bluuebunny/crossref_metadata_2025 huggingface dataset:
Stomatal Limitation of Photosynthesis in Abscisic Acid-Treated and in Water-Stressed Leaves Measured at Elevated CO2
Authors: Robinson SP , Grant WJR , Loveys BR
Published: January 2006

Feeding 10-5M (�)-abscisic acid (ABA) via the petioles of detached leaves of apricot (Prunus armeniaca) or sunflower (Helianthus annuus) decreased stomatal conductance and assimilation rate but not the calculated intercellular CO2 concentration (Ci) suggesting non-stomatal as well as stomatal inhibition of photosynthesis. Evidence for non-stomatal inhibition was not observed in spinach (Spinacia oleracea). There was no significant decrease in rates of electron transport nor ribulosebisphosphate carboxylase (Rubisco) activity in intact chloroplasts isolated from ABA-treated sunflower leaves. Oxygen evolution by leaf discs with 3% CO2 in the gas phase was inhibited in ABA- treated sunflower and apricot leaves but not in spinach; the inhibition was only half as great as the inhibition of assimilation rate at ambient CO2. The quantum yield of oxygen evolution decreased in ABA-treated sunflower leaves in proportion to the decrease in the light-saturated rate. There was no significant difference in room temperature chlorophyll fluorescence of ABA-treated leaves compared to controls.
Stomatal conductance of sunflower leaves decreased by more than 90% when the CO2 concentration was increased from 340 ppm to 1000 ppm but at much higher CO2 concentrations the stomata appeared to reopen. Stomatal conductance at 2-3% CO2 (20 000-30 000 ppm) was 50% that at ambient CO2. This reopening of stomata at high CO2 was inhibited in previously water-stressed or ABA-treated plants. In unstressed leaves, the maximum rate of oxygen evolution occurred at 0.5-2% CO2 but in ABA-treated leaves 10-15% CO2 was required for maximum rates. It is suggested that stomatal closure may limit photosynthesis in ABA-treated or previously water-stressed leaves even at the relatively high CO2 concentrations normally used in the leaf disc oxygen electrode. The inhibition of photosynthesis by ABA is largely overcome at saturating CO2. The apparent non-stomatal inhibition suggested by gas exchange measurements and the decreased quantum yield could be explained by patchy stomatal closure in response to ABA.
Growth and Development in NaCl-Treated Plants. I. Leaf Na+ and Cl- Concentrations Do Not Determine Gas Exchange of Leaf Blades in Barley
Authors: Rawson HM , Long MJ , Munns R
Published: January 2006

Plants of Hordeum vulgare cv. Clipper were grown in gravel culture in solutions containing 0, 100 or 175 mol m-3 NaCl from the seedling stage until anthesis. When salt was first applied, leaf 4 was elongating within the sheath, and leaf 7 had yet to appear on the growing apex. These two leaves were sampled most intensively. Relative to the controls, salinity reduced the final area of all leaves progressively with higher node of insertion, and in the 175 mol m-3 NaCl treatment the reduction was more than twice that in 100 mol m-3 NaCl. Because leaves were at very different stages of development when salt was applied, the progressive reduction inferred there was no evidence of a particular sensitivity to salinity at different stages of leaf expansion. The peak CO2 exchange rate (CER), which occurred shortly after ligule emergence of each leaf was reduced far more in leaf 4 than in higher positions. Indeed, CER averaged throughout the leaf's life, was actually increased by salinity in upper leaves which initiated and expanded during the salt treatment. However, the reduction in leaf area was not offset by this increase in CER. The integration of these adjustments in area per leaf and CER resulted in a reduction in carbon fixation per leaf of about 30 and 60%, by the 100 and 175 mol m-3 treatments respectively for all leaves. There was no obvious relation between CER and Na+ and Cl- concentrations in leaf 7, and between CER and Na+ concentrations in leaf 4. Regression analysis showed that chloride and sodium ions in the leaves accounted for 54% of the statistical variation in CER in leaf 4 but for an insignificant proportion of the variation in leaf 7. As leaf 7 is presumably no different from leaf 4 in its ionic response, we conclude that the high leaf concentrations of Na+ and Cl- did not directly affect the rate of photosynthesis.
Effects of Growing Wheat in Hypoxic Nutrient Solutions and of Subsequent Transfer to Aerated Solutions. II. Concentrations and Uptake of Nutrients and Sodium in Shoots and Roots
Authors: Buwalda F , Barrett-Lennard EG , Greenway H , Davies BA
Published: January 2006

