bluuebunny/crossref_metadata_2025 huggingface dataset:
A new equation for calculating osmotic potential
Authors: COCHRANE T. T.
Published: April 2006

A new equation for calculating osmotic potential was developed that is based on a molecular model of the structure of water. Its formulation involves no sub‐equations that cannot be fully derived from established physical principles. The equation was first tested by calculating the osmotic potentials of a series of aqueous NaCl solutions with concentrations ranging from 0.103 to 4.382 kmol m−3, and comparing those calculations with calculations made using equations recorded in the literature. Virtually the same results were obtained. Subsequently, it was tested successfully by comparative calculations on a selection of inorganic and organic aqueous solutions. The principles it embodies provide for the visualization of the molecular role of water in plants as a unifying mechanism. Factors that affect the size of the structure of water, and the consequent distance an individual molecule must travel to and fro across its cage, determine direction and rate of flow. It is considered that the equation will provide a new research tool for many osmotic‐potential‐related questions.
Temperature dependence of trans plasma membrane electron transport in Zea mays L. roots
Authors: DÖRING O. , BÖTTGER M.
Published: April 2006

Intact Zea mays L. cv. Golden Bantam seedlings which were not cold adapted were exposed to various temperatures. Trans plasma membrane potential difference was measured in a temperature range from 0 to 40 °C using intracellular microelectrodes. The depolarization caused by electron transfer across the PM to artificial external electron acceptors was investigated. Active membrane potential increased with temperature in the range from 0 to 15 °C but was independent of temperature above 20 °C. Depolarization caused by the non‐membrane‐permeating electron acceptors hexacyanoferrate III (HCF III) and hexabromoiridate IV (HBIIV) took place over the whole temperature range investigated. The effect of HBI IV increased up to 10 °C whereas the HCF III effects increased up to 25 °C.
Scaling sun and shade photosynthetic acclimation ofAlocasia macrorrhizato whole‐plant performance – I. Carbon balance and allocation at different daily photon flux densities
Authors: SIMS D. A. , PEARCY R. W.
Published: April 2006

We determined the carbon allocation patterns and construction costs ofAlocasia macrorrhizaplants grown at different photon flux densities (PFD) as well as the whole‐plant carbon gain of these plants at different daily PFDs. Growth at high PFD resulted in thicker leaves with a higher leaf mass per unit area, and increased biomass allocation to petioles and roots, as compared to growth at low PFD. Increased allocation to petioles may have been necessary to support the heavier leaves, whereas increased allocation to roots may have been necessary to supply sufficient water for the higher transpiration rates in high PFD. Root biomass was highly correlated with the daily, whole‐plant transpiration rate. Tissue construction costs per unit dry mass were unchanged by acclimation, but, since the mass per unit areas of leaves, roots and petioles all increased, construction costs per unit leaf area were much higher for plants grown at high PFD. On a per unit leaf area basis, daily whole‐plant carbon gain measured at high daily PFD was higher in high‐ than in low‐PFD‐grown plants. However, on a per unit leaf mass basis, low‐PFD‐grown plants had a daily carbon gain at least as high as that of high‐PFD‐grown plants at high daily PFD. At low daily PFD, low‐PFD‐grown plants maintained an advantage over high‐PFD‐grown plants in terms of carbon gain because of their larger leaf area ratios. Thus, in terms of carbon gain, low‐PFD‐grown plants performed better than sun plants at low PFD and as well as high‐PFD‐grown plants at high PFD, despite their lower photosynthetic capacities per unit area. For high‐PFD‐grown plants, the higher construction costs per unit leaf area resulted in lower leaf area ratios, which counteracted the advantage of higher photosynthetic rates per unit leaf area.
The interactive effects of light and inorganic carbon on aquatic plant growth
Authors: MADSEN T. V. , SAND‐JENSEN K.
Published: April 2006

Submerged aquatic macrophytes grow across a wide, often coupled, range of light and inorganic carbon availabilities, and each single factor influences photosynthesis and acclimation. Here we examine the interactive effects of light and inorganic carbon on the growth of Elodea canadensis and Callitriche cophocarpa. The plants were grown in the laboratory at a range of light intensities (0–108 μmol m−2s−1) and four inorganic carbon regimes in a crossed factorial design. Plant growth rates, measured over 3–4 weeks of incubation, increased in response to increasing light intensity and inorganic carbon availability, and significant interactive effects were observed. The light‐use efficiency for growth at low light increased 2‐fold for Callitriche and 6‐fold for Elodea between the lowest and highest inorganic carbon concentrations applied. Also, the growth rate at the highest light intensity increased with inorganic carbon availability, but the relative increase was smaller than at low light. Both species acclimated to the light and carbon regime such that the chlorophyll content declined at low and high light intensities and the initial slopes of the photosynthetic CO2 and HCO3− response curves declined at high levels of CO2. Callitriche responded less markedly than Elodea to changing inorganic carbon availability during growth, and the initial slope of the photosynthetic HCO3− response curve, in particular, was greatly reduced (>90%) in Elodea by high CO2. It is suggested that the coupled responses of aquatic macrophytes to light and inorganic carbon influence their ability to develop dense stands at high light in shallow water and to extend to greater depths in waters rich in inorganic carbon.
The chemical composition and anatomical structure of leaves of grass species differing in relative growth rate
Authors: VAN ARENDONK J. J. C. M. , POORTER H.
Published: April 2006

To arrive at a better understanding of variation in specific leaf mass (SLM, leaf weight per unit leaf area), we investigated the chemical composition and anatomical structure of the leaves of 14 grass species varying in potential relative growth rate. Expressed on a dry weight basis, the fast‐growing grass species with low SLM contained relatively more minerals and organic N‐compounds, whereas slow‐growing species with high SLM contained more (hemi)cellulose and lignin. However, when expressed per unit leaf area, organic N‐compounds, (hemi)cellulose, total structural carbohydrates and organic acids increased with increasing SLM.

