bluuebunny/crossref_metadata_2025 huggingface dataset:
Empirical models of the conductance of leaves in apple orchards
Authors: JONES H. G. , HIGGS K. H.
Published: April 2006

Abstract.Seasonal data on leaf conductance (gl) for three different apple cultivars grown in four separate orchards with different aged trees was studied between 1979 and 1985. A number of empirical models for predicting leaf conductance from environmental measurements were compared using this data and a general method for adapting such models for the prediction of different data sets is proposed. Although stepwise multiple regression identified relative humidity or vapour pressure as important variables, it frequently did not identify the optimal set of independent variables, which often did not include either of these. There was no advantage in regressing glagainst principal components of the environment, rather than against the raw environmental variables. A simple model involving air vapour pressure deficit, air temperature and a hyperbolic function of irradiance was found to explain between 32 and 62% of the variance in glfor the different data sets. Parameters fitted for one data set led to the effective prediction of glin other years or plots. The model fit could generally be improved significantly by including soil moisture deficit among the independent variables.
Vascular transport and soybean nodule function: nodule xylem is a blind alley, not a throughway
Authors: WALSH K. B. , McCULLY M. E. , CANNY M. J.
Published: April 2006

Abstract.The only published consideration of product removal from the soybean root nodule hypothesizes that the peripheral xylem circuit of this determinate nodule structure is flushed by the transpiration stream. However, dyes fed to the transpiration stream through a cut root distal to the nodule do not enter the nodule, and the observed movement of radio‐tracers from the root into the nodule can be explained by simple diffusion, Also, there are few xylem elements in the nodule, and these elements are of a small diameter, such that this path can not act as a functional loop of the root system. Further, in this study, nodule vascular strands were never observed to be continuous about the nodule, but were observed to end at the nodule tip in a loop within an intact, closed endodermal sac. Nodule vascular tissue was invested in a pericycle of at least three cell layers. These cells are suggested to be active in the loading of the xylem apoplast with ureides, such that the xylem of the nodule always operates in an export role. Nodule water requirements may be suppliedviathe phloem or the root cortex apoplasm.
The function, action and adaptive significance of phytochrome in light‐grown plants
Authors: CASAL J. J. , SMITH H.
Published: April 2006

Abstract.It has previously been proposed that the fundamental function of phytochrome in the natural environment is the perception of the relative proportions of red and far‐red light, i.e. the red: far‐red ratio. This paper re‐evaluates this hypothesis, for vegetative green plants, in the light of recent findings. Essentially, three issues are considered: (a) the modulation of the response to red: far‐red by fluence rate: (b) the anticipation of competition for light by perception of changes in red: far‐red that precede actual shading: and (c) characteristics of phytochrome that may be important in the mechanism of photoperception (i.e. the accumulation of photoconversion intermediates, and the stability of Pfr). We conclude: (a) the red: far‐red ratio provides a reliable signal of plant density, even before shading by neighbours occurs: (b) plants are able to perceive and respond to these signals, and that possible ambiguities due to low red: far‐red at low solar angles may be avoided by modulation of the perception process by fluence‐rate dependent mechanisms; (c) although direct experimental evidence does not yet exist, circumstantial evidence suggests that the perception of red: far‐red may confer positive adaptive advantage; and (d) plants of certain species perceive and respond to fluence rate changes, mediated perhaps by a blue‐light absorbing photoreceptor or by phytochrome, but that these responses do not necessarily lead to shade avoidance reactions and their ecological relevance is not fully understood.
An integrated portable apparatus for the simultaneous field measurement of photosynthetic CO2 and water vapour exchange, light absorption and chlorophyll fluorescence emission of attached leaves
Authors: IRELAND C. R. , LONG S. P. , BAKER N. R.
Published: April 2006

Abstract. A portable apparatus has been constructed to measure simultaneously the quantum yield of CO2 assimilation, light absorption, chlorophyll fluorescence emission and water vapour exchange of attached intact leaves in the field. The core of the instrument is a light‐integrating spherical leaf chamber which includes ports for a light source, photosynthetically active radiation sensor, fluorescence probes and gas inlet and outlet manifolds. Measurement of the quantum flux inside the empty chamber and with a leaf present allows determination of leaf absorptance. An open gas exchange system is employed using an infra‐red analyser to measure leaf CO2 exchange. Using a DC white light source the quantum yield of CO2 assimilation based on absorbed light (φabs) may be determined rapidly in either ambient air or artificial gas mixtures. Inclusion of capacitance humidity probes into the gas inlet and outlet ports allows simultaneous determination of water vapour exchange and subsequent estimation of stomatal conductance to CO2 and intercellular CO2 concentration. Measurement of fluorescence emission by the sample leaf exposed to white light is achieved by a modulated fluorescence detection system. In addition to determination of the minimal, maximal and variable fluorescence levels, a further analysis allows the photochemical and non‐photochemical components of fluorescence quenching, to be estimated. The theory and design of this apparatus is described in detail. The use of the apparatus in the field is demonstrated through a study of the photosynthetic performance of a maize and bean crop during the growing season and by analysis of the photosynthetic performance of crops subjected to nitrogen‐stress and a herbicide treatment.
Mechanism of soybean nodule adaptation to different oxygen pressures
Authors: PARSONS R. , DAY D. A.
Published: April 2006

