bluuebunny/crossref_metadata_2025 huggingface dataset:
The differential role of mechanisms for drought resistance in a Mediterranean evergreen shrub: a simulation approach
Authors: RAMBAL S.
Published: April 2006

Plants have the ability to dampen the effects of variability in water resources. Various mechanisms contribute to these properties: reduction of leaf area, increased rooting depth and stomatal conductance. To evaluate the differential roles and interactions of these mechanisms, we have built a model and simulated flows of water in Mediterranean evergreen scrub. The essential concept of this model is that the water status of the canopy is governed by the water lost by transpiration, the availability of soil water and the hydraulic resistances to water flow in soil and plant. The amount of water supplied by the roots is related to changes in water potential between the soil and the leaf. The amount of water lost to the atmosphere is regulated by an interaction between atmospheric demand and canopy water potential. Water uptake by plant is assumed equal to plant water loss. Leaf area appears to affect largely the annual water balance. The critical leaf water potential required to reduce the maximum stomatal conductance by half has a dominant effect on annual leaf water potential. Reducing rooting depth induces a new functional equilibrium for the plant. This new equilibrium is reached by decreasing leaf area and the critical leaf water potential. Our results show the complexity of interactions of these mechanisms and highlight the importance of the coordination between them. Finally, we suggest a reconsideration of these mechanisms in a context of the survival and long‐term persistence of the plant.
Field investigations of ammonia exchange between barley plants and the atmosphere. I. Concentration profiles and flux densities of ammonia
Authors: SCHJOERRING J. K. , KYLLINGSBAEK A. , MORTENSEN J. V. , BYSKOV‐NIELSEN S.
Published: April 2006

The exchange of ammonia between the atmosphere and the canopy of spring barley crops growing at three levels of nitrogen application (medium N, high N and excessive N) was studied over two consecutive growing seasons by use of micrometeorological techniques. In most cases, ammonia was emitted from the canopy to the atmosphere. The emission started around 2 weeks before anthesis, and peaked about or shortly after anthesis. The volatilization of ammonia only took place in the daytime. During the night‐time, atmospheric ammonia was frequently aborbed by the canopy. Occasionally, plants in the medium and high N treatments also absorbed ammonia from the atmosphere during the daytime. Daytime absorption of ammonia never occurred in the excessive N canopy. The loss of ammonia from the canopy amounted in both years to 0.5–1.5 kg NH3‐N ha−1 and increased with the N status of the canopy. In agreement with the small losses of ammonia, the content of 15N‐labelled nitrogen in the plants did not decline during the grain‐filling period. The experimental years were characterized by very favourable conditions for grain dry matter formation, and for re‐utilization of nitrogen mobilized from leaves and stems. Consequently, a very high part of the nitrogen in the mature plants was located in grain dry matter (80–84% in 1989; 74–80% in 1990). The efficient re‐utilization of nitrogen may have reduced the volatilization of ammonia.
Fructan contents and dry matter deposition in different tissues of the wheat grain during development
Authors: SCHNYDER H. , GILLENBERG C. , HINZ J.
Published: April 2006

The role of fructan metabolism in the assimilate relations of the grain of wheat (Triticum aestivum L.) was investigated by determination of the dry matter and fructan content of grain components at short intervals during grain filling. During the initial phase of rapid expansion, most of the assimilates entering the grain were partitioned to the outer pericarp. A large fraction of these assimilates were used for the synthesis of fructan. Dry matter deposition and fructan synthesis in the outer pericarp ceased at about 5d after anthesis. At the same time, the endosperm and the inner pericarp and testa started to accumulate dry matter at a fast rate. This was also associated with significant fructan synthesis in the latter tissues. The outer pericarp lost about 45% of its former maximum dry weight between 9 and 19 d after anthesis. This loss was due almost entirely to the near complete disappearance of water‐soluble carbohydrates, most of which was fructan. The inner pericarp and testa accumulated dry matter until about mid‐grain filling. The fructan contents of the inner pericarp and testa and the endosperm decreased slowly towards the end of grain filling. Most of the fructans in the inner pericarp and testa and the endosperm had a low molecular weight, whereas higher molecular weight fructans predominated in the outer pericarp. The embryo did not contain fructan. The presence of low molecular weight fructans in the endosperm cavity at mid‐grain filling was confirmed. It is suggested that fructan synthesis is closely linked to growth‐related water deposition in the different tissues of the wheat grain and serves to sequester the surplus of imported sucrose.
Relationships between the kinetics of NH4+ and NO3− absorption and growth in the cultivated tomato (Lycopersicon esculentum Mill, cv T‐5)
Authors: SMART* D. R. , BLOOM A. J.
Published: April 2006