We report on the effects of hypoxia (low O2 concentrations) in the nutrient solution on the net uptake of Cl-, N, P, K+ and Na+ by 26-30-day-old wheat plants (Triticum aestivum L.). Plants treated with hypoxia had been grown at 0.003 mol O2 m-3 for 10 days; four consecutive samplings were taken between 10 and 14 days to assess the uptake of nutrients. After the first sampling, one group of plants exposed to hypoxia was returned to aerated solutions containing 0.27 mol O2 m-3 .
Seminal and crown roots grown in hypoxic solutions showed remarkably few differences in ion relations, despite the high porosity and the fast growth of the crown roots. The exception was with K+ /Na+; a decrease in this ratio due to growth in hypoxic solutions was more pronounced for seminal than for crown roots.
Concentrations of elements in the shoots were nearly always lower for plants grown at 0.003 than for plants grown at 0.27 mol O2 m-3 ; the only exception was for Na+ which increased in plants grown in the hypoxic solutions. Comparisons with published critical nutrient concentrations indicate that decreased growth at 0.003 mol O2 m-3 was not due to reduced nutrition of the plants. For N, this conclusion is supported by higher concentrations of soluble amino acids in plants at 0.003 than at 0.27 mol O2 m-3 .
The reduction in net transport to the shoots of N, P, Cl- and K+ was due to a combination of decreases in root /shoot ratio and an inhibition of ion uptake per unit root weight. This inhibition was not due to permanent damage as it was reversible. A rapid restoration of concentrations of N, P, Cl- and K+ in the shoot upon transfer from 0.003 to 0.27 mol O2 m-3 was attributed to a combination of higher net rates of ion uptake by the roots compared with continuously aerated plants and a rapid expansion of the root system.
Comparison of Carbohydrate Compartmentation in Relation to Photosynthesis, Assimilate Export and Growth in a Range of Maize Genotypes
Authors: Rocher JP
Published: January 2006

A model derived from compartmental analysis of assimilate partitioning and export in maize source leaves was used for comparison of growth characters in a series of genotypes. The measurements were performed on fully expanded fourth leaves. The parameters measured were: net photosynthetic rate (PN), carbon flux to starch and size of two sucrose pools (Q1 and Q2), the transfer coefficients between the compartments, the leaf area and the shoot and root dry matter. Photosynthetic rate on a leaf area basis varied slightly (1:1.34). This variation is amplified (1:2.96) when carbon input is expressed on a leaf basis due to great genotypic variability in leaf area (1:2.33). Compartmental analysis showed that the sizes of sucrose compartments (Q1 and Q2) were also genotype dependent. Compartment Q1, which provides sucrose export, was always greater than Q2, which represents a transient storage. The transfer coefficients were rather constant among genotypes, the coefficient representing export (k01) being much higher than the other two (k12 and k21). The proportion of the assimilates which was stored as starch during the light period represented 12-17.5% of the carbon input depending upon the genotype. This percentage of allocation to starch was negatively correlated with growth rate. The correlations between growth rate or total production and each physiological parameter (PN, carbon output from Q1 and Q2, flux toward starch) were in general highly significant (0.75<r<0.94). The correlations were positive except for starch. A multilinear regression analysis showed that two parameters, net photosynthesis per leaf or net output from Q1 associated with carbon flux to starch, provided a nearly total explanation of intergenotypic variance. The results clearly demonstrate that in maize, the differences in net carbon fixation were not sufficient to totally predict growth rate and that knowledge of the additional carbon distribution and export parameters will give a better understanding of the system.
Root Signals Control Leaf Expansion in Wheat Seedlings Growing in Drying Soil
Authors: Passioura JB
Published: January 2006

Wheat plants were grown with their roots and soil in pressure chambers, so that the leaves could be kept highly turgid, even when the soil dried, by applying a pneumatic pressure to the roots. The relative leaf expansion rate (RLER) of plants in drying soil eventually fell behind that of well-watered plants, but, remarkably, the fall in RLER was the same whether or not the leaves were kept highly turgid. It is argued that the roots sensed the drying of the soil and sent signals to the leaves that controlled their behaviour, overriding any effects of turgor on the leaves. It is likely that the roots were sensing not only
the water potential of the soil but also its hardness, which increased substantially as the soil dried.
Why Measure Osmotic Adjustment?
Authors: Munns R
Published: January 2006

Osmotic adjustment (erroneously called 'osmoregulation') is generally regarded as an important adaptation to drought or salinity. Because it helps to maintain turgor and cell volume, it is often thought to promote growth, yield, or survival, of plants in dry or saline soils. However, a physiological rationale for such views is lacking. Osmotic adjustment itself cannot promote growth; the solutes which account for it must be diverted from essential processes such as protein and cell wall synthesis. Further, it now appears that turgor does not control cell expansion or stomatal conductance. Thus, osmotic adjustment cannot affect yields except via other processes, the controls of which are almost entirely unexplored. Future research in this area should test hypotheses, rather than merely measure osmotic adjustment.
Putrescine Reduces NaCl-Induced Inhibition of Germination and Early Seedling Growth of Rice (Oryza sativa L.)
Authors: Prakash L , Prathapasenan G
Published: January 2006

Changes induced by NaCl and putrescine during germination and early seedling growth of rice were examined for 120 h. Rates of germination and growth of embryo axis decreased when seeds were subjected to salt stress (100, 150 and 200 mM NaCl). NaCl at 150 mM concentration significantly reduced the water uptake of seeds and the rate of mobilisation of reserves. All concentrations of putrescine (0.001, 0.01, 0.1 and 1.0mM) applied exogenously to seeds increased the rate of germination and seedling growth when NaCl(150 mM) was also present and 0.01 mM putrescine gave maximum response. Addition of putrescine (0.01 mM) to NaCl solution (150 mM) reduced the net accumulation of sodium and chloride ions in seeds and increased their water uptake. Thus, putrescine can alleviate the adverse effects of NaCl during germination and early seedling growth of rice.
Discrimination in Carbon Isotopes of Leaves Correlates With Water-Use Efficiency of Field-Grown Peanut Cultivars
Authors: Wright GC , Hubick KT , Farquhar GD
Published: January 2006