Abstract. Soybean nodules showed the ability to adapt to oxygen pressures above and below ambient levels and this adaptation involved a decrease in cortical intercellular air‐spaces with increasing oxygen pressure. Nodules were grown in oxygen pressures from 4.7 to 75 kPa and the decrease in number and size of cortical intercellular spaces with increasing oxygen pressure was the result of a change in cell structure and the deposition of an electron dense material within intercellular spaces. Exposure to a saturating pressure of acetylene caused a similar inhibition of respiration and nitrogenase activity in nodules developed in oxygen pressures from 4.7 to 47 kPa, suggesting that putative acetylene‐induced changes in oxygen diffusion resistance occur by a different mechanism than that involved in long‐term adaptation to oxygen. However, in nodules grown at 75 kPa oxygen, the initial specific activities were lower and did not show an acetylene induced decline. The results are discussed in terms of the current theories of regulation of nitrogenase activity by oxygen availability.
Sensing of soil water status and the regulation of plant growth and development
Authors: DAVIES W. J. , MANSFIELD T. A. , HETHERINGTON A. M.
Published: April 2006

Abstract. It is now clear that drying of the soil does not always result in an early change in shoot water status. This may be because stomata close and leaf growth slows to reduce water loss. When this is the case, it is necessary to ask how the change in soil water status has been ‘sensed’by the shoot. The current view is that soil drying results in some type of chemical signalling between roots and shoots. The sensitivity of the response and experiments involving the manipulation of small parts of root systems suggest that the signalling involves more than a simple change in root activity in response to soil drying. In this paper, we consider the evidence for chemical signalling between roots and shoots and discuss the possible candidates for such signals. In some plants, root‐sourced ABA can apparently influence shoot physiology and growth in the absence of any perturbation of shoot water relations. The ABA produced is quantitatively sufficient to account for the responses observed. Applied ABA can mimic many of the effects of soil drying on plants, including effects at the plasma membrane and on gene expression. Perhaps uniquely, ABA seems to be involved in signalling between different plant organs, and in signalling at the transmembrane and genome levels. We review the effects of ABA on leaf cells with a view to gaining some understanding of how soil drying may influence plant development.
Sensing pH?
Authors: RAVEN J. A.
Published: April 2006

Abstract. pH is an all‐pervasive variable in the environment of phototrophs. Phototrophs as a whole, and even the single genus Dunaliella, can grow over essentially the whole range of pH values found in nature. Such a large range of pH values, combined with other chemical variations in the environment, impose a range of constraints on plant behaviour related to intracellular pH regulation, nutrient acquisition, and avoidance of toxic effects. No single genotype can grow well over the whole pH range compatible with growth of phototrophs as a whole, although some deliberately alter surface pH so as to create a 5–6 unit pH gradient over the surface related to nutrient acquisition and avoidance of toxic influences. The regulation of these various processes does not, on current evidence, involve pH‐sensing by any extracellular sensor which is not part of the catalytic or regulatory mechanism of a membrane protein such as a porter.
Does transpiration have an essential function in long‐distance ion transport in plants?
Authors: TANNER W. , BEEVERS H.
Published: April 2006

Abstract. Long‐term effects of transpiration on growth and on long‐distance ion transport were investigated in maize over a whole growth cycle. Maize plants were grown with nutrients supplied at adequate levels in hydroculture or in soil at 50–60% and at >95% relative humidity. Although the amount of water lost by the plants under these conditions differed by a factor 2 to 3, there was neither a decrease in growth (fresh weight and dry weight) nor in ash content of the ‘humid’plants. This was also found when the upper part of the shoot (70–150 cm) was tested separately. It is suggested that transpiration is not essential for long‐distance transport of mineral elements in plants. Alternatives are discussed.
Inhibition studies on acetyl group incorporation into chloroplast fatty acids
Authors: MASTERSON C. , WOOD C. , THOMAS D. R.
Published: April 2006

Abstract. Whilst L‐acetylcarnitine acted as a substrate for fatty acid synthesis by isolated pea leaf chloroplasts, D‐acetylcarnitine did not. This result, together with those obtained using the inhibitors D‐carnitine and deoxycarnitine, indicated that L‐acetylcarnitine was not being hydrolysed to free acetate prior to incorporation into chloroplast fatty acids. Seventy‐five per cent and 66% inhibitions of L‐acetylcarnitine incorporation into fatty acids, brought about by adding equimolar quantities of D‐carnitine and deoxycarnitine, respectively, were suggestive of competitive inhibition at two points: an integral membrane translocator in the chloroplast envelope: and the carnitine acetyltransferase enzyme of the chloroplast stroma, which converts L‐acetylcarnitine to acetyl CoA. Isotope competition experiments between acetate and L‐acetylcarnitine confirmed that L‐acetylcarnitine was the preferred substrate for pea chloroplast fatty acid synthesis.
Water balance in Hemizonia luzulifolia: the role of extracellular polysaccharides
Authors: MORSE S. R.
Published: April 2006