For the 14 grasses, no trend with SLM was found for the leaf volume per unit leaf area. Leaf density was positively correlated with SLM. Variation in density was not caused by variation in the proportion of intercellular spaces. The proportion of the total volume occupied by mesophyll and veins did not differ either. A high SLM was caused, at least partly, by a high proportion of non‐veinal sclerenchymatic cells per cross‐section. The epidermal cell area was negatively correlated with SLM.

We conclude that the differences in SLM and in the relative growth rate (RGR) between fast‐ and slow‐growing grass species are based partly on variation in anatomical differentiation and partly on chemical differences within cell types.
Changes in net photosynthesis and growth of Pinus eldarica seedlings in response to atmospheric CO2 enrichment
Authors: GARCIA R. L. , IDSO S. B. , WALL G. W. , KIMBALL B. A.
Published: April 2006

Pinus eldarica L. trees, rooted in the natural soil of an agricultural field at Phoenix, Arizona, were grown from the seedling stage in clear‐plastic‐wall open‐top enclosures maintained at four different atmospheric CO2 concentrations for 15 months. Light response functions were determined for one tree from each treatment by means of whole‐tree net CO2 exchange measurements at the end of this period, after which rates of carbon assimilation of an ambient‐treatment tree were measured across a range of atmospheric CO2 concentrations. The first of these data sets incorporates the consequences of both the CO2‐induced enhancement of net photosynthesis per unit needle area and the CO2‐induced enhancement of needle area itself (due primarily to the production of more needles), whereas the second data set reflects only the first of these effects. Hence the division of the normalized results of the first data set by the normalized results of the second set yields a representation of the increase in whole‐tree net photosynthesis due to enhanced needle production caused by atmospheric CO2 enrichment. In the solitary trees we studied, the relative contribution of this effect increased rapidly with the CO2 concentration of the air to increase whole‐tree net photosynthesis by nearly 50% at a CO2 concentration approximately 300 μmol mol−1 above ambient.
Does proline play a special role in bacteroid metabolism?
Authors: KOHL D. H. , STRAUB P. F. , SHEARER G.
Published: April 2006

Dicarboxylates are generally considered the main energy‐bearing compounds imported from plant cells into bacteroids in root nodules. Evidence reviewed herein supports the hypothesis that other compounds contribute and that proline catabolism may have particular importance in bacteroid metabolism during environmental stress.
Leaf extension in the slender barley mutant: delineation of the zone of cell expansion and changes in translatable mRNA during leaf development
Authors: SCHÜNMANN P. H. D. , OUGHAM H. J. , TURK K. S.
Published: April 2006

The slender mutant of barley (Hordeum vulgare L.) results from an alteration to a single nuclear gene. Plants homozygous for the mutant allele have long, attenuated leaves as a result of a greatly increased extension rate. Although the growth rate at any one position in the extension zone appears not to differ between slender and normal (wild‐type) barley, in slender the length of the zone over which cells extend is approximately 50% greater than that in normal barley. Epidermal cells are both longer and narrower in slender, so the whole‐plant phenotype is mirrored at the cell level. Translation in vitro of RNA extracted from successive sections of the young primary leaf, followed by one‐dimensional SDS‐PAGE separation, facilitated the alignment of equivalent developmental stages in the two genotypes, but failed to demonstrate major differences between the two genotypes. Two‐dimensional separation of translation products from total leaf tissue revealed a few small differences between normal and slender. Growth of plants at 8°C compared with 20°C caused changes in some translation products, with one (unknown) product decreasing in abundance in cold‐treated normal tissue but not in slender tissue.
Blue‐light‐induced pH changes associated with NO3−, NO2− and Cl− uptake by the green alga Monoraphidium braunii
Authors: APARICIO P. J. , WITT F. G. , RAMÍREZ J. M. , QUIÑONES M. A. , BALANDÍN T.
Published: April 2006

