomatal conductance did not recover overnight after watering water‐stressed plants. Relative water content remained above 90% in grapevine. We conclude that non‐stomatal components do not play a significant role in lupins, sunflower or eucalyptus, but could in grapevine. The effect of water‐stress on partitioning of photosynthate was investigated by measuring the amounts of sucrose and starch in leaves during a diurnal rhythm, and by measuring the partitioning of 14C‐carbon dioxide between sucrose and starch. In all four species, starch was depleted in water‐stressed leaves but sucrose was maintained at amounts similar to, or higher than, those in watered plants. Partitioning into sucrose was increased in lupins and eucalyptus, and remained unchanged in grapevine and sunflower. It is concluded that water‐stressed leaves in all four species maintain high levels of soluble sugars in their leaves, despite having lower rates of field photosynthesis, decreased rates of export, and low amounts of starch in their leaves.
Photoinhibitory responses of the epiphytic bromeliad Guzmania monostachia during the dry season in Trinidad maintain photochemical integrity under adverse conditions
Authors: MAXWELL C. , GRIFFITHS H. , BORLAND A. M. , BROADMEADOW M. S. J. , McDAVID C. R.
Published: April 2006

The C3–CAM epiphytic bromeliad Guzmania monostachia var. monostachia may be exposed to high incident photosynthetically active radiation (PAR) during the dry season in Trinidad, and resultant variations in photochemical efficiency have been investigated for ‘exposed’ (receiving ∼100% incident PAR), ‘semi‐exposed’ (∼60% PAR) and shaded populations under natural conditions. The more succulent leaves of the plants growing fully exposed within the canopy had higher overall CAM activity (measured as ΔH+, the dawn‐dusk titratable acidity), a smaller proportion of chlorenchyma and lower total chlorophyll content. There was a gradation of morphological and physiological characteristics between these and shaded leaves. Diurnal time‐courses of photosynthetic light responses (as O2 evolution) showed marked variations in apparent quantum yield (AQY) and light‐saturated rates for both exposed and semi‐exposed populations, dependent on incident PAR during the day. Similar measurements of photosystem II fluorescence characteristics showed that Fv/Fm declined from 0·70 to 0·42 at midday for exposed plants (on a day when total incident PAR was 44 mol photon m−2), indicating non‐photochemical quenching (qNP) of photosynthesis. However, in contrast to AQY determinations, Fv/Fm recovered during the afternoon. The decrease in Fv/Fm was reduced from 0·72 to 0·64 under 24 mol photon m−2 d−1. The long–term recovery of photo‐synthetic efficiency was assessed for exposed plants placed under three shading regimes (60, 30 and 3% of incident PAR) over a 17‐d period. During this time, total chlorophyll content increased from 228 to 515 and 585 μg g−1 fresh weight (for 3 and 30%, respectively) and chlorophyll a:b declined. While AQY recovery was much longer under the lowest PAR (17d), under 30% PAR both AQY and Fv/Fm had recovered after 2d shading. The differences between timing of recovery for Fv/Fm during diurnal time courses and in the long term suggest that, while quenching associated with PSII recovers rapidly, enzyme activation and/or protein synthesis of other photosynthetic components may be limiting under low PAR. However, it is suggested that the occurrence of qNP on a daily basis may preclude long‐term photoinhibitory damage under natural conditions during the dry season.
Light as a hazard for the desiccation‐resistant ‘resurrection’ fern Polypodium polypodioides
Authors: MUSLIN L. E. H. , HOMANN P. H.
Published: April 2006

Abstract. Using fluorescence measurements at 77 K, the susceptibility to photoinhibition was determined for variously dehydrated leaves of the desiccation–resistant fern Polypodium polypodioides. As expected, water loss increased the potential danger of light‐induced damage. However, the long‐term consequences of that damage differed depending on the hydration state of the leaves during the light treatment, suggesting that different targets were affected. This became evident when the rehydrated leaves were subjected to conditions conducive to recovery from typical photoinhibition. After an exposure to high photon flux densities, full restoration of normal photochemical efficiencies occurred only when the photoinhibitory treatment had occurred while the leaves were fully hydrated, or when the dehydration had reached a stage at which only the primary charge separation and storage were operative. On the other hand, lesions caused by high light intensities in slightly dehydrated leaves which had retained some electron transport activity between the two photosystems, or in desiccated leaves incapable of any measureable photosynthetic functions, were very inefficiently or not at all repaired. One unexpected hazard of high light intensities for dehydrated leaves turned out to be further desiccation which could reach a threshold at which extensive cellular damage began to ensue. In nature, P. polypodioides avoids light–induced damage by rolling its leaves during dehydration so that only the reflectant abaxial leaf surface is exposed to ambient light, and by occupying shaded habitats.
Changes in mRNA levels and polypeptide subunits of ribulose 1,5‐bisphosphate carboxylase in response to supplementary ultraviolet‐B radiation
Authors: JORDAN B. R. , HE J. , CHOW W. S. , ANDERSON J. M.
Published: April 2006