Variation in water-use efficiency (W, g of total dry matter produced/kg water used), and its correlation with cultivar isotope discrimination in leaves (Δ) was assessed in peanut plants grown in small canopies in the field. Plants were grown in separate minilysimeters that were both embedded in the ground and positioned above the crop. Differences among cultivars were found in W and � and the relationship between W and Δ was compared for plants grown in open and closed canopies. Genetic variability in W in plants grown in the field under non-limiting water conditions was demonstrated, with Tifton-8, of Virginia habit, having the highest W (3.71 g/kg) and Rangkasbitung, an Indonesian cultivar of Spanish habit, the lowest (2.46 g/ kg). Variability in W was due to variation in total dry matter production more than that of water use. A strong negative correlation was found between Δ and W, and also between Δ and total dry matter. The relationship between whole plant W, including roots, and Δ was stronger than that between shoot W, without roots and Δ. The improvement occurred because of variation among cultivars in the root to shoot ratio. This highlights the importance of taking account of root dry matter in studies concerning W. There were significant differences in W and Δ between plants in pots above-ground compared to pots in the ground, with above-ground plants having significantly lower values of both W and Δ. The ranking of W and Δ among cultivars was not affected by the contrast in environment, which suggests these parameters are under strong genetic control. Total above-ground dry matter yield at maturity was negatively correlated with Δ, while pod yield was not. It appears a negative association between harvest index and Δ may exist; however not all cultivars used in this and other studies follow this response. Both water-use efficiency, Wand total dry matter production are negatively correlated with Δ in leaves of peanut plants grown in small canopies in the field. Measurement of Δ may prove a useful trait for selecting cultivars with improved W and total dry matter yield under field conditions.
Membrane Interface of the Bradyrhizobium japonicum - Glycine max Symbiosis: Peribacteroid Units From Soyabean Nodules
Authors: Day DA , Price GD , Udvardi MK
Published: January 2006

A method for preparing intact peribacteroid units from soybean nodules is described in detail. The polypeptide compositions of the peribacteroid membrane and peribacteroid space contents are described, and the properties of these compartments are discussed In the hght of recent literature. Evidence is presented that the peribacteroid membrane is permeable to succinate and malate but not to sucrose and glutamate. A dicarboxylate transporter on the peribacteroid membrane, which is capable of transporting malate and succinate at rates sufficient to support measured nitrogenase activity, is described The properties of an ATPase found on the peribacteroid membrane are also described and compared to those described in other reports This ATPase is able to catalyse energisation of the peribacteroid membrane in an uncoupler and vanadate-sensitive manner and may play an important role in the regulation of ion movements across the membrane.
Reducing Activity at the Root Surface in Eucalyptus pilularis - Pisolithus tinctorius Ectomycorrhizas
Authors: Cairney JWG , Ashford AE
Published: January 2006

Mycorrhizal and non-mycorrhizal areas of root systems of Eucalyptus pilularis Sm. seedlings, grown in growth pouches and inoculated with Pisolithus tinctorius (Pers.) Coker & Couch, were compared with regard to their surface reducing activity. Mycorrhizal roots were shown to reduce silver bromide, ferric EDTA and a higher oxide of manganese while non-mycorrhizal roots of the same plants did not. A number of ecto- and endomycorrhizal fungi were screened for reducing ability. All fungal isolates tested, with the exception of Hebeloma crustuliniforme and a suspected ectomycorrhizal symbiont from Pisonia grandis roots, were capable of reducing a manganic oxide.
Expression of Nodulation Genes in Rhizobium and Acid-Sensitivity of Nodule Formation
Authors: Richardson AE , Djordjevic MA , Rolfe BG , Simpson RJ
Published: January 2006

The formation of nitrogen-fixing nodules on roots of leguminous plants by Rhizobium spp. involves a complex interaction between host plant and symbiont. Successful nodulation requires the coordinated expression of several nodulation (nod) genes in the bacteria. The expression of these genes is induced by flavonoid compounds present in root exudates of host plants. Growth of Rhizobium spp. and formation of nodules on roots of leguminous plants is known to be adversely affected by low pH and factors associated with soil acidity, but the basis of this acid-sensitivity is poorly understood. We consider that poor induction of nodulation gene expression in Rhizobium is a major factor contributing to the acid-sensitivity of nodulation formation. At low pH, induction of nod gene expression in R. leguminosarum biovar trifolii is markedly reduced in the presence of flavone-inducer. Furthermore, inducibility of nod gene expression in R. leguminosarum bv. trifolii is also affected by a net reduction in the concentration of nod gene-inducing factors present in the root exudates of clover seedlings grown in acidic conditions.
Mycorrhizas and Root Attributes of Plants of Coastal Sand-Dunes of New South Wales
Authors: Logan VS , Clarke PJ , Allaway WG
Published: January 2006