In M. braunii, the uptake of NO3− and NO2− is blue‐light‐dependent and is associated with alkalinization of the medium. In unbuffered cell suspensions irradiated with red light under a CO2‐free atmosphere, the pH started to rise 10s after the exposure to blue light. When the cellular NO3− and NO2− reductases were active, the pH increased to values of around 10, since the NH4+ generated was released to the medium. When the blue light was switched off, the pH stopped increasing within 60 to 90s and remained unchanged under background red illumination. Titration with H2SO4 of NO3− or NO2− uptake and reduction showed that two protons were consumed for every one NH4+ released. The uptake of Cl− was also triggered by blue light with a similar 10 s time response. However, the Cl− ‐dependent alkalinization ceased after about 3 min of blue light irradiation. When the blue light was turned off, the pH immediately (15 to 30 s) started to decline to the pre‐adjusted value, indicating that the protons (and presumably the Cl−) taken up by the cells were released to the medium. When the cells lacked NO3− and NO2− reductases, the shape of the alkalinization traces in the presence of NO3− and NO2− was similar to that in the presence of Cl−, suggesting that NO3− or NO2− was also released to the medium. Both the NO3− and Cl−‐dependent rates of alkalinization were independent of mono‐ and divalent cations.
Seasonal variation of leaf glutamine synthetase isoforms in temperate deciduous trees strongly suggests different functions for the enzymes
Authors: PEARSON J. , JI Y.‐M.
Published: April 2006

Changes in the activities of leaf glutamine synthetase (GS) isoforms were followed in four temperate deciduous trees from full leaf expansion to senescence (May to November). In the early part of the season, total GS activity was high in all species, with values ranging from 90 to 200 μmol h−1 g−1 fw. During this early period this activity comprised only the activity of the chloroplastic (GS2) isoform in all species. These high GS2 activities are consistent with the role of GS2 in the re‐assimilation of photorespired ammonia. The early high values also coincided with high nitrate reductase activity in one of the species, the highly nitrophilous species Sambucus nigra, with values of up to 16μmol h−1 g−1 fw. This indicates that GS2 is also important in the assimilation of ammonia produced from nitrate reduction. From mid‐ to late‐season, the cytosolic isoform (GS1) was detected in all four species and became increasingly more active in comparison to GS2. By the time of senescence it was the dominant enzyme of the two forms in both S. nigra and Carpinus betulus. The results provide strong support for recent findings that GS1 is an important enzyme for the mobilization of nitrogen for translocation or storage.
Negative phototropism in young gametophytes of the fern Schizaea pusilla
Authors: KISS J. Z.
Published: April 2006

Schizaea pusilla is a rare fern that occurs in acidic bogs and is one of the few fern species that maintains a filamentous gametophyte throughout its development. To expand our knowledge of the physiology of this fern, phototropic responses were examined in young gametophytes. In contrast to germ filaments of other fern species, apical protonemata of young gametophytes are negatively phototropic in continuous white, red and blue light at all fluence rates tested. The expression of phototropic curvature is not limited by time since apical protonemata are also negatively phototropic when they are given brief exposures of light and then placed in the dark. In other lower plant groups such as mosses and some algae, the direction of phototropic curvature can change depending on light quality and intensity, but in young gametophytes of Schizaea, negative phototropic curvature was observed in all conditions studied. Blue light is the most effective in promoting the negative phototropic response in Schizaea.
The influence of nitrogen availability on carbon and nitrogen storage in the biennial Cirsium vulgare (Savi) Ten. I. Storage capacity in relation to resource acquisition, allocation and recycling
Authors: HEILMEIER H. , FREUND M. , STEINLEIN T. , SCHULZE E.‐D. , MONSON R.K.
Published: April 2006

Plants of Cirsium vulgare (Savi) Ten. were cultivated under five different nitrogen regimes in order to investigate the effects of nitrogen supply on the storage processes in a biennial species during its first year of growth.

External N supply increased total biomass production without changing the relationship between ‘productive plant compartments’ (i.e. shoot plus fine roots) and ‘storage plant compartments’ (i.e. structural root dry weight, which is defined as the difference between tap root biomass and the amount of stored carbohydrates and N compounds). The amount of carbohydrates and N compounds stored per unit of structural tap root dry weight was not affected by external N availability during the season, because high rates of N supply increased the concentration of N compounds whilst decreasing the carbohydrate concentration, and low rates of N supply had the opposite effect. Mobilization of N from senescing leaves was not related to the N status of the plants. The relationship between nitrogen compounds stored in the tap root and the maximum amount of nitrogen in leaves was an increasing function with increasing nitrogen supply. We conclude that the allocation between vegetative plant growth and the growth of storage structures over a wide range of N availability seems to follow predictions from optimum allocation theory, whereas N storage responds in a rather plastic way to N availability.
Replacement of nitrate by ammonium as the nitrogen source increases the salt sensitivity of pea plants. I. Ion concentrations in roots and leaves
Authors: SPEER M. , BRUNE A. , KAISER W. M.
Published: April 2006

Pea seedlings (Pisum sativum L. cv ‘Kleine Rheinlän‐derin’) were grown hydroponically in solutions containing either nitrate (3 or 14 mol m−3) or ammonium (3 mol m−3) as the nitrogen source. Ammonium nutrition as such had no negative effect on plant biomass production, but drastically increased the sensitivity to moderate salinity (50 mol m−3 NaCl). The reasons for this effect are investigated here and in a subsequent paper. The appearance of visible symptoms of salt damage (wilting of marginal leaf areas followed by progressive necrosis) was paralleled by the development of several characteristic modifications in the solute and metabolite contents. Major changes were: (i) high salt (NaCl) accumulation in leaves; (ii) accumulation of ammonium (up to 20 mol m−3) and amino acids (up to 110 mol m−3) in leaves, but at decreased ammonium uptake rates; and (iii) decreased protein content. In a comparison paper we report on the subcellular distribution of salts, ammonium and metabolites under the above conditions.
Intra‐ and inter‐plant variation in xylem cavitation in Betula occidentalis
Authors: SPERRY J. S. , SALIENDRA N. Z.
Published: April 2006