Abstract. Pea plants (Pisum sativum L., cv. Greenfeast) were grown for 17d (150 μmol photons m−2 s−1; 12h light/12 h dark) and then exposed to moderate levels of supplementary ultraviolet‐B radiation (UV‐B: 280–320 nm) during the light cycle. The total soluble leaf protein, maximum Rubisco activity, polypeptide and mRNA transcript levels for Rubisco subunits were then determined in the mature third leaf pair from the base of the plants. Total soluble protein per unit leaf area showed little change after 1 d but declined by 33% during 3d of UV‐B exposure. However, there was no change on a unit chlorophyll basis. Total RNA per unit area declined by 15% and 37% after 1 or 3d of UV‐B treatment, respectively. Maximum Rubisco activity declined by 38% after 1 d and 71% after 3d of UV‐B exposure. Rubisco polypeptide subunits showed some decrease (∼16%) after 1d exposure, but declined by 56% over 3d. The decrease in Rubisco is probably the major reason for the reduction in soluble protein. In contrast to the relatively slow decline in total soluble protein and Rubisco, the level of the mRNA transcripts for both rbc L and rbc S showed a dramatic decrease within hours of UV‐B exposure. The mRNA transcripts for rbc S were reduced to >20% of control values after 4h of UV‐B exposure, while the rbc L transcripts were reduced by 60% after 8h. Further exposure to UV‐B reduced the mRNA transcripts to either trace or undetectable levels. The decrease in rbc S mRNA levels with the UV‐B exposure can be partially ameliorated by higher photosynthetically active irradiance during the period of UV‐B exposure. Plants that were exposed to supplementary UV‐B radiation for short periods (4h or 8h) and returned to control conditions, showed no recovery after 24h. However, after a further 2d, the rbc L and rbc S mRNA transcripts had recovered to ca. 60% of the control values, showing that the effect upon the mRNA transcripts is a reversible response.
Spontaneous oscillations of extracellular electrical potentials measured on Lepidium sativum L. roots
Authors: HECKS B. , HEJNOWICZ Z. , SIEVERS A.
Published: April 2006

Abstract. Electrical potential differences (PDs) were measured continuously at six points in line on the surface of primary roots of cress (Lepidium sativum L.). The root‐to‐hypocotyl transition was used as the reference. A gradient of extracellular potentials existed along the root axis. The most negative potentials were observed at the root tip. Whenever the growth rate declined to 0·10 mm h−1 or less, the PDs were nearly constant in space and time. Otherwise, the PDs showed different types of changes with time. Roots with growth rates between 0·65 and 1·10 mm h−1 preferably showed PDs characterized by approximately sinoidal oscillations. Maximal amplitudes appeared in the middle of the elongation zone. Roots which were exposed to a N2‐atmosphere exhibited a reversible disappearance of the electrical oscillations and a decrease in the PD‐gradient. Similar effects were obtained by the application of the metabolic inhibitor azide (N3−). These results proved that the electrical oscillations are dependent on the aerobic cell metabolism.
Kinetics of chloride transport in the cyanobacterium Synechococcus R‐2 (Anacystis nidulans, S. leopoliensis) PCC 7942
Authors: RITCHIE R. J.
Published: April 2006

The kinetics of the light‐driven Cl− uptake pump of Synechococcus R‐2 (PCC 7942) were investigated. The kinetics of Cl− uptake were measured in BG‐11 medium (pHo, 7·5; [K+]o, 0·35 mol m−3; [Na+]o, 18 mol m−3; [Cl−]o, 0·508 mol m−3) or modified media based on the above. Net36Cl− fluxes (ϕCl−o,i) followed Michaelis‐Menten kinetics and were stimulated by Na+ [18 mol m−3 Na+ BG‐11 ϕCl−max= 3·29±0·60 (49) nmol m−2 s−1 versus Na+‐free BG‐11 ϕCl−max= 1·02±0·13 (54) nmol m−2 s−1] but the Km was not significantly different in the presence or absence of Na+ at pHo 10; the Km was lower, but not affected by the presence or absence of Na+ [Km = 22·3±3·54 (20) mmol m−3]. Na+ is a non‐competitive activator of net ϕCl−o,i. High [K+]o (18 mol m−3) did not stimulate net ϕCl−o,i or change the Km in Na+‐free medium. High [K+]o (18 mol m−3) added to Na+ BG‐11 medium decreased net ϕCl−o,i [18 mol m−3K+ BG‐11; ϕCl−max= 2·50±0·32 (20) nmol m−2 s−1 versus BG‐11 medium; ϕCl−max= 3·35±0·56 (20) nmol m−2 s−1] but did not affect the Km 55·8±8·100 (40) mmol m−3]. Na+‐stimulation of net ϕCl−o,i followed Michaelis‐Menten kinetics up to 2–5 mol m−3 [Na+]o but higher concentrations were inhibitory. The Km for Na+‐stimulation of net ϕCl−o,i [K1/2(Na+)] was different at 47 mmol m−3 [Cl−]o (K1/2[Na+] = 123±27 (37) mmol m−3]. Li+ was only about one‐third as effective as Na+ in stimulating Cl− uptake but the activation constant was similar [K1/2(Li+) = 88±46 (16) mmol m−3]. Br− was a competitive inhibitor of Cl− uptake. The inhibition constant (Ki) was not significantly different in the presence and absence of Na+. The overall Ki was 297±23 (45) mmol m−3. The discrimination ratio of Cl− over Br− (δCl−/δBr−) was 6·38±0·92 (df = 147). Synechococcus has a single Na+‐stimulated Cl− pump because the Km of the Cl− transporter and its discrimination between Cl− and Br− are not significantly different in the presence and absence of Na+. The Cl− pump is probably driven by ATP.
What information is conveyed by an ABA signal from maize roots in drying field soil?
Authors: TARDIEU F. , ZHANG J. , DAVIES W. J.
Published: April 2006