Root samples of 41 sand-dune plant species in 28 families were collected from sites along the coast of New South Wales during spring 1987. Of the species, 36 had vesicular-arbuscular mycorrhizas, with vesicles and internal and external hyphae. Among these species there was great variation in the pro- portion of root length colonised by vesicular-arbuscular mycorrhizal fungi (from 1 to 96%); in 33 species over 10% of root length was infected. Of the vesicular-arbuscular mycorrhizal species, 21 showed arbuscules, and 16 had intracellular hyphal coils. In four plant species mycorrhizas were not found in the single samples examined; ericoid mycorrhizas were present in the remaining species, Leucopogon parviflorus, but its vesicular-arbuscular mycorrhizal status could not be assessed. The results, though preliminary, may reflect a high vesicular-arbuscular mycorrhizal status of vegetation of coastal sand-dunes of New South Wales. This would be likely to enhance plant nutrition and sandbinding, and to have implications for sand-dune management.
Differences in Growth in Response to Temperature of Maize Hybrids Varying in Low Temperature Tolerance
Authors: Hardacre AK , Greer DH
Published: January 2006

Two hybrids of maize (Zea mays L.) were compared for low temperature tolerance using a range of characters. The hybrid A665 × H99 (CBD) originated in the Corn Belt of the U.S.A. NZlA × 5-113 (CT) was produced in New Zealand; it is of highland tropical origins and in previous work grew faster than Corn Belt Dent (CBD) hybrids at low temperatures. The objective of this work was to determine if the two hybrids grew at different rates in two contrasting temperature environments approximating those which can occur after planting in New Zealand. Growth analysis, chlorophyll concentration and chlorophyll fluorescence were used to estimate the differences between these hybrids. The plants were grown continuously in a 22/18°C day/night temperature environment or transferred to a 16/14°C environment at the 2-mature-leaf stage. Plants were harvested immediately prior to transfer at the 2-mature-leaf stage and at the 5-mature-leaf stage of development.
Differences in dry weight and leaf area were compared with net assimilation rate, chlorophyll concentration and chlorophyll fluorescence as estimates of photosynthetic capacity.
Shoot dry weight of CBD was greater than that of CT at 22/18°C, but the reverse was true at 16/14°C, showing that CT hybrids were more tolerant of the transfer to cool temperatures than CBD hybrids. In both environments the leaf areas of CBD and CT were similar. Net assimilation rate and chlorophyll concentration was greater for CT at 16/14°C, suggesting a greater photosynthetic efficiency at lower temperatures. This argument was supported by a slower decline in chlorophyll fluorescence for CT grown at 16/14°C. The coolest temperature treatment used in this work was less effective in discriminating between hybrids for differences in growth in cool environments, compared with the longer periods at 16/6°C of previous work.
Photosynthetic Temperature Acclimation in Eucalyptus Species From Diverse Habitats, and a Comparison With Nerium oleander
Authors: Ferrar PJ , Slatyer RO , Vranjic JA
Published: January 2006

The response of net photosynthesis per unit leaf area to growth and measurement temperature was measured for Eucalyptus species from diverse natural habitats in Australia and the results compared to Nerium oleander, a species known to exhibit a large degree of temperature acclimation. Eucalyptus species did not show the same degree of temperature acclimation observed for N. oleander in this and previous studies. Photosynthetic acclimation to temperature varied with species of Eucalyptus, apparently in relation to their subgeneric groupings. The estimated parameter, the preferred temperature for photosynthesis, correlated broadly with temperature conditions in the natural habitat.
Estimates of two biochemical parameters, Rubisco (estimated from the initial slope of the A/pI curve) and RuP2 regeneration capacity (calculated from the A/pI curve at high CO2) were found to parallel changes in assimilation rate. High temperature acclimation in the Eucalyptus species differed from that of N. oleander. Low temperature grown species, except E. miniata, had a higher temperature threshold for reversible inactivation of Rubisco. In E. miniata, Rubisco, but not regeneration capacity, was sensitive to high temperatures in low temperature grown plants. High growth temperatures appeared to affect the thermolability of the photosynthetic components of the two E. pauciflora provenances. Low temperature acclimation was similar for N. oleander and most of the Eucalyptus species and was accompanied by an increase in both biochemical parameters. Regeneration capacity was sensitive to low temperatures in high temperature grown E. miniata.
The Porosity of Permeabilised Chara Cells
Authors: Shepherd VA , Goodwin PB
Published: January 2006

When charophyte cells are plasmolysed at low temperature in the presence of the chelator EGTA, they lose turgor and stop streaming. Since cyclosis can be restored with exogenous ATP, the cells are described as permeabilised. In this study, the permeabilised cell is described in terms of the measured permeability of plasmalemma and cell wall to a series of fluorescent probes of increasing molecular weight. Cells were permeabilised according to standard procedures, and cyclosis was restored with exogenous ATP. The plasmalemma was not disintegrated, but rather a limited increase in porosity occurred, permitting the passage of molecules up to 874 daltons in weight. This indicates that permeabilisation induces the formation of pores with a maximum diameter close to 3 nm. The cell wall was permeable to molecules up to 4158 Da, indicating a maximum porosity close to 4.8 nm. Thus, the plasmalemma remains an important barrier to diffusion in permeabilised cells.
Chilling Injury in Mature Leaves of Rice. II. Varietal Differences in the Response to Interactions Between Low Temperature and Light Measured by Chlorophyll Fluorescence at 77k and the Quantum Yield of Photosynthesis
Authors: Huang LK , Osmond CB , Terashima I
Published: January 2006

