bluuebunny/crossref_metadata_2025 huggingface dataset:
The Role of Transpiration in Phototropism of the Avena Coleoptile: Evidence of Stomatal Control of the Phototropic Response
Authors: Mcintyre GI
Published: January 2006

The hypothesis that phototropism is caused by the effect of light on transpiration seemed at variance with the observation that Avena coleoptiles respond phototropically while submerged in water. Further investigations, however, provided evidence that phototropism of submerged coleoptiles may be caused by the light-induced promotion of stomatal guttation. Evidence of guttation by submerged coleoptiles was provided by measuring tritium accumulation in a water jacket surrounding the coleoptile when tritiated water (THO) was supplied to the roots. It was shown that the application of nail polish to the coleoptiles induced almost immediate guttation from the stomata. This response, measured quantitatively by image analysis, occurred predominantly on the irradiated side of coleoptiles previously stimulated phototropically with blue light while submerged in water, and was much reduced and more uniformly distributed when the phototropic response had been eliminated by saturating the water with CO2. Small curvatures exhibited by coleoptiles kept in the dark at high humidity were almost invariably towards the side from which guttation occurred from the apical (hydathode) stomata. Application of petroleum jelly (Vaseline) to the stomata on the illuminated side of coleoptiles stimulated phototropically in moist air induced a negative curvature in the apical half of the coleoptile and reduced the positive curvature in the basal half. In contrast, vaseline applied to the middle of one of the broad sides of the coleoptile, where no stomata occur, had relatively little effect. These results are consistent with the hypothesis that phototropism of the Avena coleoptile is caused by light-induced loss of water from the stomata. It is also postulated that the mechanism is essentially the same whether water is lost by transpiration or guttation.
Photosynthesis Response of Sunlit and Shade Pepper (Capsicum annuum) Leaves at Different Positions in the Canopy Under Two Water Regimes
Authors: Alvino A , Centritto M , Lorenzi FD
Published: January 2006

Pepper (Capsicum annuum L.) plants were grown in 1 m2 lysimeters under two different water regimes in order to investigate differences in the spatial arrangements of the leaves and to relate this to daily assimilation rates of leaves of the canopy. The control regime (well-watered (W) treatment) was irrigated whenever the accumulated 'A' pan evaporation reached 4 cm, whereas the water-stressed (S) treatment was watered whenever the predawn leaf water potential fell below -1 MPa. During the growing cycle, equal numbers of sun and shade leaves were chosen from the apical, middle and basal parts of the canopy, corresponding to groups of leaves of increasing age. The CO2 exchange rate (CER) was measured at 0830, 1230 and 1530 hours on 8 days along the crop cycle, on leaves in their natural inclination and orientation. Leaf water potentials were measured on apical leaves before dawn and concurrently with gas exchange measurements. Control plants maintained predawn leaf water potential at -0.3 MPa, but S plants reached values lower than -1.2 MPa. Midday leaf water potentials were about twice as low in the S plants as in the controls. Water stress reduced LA1 during the period of crop growth, and dry matter production at harvest. Stressed apical leaves appeared to reduce stress by changing their inclination. They were paraheliotropic around midday and diaheliotropic at 0830 and 1530 hours. The CER values of the S treatment were significantly lower than those of the W treatment in apical and middle leaves, whereas the CER of basal leaves did not differ in either treatments. In the S treatment, reduction in the CER values of sunlit apical leaves was more evident in the afternoon than at midday or early in the morning, whereas basal leaves were less affected by water than basal stress leaves if sunlit, and negligibly in shaded conditions.
Subcellular Localisation of Copper and Partial Isolation of Copper Proteins in Roots From Rice Plants Exposed to Excess Copper
Authors: Lidon FC , Henriques FS
Published: January 2006