During two seasons, ABA concentrations were monitored in roots, leaves and xylem sap of field‐grown maize. The water status of soil and plant was also measured. Plants were grown on plots with compacted or non‐compacted soil, which were irrigated or remained unwatered. ABA concentration in the xylem sap before dawn and in the roots increases 25‐fold and five‐fold, respectively, as the soil dried, with a close correlation with the soil water status, but with no clear effect of the soil structure. In contrast to the results of several laboratory experiments, no appreciable increase in xylem [ABA] and reduction in stomatal conductance were observed with dehydration of the part of the root system located in soil upper layers. These responses only occurred when the water reserve of the whole soil profile was close to depletion and the transpiration declined. Xylem [ABA] measured during the day was appreciably higher in the compacted treatment than in non‐compacted treatment, unlike that measured before dawn. Since a mechanical message is unlikely to undergo such day‐night alterations, we suggest that this was due to a faster decrease in root water potential and water flux in the compacted treatment, linked to the root spatial arrangement. These results raise the possibility that ABA concentration in the xylem sap could be controlled by two coexisting mechanisms: (1) the rate of ABA synthesis in the roots linked to the soil or root water status, as shown in laboratory experiments; (2) the dilution of ABA in the water flow from roots, which could be an overriding mechanism in field conditions. This second mechanism would allow the plant to sense the water flux through the root system.
137Cs penetration by contact exchange through isolated plant cuticles: cuticles as asymmetric transport membranes
Authors: ERTEL J. , PARETZKE H. G. , ZIEGLER H.
Published: April 2006

The penetration of 137Cs by contact exchange through cuticular membranes from the adaxial surface of leaves of Pyrus communis and Prunus cerasus has been investigated. The resistance of the cuticles to the caesium penetration was dependent on the counter‐ions associated with the fixed negative ion exchange sites in the membrane. The mobility of hydrated potassium ions and their tenuous connection to ‐COOH−‐groups in the membrane encouraged caesium permeation in contrast to cuticular membranes with predominantly protonized ion exchange sites. Divalent calcium ions caused a strong reduction (4–20 times) of the caesium permeability which is decisive for the calculation of the caesium uptake by the intact leaf. Under these conditions, a penetration rate of the deposited caesium of 0·11±0·05% h−1 for pear cuticles and of 0·036±0·025% h−1 for cherry cuticles was measured after the adjustment to steady state conditions. Approximately 12–24% and 4·5–7·5% of initially retained caesium could be absorbed by the leaves of pear and cherry, respectively, in a rain‐free period of 7 d in the area of Munich after wet deposition of fallout from the Chernobyl reactor accident. Furthermore, the caesium penetration from the physiological inside to the outside of the membrane was found to be smaller by a factor of 100–150 compared with that of the opposite direction.
Cytokinins and tRNAs: arguments against a hypothesis of cytokinin action
Authors: JANSSON S.
Published: April 2006

Arguments against the hypothesis of cytokinin action via competitive binding of cytokinin and cytokinin‐containing tRNA to specific receptor proteins [Romanov (1990) Plant, Cell and Environment 13, 751–754] are presented. The hypothesis states that cereal genes containing a large number (4–8%) of the Leu codons TTG and TTA should be regulated by cytokinin at the translational level. In this paper, it is shown that the only genes that fulfil these hypothetical requirements are the ones encoding the heavy chain of zein. It is further shown that this is a consequence of the high Leu content of the encoded proteins and the lack of GC bias of the genes.
Induction of crown galls by Agrobacterium tumefaciens (strain C58) reverses assimilate translocation and accumulation in Kalanchoë daigremontiana
Authors: MALSY S. , BEL VAN , KLUGE M. , HARTUNG W. , ULLRICH C. I.
Published: April 2006

The mechanism of hormone‐enhanced solute accumulation was investigated in crown galls of Kalanchoë diagremontiana, induced by Agrobacterium tumefaciens (C58). Electrophysiological, cytological staining, and 14CO2‐tracer techniques were used. Intracellular auxin and zeatinriboside concentration increased in tumours by three and five times, respectively, compared with the mesophyll. In contrast, the electrical membrane potential difference, in particular the energy‐dependent component across the plasma membrane of tumours was at least 60 mV smaller than that of mesophyll cells. The detection of functional sieve tubes, metabolically active companion cells and also developed xylem vessels correlated well with evidence for reversed, long‐distance, 14C‐labelled assimilate flow from CAM–performing mesophyll to sugar and potassium‐accumulating tumours. The change in hormone production, encoded by the iaaM, iaaH and ipt T‐DNA genes, apparently induced functioning vascular bundles and established a strong sink. From a comparison with tobacco suspension cells (SR1 and SR1‐C58), the changes in the amino acid pattern in the mesophyll/tumour complex can be attributed to the enzymatic activities incited by the nos encoding gene. Differences in sugar and inorganic ion content were not apparent in the transformed suspension cells, indicating that the increase in extracellular concentration by enhanced phloem unloading may be the crucial factor for the pathological solute accumulation in the tumours.
Stomatal conductance, photosynthesis and respiration of temperate deciduous tree seedlings grown outdoors at an elevated concentration of carbon dioxide
Authors: BUNCE J. A.
Published: April 2006