Characteristics of photosynthetic CO2 exchange and 77K chlorophyll fluorescence in attached and detached leaves of rice were used to examine differences between five varieties exposed to a combination of bright light and low temperature. Preliminary studies established that attached leaves of varieties did not differ in the light dependence of photosynthesis at CO2 saturation or fluorescence properties when grown in full sunlight and measured under comparable conditions. All varieties showed a depression in maximum photosynthetic rate, but not in quantum yield or chlorophyll fluorescence when examined in the late afternoon after a period of active photosynthesis in air at 25°C. Attached leaves of all varieties showed depression of the maximum rate of photosynthesis and of quantum yield as well as an increase in Fo and a decrease in Fv/Fm of chlorophyll fluorescence at 77K following horizontal exposure to bright light in air at 25°. Similar results were obtained with detached leaves exposed to bright light while floating on water at 25°C. The rate of photosynthetic CO2 fixation in air as a function of temperature was examined in attached leaves of two representative varieties, Japonica-like cv. Hungarian- 1, and Indica-derived cv. IR-8. The former was capable to rates of photosynthesis at 10°C which were twice those of the latter, although the rates at 25°C were similar in both varieties.
When detached leaves of five varieties of rice were floated on water at 10°C while exposed to 700 �mol photons m-2 s-1 of white light, consistent differences in the response of quantum yield, light and CO2 saturated O2 evolion and 77K fluorescence properties were found. Exposure to bright light at low temperature exaggerated the effects of bright light on these parameters, and larger effects were found in the Indica-derived varieties than in Japonica-like cv. Hungarian-1. These treatments at 10°C did not influence the extent of increase in Fo, but led to a greater decline in Fv/Fm of 77K chlorophyll fluorescence compared with treatments at 25°C. Varieties in which there was a larger decline in Fv/Fm showed slower recovery of variable fluorescence in the dark at 25°C, and especially at 10°C. These responses to in vitro treatments seem capable of distinguishing varieties such as cvv. Hungarian-1 and Er Bai Ai, which are believed to be more chilling tolerant by other criteria, from chilling sensitive varieties such as Gui Chao-2 and IR-8. Even though there is potential for application of these methods in screening programs, there is little evidence that the lesions in primary photosynthetic processes indicated by these methods are significant determinants of photosynthesis under field conditions.
The Kinetics of Reactions in and Near the Cytochrome b/f Complex of Chloroplasts. II. Cytochrome b-563 Reduction
Authors: Hope AB , Liggins J , Matthews DB
Published: January 2006

The reduction of cytochrome b-563 was measured following flash-induced electron transfer from duroquinol to methyl viologen, in the presence of 3,(3,4-dichlorophenyl)-1,1-dimethyl urea and 1 �M nonactin or 1 �M valinomycin (plus 10 mM K+). The apparent rate constant of this reduction (nonactin present) increased from about 100 s-1 to 460 s-1 as the external concentration of duroquinol was varied from 0.01 to 0.5 mM. The corresponding maximum extent of reduction of cytochrome b-563 varied from 0.13 to 0.27 molecules per b/f complex. Rate constants in the presence of valinomycin were lower at all concentrations of duroquinol by a factor of about 1.5.
The mean enthalpy of activation calculated from Arrhenius plots of apparent rate constant for cytochrome b-563 reduction was 60 kJ mol-1, for temperature variation between 23 and 4°C.
The above, and further data in oxidising conditions, and with added 2-n-nonyl-4-hydroxyquinoline N-oxide, together with data on proton deposition, were compared with the predictions of a kinetic model. In this model, flash-generated oxidised plastocyanin oxidised Rieske centres during random diffusion, and plastoquinol reduced the Rieske centres and cytochrome b-563 sequentially; subsequent electron and proton transfers followed those in a Q-cycle.
Many observations were predicted by the model, in which the rate-limiting step was the first electron transfer from plastoquinol to the Rieske centre, subsequent steps being much faster. The rate and extent of reduction of cytochrome b-563 were fully consistent with a reaction between it and a radical form of plastoquinone formed after oxidation of the latter by Rieske centres (referred to as 'oxidant-induced reduction'), under a wide range of conditions.
Effects of External Mg2+ and Ph on the Photosynthetic Activities and Stromal pH of Intact Chloroplasts Isolated From C4 Plants
Authors: Boag S , Agostino A , Furbank RT , Hatch MD
Published: January 2006

Exogenous Mg2+ inhibited PGA- and OAA-dependent photosynthetic O2 evolution by isolated mesophyll chloroplasts from Zea mays and also HCO3--dependent O2 evolution by chloroplasts from Panicum miliaceum bundle sheath cells. Inhibition of 50-75% was observed with 3-5 mM Mg2+; this varied to some extent with pH but was not reversed by K+ . Inhibition of HCO3-- and PGA-dependent O2 evolution by the divalent cation ionophore A23187 was reversed by adding Mg2+ . Notably, HCO3-- dependent O2 evolution by isolated bundle sheath cells from P. miliaceum was not inhibited by [Mg2+] up to 20 mM.
Addition of Mg2+ in the light decreased the stromal pH of mesophyll chloroplasts by less than 0.2 units but reduced apparent stromal volume by as much as 25%. At least for bundle sheath chloroplasts, stromal pH varied with external pH over the range from 7 to 8 but remained about 0.3 units higher throughout this range. Oxygen evolution by isolated mesophyll chloroplasts, and bundle sheath cells and chloroplasts, was relatively insensitive to external pH in the range from 7 to 8. The results are considered in terms of likely mechanisms for the effects of exogenous Mg2+ and pH on photosynthesis by isolated chloroplasts and the physiological significance of Mg2+ effects.
PI Efflux and Influx by P-Adequate and P-Deficient Spirodela and Lemna
Authors: McPharlin IR , Bieleski RL
Published: January 2006