Rice (Oryza sativa L.) plants were grown over a 30 day period in nutrient solution containing concentrations of Cu varying from 0.002 to 6.25 mg/L. Electron microscopy of root tissues showed large amounts of electron-dense Cu-containing deposits inside the root cell vacuoles in concentrations higher than 0.05 mg/L. Cu-binding proteins, 30 and 8.5 kDa in size, were partially isolated from plants grown in Cu concentrations between 0.002 and 1.25 mg/L. Two additional Cu-binding protein peaks corresponding to sizes of 11 and 2 kDa were observed in the 6.25 mg/L Cu treatment. A positive correlation between Cu concentrations and both histidine and methionine was observed in all protein fractions. These amino acids decreased in the 30 kDa protein fractions and increased in the 8.5 kDa protein fractions, in parallel with increasing Cu concentrations. The existence of a specific subcellular localisation of Cu in rice is presented. Furthermore, Cu proteins in rice roots which appear to be related to Cu accumulation were partially isolated.
An Improved Method for Measuring the CO2/O2 Specificity of Ribulosebisphosphate Carboxylase-Oxygenase
Authors: Kane HJ , Viil J , Entsch B , Paul K , Morell MK , Andrews TJ
Published: January 2006

A simple, but very reproducible, method for measuring the relative specificity of ribulosebisphosphate carboxylase-oxygenase for CO2, as opposed to O2, is described. The method uses [1-14C]ribulose bisphosphate as substrate and combines the advantages of supplying both gaseous substrates from the gas phase with HPLC separation of the labelled products. Volumetric or gravimetric accuracy is not required at any stage of the procedure and variations in ionic strength and pH have little effect on the measurements. This leads to excellent reproducibility without the need for normalisation. The average standard deviation was 1.3% of the measured CO2/O2 specificity. Use of very low ribulose bisphosphate concentrations ensures that the gaseous substrates cannot be depleted appreciably during the reaction and enhances the attractiveness of the procedure for measurements with crippled mutant enzymes. The procedure's ability to resolve small differences in relative specificity is demonstrated by its easy detection of the 5% increase in specificity that accompanies substitution of four residues at positions 338-341 of the cyanobacterial large subunit with the analogous higher-plant residues. This resolving power is essential for detecting small differences in the specificities of higher-plant ribulosebisphosphate carboxylases which may be the signature of continuing evolutionary refinement.
Nitrogen Balance of Field Pea Crops in South Western Australia, Studied Using the 15N Natural Abundance Technique
Authors: Armstrong EL , Pate JS , Unkovich MJ
Published: January 2006

The nitrogen economies of six contrasting field pea (Pisum sativum L.) genotypes were examined at three widely separated sites in south Western Australia, using the 15N natural abundance technique to asssess proportional dependence on fixed N, harvests at peak biomass to assess total N yields and harvests at crop maturity to examine partitioning of N between seed and non harvested crop residues. The budgets for one site (Wongan Hills) included N of nodulated roots which on average comprised 12% of total plant N at peak crop biomass and 25% of recoverable plant N after harvest of seed. At this site maximum potential (residual) benefits to a following crop (peak total plant biomass N - N uptake from soil and N taken off as harvested seed) varied between genotypes from 8 to 41 g N ha-1 (mean 26; n = 6). Data for the other two sites, based solely on N budgets of above-ground parts, provided evidence of substantial site- and genotype-specific differences in N balance in terms of shoot residues (i.e. + 7 to - 24 kg N ha-1 (mean - 5) at Avondale, + 40 to - 29 (mean + 3) at Mt Barker). The results collectively indicated a general relationship between peak biomass N of a crop and its potential or otherwise to effect a net input of residue N to the ecosystem. There were, however, considerable variations between genotypes and sites due to differences in proportional dependence on fixation (range across all sites and genotypes 60-91%) and crop harvest indices for N (corresponding range 53-90%). Correlation plots were constructed from the data for N2 fixed against crop dry matter yield and residual nitrogen benefit against nitrogen harvest index. Results are discussed in relation to values for N balance of field pea and other grain legumes obtained elsewhere by other investigators.
Capacity for Energy Dissipation in the Pigment Bed in Leaves With Different Xanthophyll Cycle Pools
Authors: Demmig-Adams B , Adams WW Iii
Published: January 2006