Tomato growth was examined in solution culture under constant pH and low levels of NH4+ or NO3−. There were five nitrogen treatments: 20 mmoles m−3 NH4+, 50 mmoles m−3 NO3−, 100 mmoles m−3 NH4+ 200 mmoles m−3 NO3−, and 20 mmoles m−3 NH4++ 50 mmoles m−3 NO3−. The lower concentrations (20 mmoles m−3 NH4+ and 50 mmoles m−3 NO3−) were near the apparent Km for net NH4+ and NO3− uptake; the higher concentrations (100 mmoles m−3 NH4+ and 200 mmoles m−3 NO3−) were near levels at which the net uptake of NH4+ or NO3− saturate. Although organic nitrogen contents for the higher NO3− and the NH4++ NO3− treatments were 22.2–30.3% greater than those for the lower NO3− treatment, relative growth rates were initially only 10–15% faster. After 24 d, relative growth rates were similar among those treatments. These results indicate that growth may be only slightly nitrogen limited when NH4+ or NO3− concentrations are held constant over the root surface at near the apparent Km concentration. Relative growth rates for the two NH4+ treatments were much higher than have been previously reported for tomatoes growing with NH4+ as the sole nitrogen source. Initial growth rates under NH4+ nutrition did not differ significantly (P≥ 0.05) from those under NO3− or under combined NH4++ NO3−. Growth rates slowed after 10–15 d for the NH4+ treatments, whereas they remained more constant for the NO3− and mixed NH4++ NO3− treatments over the entire observation period of 24–33 d. The decline in growth rate under NH4+ nutrition may have resulted from a reduction in Ca2+, K+, and/or Mg2+ absorption.
NMR imaging of root water distribution in intact Vicia faba L plants in elevated atmospheric CO2
Authors: BOTTOMLEY P. A. , ROGERS H. H. , PRIOR S. A.
Published: April 2006

The effect of elevated atmospheric CO2 on water distribution in the intact roots of Vicia faba L. bean seedlings grown in natural soil was studied noninvasively with proton (1H) nuclear magnetic resonance (NMR) imaging. Exposure of 24‐d‐old plants to atmospheric CO2‐enriched air at 650 cm3 m−3 produced significant increases in water imaged in upper roots, hypogeal cotyledons and lower stems in response to a short‐term drying‐stress cycle. Above ground, drying produced negligible stem shrinkage and stomatal resistance was unchanged. In contrast, the same drying cycle caused significant depletion of water imaged in the same upper root structures in control plants subject to ambient CO2 (350 m3 m−3), and stem shrinkage and increased stomatal resistance. The results suggest that inhibition of transpiration caused by elevated CO2 does not necessarily result in attenuation of water transport from lower root structures. Inhibition of water loss from upper roots and lower stem in elevated CO2 environments may be a mitigating factor in assessing deleterious effects of greenhouse changes on crops during periods of dry climate.
Characteristics of inorganic nitrogen assimilation of plants in fire‐prone Mediterranean‐type vegetation
Authors: STEWART G. R. , PATE J. S. , UNKOVICH M.
Published: April 2006

A range of approaches was used to investigate how species within a fire‐prone Banksia woodland in South West Australia exploited inorganic soil nitrogen sources and how this changes through the development of the fire chronosequence. Nitrate and ammonium were present in soil solution throughout the chronosequence but nitrate predominated in recently burnt sites. Mean shoot nitrate reductase activities were high for all species in recently burnt sites and showed little increase when nitrate was supplied via the transpiration stream. Nitrate reductase of shoots of most species was low at sites not burnt for several years, but following transpirational induction with nitrate, developed activities similar to those at recently burnt sites. The principal amino compounds transported in the xylem were species specific, including asparagine, glutamine and citrulline‐dominated species, and changed little in relative composition across the chronosequence. Species most active in leaf nitrate reduction transported the largest amounts of nitrate in their xylem sap and proportional amounts of nitrate in xylem tended to be greatest in recently burnt sites. Most of the species examined appeared to be shoot rather than root nitrate assimilators, but marked differences were recorded in potential of leafy shoots of different species to reduce nitrate. As a general rule, shallow‐rooted herbaceous, non‐mycorrhizal or VAM‐positive species had the highest capacity to reduce nitrate, whereas woody species with ericoid mycorrhizae or combined vesicular arbuscular/ectomycorrhizal associations exhibited little capacity to reduce nitrate in roots or shoots. It seems likely that this latter group utilize ammonium or even organic forms of nitrogen rather than nitrate. Some putative nitrogen‐fixing species were active in reducing and transporting nitrate, others were virtually inactive in these respects.
Changing sensitivity to light and nitrate but not to gibberellins regulates seasonal dormancy patterns in Sisymbrium officinale seeds
Authors: DERKX M. P. M. , KARSSEN C. M.
Published: April 2006

Seeds of Sisymbrium officinale (L.) Scop, that are buried under natural conditions in soil pass annually through a seasonal pattern of changes in dormancy. Dormancy is broken in autumn‐winter and re‐induced in summer. To elucidate dormancy regulation in this species under natural conditions, a detailed analysis of the changes in sensitivity to some relevant germination factors was carried out Germination data fitted as logistic dose response curves showed that sensitivity to light and nitrate, both indispensable stimuli for germination of this species, varied with the seasons. Patterns of shifts in requirement for light and nitrate were remarkably similar. Sensitivity increased when both primary and secondary dormancy were alleviated, and it was reversed during induction of secondary dormancy. During alleviation of primary dormancy in spring 1991, the fluence response curves exhibited a biphasic character with responses occurring both in the very‐low‐fluence‐range and in the low‐fluence‐range. The nitrate dose response data could all be fitted as monophasic curves, although responses might have occurred in two distinct ranges as well. From interpretation of curve parameters, it is postulated that dormancy is regulated by changes in the number of phytochrome and nitrate receptors, in shifts in the binding characteristics of the receptors and/or in shifts in the response chain initiated by the ligand‐receptor interaction. Somewhere in this response chain, biosynthesis of gibberellins (GAs) is stimulated. By use of the GA biosynthesis inhibitor tetcyclasis, it was indirectly proven that the capacity to synthesize GAs indeed varied with the seasons. Sensitivity to GAs gradually increased from burial onwards and was not particularly related to changes in dormancy. Thus, except for the first few months of burial, GA sensitivity may not be regarded as a limiting factor in controlling dormancy in this species.
Interpretation of the electrical potential on the surface of plant roots
Authors: DUE G.
Published: April 2006