Abstract. The role of extracellular water in ameliorating drought stress was examined in Hemizonia luzulifola, an annual composite that comprises two subspecies differing significantly in the amount of extracellular polysaccharide within basal leaves. Rosette leaves of the high polysaccharide (HP) ssp were more than 30% pectin on a dry weight basis in contrast to only 4% in the low polysaccharide (LP) ssp. Concomitant with this difference in polysaccharide content was a significant difference in the dehydration response of leaves. Near full hydration, relative capacitances of HP leaves (0.7 MPa‐1) were an order of magnitude greater than the LP leaves (0.08 MPa‐1). Relative capacitance of the polysaccharide alone was 1.5 MPa‐1. The weight of water per unit leaf area was not significantly different in the two taxa. Nevertheless, the pectin‐like polysaccharides significantly altered the distribution of metabolically available water from primarily cell‐stored water in the LP ssp to apoplasmic and symplasmic capacitors in leaves of the HP ssp. The consequence of this apoplasmic water store with colloidal properties is that the Ψ‐dependence of leaf capacitance ceases to be linked directly to cell water relations. Transfer resistances for water movement between capacitors and the xylem near full hydration (0 to ‐0.5 MPa) were significantly larger in the HP leaf. This difference in transfer resistance was interpreted to be the result of a large resistance to water movement between the polysaccharide and the xylem. Because of these large transfer resistances, the apoplasmic capacitor probably buffers cells at lower water potentials under transient water fluxes than expected from laboratory measurements made during slow desiccation. Field measurements support this conclusions; HP leaves were better buffered than LP leaves at midday water potentials.
Reflection signals and the perception by phytochrome of the proximity of neighbouring vegetation
Authors: SMITH H. , CASAL J. J. , JACKSON G. M.
Published: April 2006

Abstract. Spectral photon distributions, red:far‐red ratios (i.e. R:FR) and phytochrome photoequilibria (i.e. Pfr/Ptotal) were measured at various distances from artificial canopies composed of mustard or tobacco plants. Measurements were compared for radiation propagated predominantly vertically downwards and radiation propagated predominantly horizontally. Reflection signals from the artificial canopies were computed and shown to consist of a depletion of radiation over the 400–690 nm wavelength range, and an enhancement of radiation over the 690–800 nm range. R:FR and Pfr/Ptotal increased gradually with distance from the canopies, with significant depressions of both parameters evident at least as far as 30 cm from the vegetation stands. It is concluded that, in principle at least, detection of spectral quality differences by phytochrome would allow not only the presence but also the proximity of neighbouring plants to be perceived. Proximity perception is proposed as an ecologically valuable mechanism through which plants may be able to gauge their anticipatory responses to incipient shading according to the challenge posed by the nearness of neighbouring plants.
Water‐stress‐induced xylem embolism in three species of conifers
Authors: SPERRY J. S. , TYREE M. T.
Published: April 2006

Abstract. The mechanism of water‐stress‐induced xylem embolism was studied in three species of conifers: Abies balsamea (L.) Mill., Picca rubens Sarg, and Juniperus virginiana L. Each species showed a characteristic relationship between xylem tension and the loss of hydraulic conductivity by air embolism. Abics balsamea and Picca rubens began to embolize at tensions between 2 and 3 MPa and were completely non‐conducting between 3 and 4 MPa. Juniperus virginiana was least vulnerable, beginning to embolize at 4 and still retaining approximately 10% conductivity at 10 MPa. As with a previous study of the vessel‐bearing Accr saccharum Marsh., a brief perfusion of branch segments with an oxalic acid and calcium solution (10 and 0.1 mol m−3. respectively) increased the vulnerability of the xylem to embolism; this was especially pronounced in Abies balsamea. In order to test whether embolism was caused by aspiration of air into functional tracheids from neighbouring embolized, ones (the ‘air‐seeding’hypothesis), hydrated branch segments were injected with air at various pressures and measured for embolism. Results supported the air‐seeding hypothesis because the relationship between injection pressure and embolism for both native and oxalic‐calcium‐treated segments was essentially the same as for embolism induced by xylem tension. Structural and experimental evidence suggested the air seeding occurred through inter‐tracheid pit membranes when the thickened torus region of the membrane became displaced from its normal sealing position over the pit aperture. Thus, the embolism‐inducing tension may be a function of pit membrane flexibility. This tension is of ecological significance because it reflects to some extent the range of xylem tensions to which a species is adapted.
The appearance of a new molecular species of phosphoenolpyruvate carboxylase (PEPC) and the rapid induction of CAM in Sedum telephium L.
Authors: GROENHOF A. C. , SMIRNOFF N. , BRYANT J. A.
Published: April 2006