Seedlings of temperate deciduous tree species were grown outdoors at ambient and at an elevated concentration of carbon dioxide to examine how aspects of their gas exchange would be altered by growth at elevated carbon dioxide concentration. Leaf conductances to water vapour and net carbon dioxide exchange rates were determined periodically near midday. Whole‐plant carbon dioxide efflux rates in darkness were also determined. The stomatal conductance of leaves of plants grown and measured at 700 cm3 m−3 carbon dioxide did not differ from that of plants grown and measured at 350 cm3 m−3 in Malus domestica, Quercus prinus and Quercus robur at any measurement time. In Acer saccharinum, lower conductances occurred for plants grown and measured at elevated carbon dioxide concentration only at measurement temperatures above 33°C. Photo‐synthetic adjustment to elevated carbon dioxide concentration was evident only in Q. robur. All species examined had lower rates of dark respiration per unit of mass when grown and measured at elevated carbon dioxide concentration.
Photosynthesis in flashing light in soybean leaves grown in different conditions. I. Photosynthetic induction state and regulation of ribulose‐1,5‐bisphosphate carboxylase activity
Authors: PONS T. L. , PEARCY R. W. , SEEMANN J. R.
Published: April 2006

The photosynthetic induction state under conditions of different lightfleck frequencies or durations, or different shade periods was studied in soybean leaves in order to examine how it might limit utilization of sunflecks in leaf canopies. Induction following an increase in photon flux density (PFD) from strongly limiting to saturating PFDs exhibited two phases; a fast‐inducing one, requiring about 1 min and a slow one, requiring up to 60 min for completion. Transfer of fully induced leaves to low light resulted in a rapid decrease in the fast‐inducing component, a slower decrease in the slow‐inducing component and an even slower decrease in stomatal conductance. Therefore, the decreases in extent of induction appeared to be due to biochemical factors and not to stomatal closure. Under flashing light regimes consisting of 1‐s lightflecks given at different frequencies for long periods, a constant induction state was achieved, the measure of induction state increased with the frequency of the lightflecks. This constant induction state also depended on the growth conditions, with shade leaves having a higher value than those grown at high light at any particular lightfleck frequency. The measure of induction state was mostly lower in flashing light as compared to constant light of the same mean PFD, particularly in leaves with a low light saturation point and in short lightflecks. Initial activities of ribulose‐1,5‐bisphosphate carboxylase (rubisco) were also higher in continuous light and were highly correlated with the measure of induction state. The rapid decrease in extent of induction of soybean leaves during shade periods is an important limitation to the ability of the leaves to respond to light increases similar to those occurring with sunflecks. At least part of the limitation on carbon assimilation during sunflecks due to photosynthetic induction is based on regulation of rubisco activity.
Photosynthesis in flashing light in soybean leaves grown in different conditions. II. Lightfleck utilization efficiency
Authors: PONS T. L. , PEARCY R. W.
Published: April 2006

Leaves of soybean plants grown in contrasting light and nutrient availability conditions were exposed to constant and to flashing light regimes with lightflecks of different frequencies, durations and photon flux density (PFD). The lightfleck characteristics were selected to be representative of the range of variation found for sunflecks in a soybean canopy. CO2 fixation rates were measured using a fast‐response gas‐exchange apparatus. The net CO2 fixation due to 1‐s‐duration lightflecks was 1·3 times higher than predicted from steady‐state measurements in constant light at the lightfleck and background PFD. This lightfleck utilization efficiency (LUE) was somewhat higher at a high than at a low frequency of one second lightflecks. LUE in flashing light with very short lightflecks (0·2s) and single 1 s lightflecks was as high as 2, but decreased sharply with increasing duration of lightflecks. This decrease occurred because CO2 fixation rates during lightflecks were constrained by carbon metabolism and induction limitations, and because the contribution of post‐illumination CO2 fixation to total CO2 fixation decreased with increased duration of lightflecks. LUE increased with increased PFD during the lightflecks, particularly in leaves from plants grown in high‐light, high‐nutrient conditions. Saturation PFDs were much higher in flashing light than in constant light. Only small differences in LUE were apparent between leaves from the three growth conditions.
X‐ray microanalytical (EDX) investigation of potassium distributions in mesophyll cells of non‐acclimated and cold‐acclimated rye leaves
Authors: PIHAKASKI‐MAUNSBACH K. , HARVEY D. M. R.
Published: April 2006