The electrical potential difference (p.d.) between two points on the surface of a plant root is shown to be a measure of the difference between the transmembrane potentials at those two points. More precisely, it is shown that axial differences in electrical potential on the surface of the root, or within several tenths of millimeters of it in the rhizosphere, are primarily a result of axial differences in p.d. across the plasmalemma of cells in the cortex, with an additional small effect from axial differences in p.d. across the plasmalemma of cells in the stele. This conclusion results from a model of the root as a three‐conductor electrical transmission line. The model requires the solution of a set of differential equations, but simple algebraic approximations are found to apply over a range of model parameters derived from published data. Given the predictions of the model, it follows that, for many research purposes, microelectrode measurements could be replaced with measurements of p.d. on the surface of the root. Such measurements offer substantial advantages over microelectrode measurements: they are non‐invasive; they do not include the potential difference across the tonoplast; they measure a spatial average of many cells, not just a single cell; and the measurement is physically robust. Surface potential measurements do not, however, measure trans‐membrane potential at one point, but measure differences in trans‐membrane potential.
Rapid appearance of 13C in biogenic isoprene when 13CO2 is fed to intact leaves
Authors: DELWICHE C. F. , SHARKEY T. D.
Published: April 2006

Biogenic isoprene substantially affects atmospheric chemistry, but it is not known how or why many plants, especially trees, make isoprene. We fed 13CO2 to leaves of Quercus rubra and monitored the incorporation of 13C into isoprene by mass spectrometry. After feeding 13CO2 for 9 min we found all possible labelling patterns from completely unlabelled to fully labelled isoprene. By 18 min, 84% of the carbon atoms in isoprene were 13C. Labelling of the last 20% of the carbon atoms was much slower than labelling of the first 80%. The rate of labelling of isoprene was similar to that reported for phosphoglyceric acid indicating that there is a close linkage between the carbon source for isoprene synthesis and the photosynthetic carbon reduction pathway.
The use of transgenic and naturally occurring mutants to understand and manipulate tomato fruit ripening
Authors: GRAY J. E. , PICTON S. , GIOVANNONI J. J. , GRIERSON D.
Published: April 2006

In the years since we last reviewed the use of mutants to study tomato fruit ripening (Grierson et al. 1987), considerable information has been gained by the cloning, sequencing and identification of many mRNAs implicated in this developmental process. Genes involved in cell wall degradation, colour change and ethylene synthesis have been cloned, and antisense techniques have been developed and used to produce genetically engineered mutant fruit deficient in these aspects of ripening (see Gray et al. 1992). Recently, a previously cloned ripening gene has been used to complement a naturally occurring fruit colour mutant, yellow flesh (Fray & Grierson 1993a), and a ripening impaired mutant, ripening inhibitor, has been used to identify several new ripening‐related mRNAs (Picton et al. 1993b). The chromosomal region bearing the ripening inhibitor mutation has been subjected to high‐resolution mapping (Churchill, Giovannoni & Tanksley 1993) and chromosome walking experiments are in progress to identify this gene.
Action and function of phytochrome family members revealed through the study of mutant and transgenic plants
Authors: WHITELAM G. C. , HARBERD N. P.
Published: April 2006

The adaptation of plant growth and development to changes in the light environment is dependent upon photoperception by information transducing photoreceptors. The red/far‐red light‐absorbing phytochromes are perhaps the best characterized of these regulatory photoreceptors. Higher plants possess multiple, discrete phytochromes, the apoprotein components of which are the products of a small, divergent gene family. Different phytochromes have different biochemical and physiological properties, and are differentially expressed in the growing plant. This has led to the proposal that different phytochromes have different physiological roles. Mutations that disrupt the normal perception of light signals have proved to be a valuable resource in assigning physiological roles to different phytochromes as well as in identifying residues/domains critical for phytochrome function and in attempting to elucidate the signal transduction pathway(s) downstream of phytochromes. This article reviews some recent progress in these areas from the study of conventional and transgenic photomorphogenic mutants.
Inhibition of photosynthesis by osmotic stress in pea (Pisum sativum) mesophyll protoplasts is intensified by chilling or photoinhibitory light; intriguing responses of respiration
Authors: SARADADEVI K. , RAGHAVENDRA A. S.
Published: April 2006