Abstract. When detached leaves of Sedum telephium are incubated in the absence of water, a rapid switch from C3 photosynthesis to CAM (as indicated by the onset of day‐to‐night fluctuations in titratable acidity. ΔH+) occurs within the first dark period. The C3‐CAM switch in intact plants occurs within 3 5d. Extractable activity of phosphoenolpyruvate carboxylase (PEPC) increases five‐fold in intact plants during CAM induction; however, during rapid CAM induction in detached leaves, there is only a very small increase in PEPC activity. Fractionation by anion exchange chromatography of crude extracts from leaves of intact plants subjected to water deficit shows that CAM induction is associated with the appearance of a molecular species of PEPC termed PEPC I. PEPC I is barely detectable in well‐watered plants which are not performing CAM. The major form in these plants is termed PEPC II. In leaves from intact plants, there is a significant positive correlation between PEPC I activity and ΔH+ during a period of increasing water deficit. PEPC I exhibits day to night fluctuations in malate sensitivity, being less sensitive during the dark period. In contrast, PEPC II is more sensitive to inhibition by malate and has no day to night fluctuation in sensitivity. In detached leaves deprived of water, a small increase in PEPC I capacity is detected at the end of the first dark period (20 h after the start of treatment). The results suggest that PEPC I is required for attainment of maximum nocturnal malic acid synthesis. There is a significant correlation between leaf water status (relative water content), ΔH+, total PEPC and PEPC I activity suggesting that the internal water status of the plant may be a trigger for CAM induction. Abscisic acid applied to detached leaves does not cause nocturnal acidification.
Rehydration‐induced changes in pressure‐volume relationships of Artemisia tridentata Nutt. ssp. tridentata
Authors: EVANS R. D. , BLACK R. A. , LINK S. O.
Published: April 2006

Abstract. Pressure‐volume measurements were made on Artemisia tridentata Nutt. ssp. tridentata samples rehydrated for 0, 1.5, 3, 6 or 24 h. Increasing rehydration time caused a significant increase in osmotic potential at turgor loss, cell elasiticity, and the relative water content at turgor loss, and a significant decrease in pressure potential at saturation. Osmotic potential at saturation was changed significantly by rehydration, but no consistent trend was observed. The symplastic water fraction did not differ significantly among treatments. The increase in the osmotic potential at turgor loss did not correspond with decreasing cell elasticity or synthesis of solutes. Instead, the leaf solute content remained constant suggesting a redistribution of solutes between the symplast and apoplast. Using non‐rehydrated samples for pressure‐volume analysis introduced errors in estimates of the symplastic water fraction, osmotic potential at full turgor, and the relative water content at turgor loss. These errors are due to uncertainties in the determination of saturated weights.
Ca‐Cd interaction in the prymnesiophyte Cricosphaera elongata
Authors: KAREZ C. S. , ALLEMAND D. , DE RENZIS G. , GNASSIA‐BARELLI M. , ROMEO M. , PUISEUX‐DAO S.
Published: April 2006

Abstract. 45Ca and 109Cd uptake were followed in Criscosphaera elongata Prymnesiophyceae. In both cases, after a rapid increase for the first 5 min, the incorporation rate slowed during the hour of observation. Verapamil, a blocker of voltage‐dependent slow calcium channels, inhibited 45Ca uptake except during the first rapid phase when adsorption should predominate. Cadmium also decreased 45Ca labelling, suggesting that the two metals are antagonistic. However, verapamil was shown to augment 109Cd incorporation, contrary to what occurs in animals cells; this effect is detectable in continuous labelling as well as in pulse experiments. The data support the presence of calcium channels in the alga and suggest several processes in Cd accumulation: adsorption on peripheral envelopes and diffusion of uncharged Cd.
RuBPCO kinetics and the mechanism of CO2 entry in C3 plants
Authors: MÄCHLER F. , MÜLLER J. , DUBACH M.
Published: April 2006

Abstract. The CO2 partial pressure in the chloroplasts of intact photosynthetic C3 leaves is thought to be less than the intercellular CO2 partial pressure. The intercellular CO2 partial pressure can be calculated from CO2 and H2O gas exchange measurements, whereas the CO2 partial pressure in the chloroplasts is unknown. The conductance of CO2 from the intercellular space to the chloroplast stroma and the CO2 partial pressure in the chloroplast stroma can be calculated if the properties of photosynthetic gas exchange are compared with the kinetics of the enzyme ribulose 1,5‐bisphosphate carboxylase/oxygenase (RuBPCO). A discrepancy between gas exchange and RuBPCO kinetics can be attributed to a deviation of CO2 partial pressure in the chloroplast stroma from that calculated in the intercellular space. This paper is concerned with the following: estimation of the kinetic constants of RuBPCO and their comparison with the CO2 compensation concentration; their comparison with differential uptake of 14CO2 and 12CO2; and their comparison with O2 dependence of net CO2 uptake of photosynthetic leaves. Discrepancy between RuBPCO kinetics and gas exchange was found at a temperature of 12.5 °C, a photosynthetic photon flux density (PPFD) of 550 μmol quanta m−2 s−1, and an ambient CO2 partial pressure of 40 Pa. Consistency between RuBPCO kinetics and gas exchange was found if CO2 partial pressure was decreased, temperature incresed and PPFD decreased. The results suggest that a discrepancy between RuBPCO kinetics and gas exchange is due to a diffusion resistance for CO2 across the chloroplast envelope which decreases with increasing temperature. At low CO2 partial pressure, the diffusion resistance appears to be counterbalanced by active CO2 (or HCO3) transport with high affinity and low maximum velocity. At low PPFD, CO2 partial pressure in the chloroplast stroma appears to be in equilibrium with that in the intercellular space due to low CO2 flux.
Alteration of thylakoid membrane structure in Brassica rapa ssp. oleifera during ageing in high and low light
Authors: NURMI A. H.
Published: April 2006