Net Pi influx by P-adequate (+P) and P-deficient (-P) Spirodela and Lemna plants was determined by gross PI influx and PI efflux. Rates of PI influx (gross) and PI efflux were sensitive to both internal [P] and external [PI]. The different responses of these two fluxes to external [PI] resulted in their rates converging at low external [PI], from PI efflux being 9-10% of gross PI influx at 1000 �M PI to being over 70% at low (1 �M) external PI in + P plants. At very low external Pi (0.34 �M, 0.22 �M Pi for + P Spirodela and Lemna, respectively), rates of gross Pi influx and Pi efflux were assumed to be either equal or zero (referred to as the equilibrium [Pi]ext), such that net Pi influx was zero. There was assumed to be insufficient Pi diffusing to the carrier site on the root surface at very low external [Pi] (i.e. equilibrium [Pi]ext) for the phosphate pump to operate and Pi influx was zero. P deficiency resulted in a simultaneous reduction in rates of Pi efflux and an increase in gross Pi influx such that the equilibrium [Pi]ext was reduced 3-4 times (0.8, 0.07 �M Pi for -P Spirodela and Lemna respectively) compared with +P plants. Some of the limitations of Pi diffusion to the carrier sites in P-adequate plants are overcome as internal [P] decreases during P deficiency. As a consequence the ability of Spirodela and Lemna to extract Pi from solution during P deficiency was considerably greater than for plants with adequate P status.
Inhibitors of metabolism (low temperatures, arsenate, azide, cyanide and CCCP) reduced rates of gross Pi influx but increased rates of Pi efflux such that net Pi influx was reduced. These findings are discussed in relation to previous work on the kinetics of Pi influx in - P versus + P plants and the role of changes in membrane potentials in the enhancement of Pi influx and reduction of Pi efflux capacity in P-deficient Spirodela and Lemna.
Leaf Gas Exchange and Water Relations of Lupins and Wheat. I. Shoot Responses to Soil Water Deficits
Authors: Henson IE , Jensen CR , Turner NC
Published: January 2006

The effects of a progressive increase in soil water deficit on the leaf conductance and gas exchange of lupin (Lupinus cosentinii) and wheat (Triticum aestivum) were investigated in pot experiments in a temperature-regulated glasshouse, using a coarse, sandy soil characteristic of the Western Australian wheatbelt.
Transpiration rates decreased rapidly in both species after water was withheld, mainly as a result of stomatal closure. Photosynthesis declined also, but to a lesser extent than conductance. Leaf extension in lupin was equally as sensitive to a decrease in leaf water potential and soil water potential as stomatal conductance. Stomatal closure served to maintain the water potential of lupin leaves to within 0.1 MPa of that of control (watered) plants as the soil water content decreased from 0.14 to 0.06 m3 m-3 and as the leaf conductance and the relative transpiration rate fell to less than 50% of control values. Maintenance of leaf water potential with decreasing soil water content and stomatal conductance was less evident in wheat. In both lupin and wheat, leaf conductance decreased linearly with soil water content and curvilinearly with bulk soil matric potential, indicating that water uptake was restricted at similar water contents and matric potentials in both species. Diurnal measurements on lupin indicated a substantial reduction in stomatal conductance after water was withheld, even when the leaf water potential at midday was reduced by only 0.1 MPa and no change could be detected in the bulk leaf turgor pressure. Conductance in lupin was reduced even though the soil matric potential decreased in only part of the rooting zone. This, together with the absence of any significant change in the leaf water potential, turgor pressure, or relative water content in lupin during the initial stages of stomatal closure, suggests that a soil or root factor initiates the reduction in leaf conductance - and hence regulates the shoot water status - in response to soil drying.
Why Are Bonsai Plants Small? A Consideration of Cell Size
Authors: Korner C , Pelaez Menendez-Riedl S , John PCL
Published: January 2006

Size determinants of organisms are still a great puzzle. Analysis of dwarfs or giants within a certain taxon is a promising tool for a functional understanding of size control. Bonsai cultivation techniques lead to the most extreme forms of environmentally induced plant dwarfism. It is shown that no such changes take place at the level of cell size of leaves. Leaves of four out of six species of Bonsai had even bigger cells than leaves of normal trees, which is similar to what was observed in genetically dwarf plants from high altitudes. These results illustrate the insignificance of intraspecific cell size variations for explanations of differences of size and growth of plants. Genetic control of cell size is stronger than the most severe environmental impact as it is received by Bonsais. The relevance of concepts developed for the control of cell size and cell division in unicellular organisms for meristematic plants is discussed.
Characterisation of Leaf Fluorescence of Sodium-Deficient C4 Plants: Kinetics of Emissions From Whole Leaves and Fluorescence Properties of Isolated Thylakoids
Authors: Grof CPL , Richards DBC , Johnston M , Brownell PF
Published: January 2006