Photosynthetic capacities, xanthophyll cycle components, and the capacity for photoprotective dissipation of excess excitation in the chlorophyll pigment bed were compared in two groups of species acclimated to different growth irradiance. These were sun leaves of three species with moderately high maximal photosynthetic capacities, tulip, rose, and periwinkle (Vinca minor), and leaves which had developed under very low irradiance from three shade-tolerant species, Monstera deliciosa, Philodendron caudatum and Schefflera arboricola. All sun leaves possessed larger xanthophyll cycle pools and greater maximal zeaxanthin (and antheraxanthin) contents and also displayed a greater maximal capacity for photo-protective energy dissipation in the pigment bed than the leaves acclimated to very low irradiance. The sun leaves also maintained a considerably lower reduction state of photosystem II at full sunlight than the leaves acclimated to very low irradiance. Thus, during exposure to full sunlight, these sun leaves were able to dissipate a major portion of the absorbed light through the combination of photosynthesis and energy dissipation in the pigment bed. Under the same conditions, these two processes combined were able to dissipate only a small fraction of the absorbed light in the leaves acclimated to very low irradiance. Most of the above differences between sun-acclimated leaves and leaves acclimated to very low irradiance existed not only between species but also within a species. Acclimation of Monstera deliciosa to a high growth irradiance resulted in leaves with a larger xanthophyll cycle pool, a greater maximal zeaxanthin (and antheraxanthin) content, and a greater capacity for energy dissipation in the pigment bed.
Adenylyl Cyclase from the Green Alga Volvox carteri f . nagariensis: Partial Purification and Characterisation
Authors: Nass N , Moka R , Jaenicke L
Published: January 2006

In order to understand changes in cyclic adenylate levels of Volvox carteri during the process of sexual induction, we investigated the biochemical properties of its membrane-bound adenylyl cyclase. Membrane preparations possess low levels of Mg2+ -dependent or Mn2+ -dependent adenylyl cyclase activity. This activity was solubilised and then purified 7800-fold. The enzyme detergent complex has an apparent molecular mass of 100 kDa. Purified preparations contain a major ATP-binding protein of 33 kDa as shown by affinity labelling. The Mg2+ -dependent basal enzyme activity is regulated by Ca2+, and is highest in the presence of 10-7 M Ca2+, but is inhibited by Ca2+ above 10-5 M. La3+ at 10-4M also blocks activity. Neither calmodulin nor its antagonists affect the enzyme activity, nor do the purified preparations interact with immobilised calmodulin. Further mediators of G-protein action (NaF, or GTP and its derivatives) and forskolins have no influence on the basal activity of this plant enzyme. The function of adenylyl cyclase in sexual induction of Volvox is discussed.
Testing a Mechanistic Model of Forest-Canopy Mass and Energy Exchange Using Eddy Correlation: Carbon Dioxide and Ozone Uptake by a Mixed Oak-Maple Stand
Authors: Amthor JS , Goulden ML , Munger JW , Wofsy SC
Published: January 2006

A big-leaf model of C3-canopy mass and energy exchange was used to predict hourly CO2 and O3 uptake by a mixed deciduous Quercus-Acer (oak-maple) stand in central Massachusetts, USA. The model is based on canopy-radiation interactions, leaf mesophyll metabolism (photosynthetic electron transport, carboxylation and oxygenation of ribulose 1,5-bisphosphate [RuP2] by RuP2 carboxylase/oxygenase [Rubisco], and respiration), physical transport conductances of mass and heat above and within the canopy, conductances of mass at the leaf surface and in the mesophyll, and mass and energy exchange at the soil surface (forest floor). Predictions of hourly CO2 and O3 uptake were compared to independent whole-forest CO2 and O3 exchange measurements made by the eddy correlation method during a 68 day period in the summer and early autumn of 1992. Predicted hourly CO2 exchange rate was strongly correlated (r ≈ +0.91) with measured hourly CO2 exchange, but mean day-time predicted whole-forest CO2 uptake was c. 13% (c. 1.13 μmol CO2 m-2 s-1) greater than CO2 uptake measured by eddy correlation. The model tended to overpredict CO2 uptake during late afternoon, but was accurate during the rest of the day. Predicted and measured O3 uptake rates also were positively correlated (r ≈ +0.76). The diurnal patterns of predicted and measured O3 uptake indicated that stomata1 conductance (gs) was accurately predicted during the morning, but in the afternoon the model overpredicted gs. This pattern was consistent with the overprediction of afternoon CO2 uptake, and suggested that a feedback inhibition of photosynthesis occurred in the afternoon. This might have been related to source-sink imbalance following several hours of photosynthesis. On the whole, and in spite of the simplifications inherent in the big-leaf representation of the canopy, the model is useful for predicting forest-environment interactions and for interpreting mass and energy exchange measurements.
Influence of Rising Atmospheric CO2 Concentrations and Temperature on Growth, Yield and Grain Quality of Cereal Crops
Authors: Conroy JP , Seneweera S , Basra AS , Rogers G , Nissen-Wooller B
Published: January 2006