The effects of reduced osmotic potential on photosynthesis and respiration were studied in mesophyll protoplasts of pea (Pisum sativum). Osmotic stress was induced by increasing the sorbitol concentration in the medium from 0·4 kmol m−3 (‐1·3 MPa) to 1·0 kmol m−3 (‐3·1 MPa). Protoplasts lost up to 35% of the maximum capacity of photo‐synthetic carbon assimilation (but not PS II mediated activity) soon after exposure to 1·0 kmol m−3 sorbitol. The response of protoplast respiration to osmotic stress was intriguing. Respiration was stimulated if stress was induced at 25°C, but was inhibited when protoplasts were subjected to osmotic stress at 0°C. Photosynthesis was also much more sensitive to osmotic stress at 0°C than at 25°C. The inhibitory effects of osmotic stress on photosynthesis as well as respiration were amplified by not only chilling but also photoinhibitory light. The photosynthetic or respiratory activities of protoplasts recovered remarkably when they were transferred from hyperosmotic (1·0 kmol m−3 sorbitol) back to iso‐osmotic medium (0·4 kmol m−3 sorbitol), demonstrating the reversibility of osmotic‐stress‐induced changes in protoplasts. Respiration was more resistant to osmotic stress and was quicker to recover than photosynthesis. We suggest that the experimental system of protoplasts can be useful in studying the effects of osmotic stress on plant tissues.
Blue‐green fluorescence excited by UV laser on leaves of different species originates from cutin and is sensitive to leaf temperature
Authors: BONGI G. , PALLIOTTI A. , ROCCHI P. , MOYA I. , GOULAS Y.
Published: April 2006

Under ultra‐violet excitation, intact leaves generate a strong blue‐green fluorescence emission with several bands. Their integrated energy is 6 to 11 times the energy released by chlorophyll a bands (Chappelle et al. 1984, Applied Optics 23, 134–138). This paper provides evidence that the blue‐green fluorescence emission comes mainly from outer epidermal layers of the leaves and can be transferred on a quartz lamina by quickly dipping the leaves in organic solvents with subsequent solvent evaporation. Blue‐green fluorescence displays a diffusion‐controlled quenching of fluorescence intensity between 4°C (high fluorescence) and 37°C (low fluorescence). The blue‐green fluorescence emissivity is not linked to short‐term metabolic effects other than leaf temperature, but epidermis adaptations both to drought and to excessive radiation increase emissivity.
Far‐red light reflected from neighbouring vegetation promotes shoot elongation and accelerates flowering in spring barley plants
Authors: DAVIS M. H. , SIMMONS S. R.
Published: April 2006

Field experiments were conducted in St Paul, MN, USA, to test the hypothesis that early season declines in the red:far‐red ratio (R:FR) associated with FR reflection from neighbouring leaves have a role in regulating barley development. Treatment plants were grown adjacent to densely sown border rows of barley. The borders functioned to reflect far‐red (FR), which reduced R:FR within the treatment plant light environment without shading treatment plants. Barriers were set in the soil to minimize root interactions between treatment plants and borders. Treatment plants were spaced either 2 or 16 cm apart. The presence of borders significantly increased shoot leaf and internode lengths at both plant spacings. Leaf sheath length data suggest that interactions between 2 cm spaced treatment plants enhanced plant responsiveness to the presence of borders. Border treatments shortened the period of vegetative growth prior to initiation of main shoot floral primordia. Bordered plants formed fewer main shoot leaves, initiated internode elongation at a lower node, and had slightly earlier heading dates than unbordered controls. Leaf appearance rate was not influenced by border treatments. We conclude that barley shoot development is photomorphogenically modulated by R:FR. Early season shifts in R:FR could have a significant influence on shoot development given that barley has the capacity to detect and developmentally respond to declines in R:FR associated with FR reflection from neighbours.
Molecular evolution of phosphoeno/pyruvate carboxylase
Authors: TOH H. , KAWAMURA T. , IZUI K.
Published: April 2006

Phosphoenolpyruvate carboxylase is an enzyme involved in a wide variety of important metabolic pathways of plants such as anaplerotic reactions and C4 and CAM photosynthetic pathways. The accumulation of molecular sequence data of phosphoenolpyruvate carboxylases has enabled us to investigate the function and molecular evolution of the enzymes by computer‐assisted sequence comparison. Here we report the results of sequence comparison of phosphoenolpyruvate carboxylases: (1) Phosphoenofpyruvate carboxylases were classified into four groups; a subgroup of bacterial enzymes and three subgroups of plants enzymes. (2) The divergence time of the monocot enzymes involved in the C4 pathways was roughly estimated to be 150—300 million years. On the other hand, the phylogenetic tree of the enzymes suggested that those for the dicot enzymes involved in the C4 and CAM pathways might be close to the divergence time between the monocots and the dicots. (3) The evolutionary positions of the enzymes prevalent in roots or root nodules were identified. (4) Although sorghum and maize contained at least three genes for the enzymes in their genomes, the rates of amino acid substitution of the enzymes were different from gene to gene. The difference could not be explained by either lineage effects nor bias in base contents.
The effect of ultraviolet‐B radiation on gene expression and pigment composition in etiolated and green pea leaf tissue: UV‐B‐induced changes are gene‐specific and dependent upon the developmental stage
Authors: JORDAN B. R. , JAMES P. E. , STRID Å. , ANTHONY R. G.
Published: April 2006