A modified version of a method that uses positive air pressures to determine the complete cavitation response of a single axis is presented. Application of the method to Betula occidentalis Hook, gave a cavitation response indistinguishable from that obtained by dehydration, thus verifying the technique and providing additional evidence that cavitation under tension occurs by air entry through interconduit pits. Incidentally, this also verified pressure‐bomb estimates of xylem tension and confirmed the existence of large (i.e. >0·4 MPa) tensions in xylem, which have been questioned in recent pressure‐probe studies. The air injection method was used to investigate variation within and amongst individuals of B. occidentalis. Within an individual, the average cavitation tension increased from 0·66±0·27 MPa in roots (3·9 to 10·7 mm diameter), to 1·17±0·10 MPa in trunks (12 to 16 mm diameter), to 1·36±0·04 MPa in twigs (3·9 to 5 mm diameter). Cavitation tension was negatively correlated with the hydraulically weighted mean of the vessel diameter, and was negatively correlated with the conductance of the xylem per xylem area. Native cavitation was within the range predicted from the measured cavitation response and in situ maximum xylem tensions: roots were significantly cavitated compared with minimal cavitation in trunks and twigs. Leaf turgor pressure declined to zero at the xylem tensions predicted to initiate cavitation in petiole xylem (1·5 MPa). Amongst individuals within B. occidentalis, average cavitation tension in the main axis varied from 0·90 to 1·90 MPa and showed no correlation with vessel diameter. The main axes of juveniles (2–3 years old) had significantly narrower vessel diameters than those of adults, but there was no difference in the average cavitation tension. However, juvenile xylem retained hydraulic conductance to a much higher xylem tension (3·25 MPa) than did adult xylem (2·25 MPa), which could facilitate drought survival during establishment.
Long‐term responses of Melilotus segetalis to salinity. I. Growth and partitioning
Authors: ROMERO J. M. , MARAÑÓN T.
Published: April 2006

Annual sweetclover plants [Melilotus segetalis (Brot) Ser.] were grown for a complete life cycle with and without saline (NaCl treatment of CE=15 dS m−1). Growth and partitioning analyses were performed. Sequential harvests (every 15 d) during the life cycle, and separation of plant material into roots, stems, petioles, leaves and reproductive structures were carried out Salt treatment reduced growth during the early and middle stages of the life of the plant, but did not significantly affect RGR in the reproductive phase. The root–shoot allometric coefficient of salinized plants in the generative phase decreased more than in control plants. We suggest that salinity‐induced growth reduction in M. segetalis was primarily a result of a lower unit leaf rate (ULR) despite an increased leaf area ratio (LAR). Earlier flowering, higher biomass allocation to shoot and greater reproductive investment, but similar relative growth rate (RGR), were some of the main characteristics of salt‐stressed plants compared to controls during the reproductive phase, these apparently being associated with increased sink strength caused by developing flowers and fruits.
Freezing tolerance and avoidance in high‐elevation Hawaiian plants
Authors: LIPP C. C. , GOLDSTEIN G. , MEINZER F. C. , NIEMCZURA W.
Published: April 2006

Freezing resistance mechanisms were studied in five endemic Hawaiian species growing at high elevations on Haleakala volcano, Hawaii, where nocturnal subzero (°C) air temperatures frequently occur. Extracellular freezing occurred at around ‐5°C in leaves of Argyroxiphium sandwicense and Sophora chrysophylla, but these leaves can tolerate extracellular ice accumulation to ‐15°C and ‐12°C, respectively. Mucilage, which apparently acted as an ice nucleator, comprised 9 to 11% of the dry weight of leaf tissue in these two species. Leaves of Vaccinium reticulatum and Styphelia tameiameiae were also found to tolerate substantial extracellular freezing. Dubautia menziesii, on the other hand, exhibited the characteristics of permanent supercooling; a very rapid decline in liquid water content associated with simultaneous intracellular and extracellular freezing. However, in those species that tolerate extracellular freezing, the decline in liquid water content during freezing is relatively slow. Osmotic potential was lower at pre‐dawn than at midday in four of the species studied. Nocturnal production of osmotically active solutes may have helped to prevent intracellular freeze dehydration as well as to provide non‐colligative protection of cell membranes. Styphelia tameiameiae supercooled to ‐9·3°C and tolerated tissue freezing to below ‐15°C, a unique combination of physiological characteristics related to freezing. Tolerance of extracellular ice formation after considerable supercooling may have resulted from low tissue water content and a high degree of intracellular water binding in this species, as determined by nuclear magnetic resonance studies. The climate at high elevations in Hawaii is relatively unpredictable in terms of the duration of subzero temperatures and the lowest subzero temperature reached during the night. It appears that plants growing in this tropical alpine habitat have been under selective pressures for the evolution of freezing tolerance mechanisms.
A theoretical explanation of the Piper‐Steenbjerg effect
Authors: WIKSTRÖM F.
Published: April 2006