A possible scenario for the end of the 21st century is that the atmospheric CO2 concentration will be in the range of 510-760 μL L-1 and that the mean global temperature will be 1.5-4.5�C higher. Further, there may be greater incidences of extreme climatic events, which together with the CO2 and temperature changes will influence development, growth and grain yield of cereals such as rice and wheat. For these C3 plants, the driving force for the growth response to elevated CO2 is higher leaf CO2 assimilation rates (A). However, the response of A to CO2 depends on temperature with maximum absolute increases occuring at temperatures which do not cause flower abortion, while negligible increases are observed at low temperatures. At high temperatures, where A is reduced because of partial inactivation of photosynthetic enzymes, the increase in A due to CO2 enrichment is still observed. Other factors, such as changes in shoot water relations or hormone concentrations, may influence growth at elevated CO2 concentrations. Wheat and rice development is accelerated by high temperature and consequently grain yield is reduced because there is less time for radiation to be intercepted during the vegetative phase. Although high CO2 also accelerates development in rice and, to a lesser extent in wheat, the extra carbohydrate produced by increases in A results in at least a 40% increase in grain yield at temperatures which do not cause flower abortion. This is due mainly to increased tiller numbers rather than increases in the number or weight of individual grains. However, the yield enhancement due to high CO2 will not necessarily compensate for decreases in yield caused by accelerated development at high temperatures. As predicted by the response of A to high CO2, the relative increase in yield, due to rising CO2 concentrations, is smaller at lower temperatures. Elevated atmospheric CO2 may improve the tolerance of plants to heat-induced drought stress by facilitating the maintenance of cell volume and photosynthetic function in the leaves. Increased carbohydrate storage in the stems may also be an advantage during grain filling if the flag leaves senesce prematurely. However, it is unlikely that the effect of very high temperatures on flower abortion will be ameliorated by high CO2. For bread making, the quality of flour produced from grain developed at high temperatures is poorer. High CO2 may also have an effect through a reduction in the protein content of wheat grain. For rice, the amylose content of the grain, a major determinant of cooking quality is increased under elevated CO2.
Stem Reserve Mobilisation Supports Wheat-Grain Filling Under Heat Stress
Authors: Blum A , Sinmena B , Mayer J , Golan G , Shpiler L
Published: January 2006

The grain filling of wheat (Triticum aestivum L.) is seriously impaired by heat stress due to reductions in current leaf and ear photosynthesis at high temperatures. An alternative source of carbon for grain filling is stored stem reserves. Two spring wheat cultivars (V5 and V2183) of very similar phenology and plant stature, which had previously been found to differ in grain shrivelling under drought and heat stress conditions in the field, were used to evaluate the hypothesis that the mobilisation of stored stem reserves into the growing grain is an important source of carbon for supporting grain filling under heat stress. In two experiments in Israel (1990 and 1991), the rates of stem dry matter (DM) and stem total non-structural carbohydrates (TNC) loss, grain growth and leaf senescence were monitored under optimal (control) and high (stressed) temperatures in the glasshouse (1990) and the growth chamber (1991). Cultivar V5 always sustained a smaller reduction in grain dry weight under heat stress, than V2183. Irrespective of temperature, V5 had a higher stem DM and TNC content at the onset of grain filling, greater depletion of stem dry matter (or TNC) during grain filling, and longer duration of grain filling, than V2183. During grain filling V5 generally exported about two to three times more DM from the stems than V2183, under both non-stressed and stressed conditions. On the other hand, V5 was more heat-susceptible than V2183 in terms of leaf longevity, in vivo chlorophyll stability and grain abortion under heat stress. In a third experiment (1992) five cultivars (including V5 and V2183) were subjected to chemical desiccation (0.3% potassium iodide) of the canopy in the field in order to destroy the photosynthetic source ofthe plant after anthesis. The same cultivars were subjected to heat stress (35/25�C) or non-stressed (25/15�C) conditions after anthesis in the growth chamber. It was found that grain dry weight reduction by chemical desiccation was highly correlated with grain dry weight reduction by heat stress (r2 = 0.89). Therefore, the superior capacity of V5 for grain filling from mobilised stem reserves is a consti- tutive trait which supports grain filling under heat stress which can be tested for by chemical desiccation of plants under non-stressed conditions.
The Effect of High Temperature on Starch Synthesis and the Activity of Starch Synthase
Authors: Denyer K , Hylton CM , Smith AM
Published: January 2006