The effect of ultraviolet‐B radiation (UV‐B: 280–320nm) on gene expression and pigment composition has been investigated in pea tissue at different stages of development. Pea (Pisum sativum L., cv. Feltham First) seedlings were grown for 17d and then exposed to supplementary UV‐B radiation. Chlorophyll a per unit fresh weight decreased by more than 20% compared with control levels after exposure to UV‐B radiation for 7d. In contrast, chlorophyll b content remained the same or increased slightly. Leaf protein biosynthesis, as determined by 35S‐methionine incorporation, was rapidly inhibited by UV‐B radiation, although the steady‐state levels of proteins were either unchanged or only slightly altered. RNA transcripts for the chlorophyll a/b binding protein (cab) were also rapidly reduced to low or even undetectable levels in the expanded third leaf or younger leaf bud tissue after exposure to UV‐B radiation. In contrast, cab RNA transcripts were either low or undetectable in etiolated pea tissue, but increased substantially in light and during exposure to UV‐B radiation. The cab RNA transcripts were still present at control levels in pea plants after 7d of greening under supplementary UV‐B radiation or UV‐B alone. The protein composition changed significantly over the 7d of greening, but no differences could be detected between the light treatments. The increase in chlorophyll content was slightly greater during de‐etiolation under supplementary UV‐B radiation than under control irradiance. Under UV‐B radiation alone, chlorophyll was synthesized at a greatly reduced rate. Changes in protective pigments were also determined. Anthocyanins did not change in either etiolated or green tissue exposed to UV‐B radiation. However, other flavonoids increased substantially in either tissue during exposure to light and UV‐B radiation. The RNA levels for chalcone synthase were measured in green and etiolated tissue exposed to UV‐B radiation. The chs RNA transcripts were present in low or undetectable amounts in control tissues. In green leaf tissue exposed to supplementary UV‐B, a transient increase was detected. The transcripts for chs reached a maximum level after approximately 8 h UV‐B exposure, and then declined to lower levels over subsequent days of diurnal photoperiods. However, a constant increase in chs was found after continuous exposure to UV‐B for up to 30 h. In etiolated tissue, either white‐light, supplementary UV‐B or UV‐B alone gave small increases in chs, only 8 h of UV‐B radiation alone gave any substantial increase in chs expression. Overall, these results clearly demonstrate that the response to increased levels of UV‐B radiation is dependent upon the developmental stage of the tissue and involves complex changes in gene expression.
The CO2‐concentrating mechanism is absent in the green alga Coccomyxa: a comparative study of photosynthetic CO2 and light responses of Coccomyxa, Chlamydomonas reinhardtii and barley protoplasts
Authors: PALMQVIST K. , ÖGREN E. , LERNMARK U.
Published: April 2006

Photosynthesis was characterized for the unicellular green alga Coccomyxa sp., grown at low inorganic carbon (Ci) concentrations, and compared with Chlamydomonas reinhardtii, which had been grown so that the CO2 concentrating mechanism (CCM) was expressed, and with protoplasts isolated from the C3 plant barley (Hordeum vulgare). Chlamydomonas had a significantly higher Ci‐use efficiency of photosynthesis, with an initial slope of the Ci‐response curve of 0.7 mol(gChl)−1 h−1 mmol Cim−3)−1, as compared to 0.3 and 0.23 mol(gChl)−1 h−1 (mmol Cim−3)−1 for Coccomyxa and barley, respectively. The affinity for Ci was also higher in Chlamydomonas, as the half maximum rate of photosynthesis [K0.5 (Ci)] was reached at 0.18 mol m−3, as compared to 0.30 and 0.45 mol m−3 for Coccomyxa and barley, respectively. Ethoxyzolamide (EZ), an inhibitor of the enzyme carbonic anhydrase (CA) and the CCM, caused a 17‐fold decrease in the initial slope of the photosynthetic Cj‐response curve in Chlamydomonas, but only a 1.5‐ to two‐fold decrease in Coccomyxa and barley. The photosynthetic light‐response curve showed further similarities between barley and Coccomyxa. The rate of bending of the curve, described by the convexity parameter, was 0.99 (sharp bending) and 0.81–0.83 (gradual bending) for cells grown under low and high light, respectively. In contrast, the maximum convexity of Chlamydomonas was 0.85. The intrinsically lower convexity of Chlamydomonas is suggested to result from the diversion of electron transport from carbon fixation to the CCM. Taken together, these results suggest that Coccomyxa does not possess a CCM and due to this apparent lack of a CCM, we propose that Coccomyxa is a better cell model system for studying C3 plant photosynthesis than many algae currently used.
The origin and theoretical correction of thermal gradients in thermocouple psychrometers
Authors: VANDERCHMITT E. , DAUDET F.‐A.
Published: April 2006

Temperature‐compensated psychrometers are explained and a theoretical correction for temperature gradients, with its boundaries, is given. The correction was elaborated for in situ determination of water potential on soybean and tomato stems where it was indispensable. The origin of the thermal gradients in the psychrometric chamber is then discussed. Cooling of the conducting tissue by the circulation of xylem sap seemed to be the major cause for these gradients, while the heating of the psychrometer (thermally insulated) by the surroundings through radiative or conductive transfer was negligible.
Photon requirement for O2‐Revolution in red (λ= 680 nm) light for some C3 and C4 plants and a C3–C4 intermediate species*
Authors: OSBORNE B. A.
Published: April 2006

Photon requirements for O2‐evolution in red (λ=680nm) light (Фr) were measured for six C3 species, one C3‐like, C3–C4 intermediate species, and three C4 species, including examples of NADP‐malic enzyme and PEP‐carboxykinase C4 sub‐groups. Variation in Фr within the C3 species was small with a mean value of 7.96 ±0.12 mol photon mol−1 O2, whereas the mean value for the C4 species was 12.27± 1.53 mol photon mol−1 O2, with the lowest value, 9.24 ±0.13 mol photon mol−1 O2, for the PEP‐carboxykinase C4 species Spartina townsendii. The C3–C4 intermediate species Panicum milioides had a value of 9.05 ±0.29 mol photon mol−1 O2, approximately 1 mol photon mol−1 O2 greater than the C3 species. The possibility that this extra cost is due to PEP‐carboxylase‐dependent recycling of CO2 is discussed. No correlation was found between Фr and chlorophyll content or leaf absorptance. Based on white (ФW) and red light measurements of the photon requirement, values in red light were approximately 20% higher than white‐light estimates. These results are discussed with reference to accepted mechanisms of energy transduction in thylakoid membranes (Z‐scheme), expected inefficiencies and losses during light‐harvesting and electron transport reactions, and the influence of respiratory processes.
Spectral balance and UV‐B sensitivity of soybean: a field experiment
Authors: CALDWELL M. M. , FLINT S. D. , SEARLES P. S.
Published: April 2006