The relation between plant yield and plant nutrient concentration is sometimes found to be negative, a phenomenon called the Piper‐Steenbjerg (PS) effect. A model was used to examine the underlying causes of the PS effect, and the conditions under which it is most likely to occur. The model uses the nutrient productivity concept for plant growth and a nutrient uptake equation in which root growth rate and external nutrient concentration determine the uptake rate. The study suggests that the PS effect occurs when the fast growth of plants grown in an initially higher nutrient medium eventually leads to a more rapid depletion of external nutrients than the slow growth of plants grown in an initially lower nutrient medium. The fast growth of plants combined with a rapid decrease of nutrient uptake leads to a fall in plant nutrient concentration. When these large plants with very low nutrient concentrations are compared with the smaller, slow‐growing plants, a PS effect may be found depending on the time at which the plants are harvested, and on the range of initial values of the external nutrient content. When it occurs, the effect is greatest when the depletion volume per unit new root (Vd) is lowest, and when the mobility of nutrients in the medium is highest (α=1). The results are sufficiently general to apply to a variety of nutrients, plant species and growth media.
Integrated tolerance mechanisms: constitutive and adaptive plant responses to elevated metal concentrations in the environment
Authors: MEHARG A. A.
Published: April 2006

Isolation and study of metal tolerant and hypersensitive strains of higher plant (and yeast) species has greatly increased our knowledge of the individual pathways that are involved in tolerance. Plants have both constitutive (present in most phenotypes) and adaptive (present only in tolerant phenotypes) mechanisms for coping with elevated metal concentrations. Where studies on the mechanisms of tolerance fall down is in their failure to integrate tolerance mechanisms within cell or whole‐plant function by not relating adaptive mechanisms to constitutive mechanisms. This failure often distorts the relative importance of a proposed tolerance mechanism, and indeed has confused the search for adaptive mechanisms. The fundamental goal of both constitutive and adaptive mechanisms is to limit the perturbation of cell homeostasis after exposure to metals so that normal or near‐normal physiological function may take place. Consideration of the response to metals at a cellular rather than a biochemical level will lead to a greater understanding of mechanisms to withstand elevated levels of metals in both contaminated and uncontaminated environments. Recent advances in the study of Al, As, Cd, and Cu tolerance and hypersensitivity are reported with respect to the cellular response to toxic metals. The role of genetics in unravelling tolerance mechanisms is also considered.
Effects of patchy stomatal closure on gas exchange measurements following abscisic acid treatment
Authors: MOTT K. A.
Published: April 2006

Gas exchange data and images of leaf fluorescence were collected concurrently as stomata responded to abscisic acid (ABA) application. When 10−5kmolm−3 ABA was applied to the transpiration stream in a short pulse, stomatal conductance (gs), photosynthesis (A) and intercellular CO2 concentration (Ci) decreased rapidly after a short lag period and became approximately constant after 2h. There was an apparent reduction in the A versus c1 relationship as stomata closed, but the data returned to the A versus C1 curve while stomatal conductance was constant or slowly rising during the second hour after ABA treatment. Larger amounts of ABA administered during the pulse caused larger deviations from the A versus c1 relationship. When 10−7kmolm−3 ABA was applied continuously through the transpiration stream, gs, A and Ci decreased, but there was no substantial deviation from the A versus c{ curve. Fluorescence images were patchy as stomata closed for all experiments, but became slowly more uniform during the time that gas exchange was returning to the A versus Cj curve. The distribution of con‐ductance among patches was not bimodal, and larger devi‐ations from the A versus ct curve had greater ranges of pixel values and more pixel values representing low values of Cj during stomatal closure than did experiments show‐ing small or no deviation. Estimates of A and gs from fluo‐rescence images compared favourably with measured val‐ues in most cases, suggesting that the patchy distributions of fluorescence were caused by patchy distributions of stomatal conductance and that apparent reductions in the A versus ct relationship were the result of these patchy stomatai distributions and not direct effects of ABA on mesophyll functioning. The data show that stomatal patches can be temporary and that patchiness may not be reflected in gas exchange data if the range of stomatal con‐ductances is not large. These observations may explain some of the discrepancies among previous studies concerning the effect of ABA on the A versus Ci relationship.
A comparison of direct and indirect methods for measuring leaf and surface areas of individual bushes
Authors: BRENNER A. J. , CUETO ROMERO M. , GARCIA HARO J. , GILABERT M. A. , INCOLL L. D. , MARTINEZ FERNANDEZ J. , PORTER E. , PUGNAIRE F. I. , YOUNIS M. T.
Published: April 2006

Indirect estimates of leaf area from measurements with three commercially available instruments (DEMON, LAI‐2000 and Sunfleck Ceptometer) were compared with directly measured areas of individual Retama sphaerocarpa bushes. The three indirect methods gave good estimates of the total surface area of individual bushes. For the DEMON, the method of log‐linear averaging of transmitted radiation gave estimates closer to directly measured surface area than the method of averaging transmission linearly. For the LAI‐2000, estimated surface area index multiplied by canopy projected area gave the best agreement with directly measured values. For measurements with the Sunfleck Ceptometer, values of surface area estimated from the transmission of photosynthetic quantum flux density, without correcting for diffuse radiation, gave the best agreement with directly measured values. Surface areas estimated by the three instruments were not significantly different from directly measured total (leaf + branch + stem) surface areas. Leaf surface area could be calculated from estimated total surface area minus directly measured branch surface area. Measured branch surface area was linearly related to canopy projected area.
NaCI stress enhances proteolytic turnover of the tonoplast H+‐ATPase of Citrus sinensis: appearance of a 35 kDa fragment of subunit A still exhibiting ATP‐hydrolysis activity?
Authors: BAÑULS J. , RATAJCZAK R. , LÜTTGE U.
Published: April 2006