Electron probe X‐ray microanalysis was used to analyse the effects of sub‐zero temperatures on K+ distribution in compartments within non‐acclimated and cold acclimated rye (Secale cereale L. cv Voima) leaf cells and to evaluate membrane leakage of ions caused by freezing‐injury. The specimens were rapidly frozen from growing temperatures and from two different sub‐zero temperatures (LT50 and LT100) to which the leaves had already been slowly cooled. Measurements were made in the cytoplasm, vacuole and cell walls in freeze‐substituted mesophyll cells. At ambient temperatures, the mean K+ concentration in the cytoplasm (100 mol m−3) differed significantly from that of the vacuole (49 mol m−3) in the non‐acclimated (NA) cells, while in cold acclimated (A) cells, the concentrations were similar (109 vs 93 mol m−3, respectively). At LT50 temperatures, the K+ concentration in NA‐cells decreased significantly in the cytoplasm (59 mol m−3) but increased in the cell walls. In the A‐cells, on the other hand, the mean K+ concentration increased significantly (about three‐fold) in all major compartments. At LT100 temperatures, K+ concentrations in the cytoplasm and cell walls decreased when compared with corresponding LT50 values in the A‐cells but increased in the NA‐cells. The increased potassium concentration in the cytoplasm of A‐cells at LT50 temperature is compatible with the observed cell shrinkage and an absence of plasma membrane damage. The decreased potassium concentration in the cytoplasm of NA‐cells at LT50 temperature is compatible with the slight cell shrinkage and suggests that the plasma membrane in these cells shows increased permeability due to freeze injury.
The influence of elevated CO2 and temperature on biomass production of continuously defoliated white clover
Authors: RYLE G. J. A. , POWELL C. E.
Published: April 2006

Clonal plants of white clover (Trifolium repens L.), grown singly in pots of Perlite and solely dependent for nitrogen on root nodule N2 fixation, were maintained in controlled environments which provided four environments: 18/13 °C day/night temperature at 340 and 680 μmol mol−1 CO2 and 20·5/15·5°C day/night temperature at 340 and 680 μmol mol−1 CO2. The daylength was 12 h and the photon flux density 500±25 μmol m−2 s−1 (PFD). All plants were defoliated for about 80d, nominally every alternate day, to leave the youngest expanded leaf intact on 50% of stolons, plus expanding leaves (simulated grazing). Elevated CO2 increased the yield of biomass removed at defoliation by a constant 45% during the second 40d of the experiment and by a varying amount in the first half of the experiment. Elevated temperature had little effect on biomass yield. Nitrogen, as a proportion of the harvested biomass, was only fractionally affected by elevated CO2 or temperature. In contrast, N2 fixation increased in concert with the promoting effect of elevated CO2 on biomass production. The increased yield of biomass harvested in 680 μmol mol−1 CO2 was primarily due to the early development and continued maintenance of more stolons. However, the stolons of plants grown in elevated CO2 also developed leaves which were heavier and slightly larger in area than their counterparts in ambient CO2. The conclusion is that, when white clover plants are maintained at constant mass by simulated grazing, they continue to respond to elevated CO2 in terms of a sustained increase in biomass production.
Do phospholipids and sucrose determine membrane phase transitions in dehydrating pollen species?
Authors: HOEKSTRA F. A. , CROWE J. H. , CROWE L. M. , VAN ROEKEL T. , VERMEER E.
Published: April 2006

Gel‐to‐liquid crystalline transition temperatures (Tm) of phospholipids of five desiccation tolerant pollen species in vivo were compared at various levels of dehydration. In an attempt to explain chemically the differences in Tm between these species, phospholipids and soluble carbohydrate contents were examined. We observed a negative correlation between the number of double bonds per phospholipid and Tm values for the intact pollen. This negative correlation also applied to Tm and the relative amount of linolenic acid. For the purpose of comparing Tm values of pollen and of unsaturated PCs (from the literature), the relative amounts and degree of unsaturation of pollen PCs were determined. A discrepancy between Tm values of individual PCs and intact pollen is discussed. Sucrose is the major soluble carbohydrate in 15 pollen species tested, generally making up a considerable part of the dry weight. A positive correlation between sucrose content (either as a percentage of the dry weight or as grams sucrose per grams phospholipid) and Tm was established. This unexpected result was explained in terms of alternative adaptive strategies. We suggest that, for desiccation tolerance, pollen either has to contain sucrose for the protection of its phospholipids, or have a high degree of fatty acid unsaturation in its phospholipids. The advantages and disadvantages of the two options are discussed.
Effect of a pretreatment of seed coats with a low‐osmolality solution on subsequent [14C] sucrose transport into attached empty ovules of pea
Authors: WOLSWINKEL P. , KOERSELMAN‐KOOIJ J. W.
Published: April 2006

This paper discusses the question as to whether or not the seed coat tissues can‘adapt’to a treatment with a solution containing a low osmoticum concentration, representing an environment which is sub‐optimal for assimilate transport into attached surgically modified ovules. Before the start of a pulse‐labelling procedure, in experiments on [14C] sucrose transport into fruits of pea (Pisum sativum) with four empty ovules, two empty ovules were filled with a low‐osmolality solution (a 200 mol m−3 mannitol medium or a solution without mannitol) and the other two ovules were filled with a 400 mol m−3 mannitol medium. Pretreatment with a low‐osmolality medium, during a period of 2–3 h, enhanced subsequent transport of [14C] sucrose into empty ovules filled with a low‐osmolality medium, in comparison with [14C] sucrose transport into empty ovules filled with a 400mol m−3 mannitol medium during the pretreatment period. This partial recovery of sink strength of attached empty ovules can be explained as the result of a stimulation of solute efflux from seed coat cells at high cell turgor.
The use of natural abundance of nitrogen isotopes in plant physiology and ecology
Authors: HANDLEY L. L. , RAVEN J. A.
Published: April 2006