Soybean [Glycine max (L.) Merr.] cultivar Essex was grown and tested for sensitivity to UV‐B radiation (280–320 nm) under different combinations of UV‐A (320–400 nm) and PFD (400–700 nm) radiation in four simultaneous field experiments. The radiation conditions were effected with combinations of filtered solar radiation and UV‐B and UV‐A lamps electronically modulated to track ambient radiation. Significant UV‐B‐caused decreases in total aboveground production and growth were seen only when PFD and UV‐A were reduced to less than half their flux in sunlight. When PFD was low, UV‐A appeared to be particularly effective in mitigating UV‐B damage. However, when PFD was high, substantial UV‐A did not appear to be required for UV‐B damage mitigation. Leaf chlorophyll fluorescence did not indicate photosynthetic damage under any radiation combination. With UV‐B, leaves in all experiments exhibited increased UV‐absorbing pigments and decreased whole‐leaf UV transmittance. Results of these field experiments indicate difficulties in extrapolating from UV‐B experiments conducted in glasshouse or growth cabinet conditions to plant UV‐B sensitivity in the field. Implications for UV radiation weighting functions in evaluating atmospheric ozone reduction are also raised.
Hardening of root cell walls: a growth inhibitory response to salinity stress
Authors: NEUMANN P. M. , AZAIZEH H. , LEON D.
Published: April 2006

Primary roots of intact maize plants (Zea mays L.) grown for several days in nutrient solutions containing 100 mol m−3 NaCl and additional calcium, had relatively inhibited rates of elongation. Possible physical restraints underlying this salt induced inhibition were investigated. The inhibition did not involve reductions in osmotic potential gradients and turgor in the tip tissues responsible for root elongation growth. The apparent yield threshold pressure, which is related to capacity of cell walls to undergo loosening by stress relaxation, was estimated psychrometrically in excised root tips. Salinity increased yield threshold values. Comparative root extensibility values were obtained for intact plants by determining the initial (1 min) increase in root elongation rate induced by an 0.1 MPa osmotic jump. Comparative extensibility was significantly reduced in the salinized root tips. Salinity did not reduce capacities for water efflux and associated elastic contraction in root tip tissues of intact plants exposed to hypertonic mannitol. We conclude that cell wall hardening in the elongating root tips is an important component of root growth inhibition induced by long‐term salinization.
Day‐to‐day variation in nitrogenase activity of Alnus incana explained by weather variables: a multivariate time series analysis
Authors: EKBLAD A. , LUNDQUIST P.‐O. , SJÖSTRÖM M. , HUSS‐DANELL K.
Published: April 2006

A modelling system is described that indicates the extent to which day‐to‐day variations in nitrogenase activity in young Alnus incana (L.) Moench, grown in defined conditions in the field, may be affected by weather conditions both during and prior to the day of measurement. Nitrogenase activity (acetylene reduction activity, ARA) was measured weekly on intact field‐grown grey alder (A. incana) plants, 0.15–0.42 m tall at planting, nodulated with Frankia. The measurements were done at noon on two groups of plants in 1987 and on two other groups in 1988. Each group was made up of five or six plants. Seven weather variables: daily sunshine hours, daily mean, maximum and minimum air temperature, daily mean and 1300 h relative humidity, and daily rainfall were used. The relation between log(ARA/leaf area) and the weather variables were analysed using a PLS model (partial least squares projection to latent structures). The advantage of PLS is that it can handle x‐variables that are correlated. Data from 1987 were chosen as a training set. Multivariate PLS time series analysis was made by adding, in a stepwise manner, the weather data up to 5 d before the day of measurement. This procedure gave six models with n * 7 x‐variables (n= 1–6). With the models from the time series analysis of 1987 data, true predictions of ARA per leaf area were made from weather data 1988 (test set 1) and from ‘early‐season’ weather data from 1987 and 1988 (test set 2). The variation in ARA/leaf area could be predicted from the weather conditions. The predictions of the two test sets improved when the weather conditions one and two days before the day of measurements were added to the model. The further addition of weather data from 3 to 5 d before the day of measurement did not improve the model. The good predictions of ARA/leaf area show that the alders responded to the variable weather conditions in the same way in 1988 as in 1987, despite the ten‐fold difference in size (leaf area) at the end of the growing season. Among the weather variables, air temperature and the daily sunshine hours were positively correlated to ARA, while relative air humidity and rainfall were negatively correlated to ARA. The daily minimum temperature and rainfall appeared to have least impact on ARA. By use of PLS, we could extract information out of a data set containing highly correlated x‐variables, information that is non‐accessible with conventional statistical tools such as multiple regression. When making measurements of nitrogenase activities under field conditions, we propose that attention should be paid to the weather conditions on the days preceding the day of measurement. The day‐to‐day variation in nitrogenase activity is discussed with reference to known effects of stress factors under controlled conditions.
Gas exchange and growth responses to elevated CO2 and light levels in the CAM species Opuntia ficus‐indica
Authors: CUI M. , NOBEL P. S.
Published: April 2006