Salinity stress caused a decrease in the relative amount of the subunits of the hydrophilic head of the tonoplast H±ATPase from leaf cells of Valencia orange [Citrus sinensis (L.) Osbeck]. In parallel, a 35 kDa polypeptide appeared in the tonoplast, which could be separated from the tono‐plast H±ATPase and cross‐reacted with an antiserum against the catalytic subunit A of the tonoplast H±ATPase. This polypeptide seems to show ATP hydrolysis activity and is considered to be a product of proteolytic breakdown of subunit A. This indicates an increased turnover of the tonoplast H±ATPase of Valencia orange under salinity stress.
Stress‐dependent redistribution of abscisic acid (ABA) in Hordeum vulgare L leaves: the role of epidermal ABA metabolism, tonoplastic transport and the cuticle
Authors: DAETER W. , HARTUNG W.
Published: April 2006

When 14C‐labelled abscisic acid ([14C]ABA) was supplied to isolated protoplasts of the barley leaf at pH 6, initial rates of metabolism were about five times higher in epidermal cell protoplasts than in mesophyll cell protoplasts if equal cytosolic volumes were considered. In spite of the fact that epidermal cells make up only about 35% of the total water space in barley leaves, and despite the small cytosolic volume of these cells, in intact leaves all epidermal cells would thus metabolize half as much ABA per unit time as the mesophyll cells (0–27 and 0–51 mmol h−1 m−3 leaf water). Therefore, under these conditions epidermal cells seem to be a stronger sink than mesophyll cells for ABA that arrives via the transpiration stream. However, at an apoplastic pH of 7–25, which occurs in stressed leaves, the proportion of total metabolized ABA would be much smaller in epidermal than in mesophyll cells (0–029 and 0–204 mmolh−l m−3 leaf water). Our results indicate that under conditions of slightly alkaline apoplastic pH the epidermis may serve as the main source for fast stress‐dependent ABA redistribution into the guard cell apoplast. This is partly the result of ABA transport across the epidermal tonoplast, which is dependent on the apoplastic pH and possibly on the cytosolic calcium concentration. The cuticle seems to be of no particular importance in stress‐induced apoplastic ABA shifts and cannot be regarded as a significant sink for high ABA concentrations under stress.
Temporal and spatial variations in the oxygen‐18 content of leaf water in different plant species
Authors: WANG X.‐F. , YAKIR D.
Published: April 2006

Temporal variations in the δ18 oxygen (δ18O) content of water transpired by leaves during a simulated diurnal cycle fluctuated around the δ18O content of the source water. Reconstructed variations in the δ18O values of leaf water differed markedly from those predicted by conventional models. Even when transpiring leaves were maintained under constant conditions for at least 3 h, strict isotopic steady‐state conditions of leaf water (equality of the 18O/16O ratios in the input and transpired water) were rarely attained in a variety of plant species (Citrus reticu‐lata, Citrus paradisi, Gossypium hirsutum, Helianthus annuns, Musa musaceae and Nicotinia tabacum). Isotopic analysis of water transpired by leaves indicated that leaves approach the isotopic steady state in two stages. The first stage takes 10 to 35 min (with a rate of change of about 3–3%h−1), while in the second stage further approach to the isotopic steady state is asymptotic (with a rate of change of about 0–4% h−1), and under conditions of low transpiration leaves can last for many hours. Substantial spatial isotopic heterogeneity was maintained even when leaves were at or near isotopic steady state. An underlying pattern in this isotopic heterogeneity is often discerned with increasing 18O/16O ratios from base to tip, and from the centre to the edges of the leaves. It is also shown that tissue water along these spatial isotopic gradients, as well as the average leaf water, can have 18O/16O ratios both lower and higher than those predicted by the conventional Craig and Gordon model. We concluded, first, that at any given time during the diurnal cycle of relative humidity the attainment of an isotopic steady state in leaf water cannot be assumed a priori and, secondly, that the isotopic enrichment pattern of leaf water reflects gradual enrichment along the water‐flow pathway (e.g. as in a string of pools), rather than a single‐step enrichment from source water, as is normally assumed.
Photoinhibition in the Antarctic moss Grimmia antarctici Card when exposed to cycles of freezing and thawing
Authors: LOVELOCK C. E. , OSMOND C.B. , SEPPELT R. D.
Published: April 2006