Abstract Alterations in the composition and structure of thylakoids were studied in Brassica rapa ssp. oleifera grown under high and low irradiance (800 μmol m−2 s−1 and 80 μmol m−2 s−1). During ageing, both high and low light induced a decrease in total protein particle density and in the relative amount of 80–90 Å cytochrome b6/f and 90–100 Å ATP‐synthetase. The density of PSII complexes in stacked (EFs) and unstacked (EFu) thylakoids also decreased. In high light, a shift was noted towards smaller PSII complexes in the EFs face with decreasing attached antenna complex CP29, but the relative amount of the antenna chlorophyll a‐protein complexes of photosystem II (CPa) remained stable. In contrast, the proportion of peripheral LHCH on the PFs face and the density of PFs particles increased together with an increase in grana size. In low light, a shift occurred towards larger PSII complexes on the EFs face, along with a decrease in the proportion of CPa complexes and the PFs particle density (peripheral LHCH), though a marked increase was observed in the proportion of chlorophyll a/b‐protein complexes in SDS‐PAGE. The amount of photosystem I in green gel remained fairly stable, although the density of PFu particles (including PSI) increased in low and slightly diminished in high light. The results indicate that the organization of thylakoid components depends strongly on the light conditions and stage of development.
Plant respiration in relation to growth, maintenance, ion uptake and nitrogen assimilation
Authors: JOHNSON I. R.
Published: April 2006

Abstract Respiration in plants is generally observed to comprise two components: one proportional to the growth rate and the other to the plant dry mass. These components are usually interpreted as being related to the growth of new plant material and maintenance of existing plant material, respectively. By analysing data in this way, the respiratory costs of both structural synthesis and maintenance are observed to be greater in the root than the shoot. This contradicts current understanding of the biochemistry of the processes involved. The basic model is developed to incorporate three additional processes. The first is the cost of ion uptake for plant growth. The second allows for the fact that the site of nitrogen assimilation into amino acids may differ from the site of utilization for protein synthesis: when ammonium is supplied, this is incorporated immediately into amino acids owing to its toxicity to the plants; when nitrate is supplied it may be reduced either in the shoot or root, or both, and subsequently transported around the plant for utilization. The third process to be included is an energy cost for the uptake of ions to balance efflux from the root system. The theory is consistent with experimental observation and provides a means of understanding and interpreting respiration and nitrogen metabolism in plants.
Stomatal and hydraulic conductance in growing sugarcane: stomatal adjustment to water transport capacity*
Authors: MEINZER F. C. , GRANTZ D. A.
Published: April 2006

Abstract Stomatal conductance per unit leaf area in well‐irrigated field‐ and greenhouse‐grown sugarcane increased with leaf area up to 0.2 m2 plant 1, then declined so that maximum transpiration per plant tended to saturate rather than increase linearly with further increase in leaf area. Conductance to liquid water transport exhibited parallel changes with plant size. This coordiantion of vapour phase and liquid phase conductances resulted in a balance between water loss and water transport capacity, maintaining leaf water status remarkably constant over a wide range of plant size and growing conditions. The changes in stomatal conductance were not related to plant or leaf age. Partial defoliation caused rapid increases in stomatal conductance, to re‐establish the original relationship with remaining leaf area. Similarly, pruning of roots caused rapid reductions in stomatal conductance, which maintained or improved leaf water status. These results suggest that sugarcane stomata adjusted to the ratio of total hydraulic conductance to total transpiring leaf area. This could be mediated by root metabolites in the transpiration stream, whose delivery per unit leaf area would be a function of the relative magnitudes of root system size, transpiration rate and leaf area.
Aerenchyma formation and associated oxygen movement in seminal and nodal roots of wheat
Authors: THOMSON C. J. , ARMSTRONG W. , WATERS I. , GREENWAY H.
Published: April 2006

Abstract The present paper describes the effects of growth of roots of wheat (Triticum aestivum cv. Gamenya) in hypoxic nutrient solutions on acrenchyma formation and O2 movement from shoots to roots. Two types of roots were investigated: (1) seminal roots of 4–7‐d‐old seedlings, and (2) seminal and nodal roots of 10–28‐d‐old plants. Gas‐filled porosity of seminal and nodal roots increased from 3 to 12% and from 5–7 to 11–15%, respectively, when the roots emerged in stagnant or N2‐flushed solutions (0.003 mol m −3 O2) compared with growth in continuously acrated solutions (0.26 mol m −3 O2). However, neither root type increased in porosity when they were longer than 100–200 mm at the start of the exposure to these stagnant or N2‐flushed treatments. A vernier microscope and cylindrical platinum‐electrode were used to examine the relationship between root extension and transport of O2 from shoots to roots via the gas spaces. Measurements were made when the roots were in an anoxic medium and were dependent solely on O2 supplied from the shoots. For seminal roots of 5–7‐d‐old seedlings raised in stagnant solutions (90–100 mm), internal O2 transport was sufficient to support a rate of root elongation in the O2‐free medium of between 0.03 and 0.17 mm h−1. When the O2 pressure around the shoots was increased from 20 to 100 kPa O2, the O2 concentrations at the walls of the expanding zone (2–7 mm from the tip) of these roots increased from 0.006 mol m−3 to between 0.04 and 0.26 mol m−3, and the rate of root extension increased five‐fold. Oxygen transport to roots grown continuously in acrated solutions was considerably less than for roots raised in stagnant solutions; this difference was greater for seminal than for nodal roots. When the acrated seminal roots were longer than 100 mm and transferred to an O2‐free root medium, O2 concentration became zero at the root tip causing elongation to cease. After 24 h of anoxia, none of these roots were able to resume elongation following a return to acrated solutions.
Diel water movement between parenchyma and chlorenchyma of two desert CAM plants under dry and wet conditions
Authors: TISSUE D. T. , YAKIR D. , NOBEL P. S.
Published: April 2006