Gas exchange and dry‐weight production in Opuntia ficus‐indica, a CAM species cultivated worldwide for its fruit and cladodes, were studied in 370 and 750 μmol mol−1 CO2 at three photosynthetic photon flux densities (PPFD: 5, 13 and 20 mol m−2 d−1). Elevated CO2 and PPFD enhanced the growth of basal cladodes and roots during the 12‐week study. A rise in the PPFD increased the growth of daughter cladodes; elevated CO2 enhanced the growth of first‐daughter cladodes but decreased the growth of the second‐daughter cladodes produced on them. CO2 enrichment enhanced daily net CO2 uptake during the initial 8 weeks after planting for both basal and first‐daughter cladodes. Water vapour conductance was 9 to 15% lower in 750 than in 370 μmol mol−1 CO2. Cladode chlorophyll content was lower in elevated CO2 and at higher PPFD. Soluble sugar and starch contents increased with time and were higher in elevated CO2 and at higher PPFD. The total plant nitrogen content was lower in elevated CO2. The effect of elevated CO2 on net CO2 uptake disappeared at 12 weeks after planting, possibly due to acclimation or feedback inhibition, which in turn could reflect decreases in the sink strength of roots. Despite this decreased effect on net CO2 uptake, the total plant dry weight at 12 weeks averaged 32% higher in 750 than in 370 μmol mol−1 CO2. Averaged for the two CO2 treatments, the total plant dry weight increased by 66% from low to medium PPFD and by 37% from medium to high PPFD.
Regulation of transpiration in coffee hedgerows: covariation of environmental variables and apparent responses of stomata to wind and humidity
Authors: GUTIÉRREZ M. V. , MEINZER F. C. , GRANTZ D. A.
Published: April 2006

Stomatal regulation of transpiration was studied in hedgerow coffee (Coffea arabica L.) at different stages of canopy development encompassing a range of leaf area indices (L) from 0·7 to 6·7. Stomatal (gs) and crown (gc) conductance attained maximum values early during the day and then declined as both leaf‐to‐bulk air water vapour mole fraction difference (Va) and photosynthetically active photon flux density (I) continued to increase. Covariation of environmental variables during the day, particularly V, I, and wind speed (u), obscured stomatal responses to individual variables. This also caused diurnal hysteresis in the relationship between gc and individual variables. Normalization of gs and gc by I removed the hysteresis and revealed a strong stomatal response to humidity. At the crown scale, transpiration (E) increased linearly with net radiation (Rn) and seemed to increase with increasing wind speed. Increasing wind speed imposed higher leaf interior to leaf surface water vapour mole fraction differences (Vs) at given levels of Va. However, strong relationships between declining gc and E and increasing wind speed were obtained when gc and E were normalized by I and Rn, respectively, without invoking additional potential interactions involving temperature or CO2 concentration at the leaf surface. Apparent stomatal responses to wind were thus at least partially a reflection of the stomatal response to humidity.
Investigations of mechanisms of resistance to bipyridyl herbicides in Arctotheca calendula (L.) Levyns
Authors: PRESTON C. , BALACHANDRAN S. , POWLES S. B.
Published: April 2006

The mechanism of resistance to diquat and paraquat was investigated in a bipyridyl‐herbicide‐resistant biotype of Arctotheca calendula (L.) Levyns. No differences were observed in the interactions of these herbicides with Photo‐system I, the active site, in thylakoids isolated from resistant and susceptible biotypes. Likewise, absorption of herbicide through the cuticle and gross translocation were identical in plants of the two biotypes. Foliar application of either 25 g ha−1 diquat or 200 g ha−1 paraquat rapidly inhibited CO2‐dependent O2 evolution of leaf segments of the susceptible biotype. O2 evolution of leaf segments of the resistant biotype was less affected by these treatments. Fluorescence imaging was used to observe visually, as fluorescence quenching, the penetration of herbicide to the active site. These experiments demonstrated that diquat appears at the active site more slowly in the resistant biotype compared to the susceptible biotype. HCO3‐dependent O2 evolution of thin leaf slices was less inhibited by diquat in the resistant biotype than in the susceptible biotype. The mechanism of resistance to the bipyridyl herbicides in this biotype of A. calendula is not a result of changes at the active site, decreased herbicide absorption or decreased translocation, but appears to be due to reduced herbicide penetration to the active site.
Xylem water transport: is the available evidence consistent with the cohesion theory?
Authors: ZIMMERMANN U. , MEINZER F. C. , BENKERT R. , ZHU J.J. , SCHNEIDER H. , GOLDSTEIN G. , KUCHENBROD E. , HAASE A.
Published: April 2006