Examination of whole-leaf fluorescence kinetics by means of a second-degree polynomial function showed a decrease in the rate of the rise from Fd to Fp in sodium-deficient compared with normal leaves of the C4 species Kochia childsii and Amaranthus tricolor. This suggests a decreased efficiency in light harvesting and/or utilisation in sodium-deficient plants.
Fluorescence ratios (Fv/Fo) of separated mesophyll and bundle sheath thylakoids were both lower from leaves of sodium-deficient compared with normal plants of K. childsii, Chloris gayana, A. edulis and A. tricolor.
Osmotic and Turgor Relations of Three Mangrove Ecosystem Species
Authors: Rada F , Goldstein G , Orozco A , Montilla M , Zabala O , Azocar A
Published: January 2006

The water relations of three mangrove ecosystem species, Rhizophora mangle, Conocarpus erectus and Coccoloba uvifera were investigated in an intertidal zone of the Venezuelan coast. The influence of a salinity gradient (from seashore to inland) and seasonal fluctuations in salinity were studied. Daily courses of leaf water and osmotic potentials and other water relations characteristics were estimated from pressure-volume curves. All species exhibited a decrease in osmotic potential of leaf tissues during the dry season when salinity levels increased in ground water. This osmotic adjustment was due to changes in either the symplasmic water fraction, the osmotically active solutes in leaf cells, or both. Changes in cell wall elasticity were also observed. The cell walls were more rigid during the dry season in all three species. Rhizophora mangle, the species that grows closest to the sea, had much more rigid cell walls than the other two species. The adaptive significance of these mechanisms for turgor maintenance and water uptake in mangrove habitats is discussed.
Rate Limitation of Non-Steady-State Photosynthesis by Ribulose-1,5-Bisphosphate Carboxylase in Spinach
Authors: Woodrow IE , Mott KA
Published: January 2006

A mathematical framework was developed to analyse rate limitation of non-steady-state photosynthesis following an increase in photon flux density (PFD). This analysis was employed to resolve an exponential phase of the photosynthetic response of Spinacia oleracea L. to a step increase from darkness to moderate PFD. This phase had a relaxation time of approximately 5 min, similar to the relaxation time for the activation of ribulose-1,5-bisphosphate carboxylase (Rubisco) as determined by freeze-clamp experiments following the same change in PFD. Furthermore, as the time in darkness prior to illumination was increased, the exponential phase contributed more to the overall trajectory of photosynthesis following the increase in light. The relaxation time for the increase in the contribution of this phase was 24 min. Freeze-clamp studies showed a relaxation time of 28 min for Rubisco deactivation in the dark. These results, together with measurements of RuP2 levels, suggest that the exponential phase resolved from gas exchange experiments was limited by activation of Rubisco and that Rubisco deactivation in the dark was the reason that this phase contributed more to the overall photosynthetic trajectory as time in darkness increased.
A quantitative expression for the amount of extra photosynthesis that could have been obtained had Rubisco activated instantly following an increase in PFD was derived and discussed in relation to optimal functioning of the system under different environmental conditions.
Water Relations of the Banana. I. Predicting the Water Relations of the Field-Grown Banana Using the Exuding Latex
Authors: Milburn JA , Kallarackal J , Baker DA
Published: January 2006

The free exudation of latex from wounded banana plants renders conventional methods for studying water potential impracticable or suspect. Accordingly, several methods have been devised to provide supportive semi-quantitative data on water relations including measurements of leaf angle, tissue turgor and pseudostem girth throughout many diurnal cycles.
Latex exudation itself has been exploited in a new technique for investigating the internal water relations of the banana. The laticifers behave like giant unbranched cells embedded deeply within the tissues of the plant. On wounding, the milky sap escapes at a rate which decreased exponentially. We argue that, as a consequence of pressure-release, the latex is diluted by water influx until equilibrium is reached with the rest of the plant. Hence at equilibrium, the osmotic potential of this diluted latex equals the water potential of the tissues; thus the final osmotic potential can be used to monitor water potential changes of banana tissues. In this study, the water potential of well-watered plants was found to cycle diurnally within the remarkably narrow range of 0 to -0.35 MPa (0 to -3.5 bar). Turgor pressures of intact laticifers were computed from our data, which also exhibit the diurnal fluctuations predictable from our supportive data and reports for other species.
Water Relations of the Banana. III. Effects of Controlled Water Stress on Water Potential, Transpiration, Photosynthesis and Leaf Growth.
Authors: Kallarackal J , Milburn JA , Baker DA
Published: January 2006

Banana suckers were grown in the glasshouse under controlled environmental conditions. Water potentials were measured on plants 6-12 months old using the solute potential of the latex as a guide. The diurnal fluctuations in water potential were monitored along with measurements of transpiration rate, leaf conductance, leaf temperature, assimilation rate and leaf growth rate.
Concurrent measurements were made of water potential, transpiration and other gas exchange parameters under rapidly and slowly applied water stress. Stomatal closure was almost complete after 10 days of rapid stress whereas, under slow stress, closure was delayed for a total of 30 days. Stomatal closure was accompanied by a reduction in transpiration and photosynthesis. However, the water potential of the plant was never more negative than -0.35 MPa even when the water stress was severe. The growth of the plant, as evaluated from the emergence of new leaves, was stopped at the same time as photosynthesis. The implications of these observations on the ability of banana to utilise dwindling supplies of water are discussed. The use of latex solute potential as a tool to understand the water status of the plant is further explained.
Influence of Osmotic Adjustment on the Growth, Stomatal Conductance and Light Interception of Contrasting Sorghum Lines in a Harsh Environment
Authors: Flower DJ , Rani AU , Peacock JM
Published: January 2006