Abstract. Electric‐circuit analogue models of the water relations of crassulacean acid metabolism (CAM) succulents such as Agave deserti and Ferocactus acanthodes have predicted diel movement of water between the water‐storage parenchyma and the photo‐synthetic chlorenchyma. Injection of tritiated water into either tissue in the laboratory confirmed substantial and bidirectional water movements, especially under conditions of wet soil. For A. deserti, water movement from the water‐storage parenchyma to the chlorenchyma increased at night as the chlorenchyma osmotic pressure increased. Although nocturnal osmotic pressure increases and transpiration for both species were minimal in the field under dry conditions, diel changes in the deuterium: hydrogen ratio (expressed as ΔD) were similar for the water‐storage parenchyma and the chlorenchyma. Such indication of [substantial mixing of water between the tissues over a 24‐h cycle was more evident under wet conditions in the field. For A. deserti, ΔD then increased by 32%o from the afternoon to midnight and was essentially identical in the water‐storage parenchyma and the chlorenchyma. For F. acanthodes, the diel changes in ΔD were one‐third those of A. deserti, and ΔD was always slightly higher for the chlorenchyma than for the water‐storage parenchyma, apparently reflecting the lower surface‐to‐volume ratio of A. deserti. In summary, data obtained using radioactive and stable isotopes strongly supported model predictions concerning diel cycles of internal water distribution for these CAM species.
Self‐incompatibility in Papaver rhoeas: there is no evidence for the involvement of stigmatic ribonuclease activity
Authors: FRANKLIN‐TONG V. E. , ATWAL K. K. , HOWELL E. C. , LAWRENCE M. J. , FRANKLIN F. C. H.
Published: April 2006

Abstract. Ribonuclease assays have revealed, in contrast to the self‐incompatibility (SI) system of Nicotiana alata, there is no detectable ribonuclease activity that correlates with the presence of the functional stigmatic S‐gene product in Papaver rhoeas. Thus, we have shown that the inhibition of incompatible pollen tube growth in P. rhoeas is not associated with ribonuclease activity. Furthermore, the finding that pollen from P. rhoeas, unlike that from N. alata, is insensitive to purified bovine pancreatic ribonuclease A at very high concentrations, suggests that the involvement of ribonucleases in the inhibition reaction of the SI response in P. rhoeas is unlikely. In addition, the level of ribonuclease activity in mature stigmas of P. rhoeas is very much lower than that in N. alata and significantly, the level of ribonuclease activity did not rise in conjunction with the developmental expression of SI. Therefore, as a result of these studies, we believe that SI in P. rhoeas does not involve ribonuclease activity.
Growth‐induced water potentials may mobilize internal water for growth
Authors: MATYSSEK R. , MARUYAMA S. , BOYER J. S.
Published: April 2006

Abstract. Wphen there is no external source of water, plants can grow by mobilizing internal water from nongrowing tissues. We investigated how this internal water moves by measuring continuously and simultaneously the water potential (ψw) of soybean (Glycine max L. Merr.) seedlings in the upper, growing stem tissues and the lower, non‐growing stem tissues. When external water was available to the roots, the stems grew rapidly and the ψw of the growing tissue was continually below that of the nongrowing tissue and the medium around the roots. This indicated that a growth‐induced gradient in ψw favoured water movement from the external source to the growing cells. When the external source was removed, the ψw of the growing tissue remained constant for a time and the ψw of the nongrowing tissue decreased somewhat. Growth took place slowly as water was withdrawn from the nongrowing tissue but ψw gradients continued to favour water transport to the growing cells. On the other hand, if this internal source was removed by excision, growth ceased abruptly. In this case, the cell walls relaxed and the ψw of the growing tissue decreased by about 0.1 MPa instead of remaining constant. The ψw of the detached nongrowing tissues remained constant instead of decreasing. This indicates not only that water mobilization required attached nongrowing or slowly growing tissues but also that mobilization affected wall relaxation. Thus, ψw differences may mobilize internal water, may explain the continued growth of plants and plant parts removed from external sources of water, and may account for discrepancies in measurements of cell wall properties in growing tissues.
Photosynthetic productivity of an immature maize crop: changes in quantum yield of CO2 assimilation, conversion efficiency and thylakoid proteins
Authors: STIRLING C. M. , NIE G. Y. , AGUILERA C. , NUGAWELA A. , LONG S. P. , BAKER N. R.
Published: April 2006