Nitrogen stable isotope (15N, 14N) natural abundance has been much less used than carbon isotopes (13C, 12C) in plant physiology and ecology. Analytical problems, the lower fractional abundance of 15N than of 13C in the biosphere, the greater complexity of the N cycle relative to the C cycle, and smaller expressed discriminations in nature, are contributing factors. The major N pools, globally, have different isotope signatures: atmospheric N2 is 15N‐depleted relative to organic N (including sedimentary N), a situation resulting from a greater expressed discrimination in the organic N to N2 (via denitrification) reaction than of diazotrophy during accumulation of the reduced N. Essentially all of the enzymes except nitrogenase which transform N compounds show discrimination against 15N, although for glutamine synthetase, and the amination of 2‐oxoglutarate and pyruvate, this is only seen in terms of NH4+ rather than the true substrate, NH3. Discrimination is expressed in various N interconversions within plants, leading to substantial differences in δ15N (up to 12‰) among N compounds and macroscopic plant parts. N isotope fractionation during assimilation of exogenous combined N is often much lower than that expected from studies of isolated enzymes due to processes which show very little discrimination, such as limitation by transport through aqueous solution and membranes. Application of 15N/14N discrimination studies to plant ecology have concentrated largely on distinguishing diazotrophy from N supplied from combined N, based on the lower 15N/14N in diazotrophs due to the higher 15N/14N of combined N sources not being offset by fractionation during uptake. While potentially very useful, a number of pitfalls are discussed in its ecological use in both terrestrial and aquatic systems. N isotope discrimination is also useful in tracking N through food webs, and hence, back to combined N sources for plants.
Carbon: terrestrial C4 plants
Authors: PEISKER M. , HENDERSON S. A.
Published: April 2006

The carbon isotope composition of terrestrial C4 plants depends on the primary carboxylation of phosphoenolpyruvate (PEP) and on the diffusion of CO2 to the carboxylation sites, but is also influenced by the final carboxylation of ribulose‐1,5‐bisphosphate (RuBP). Several models have been used for reproducing this complex situation. In the present review, a particular model is applied as a means to interpret the effects of environmental and genetically determined factors on carbon isotope discrimination during C4 photosynthesis. As a new feature, the model considers four types of limitation of the overall CO2 assimilation rate. Both carboxylation reactions are assumed to be limited by either maximum enzyme activity or maximum substrate regeneration rate. The model is applied to experimental data on the effects of CO2, irradiance and water stress on short‐term discrimination by leaves of several C4 species measured simultaneously with CO2 gas exchange characteristics. In particular, different patterns of the influence of low irradiances on carbon isotope discrimination are interpreted as due to variations in that irradiance at which a transition from limitation by PEP regeneration rate and RuBP carboxylase activity to limitation by the regeneration rates of both substrates occurs. After discussing literature data on the effects of environmental conditions on carbon isotope discrimination by C4 plants seasonal and developmental changes in carbon isotope composition, studies on the systematic and geographic distribution of C4 plants, evolutionary and genetical aspects, and some ecological implications are reviewed.
Carbon: freshwater plants
Authors: KEELEY J. E. , SANDQUIST D. R.
Published: April 2006

δ13C values for freshwater aquatic plant matter varies from −11 to −50‰ and is not a clear indicator of photosynthetic pathway as in terrestrial plants. Several factors affect δ13C of aquatic plant matter. These include: (1) The δ13C signature of the source carbon has been observed to range from +1‰ for HCO3− derived from limestone to −30‰ for CO2 derived from respiration. (2) Some plants assimilate HCO3−, which is –7 to –11‰ less negative than CO2. (3) C3, C4, and CAM photosynthetic pathways are present in aquatic plants. (4) Diffusional resistances are orders of magnitude greater in the aquatic environment than in the aerial environment. The greater viscosity of water acts to reduce mixing of the carbon pool in the boundary layer with that of the bulk solution. In effect, many aquatic plants draw from a finite carbon pool, and as in terrestrial plants growing in a closed system, biochemical discrimination is reduced. In standing water, this factor results in most aquatic plants having a δ13C value similar to the source carbon. Using Farquhar's equation and other physiological data, it is possible to use δ13C values to evaluate various parameters affecting photosynthesis, such as limitations imposed by CO2 diffusion and carbon source.
Site‐specific natural isotope fractionation of hydrogen in plant products studied by nuclear magnetic resonance
Authors: MARTIN G. J. , MARTIN M. L. , ZHANG B.‐L.
Published: April 2006

Deuterium NMR at the natural abundance was used to determine the site‐specific isotope ratios (D/H)i of the non‐equivalent isotopomers of various chemical species which exist in plant products. The deuterium distribution in glucose, galactose and mannitol samples from different botanical and compartmental origins is discussed in terms of the influence of plant metabolism and environmental factors. Particular emphasis is given to the potential versatility of deuterium NMR in the study of natural isotopic distribution in pro‐chiral situations. Typical examples of chiral recognition are given in the field of glycolysis metabolites (ethanol, amino‐acids) and of monoterpene biosynthesis.
Carbon isotope discrimination and the integration of carbon assimilation pathways in terrestrial CAM plants
Authors: GRIFFITHS H.
Published: April 2006