The decrease in yield which is observed when developing storage organs such as cereal grains or potato tubers are exposed to high temperatures is due to a lower final starch content. The rate of starch synthesis during the development of these storage organs at high temperature, is either reduced or fails to increase sufficiently to compensate for the shorter developmental period. This effect on the rate of starch synthesis does not seem to be due to a reduction in the supply of photosynthate. One of the enzymes in the pathway of starch synthesis, soluble starch synthase, is susceptible to heat inactivation at unusually low temperatures and may also have a low optimum temperature for maximum activity. In some storage organs, the maximum catalytic activity of soluble starch synthase is not very much greater than the rate of starch synthesis. A decrease in the activity of this enzyme is therefore, likely to affect the rate of starch synthesis. Thus, the effect of high temperature on the rate of starch synthesis may be due, at least in part, to the properties of this enzyme. This review discusses the unusual heat-sensitivity of starch synthase in the context ofthe effects of high temperature on starch synthesis in storage organs.
Heat Stress During Grain Filling in Maize: Effects on Carbohydrate Storage and Metabolism
Authors: Singletary GW , Banisadr R , Keeling PL
Published: January 2006

Heat stress during maize seed development can interfere with endosperm starch biosynthesis and reduce seed size, an important component of yield. Our objectives were to evaluate the direct influence of temperature during grain filling on kernel growth, carbohydrate accumulation, and corresponding endosperm metabolism. Kernels of maize were grown in vitro at 25�C until 15 or 16 days after pollination and then subjected to various temperatures for the remainder of their development. Mature kernel dry weight declined 45% in a linear fashion between 22 and 36�C. The rate of starch accumulation reached a maximum at approximately 32�C, and when measured at frequent intervals, declined only slightly with further temperature increase to 35�C. Reduced seed size resulted from an abbreviated duration of starch-related metabolism, which did not appear to be limited by endogenous sugars. Instead, a survey of 12 enzymes of sugar and starch metabolism indicated that ADP glucose pyrophosphorylase and soluble starch synthase were unique in displaying developmental peaks of activity which were compressed both in amount and time, similar to the effect of temperature on starch accumulation. We conclude that decreased starch synthesis in heat-stressed maize kernels results from a premature decline in the activity of these enzymes.
Heat-Shock Proteins: a Search for Functions
Authors: Harrington HM , Dash S , Dharmasiri N , Dharmasiri S
Published: January 2006

Heat-shock proteins (hsps) are synthesised in response to increased growing temperature and are thought to result in the acquisition of thermotolerance to otherwise nonpermissive temperatures. This phenomenon, called the heat-shock response (HSR), is conserved among all biological organisms. While much information is available on the regulation of heat-shock genes and the synthesis of hsps, comparatively little is known of hsp functions in cells. The current thinking is that hsps provide the molecular basis for thermotolerance although there is limited evidence to support this view. Research in our laboratory is focused on the characterisation of hsps that function in signal transduction including protein kinases and heat-shock-induced calmodulin-binding proteins. Such enzymes and proteins are likely to be involved in key regulatory mechanisms that operate during the HSR and may help to identify specific functions for HSPs.
Xylem Permeability and Embolism Susceptibility in Seedlings of Eucalyptus camaldulensis Dehnh. From Two Different Climatic Zones
Authors: Franks PJ , Gibson A , Bachelard EP
Published: January 2006

We examined the hydraulic architecture of Eucalyptus camaldulensis seedlings from two climatically different provenances: Petford in north Queensland, which is humid; and Tennant Creek in the Northern Territory, which is semi-arid. Xylem liquid specific permeability (Kg) was calculated from measurements of xylem air permeability (Kg) and hydraulic resistances (R) were also calculated. Embolism susceptibility curves were constructed by measuring Kg in the stems of young seedlings. Canopy leaf areas were mapped and sap flows through the Tennant Creek and Petford seedlings were simulated. Results showed that the permeability of stem wood in the Tennant Creek seedling was about 40% higher than in Petford, and the permeability of its branch wood was about 25% higher. The threshold water potential for initiation of embolisms (Ψt) was slightly lower in Tennant Creek seedlings (- 3.3 MPa compared with - 3.0 MPa) and the number of embolisms with decreasing Ψ was lower for Tennant Creek seedlings. Sap flow simulations showed differences in the distribution of water throughout the seedling canopies. The results are discussed in terms of the ecology of the two eucalypt provenances.
Transpiration and Leaf Area Under Elevated CO2: Effects of Soil Water Status and Genotype in Wheat
Authors: Samarakoon AB , Muller WJ , Gifford RM
Published: January 2006