Abstract. The effect of growth temperatures on quantum yield (φ) was examined for leaves at different stages of development within the immature canopies of two crops of field grown maize (Zea mays cv. LG11) sown on 3 May and 20 June 1990. During the period of 23 to 49d after sowing, the crop sown on the 3 May experienced temperatures below 10°C on 19 occasions compared with only two for the crop sown on 20 June. A period of severe chilling at the end of May and the beginning of June was associated with a marked reduction in φ for all leaves in the early‐sown crop. This chill‐induced depression in φ was greater in recently emerged than more mature leaves in the canopy and was found to be accompanied by modifications in the polypeptide profiles of thylakoids isolated from the leaves. During the chilling period, decreases in some polypeptides, notably in the range of 41–42 and 20kDa apparent molecular size, and increases of polypeptides of c. 15–16kDa were observed compared with leaves developing at warmer temperatures in July. The efficiency of converting intercepted radiation into dry matter (conversion efficiency) was 42% lower in the early‐ than late‐sown crop, but no significant relationship between conversion efficiency and quantum yield was found in either treatment.
Functional interaction between leaf trichomes, leaf wettability and the optical properties of water droplets
Authors: BREWER C. A. , SMITH W. K. , VOGELMANN T. C.
Published: April 2006

Abstract. Because CO2 diffuses 10000 times more slowly through water than air, there may be strong selective pressure for increased water repellency in terrestrial plant leaves. In the present study, leaf trichomes appeared to have a strong influence on leaf water repellency (i.e. degree of water droplet formation on the leaf surface) as well as the retention of droplets on the leaf. Based upon evaluation of 38 plant species from 21 families, we found that leaves with trichomes were more water repellent, especially where trichome density was greater than 25mm2. However, droplet repellency and retention were both high in some species where trichomes entrapped droplets. Finally, the lensing effects of water droplets on leaf surfaces increased incident sunlight by over 20‐fold directly beneath individual droplets. These results may have important implications for such processes as stomatal function, whole leaf photosynthesis, and transpiration for a large variety of plant species.
Responses of stomata of senescing and nonsenescing leaves of Nicotians glauca to changes in intercellular concentrations of leaf carbon dioxide
Authors: THOMAS C. , DAVIS S. D. , TALLMAN G.
Published: April 2006

Abstract. Gas exchange measurements were performed to test the hypothesis that failure of stomata to open in senescing leaves of Nicotiana glauca is caused by elevated concentrations of carbon dioxide in the intercellular spaces of leaf mesophyll tissue (ci). Senescing leaves selected for experiments were completely chlorotic and lacked positive rates of photosynthesis. When stomata in detached epidermis from senescing leaves were illuminated in CO2‐free air, they opened to similar apertures as those in detached epidermis from nonsenescing leaves. To compare the effects of changes in ci on stomatal responses of the two leaf types, leaf ‘flags’ of either nonsenescing or senescing leaves were illuminated at a photosynthetic photon flux density of 500 μmol m−2 s−1 in a gas exchange cuvette. Leaf temperatures were maintained at 23.5 ± 0.5°C, and vapour pressure differences between leaves and the air were maintained between 0.70 and 0.75kPa. Ci was adjusted by changing external concentrations of carbon dioxide in air circulating through the cuvette. Conductances and photosynthetic rates of nonsenescing leaves changed in response to changes in ci, but neither the conductances nor the photosynthetic rates of senescing leaves were affected significantly by changes in q. We conclude that guard cells of senescing leaves of Nicotiana glauca do not lose the capacity to respond to changes in carbon dioxide concentration and that increases in ci resulting from declining rates of mesophyll photosynthesis are not the sole cause of maintenance of stomatal closure during leaf senescence. The data suggest that factors external to guard cells may prevent them from responding to changes in carbon dioxide concentrations in intact senescing leaves.
Long‐term functioning of enucleate sieve elements: possible mechanisms of damage avoidance and damage repair
Authors: RAVEN J. A.
Published: April 2006

Abstract. The sieve‐tube elements of long‐lived arborescent monocotyledons which lack secondary thickening remain functional for many decades despite lacking a nucleus. A minimal requirement for transport by mass flow powered by the Munch mechanism is maintenance of semi‐permeability of the plasmalemma of the sieve tube elements; loading and re‐loading could be deputed to nucleate cells symplastically linked to the sieve elements. An additional requirement in the long‐term relates to replacement of components damaged by mechanical, chemical or radiation intrusions. Minimizing the damage from radiation and chemical agents can be related to a number of commonly observed features of sieve tubes. Damage from O2 and radicals derived there from is minimized in these essentially aerobic cells by (1) the absence of intercellular gas spaces in the phloem combined with the lower O2 solubility and diffusivity in concentrated disaccharide (or sugar alcohol) solutions, (2) the absence of photosynthetic machinery which could generate singlet oxygen, and (3) the presence of at least some components of scavenging mechanisms (glutathione, peroxidase, abscorbate). Non‐enzymic glycosylation (exacerbated by O2) of proteins is minimized by the low concentrations of reducing sugars in sieve‐tubes. Ultraviolet damage is minimized by UV‐absorbing materials between the plant surface and the sieve tubes, including the selerenchymatous cap of fibres on the vascular bundles. The extent to which repair involves symplastic import of polypeptides from nucleate companion cells, thus breaching the −800‐Da limit on symplastic transport, is unclear, but it could occur in fully differentiated companion cell‐sieve element associations without necessarily upsetting development.
Oleosomes (lipid bodies) in nitrogen‐fixing peanut nodules
Authors: JAYARAM S. , BAL A. K.
Published: April 2006