Measurement of carbon isotope discrimination (Δ) of organic plant material integrates the combination of C4 and C3 carboxylation processes during the phases of CAM through dark and light periods. These processes are tempered by environmental conditions which regulate CAM activity at the molecular, biochemical and ecological level. The factors contributing to short‐term changes in Δ are discussed in terms of the day‐night changes in metabolite pools and integration via on‐line, instantaneous discrimination techniques. Thus, the isotope signature of newly fixed carbon in malic acid reflects the balance between diffusion and carboxylation limitation together with direct and indirect effects of respiratory metabolism. Leakage of CO2 during decarboxylation leads to greater discrimination being expressed than is predicted from existing models. Over the timescales of seasonal growth and productivity, most constitutive CAM and C3‐CAM intermediate plants show little variation in Δ (2–4‰). The changes induced by developmental and environmental signals and genetic regulation of CAM are compared for stem and leaf succulents. The role of CAM as a potentially highly productive photosynthetic pathway is contrasted with the induction of CAM as a maintenance mechanism in response to environmental stresses. Analyses of Δ have already contributed much to our understanding of the distribution and regulation of CAM, and in turn can also be used to analyse phylogenetic relationships and the origins of CAM as determined from palaoecological evidence.
Carbon isotope discrimination in C3‐C4 intermediates
Authors: von CAEMMERER S.
Published: April 2006

Carbon isotope discrimination in C3–C4 intermediates is determined by fractionations during diffusion and the biochemical fractionations occurring during CO2 fixation. These biochemical fractionations in turn depend on the fractionation by Rubisco in the mesophyll, the amount of CO2 fixation. These biochemical fractionations in turn depend on the fractionation by Rubisco in the mesophyll, the amount of CO2 fixation occurring in the bundle sheath, the extent of bundle‐sheath leakiness and the contribution which C4‐cycle activity makes to the CO2 pool there. In most instances, carbon isotope discrimination in C3–C4 intermediates is C3‐like because only a small fraction of the total carbon fixed is fixed in the bundle sheath. In particular, this must be the case for Flaveria intermediates which initially fix substantial amounts of CO2 into C4‐acids. In C3–C4 intermediates that refix photorespiratory CO2 alone, it is possible for carbon isotope discrimination to be greater than in C3‐species, particularly at low CO2 pressures or at high leaf temperatures. Short‐term measurements of carbon isotope discrimination and gas exchange of leaves can be used to study the photosynthetic pathways of C3‐C4 intermediates and their hybrids as has recently been done for C3 and C4 species.
Water uptake by plants: perspectives from stable isotope composition
Authors: EHLERINGER J. R. , DAWSON T. E.
Published: April 2006

Stable isotope studies of hydrogen and oxygen stable isotope ratios of water within plants are providing new information on water sources, competitive interactions and water use patterns under natural conditions. Variation in the utilization of summer rain by aridland species and limited use of stream water by mature riparian trees are two examples of how stable isotope studies have modified our understanding of plant water relations. Analyses of xylem sap and tree rings have the potential of providing both short‐term and long‐term information on plant water use patterns.
Growth and shoot:root partitioning of spinach plants as affected by nitrogen supply
Authors: SMOLDERS E. , MERCKX R.
Published: April 2006

Spinach plants (Spinacia oleraceaL.) were grown hydroponically in fixed environmental conditions either at full nitrate availability (11·8mol m‐3) or at a suboptimum relative nitrate addition rate of 0·20d‐1, 0·15d‐1or 0·10d‐1respectively, the other nutrients being adequately provided. The relative growth rate (RGR) of the plants varied significantly with the nutrition treatment and decreased during development in all treatments. The concentration of reduced nitrogen in the plants grown at full nitrate availability did not change significantly during the experimental growth period and nitrate accumulation was substantial. After an adaptation period, the concentration of reduced nitrogen in the plants at the suboptimum nitrate addition rates increased during growth and was lowest at the lowest relative nitrate addition rate. Nitrate uptake was almost complete in the suboptimum treatments and nitrate accumulation was negligible as long as the concentration of reduced nitrogen was below 2·0 mmol (g dry weight)‐1. The RGR of all plants was proportional to the concentration of reduced nitrogen in the plant minus a minimal tissue concentration required for growth. However, the proportionality factor was inversely related to the plant mass. This relationship was summarized in an empirical model which explained 98·7% of the variance of the dry weight (log scale) data of all treatments at all harvests. The model was compared with other growth models found in the literature. The shoot/root weight ratio increased from 2 to 4 if nitrate provision was not limiting, and initially, this ratio decreased at suboptimum nitrate provision but increased at higher growth stages. Possible explanations of the dynamics of dry matter partitioning are discussed in relation to models.
Differential effects of drought and light levels on accumulation of citric and malic acids during CAM in Clusia
Authors: FRANCO A. C. , BALL E. , LÜTTGE U.
Published: April 2006