Since its introduction in the late 19th century, the so‐called cohesion theory has become widely accepted as explaining the mechanism of the ascent of sap. According to the cohesion theory, the minimum standing vertical xylem tension gradient should be 0·01 MPa m−1. When transpiration is occurring, frictional resistances are expected to make this gradient considerably steeper. The results of numerous pressure chamber measurements reported in the literature are generally regarded as corroborating the cohesion theory. Nevertheless, several reports of pressure chamber measurements in tall trees appear to be incompatible with predictions of the cohesion theory. Furthermore, the pressure chamber is an indirect method for inferring xylem pressure, which, until recently, has not been validated by comparison against a direct method. The xylem pressure probe provides a means of testing the validity of the pressure chamber and other indirect techniques for estimating xylem pressure. We discuss here the results of concurrent measurements made with the pressure chamber and the xylem pressure probe, particularly recent measurements made at the top of a tall tropical tree during the rainy season. These measurements indicate that the pressure chamber often substantially overestimates the tension previously existing in the xylem, especially in the partially dehydrated tissue of droughted plants. We also discuss other evidence obtained from classical and recent approaches for studying water transport. We conclude that the available evidence derived from a wide range of independent approaches warrants a critical reappraisal of tension‐driven water transport as the exclusive mechanism of long‐distance water transport in plants.
Predicting responses of photosynthesis and root fraction to elevated [CO2]a: interactions among carbon, nitrogen, and growth*
Authors: LUO Y. , FIELD C. B. , MOONEY H. A.
Published: April 2006

At elevated atmospheric CO2 concentrations ([CO2]a), photosynthetic capacity (Amax) and root fraction (ηR, the ratio of root to plant dry mass) increased in some studies and decreased in others. Here, we have explored possible causes of this, focusing on the relative magnitudes of the effects of elevated [CO2]a on specific leaf (nm) and plant (np) nitrogen concentrations, leaf mass per unit area (h), and plant nitrogen productivity (α). In our survey of 39 studies with 35 species, we found that elevated [CO2]a led to decreased nm and np in all the studies and to increased h and α in most of the studies. The magnitudes of these changes varied with species and with experimental conditions.

Based on a model that integrated [CO2]a‐induced changes in leaf nitrogen into a biochemically based model of leaf photosynthesis, we predicted that, to a first approximation, photosynthesis will be upregulated (Amax will increase) when growth at increased [CO2]a leads to increases in h that are larger than decreases in nm. Photosynthesis will be downregulated (Amax will decrease) when increases in h are smaller than decreases in nm. The model suggests that photosynthetic capacity increases at elevated [CO2]a only when additional leaf mesophyll more than compensates the effects of nitrogen dilution.

We considered two kinds of regulatory paradigms that could lead to varying responses of ηR to elevated [CO2]a, and compared the predictions of each with the data. A simple static model based on the functional balance concept predicts that ηR should increase when neither np nor h is very responsive to elevated [CO2]a. The quantitative and qualitative agreement of the predictions with data from the literature, however, is poor. A model that predicts ηR from the relative sensitivities of photosynthesis and relative growth rate to elevated [CO2]a corresponds much more closely to the observations. In general, root fraction increases if the response of photosynthesis to [CO2]a is greater than that of relative growth rate.
Interacting effects of CO2 concentration, temperature and nitrogen supply on the photosynthesis and composition of winter wheat leaves
Authors: DELGADO E. , MITCHELL R. A. C. , PARRY M. A. J. , DRISCOLL S. P. , MITCHELL V. J. , LAWLOR D. W.
Published: April 2006

Winter wheat (Triticum aestivum L., cv. Mercia) was grown at two different atmospheric CO2 concentrations (350 and 700 μmol mol−1), two temperatures [ambient temperature (i.e. tracking the open air) and ambient +4°C] and two rates of nitrogen supply (equivalent to 489 kg ha−1 and 87 kg ha−1). Leaves grown at 700 μmol mol−1 CO2 had slightly greater photosynthetic capacity (10% mean increase over the experiment) than those grown at ambient CO2 concentration, but there were no differences in carboxylation efficiency or apparent quantum yield. The amounts of chlorophyll, soluble protein and ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco) per unit leaf area did not change with long‐term exposure to elevated CO2 concentration. Thus winter wheat, grown under simulated field conditions, for which total biomass was large compared to normal field production, did not experience loss of components of the photosynthetic system or loss of photosynthetic competence with elevated CO2 concentration. However, nitrogen supply and temperature had large effects on photosynthetic characteristics but did not interact with elevated CO2 concentration. Nitrogen deficiency resulted in decreases in the contents of protein, including Rubisco, and chlorophyll, and decreased photosynthetic capacity and carboxylation efficiency. An increase in temperature also reduced these components and shortened the effective life of the leaves, reducing the duration of high photosynthetic capacity.
Effective stomatal and boundary‐layer resistances of heterogeneous surfaces
Authors: McNAUGHTON K. G.
Published: April 2006

In nature surfaces are rarely uniform, so terms such as ‘surface’, ‘stomatal’ or ‘canopy’ resistance usually indicate some kind of average over a population of sub‐areas, each with its own separate resistance. Questions then arise as to how gross measurements of these resistances should be interpreted in terms of the components, or how components should be aggregated into representative single values. Aggregation schemes have been published by Raupach (1991, Vegetatio 91, 105–120) and Lhomme (1992, Agriculture and Forest Meteorology 61, 11–21), but these are different for reasons that were not explained. This paper develops the idea that averaging schemes should be designed to serve particular purposes, and that they can be varied to suit these purposes. It is shown that the ‘effective’ resistances defined by Raupach and Lhomme preserve different quantities. A further averaging scheme is developed which preserves both correct transpiration rate and CO2 flux when used in the Penman‐Monteith equation and an equation describing assimilation. All of these schemes are fairly complex, so the work provides a warning against naive use of effective variables.