Abstract. Nitrogen‐fixing peanut root nodules have oleosomes (lipid bodies) in the infected cells. The oleosomes have been characterized and compared with their counterparts in the seed tissue. Eighty per cent of the nodule oleosomes were found to be of smaller size (0.16–1.0 μm diameter [dia.]) while in the seeds the larger size (2.0–5.5 μm dia.) dominated. The larger oleosomes were exclusively found in the uninfected three layers of cortical cells adjacent to the infected zone. Morphometric analyses have revealed significantly higher numbers of oleosomes covering about 8% of the infected cell area in immature (white) nodules, whereas the mature/old (pink) nodules had lesser numbers occupying about 4–5% of the cell area. The decrease in the amount of oleosomes possibly reflects their utilization in mature/old nodules which effectively fix nitrogen. The oleosomes were distinctly stained by p‐phenylenediamine (pPD) at both light and electron microscopic levels. An electron‐dense rim was observed around the nodule oleosomes; where lipolytic activity was also demonstrated using cytochemical methods. The rim was absent in the seed oleosomes. The defatted oleosomes were found to be surrounded by a ‘half unit membrane’ and a non‐extractable rim of possible pro‐teinaceous substance. Gas chromatographic analyses of the lipid from the isolated oleosomes indicated the presence of higher amounts of saturated fatty acids in the nodule oleosomes than the ones in the seed. The study indicates that the nodule oleosomes differ from seed oleosomes, with respect to the presence of (1) an electron‐dense rim showing (2) lipolytic activity and (3) higher amounts of saturated fatty acids. Nodule oleosomes seem to be transient storage organelles to be metabolized, while in the seed they are meant for long‐term storage.
The influence of water deficits on mRNA levels in tomato
Authors: HO T.‐Y. , MISHKIND M. L.
Published: April 2006

Abstract. The levels of particular mRNAs in tomato leaves are altered in response to water stress. Certain transcripts appear as a consequence of the stress whereas others diminish in abundance. These changes were analysed by cell‐free translation of leaf RNA followed by two‐dimensional polyacrylamide gel electrophoresis of the translation products. Within 20 min of the onset of reduced leaf water potential in detached leaflets, stress‐induced mRNAs appear in the transcript population. Other mRNAs accumulate more slowly and include among their in vitro translation products a prominently labelled group of polypeptides of about 18–19 kD. These transcripts are recovered from detached leaflets incubated for 2 and 8h at low water potential and from leaflets of intact plants that were stressed in situ for 10d. An additional water stress‐induced mRNA that encodes a 64‐kD polypeptide is detected in the transcript population from detached leaflets when [3H]glycine is used in place of [35S]methionine in the cell‐free translations. Among the transcripts that decline in abundance as a consequence of water stress is an mRNA that encodes the small subunit of ribulose‐1,5‐bisphosphate carboxylase. Within 2h of rewatering stressed plants, certain mRNAs that were lost from the population return and those in the 18–19 kD range that are associated with the stress are not detectable.
Quantitative determination of water sorption by plant cuticles
Authors: CHAMEL A. , PINERI M. , ESCOUBES M.
Published: April 2006

Abstract. The water sorption by cuticular membranes (CM) isolated from both leaves and fruit and representing different structural and chemical cuticular types, was measured over the whole range of relative humidities using a magnetic suspension microbalance. The sorption isotherms were generally not linear and sorption increased more rapidly at the highest values of relative humidity. The highest values of water content, at 80–99% r.h., measured for the seven species ranged from 1.1 to 7.7% of the dehydrated weight. Extraction of the soluble cuticular lipids did not result in a decrease of sorption, but only in a lower interspecific dispersion. There was a drastic reduction (63%) in water sorption when polymer matrix membranes (MX) were partially acid‐hydrolysed; but methylation or charging with iron of (MX) did not have any significant effect on water sorption. The data obtained are discussed in relation to cuticular permeability. Two determinants of water permeability were determined: the partition coefficient (K) relating the equilibrium Water concentration of the cuticle to that of the surrounding atmosphere; and the diffusion coefficient (D), calculated from the half‐times of the sorption process in kinetic measurements.
The effects of increased atmospheric carbon dioxide and temperature on carbon partitioning, source‐sink relations and respiration
Authors: FARRAR J. F. , WILLIAMS M. L.
Published: April 2006

Abstract. Herbaceous C3 plants grown in elevated CO2 show increases in carbon assimilation and carbohydrate accumulation (particularly starch) within source leaves. Although changes in the partitioning of biomass between root and shoot occur, the proportion of this extra assimilate made available for sink growth is not known. Root:shoot ratios tend to increase for CO2‐enriched herbaceous plants and decrease for CO2‐enriched trees. Root:shoot ratios for cereals tend to remain constant. In contrast, elevated temperatures decrease carbohydrate accumulation within source and sink regions of a plant and decrease root:shoot ratios. Allometric analysis of at least two species showing changes in root: shoot ratios due to elevated CO2 show no alteration in the whole‐plant partitioning of biomass. Little information is available for interactions between temperature and CO2. Cold‐adapted plants show little response to elevated levels of CO2, with some species showing a decline in biomass accumulation. In general though, increasing temperature will increase sucrose synthesis, transport and utilization for CO2‐enriched plants and decrease carbohydrate accumulation within the leaf. Literature reports are discussed in relation to the hypothesis that sucrose is a major factor in the control of plant carbon partitioning. A model is presented in support.