Night‐time citrate accumulation has been proposed as a response to stress in CAM plants. To address this hypothesis, gas exchange patterns and nocturnal acid accumulation in three species of Clusia were investigated under controlled conditions with regard to water stress and responses to low and high photosynthetic photon flux density (PPFD). Under high PPFD, leaves of Clusia nocturnally accumulated large amounts of both malic and citric acids. Under low PPFD and well‐watered conditions, substantial night‐time citrate accumulation persisted, whereas malate accumulation was close to zero. Malate accumulation and night‐time CO2 uptake from the atmosphere declined in all three species during prolonged drought periods, whereas citrate accumulation remained similar or increased. Recycling of respiratory CO2 was substantial for both well‐watered and water‐stressed plants. The suggestion that citrate accumulation is energetically more favourable than malate accumulation is not supported if the source of CO2 for the formation of malate is respiratory CO2. However, the breakdown of citric acid to pyruvate in the light period releases three molecules of CO2, while the breakdown of malic acid releases only one CO2 per pyruvate formed. Thus, citric acid should be more effective than malic acid as a mechanism to increase CO2 concentration in the mesophyll and may help to prevent photoinhibition. Organic acid accumulation also affected the vacuolar pH, which reached values of 2·6–3·0 at dawn. At these pH values, the transport of 2H+/ATP is still feasible, suggesting that it is the divalent form of citrate which is being transported in the vacuoles. Since citrate is a well‐known buffer, and Clusia spp. show the largest day‐night changes in organic acid levels measured in any CAM plant, it is possible that citrate increases the buffer capacity of the vacuoles. Indeed, malate and titratable acidity levels are positively related to citrate levels. Moreover, Clusia species that show the highest nocturnal accumulation of organic acids are also the ones that show the greatest changes in citric acid levels.
Photosynthetic gas exchange and the stable isotope composition of leaf water: comparison of a xylem‐tapping mistletoe and its host
Authors: FLANAGAN L. B. , MARSHALL J. D. , EHLERINGER J. R.
Published: April 2006

Photosynthetic gas exchange and the stable isotopic composition of foliage water were measured for a xylem tapping mistletoe, Phoradendron juniperinum, and its host tree, Juniperus osteosperma, growing in southern Utah. The observed isotopic composition of water extracted from foliage was compared to predictions of the Craig‐Gordon model of isotopic enrichment at evaporative sites within leaves. Assimilation rates of juniper were higher and stomatal conductance was lower than the values observed for the mistletoe. This resulted in lower intercellular/ ambient CO2 values in the juniper tree relative to its mistletoe parasite. For mistletoe, the observed foliage water hydrogen and oxygen isotopic enrichment was less than that predicted by the model. In juniper, foliage water hydrogen isotopic enrichment was also lower than that predicted by the evaporative enrichment model. In contrast, the oxygen isotopic enrichment in juniper foliage water was slightly greater than that predicted for the evaporative sites within leaves. Hydrogen isotopic enrichment in mistletoe foliage shows systematic variation with stem segment, being highest near the tips of the youngest stems and decreasing toward the base of the mistletoe, where isotopic composition is close to that of stem water in the host tree. In a correlated pattern, mid‐day stomatal conductance declined abruptly in mistletoe foliage of increasing age.
The coupling between extra‐ and intracellular electric potentials in Bidens pilosa L.
Authors: FRACHISSE‐STOILSKOVIĆ J. M. , JULIEN J. L.
Published: April 2006

The extracellular electrical potential in Bidens pilosa has been compared with the transmembrane potential in hypocotyl cells. We found that variations in the extracellular electrical potential, induced by different KCI concentrations, were identical to the variations of the transmembrane potential. In order to explain the strong correlation between the extracellular potential and the transmembrane potential, an interpretative model based on the electrical diagram of the plant was tested. From our data, we found that: (1) the symplast behaves as a low resistance pathway compared with the apoplast; and (2) the cell wall contributes weakly (20% at the most) to the transmembrane potential. It is concluded that each hypocotyl cell is strongly coupled electrically with its neighbour. This property, together with the high apoplast resistance, accounts for the similarity in the extracellular and transmembrane potential measurements. These properties, which are characteristic of excitable organs, support the existence of action potentials in Bidens.
Effects of ozone on the photosynthetic apparatus and leaf proteins during leaf development in wheat
Authors: NIE G.‐Y. , TOMASEVIC M. , BAKER N. R.
Published: April 2006

Leaves of Triticum aestivum cv. Avalon were grown in an atmosphere that contained 150 nmole mol‐1 ozone for 7h each day. After leaves had reached maximum size, the leaf blade was divided into three sections to provide tissue of different age, the youngest at the base of the blade and the oldest at the leaf tip. The ozone treatment was found to decrease significantly the light‐saturated rate and quantum yield of CO2 assimilation and the maximum quantum yield of photosystem II photochemistry in the oldest leaf section. No effects were found on the basal and middle sections of the leaf. These ozone‐induced decreases in the photosynthetic parameters were associated with decreases in the efficiency of utilization of light for CO2 assimilation at the photon flux density under which the leaves were grown. The depression in photosynthetic performance of tissue near the leaf tip was accompanied by large decreases in the contents of total, soluble and thylakoid proteins and chlorophyll. There was also found to be a preferential loss of ribulose‐1,5‐carboxylase‐oxygenase. These ozone‐induced changes in chlorophyll and protein contents and the photosynthetic activities of the leaf tissue were similar to changes normally associated with leaf senescence. Two‐dimensional polyacrylamide gel analyses of leaf proteins demonstrated the loss of some minor, and unidentified, proteins, whilst another group of minor proteins appeared. It is concluded that daily exposure of the leaf to 150 nmol mol‐1 ozone for 7h had no effect on the development of the photosynthetic apparatus and its activities during leaf expansion, but it did promote the onset of premature senescence in fully expanded tissue that resulted in a loss of pigments, proteins and photosynthetic capacity and efficiency.