Transpiration rate, leaf area expansion, water use and water-use efficiency (WUE) of spaced plants of wheat (cvv. Matong and Quarrion), were examined at ambient and twice ambient CO2 concentrations in wet and drying soil regimes. A hypothesis tested was that both stomatal conductance (gs) and leaf area development are so regulated by the plant in relation to soil water status that the reduction of approximately 40% in gs in high CO2 has no permanent impact on whole-plant water use. Whereas, during a soil drying cycle, leaf area increase under elevated CO2 counterbalanced closely for reduced gs in terms of soil water depletion as reported elsewhere, this counterbalance was neither exact at all times, nor did it apply when the soil was continuously wet. In wet soil, leaf area was not enhanced much by elevated CO2, probably because, under the high radiation and nutritional conditions used, the tillering rate was almost maximal anyway. Quarrion, having a 40% lower gs than Matong genetically, did not counter-balance a reduced transpiration rate with a larger leaf area under either drying or wet soil conditions. These results support rejection, for wheat, of the hypothesis posed; elevated CO2 increased leaf area mainly by virtue of the direct photosynthetic increase rather than changed soil water status. In wet soil, low gs Quarrion had a higher CO2 effect on WUE (+ 73 to 82%) than did Matong (+54 to 65%). In drying soil, both cultivars had a similar increase in WUE at high CO2 (+60 to 68%).
13C Natural Abundance in Plant Communities Along a Rainfall Gradient: a Biological Integrator of Water Availability
Authors: Stewart GR , Turnbull MH , Schmidt S , Erskine PD
Published: January 2006

Carbon isotope natural abundance (δ13C) has been previously used as a powerful tool in the study of water-using processes at the leaf, individual and within-community levels. We analysed 348 species from 12 plant communities along a 900 km-long rainfall gradient in southern Queensland. Although the range of δ13C values found in a given community was large, variability in the δ13C signature of plants within a community was relatively small given the large numbers of species sampled (mean n per site of 29) and the taxonomic diversity in each. The community-averaged δ13C signature ranged from -25.60 in a brigalow (Acacia harpophylla) dominated community in western Queensland to - 31.20 in subtropical rainforest in eastern Queensland. A strong relationship was found between the δ13C value averaged for each site and rainfall within the range 350-1700 mm per annum. Foliar δ13C was also related to the number of rain days per annum and moisture balance (rainfall - evaporation). The strength of these relationships varied only slightly according to the rainfall parameter used, with values for r2 of 0.78, 0.70, 0.70 and 0.74 for the relationship between δ13C and long-term rainfall average, 5-year rainfall average, number of rain days and moisture balance, respectively. Despite considerable taxonomic variability within a given plant community, the averaged δ13C signature for that community gives a strong indication of moisture availability.
Cytokinins Associated With Metamorphic Vegetative Growth in Elaeocarpus hookerianus
Authors: Day JS , Jameson PE , Gould KS
Published: January 2006

Elaeocarpus hookerianus, a species endemic to New Zealand, exhibits metamorphic vegetative growth. The juvenile characteristically has small, thin leaves and an entangled branch habit, whereas the adult is arborescent with larger leaves. A transitional stage with branch pattern and leaf morphology intermediate between the juvenile and adult forms is also discernible. The three developmental stages exhibited by this species provide an appropriate system for examining possible correlations between growth habit and cytokinin content. Except for an unknown non-hydroxylated cytokinin, the cytokinins detected in E. hookerianus were similar to those measured by others in various plants. A trend of decreasing concentration of active cytokinin and of increasing concentration of storage cytokinin as a percentage of total cytokinin was measured between juvenile, transitional and adult forest-grown leaves. This trend was confirmed by measuring cytokinins from leaves of juvenile and adult plants propagated and grown in a glasshouse. While cytokinin concentration changes were correlated with modifications in branching pattern and leaf morphology, further work is needed to establish if these are causally linked.
Fluorescence and Spectroscopic Studies of Exciton Trapping and Electron Transfer in Photosystem II of Higher Plants
Authors: Renger G , Eckert HJ , Bergmann A , Bernarding J , Liu B , Napiwotzki A , Reifarth F , Eichler HJ
Published: January 2006