Freezing and thawing of the endemic moss species Grimmia antarctici Card, caused photoinhibition. When snow cover was removed from moss in the field, resulting in exposure to fluctuating temperatures and light conditions, photoinhibition, measured as a reduction in the ratio of variable to maximum chlorophyll a fluorescence (Fv/Fm), was observed. The extent of photoinhibition was highly variable and appeared to be reversible during periods of warmer temperatures. A series of controlled laboratory studies found that the light conditions that prevail between freezing and thawing events influenced the recovery from photoinhibition observed during freezing and thawing, with low light conditions facilitating the greatest rates of recovery. After four cycles of freezing and thawing, recovery from photoinhibition in hydrated moss was achieved within 12 h of transfer to 5°C and 15 μmol quanta m−2 s−1. These results favour the hypothesis that photoinhibition observed during freezing represents a protective process involving the down‐regulation of photo‐system II when photosynthetic carbon assimilation is limited by low temperatures.
Xylem cavitation in two mature Scots pine forests growing in a wet and a dry area of Britain
Authors: JACKSON G. E. , IRVINE J. , GRACE J.
Published: April 2006

The xylem cavitation rate, stem water content, stomatal conductance and leaf water potential of mature Scots pine (Pinus sylvestris L.) were compared over a period of 18 months at two sites in Britain where trees were planted from the same seed source. The sites were at Thetford in south‐east England, where the climate is relatively warm and dry (average rainfall of 600mm per year), and Aberfoyle in central Scotland, where it is relatively cool and wet (average rainfall of 1500 mm year−1). In the first year of study (1992) the natural difference between the sites was amplified by a severe drought affecting south‐east England. Acoustic emissions (as a result of cavitation) were detected at both sites, but were not an everyday occurrence, and rates depended on short‐term meteorological variation. The relative water content (RWC) of the trunk at breast height at Thetford was significantly lower than at Aberfoyle, and declined in parallel with increasing severity of drought. Following the release from drought a gradual increase in RWC was found, but the pre‐drought RWC was not attained. The same xylem water potential gradient and stomatal conductance was found at the two sites; but on a diurnal basis, as water potential declined, cavitation rates increased. There was no difference in vulnerability to cavitation or in hydraulic conductance between the sites.
UV‐B increases the harvest index of bean (Phaseolus vulgaris L.)
Authors: DECKMYN G. , IMPENS I.
Published: April 2006

The effects of small changes in natural UV‐B on the photosynthesis, pigmentation, flowering and yield of bean plants (Phaseolus vulgaris L. var. Label) were studied. To obtain a relatively natural growth environment, the plants were grown in small, half‐open greenhouses of UV‐transmitting Plexiglas of different thickness (3 and 5 mm), resulting in an 8% difference in the weighted UV‐B reaching the plants. Although the UV‐B doses used did not significantly influence photosynthesis on a leaf area basis during vegetative growth, important changes in biomass allocation were noted. A UV‐B‐O induced reduction in leaf area during the period of vegetative growth resulted in decreased dry weight after 57 d. During the flowering and pod‐filling stages (57–79.d after planting), however, plants grown at high UV‐B retained their photosynthetic capacity longer: maximal photosynthesis, chlorophyll and N content of the leaves were higher under the higher UV‐B dose at a plant age of 79 d. Combined with an increased allocation under the higher UV‐B dose of both N and biomass to the pods, this resulted in a small increase in yield and an important increase in harvest index with increased UV‐B.
The gravitropic set‐point angle (GSA): the identification of an important developmentally controlled variable governing plant architecture*
Authors: DIGBY J. , FIRN R. D.
Published: April 2006

The angle at which an organ is maintained by gravit‐ropism is characteristic of the organ, its developmental state and the prevailing environmental conditions. We propose that this angle be called the gravitropic set‐point angle (GSA), defined as the angle with respect to the gravity vector (with a vertically downward orientation being 0°) at which an organ is maintained as a consequence of gravitropism. Studies of the gravitropic behaviour of organs from trailing plants show that the GSA is subject to developmental regulation. Depending on the developmental age and prevailing environmental conditions, the GSA of an organ can he set at any value between 0° and 180° The previously reported reversal of the sign of the gravitropic response in such organs, whether this is brought about developmentally or induced by light, represents the change from one common extreme (GSA = 180°, conventionally referred to as negative orthogravitropism) to another (GSA = 0°, or positive orthogravitropism). The concept of a variable gravitropic set‐point offers a more unified view of all forms of gravitropic behaviour than has been advanced previously, and places a new constraint on models of gravitropism. Current models of gravitropism appear to be unable to explain either the ability of organs to change their orientation with respect to gravity as they develop, or the re‐orientation that can be observed when some organs are exposed to new environmental conditions.
Transpiration inhibition by stored xylem sap from well‐watered maize plants
Authors: SINCLAIR T. R. , VALLERANI C. , SHILLING D. G.
Published: April 2006