Two drought resistant lines of sorghum, IS 13441 and IS 1347, with a high capacity for osmotic adjustment and two susceptible lines, IS 12739 and IS 12744, were subjected to drought in the field during a summer season in the semi-arid tropics in India. During this season there is little rain, air temperatures reach 42°C, and pan evaporation rates may reach 20 mm d-1 providing a harsh environment for crop growth. Most of the osmotic adjustment occurred within 3 weeks after withholding water and at high predawn leaf water potentials, i.e. at values above - 1.0 MPa. As a result, resistant lines were able to maintain a positive turgor to lower leaf water potentials (- 2.8 MPa) than susceptible lines (- 2.0 MPa). Nevertheless, dry matter production was negligible in both resistant and susceptible lines when predawn leaf water potentials fell to - 0.55 MPa. Furthermore, throughout the drought period the leaf area of all water-stressed plants was similar when expressed relative to the control regardless of the level of osmotic adjustment. Resistant lines had similar stomatal response to leaf water potential as susceptible lines. Large changes occurred in stomatal conductance and leaf rolling soon after withholding water while there was considerable osmotic adjustment in the leaves. Leaf rolling coincided with a reduction in the ability of the plant to utilise radiation. Therefore, even if osmotic adjustment had delayed leaf rolling, the gain in productivity would be small. On the basis of these turgor related processes it can be concluded that there would be little advantage in selecting for plants with a higher capacity for osmotic adjustment in this harsh environment.
Diffusion of Water in the Wheat Grain: Nuclear Magnetic Resonance and Radioisotopic Methods Provide Complementary Information
Authors: Jenner CF , Jones GP
Published: January 2006

Diffusion of water within developing grains of wheat has been evaluated by two independent methods: by kinetic analysis of the movement of tritiated water (THO) through the grain, and by n.m.r. procedures. Both methods provided different but complementary information.
Movement of THO basipetally through grains attached to the ear was faster than diffusion could explain, and a reversal of the gradient of THO at the base of the grain also could not be accounted for by diffusion. These observations are adduced as evidence for recirculation of water within the grain.
The average diffusion coefficient measured by both methods indicated that the diffusive motion of water was substantially hindered. Freezing and thawing resulted in an increase in diffusion but did not increase diffusion to values expected of free water. Starch, the major polymeric component of the grain, was not responsible for the residual reduction in diffusion coefficient in frozen and thawed tissue.
Control of Leaf Expansion in Wheat Seedlings Growing in Drying Soil
Authors: Passioura JB , Gardner A
Published: January 2006

The relative leaf expansion rate (RLER) of plants in drying soil eventually falls behind that of well watered plants. There is increasing evidence that the roots sense the drying of the soil and send signals to the leaves that control their behaviour independently of any effects of the soil's drying on the water status of the leaves. The roots could be sensing the falling availability of phosphorus, the falling water status, or the hardening of the soil. This paper explores these possibilities and concludes that, in the soil used, the roots were sensing both the water status and the strength of the soil but not the availability of phosphorus. Some of the experiments were done with a high evaporative demand on the leaves to see if the RLER would then be sensitive to the leaves' water status; it was not.
Phosphate Uptake and Arbuscular Activity in Mycorrhizal Allium cepa L.: Effects of Photon Irradiance and Phosphate Nutrition
Authors: Smith SE , Gianinazzi-Pearson V
Published: January 2006

The effect of photon irradiance on phosphate uptake and growth of onion plants was investigated in an experiment aimed at determining whether development and activity of mycorrhizal infection and arbuscules are linked to physiological effects.
Mycorrhizal (inoculated with Glomus mosseae (Nicol. & Gerd.) Gerdemann & Trappe) and non- mycorrhizal plants of Allium cepa. L. were grown at three soil phosphate levels and two photon irradiances (410 and 190 �mol m-2 s-1). There was one harvest at 6 weeks. Data were obtained for phosphate uptake (specific uptake rate), plant growth and mycorrhizal infection (total and active infection by trypan blue staining and succinate dehydrogenase (SDH) activity respectively).
There was a positive growth response to mycorrhizal infection at the lower P levels (P0 and P1) but not at P2 with high irradiance. This response was smaller at low irradiance. Furthermore, data for root/shoot and fresh-weight/dry-weight ratios indicated that allocation of dry matter was altered in mycorrhizal plants in accord with their increased demand for photosynthate. Reduced irradiance was associated with lower rates of P uptake in all treatments, with the greatest effect in mycorrhizal plants at P0 and P1. These reductions were not associated with reductions in incidence or intensity of mycorrhizal infection nor in the development of arbuscules. At P2 the incidence of infection was reduced under low irradiance, but development of arbuscules within infected regions of the roots remained the same as in other treatments.
The use of nitroblue tetrazolium chloride as a vital stain for SDH activity showed that during the 6-week period of the experiment, all the infection revealed by trypan blue staining was physiologically active. Branched arbuscules (counted in freehand sections of fresh roots) always had SDH and alkaline phosphatase activity. Thus, in young plants, observations of arbuscules gives a good indication of the amount of active infection. This may not be so in older roots.