Measurements of time-resolved fluorescence decay, laser-flash-induced absorption changes in the UV and at 820 nm and of the relative fluorescence quantum yield in different preparations (thylakoids, photosystem II (PSII) membrane fragments and PSII core complexes) from spinach led to a number of conclusions. (1) Light is transformed into Gibbs energy with trapping times of 250 ps and 130 ps in open reaction centres of PSII membrane fragments and PSII core complexes, respectively. Assuming rapid Boltzmann distribution of excitation energy and taking into account the antenna properties (size and spectral distribution), the molecular rate constant of primary charge separation is estimated to be about (3 ps)-1. (2) The electron transfer from Pheo- to QA is characterised by a rate constant of (300 ps)-1. (3) The QA- reoxidation kinetics are significantly retarded in D2O suspensions. These H/D isotope effects are interpreted as to reflect hydrogen-bond dependent changes in the protein dynamics that are relevant to electron transfer. (4) In PSII reaction centres closed for photochemical trapping the yield of a primary radical pair with lifetimes exceeding 1 ns is comparatively small (c 30%) at room temperature. Short illumination in the presence of Na2S2O4 changes the radical pair dynamics. (5) Photoinhibition under aerobic conditions impairs the primary charge separation and leads to formation of quencher(s) of excitation energy.
Quantitative Mapping of Leaf Photosynthesis Using Chlorophyll Fluorescence Imaging
Authors: Genty B , Meyer S
Published: January 2006

A method has been developed for routine, non-invasive monitoring of the topography of leaf photochemistry. The method uses video images of leaf chlorophyll fluorescence, taken during steady-state photosynthesis and during a transitory saturation of photochemistry, to construct, pixel by pixel, an image of the photochemical yield of photosystem II (PSII). The photochemical yield of PSII was estimated according to Genty et al. (1989) (Biochimica et Biophysica Acta 990, 87-92). The effectiveness of the method was shown by mapping the heterogeneous distribution of photosynthetic activity after treatment with either a herbicide (DCMU), abscisic acid, or during the course of the induction of photosynthesis. Leaf CO2 assimilation was simultaneously monitored under non- photorespiratory conditions to estimate the average quantum yield of linear electron transport. A unique proportional relationship was found between the mean photochemical yield of PSII calculated from images of the photochemical yield of PSII, and the average quantum yield of linear electron transport in three plant species exposed to a wide range of treatments or conditions. This new ability to quantitatively visualise leaf photochemistry provides a powerful tool to probe the spatial distribution of leaf photosynthesis. Possible errors in estimating the photochemical yield of PSII from mean fluorescence measurements are discussed.
Fluorescence Transients in Response to Nutrient Enrichment of Nitrogen- and Phosphorus-limited Microcystis aeruginosa Cultures and Natural Phytoplankton Populations: a Measure of Nutrient Limitation
Authors: Wood MD , Oliver RL
Published: January 2006

This study presents the results of an investigation of the application of chlorophyll fluorescence perturbations as a bioassay for nutrient limitation in microalgae. The variable chlorophyll fluorescence (ΔF) response of Microcystis aeruginosa, chemostatically cultured under different nitrogen (N) and phosphorus (P) nutrient regimes, to spiking with NO3-, NH4+ and PO43-, was monitored. Differences were observed in the short-term (<10 min) fluorescence response (F) which related to the original nutrient status of the test organism. Cells growing exponentially in full nutrient media showed no transient changes in ΔF. The addition of NH4+ or NO3- to N-limited cells resulted in transient changes to ΔF, while no response was observed following the addition of PO43-. Spiking of P-limited cells with either PO43- or NH4+ resulted in transient changes to ΔF, while no response was observed for NO3-. Thus a simple, rapid, nutrient-specific assay for nutrient limitation was available for cultured cells of M. aeruginosa. When the assay was tested on field samples from the Maude Weir Pool (34�28'S,144�18'E), Murrumbidgee River, NSW, Australia, apparent nutrient limitation, either N or P, was detected. Use of microalgal fluorescence responses to nutrient spiking may allow the rapid and specific detection of nutrient limitation in both cultures and field populations of phytoplankton.
Variations in Chlorophyll Fluorescence Yields in Phytoplankton in the World Oceans
Authors: Falkowski PG , Kolber Z
Published: January 2006