There is increasing evidence that a chemical signal exists in xylem sap of plants subjected to water deficits which influences physiological responses in plant shoots. An important method of studying this signal is the transpiration response of excised leaves exposed to xylem sap collected from plants. However, Munns et al [Plant, Cell & Environment 16, 867–877] cautioned that transpiration inhibition is observed when xylem sap collected from wheat and barley is stored before determining physiological activity. The objective of the study reported here was to determine if transpiration inhibition develops in maize sap collected from well‐watered plants when the sap is stored under various conditions. It was found that storage of maize sap collected from well‐watered plants for only 1 d at ‐20°C resulted in the development of substantial transpiration inhibition in bioassay leaves. Storage of sap at 4°C resulted in the development of the effect after 2 weeks, while storage at −86°C showed only small transpiration inhibition after 3 weeks. The major source of the transpiration inhibition was the development of a substance in the stored sap that resulted in physical blockage of the transpiration stream in bioassay leaves. However, a small signal component may also have developed in the stored sap. Because of the possibility of ionic activity under freezing conditions at −20°C, calcium was studied for its potential involvement in the transpiration inhibition. However, the calcium concentrations found to inhibit transpiration were nearly an order of magnitude larger than the calcium concentrations observed in xylem sap.
The expression of a small heat shock gene is activated during induction of tobacco pollen embryogenesis by starvation*
Authors: ZARSKY V. , GARRIDO D. , ELLER N. , TUPY J. , VICENTE O. , SCHÖFFL F. , HEBERLE‐BORS E.
Published: April 2006

A tobacco (Nicotiana tabacum L.) cDNA clone, Nthsp 18P, encoding a class I low‐molecular‐weight heat shock protein (LMW HSP), has been isolated from a mid‐biccllular pollen cDNA library by hcterologous hybridization with a low‐molecular‐weight heat shock cDNA clone of alfalfa. Northern analysis showed that Nthsp 18P expression is activated at normal temperature during the dehydration phase of in situ pollen development, just before anthesis. Induction of pollen embryogenesis by an in vitro starvation treatment of mid‐bicellular tobacco pollen was accompanied by a dramatic increase in the levels of the Nthsp 18P transcript, which accumulated in embryogenic pollen grains at 25°C; lower levels of this transcript were also detected in early pollen‐derived embryos. A similar pattern of expression was observed in transgcnic tobacco plants containing the promoter of Gmhsp 17.3‐B, a soybean homologue of the Nthsp 18P gene, fused to the GUS reporter gene. This is the first characterized gene shown to be transcriptionally activated during the induction of pollen embryogenesis.
On the relationship between leaf anatomy and CO2 diffusion through the mesophyll of hypostomatous leaves
Authors: SYVERTSEN J. P. , LLOYD J. , McCONCHIE C. , KRIEDEMANN P. E. , FARQUHAR G. D.
Published: April 2006

Internal conductances to CO2 transfer from the stomatal cavity to sites of carboxylation (gi) in hypostomatous sun‐and shade‐grown leaves of citrus, peach and Macadamia trees (Lloyd et al. 1992) were related to anatomical characteristics of mesophyll tissues. There was a consistent relationship between absorptance of photosynthetically active radiation and chlorophyll concentration (mmol m−2) for all leaves, including sclerophyllous Macadamia, whose transmittance was high despite its relatively thick leaves. In thin peach leaves, which had high gi, the chloro‐plast volume and mesophyll surface area exposed to intercellular air spaces (ias) per unit leaf area were similar to those in the thicker leaves of the evergreen species. Peach leaves, however, had the lowest leaf dry weight per area (D/a), the lowest tissue density (Td) and the highest chloro‐plast surface area (Sc) exposed to ias. There were negative correlations between gi and leaf thickness or D/a, but positive correlations between gi and Sc or Sc/Td.

We developed a one‐dimensional diffusion model which partitioned gi into a gaseous diffusion conductance through the ias (gias) plus a liquid‐phase conductance through mesophyll cell walls (gcw). The model accounted for a significant amount of variation (r2=0.80) in measured gi by incorporating both components. The gias component was related to the one‐dimensional path‐length for diffusion across the mesophyll and so was greater in thinner peach leaves than in leaves of evergreen species. The gcw component was related to tissue density and to the degree of chloroplast exposure to the ias. Thus the negative correlations between gi and leaf thickness or D/a related to gias whereas positive correlations between gi and Sc or Sc/Td, related to gcw. The gcw was consistently lower than gias, and thus represented a greater constraint on CO2 diffusion in the mesophylls of these hypostomatous species.
Nodule gas exchange and water potential response to rapid imposition of water deficit
Authors: PURCELL L. C. , SINCLAIR T. R.
Published: April 2006

The permeability (P) of the gaseous diffusion barrier in the nodules of soybean [Glycine max (L.) Merr.] decreases when water deficits are extended over a 7 to 10 d period. The mechanism controlling P changes is unclear, but may result from the release of water to intercellular pathways, and an associated change in the nodule water potential. The purpose of these experiments was to impose water deficit treatments rapidly in order to determine the early sequence of the responses of nodule water potential and nodule gas exchange without the complications that arise from long‐term water deficit treatments. A vertical, split‐root system was used to separate nodule drying effects from plant water deficits by replacing humidified air that was passed over upper root nodules in well‐watered plants with dry air, or by replacing the nutrient solution that surrounded lower roots with ‐1.0 MPa polyethylene glycol (PEG) solution, or by a combination of the dry air and PEG treatments. The PEG treatment caused large decreases in both the components of nodule water potential and nodule relative water content, but there was no indication that these factors had immediate, direct effects on either nitrogenase activity or P. After 7 h of the PEG treatment a significant decrease in nitrogenase activity was found but no decrease in P was detected. These results indicate that changes in nitrogenase activity in response to water deficits precede decreases in P. Exposure of nodules to dry air in well‐watered plants had no significant effect on either nitrogenase activity or P during the 7 h treatment.