The ocean is optically thin and lends itself to large-scale measurements of in vivo chlorophyll fluorescence. In the open ocean, however, phytoplankton chlorophyll concentrations average only 0.2 μg L-1, and hence high sensitivity is required for precise measurements of the fluorescence yields. Over the past decade, we have developed two approaches to achieve the required sensitivity; these are the pump- and probe-technique and a fast repetition rate (FRR) method. Both methods have been adapted for in situ studies and are used to rapidly measure the maximum change in the quantum yield (Δ�max) of photosystem II (PSII), as well as the effective absorption cross-section of PSII (σPSII). Sections of variable fluorescence across the Pacific and Atlantic Oceans reveal the influence of geophysical processes in controlling the quantum yields of phytoplankton photosynthesis. Areas of upwelling, such as off the coast of north-westem Africa, have Fv/Fm values of 0.65, which are close to the maximum achievable values in nutrient-replete cultures. Throughout most of the nutrient-deficient central ocean basins, this quantum efficiency is reduced by more than 50%. In high-nutrient, low- chlorophyll regions of the eastern Equatorial Pacific, the deliberate, large-scale addition of nanomolar iron directly to the ocean leads to a rapid increase in quantum efficiency of the natural phytoplankton community, thereby revealing that in these regions phytoplankton photosynthetic energy conversion efficiency is iron limited. Diel patterns of variation in the upper ocean display midday, intensity- dependent reductions in both upsII and A�max. We interpret the former as indicative of non- photochemical quenching in the antenna, while the latter is a consequence of both rapidly reversible and slowly reversible damage to reaction centres. From knowledge of the incident spectral irradiance, Δ�max, σPSII, and photochemical quenching, the absolute photosynthetic electron transport rate can be derived in real-time. Using unattended, moored continuous measurements of in vivo fluorescence parameters, the derived in situ electron transport rates can be related to satellite observations of the global ocean with basin-scale, seasonal estimates of phytoplankton carbon fixation. Thus, unlike any other photosynthetic parameter, chlorophyll fluorescence can be used to bridge the scales of biophysical responses to ecosystem dynamics.
Developmental Responses by Tall and Dwarf Isogenic Lines of Spring Wheat to Applied Gibberellins.
Authors: Evans LT , Blundell C , King RW
Published: January 2006

Tall (rht) and dwarf (Rht) isogenic lines of two Mexican spring wheat cultivars were grown at 25/20�C day/night in short (8 h) days of either natural or artificial light to determine their relative stem, leaf and flowering responses to one or two long days, 11 different gibberellins (GAs) and the growth retardant Cycocel (CCC). Although stem and leaf elongation in the dwarf lines did not respond to a GA application, inflorescence initiation and development was enhanced by several GAS, reduced by CCC, and restored by subsequent GA application. The floral response of tall lines to GAs was greater than that of the dwarf lines, whereas the reverse was the case with their exposure to long days. These results suggest that endogenous GAs are involved in the floral induction of wheat, and that although the stems and leaves of dwarf lines may not be able to respond to exogenous GAs, their shoot apices can do so and are sensitive to endogenous GA levels.
The Response of Wheat to High Temperature Following Anthesis. Ii. Sucrose Accumulation and Metabolism by Isolated Kernels.
Authors: Wardlaw IF , Moncur L , Patrick JW
Published: January 2006

The effect of temperature on [14C]sucrose accumulation and the incorporation of 14C-label into starch by isolated kernels was followed in a range of wheat (Triticum aestivum L.) cultivars. To facilitate uptake the outer pericarp and embryo were removed from each kernel which was then split along the crease region to expose the starchy endospem directly to a 4% [14C]sucrose solution. There was an initial high rate of 14C-accumulation by the kernel followed by a slower linear increase in activity from 6 to 24 h after the start of the uptake period. The rate of accumulation of 14C by the endosperm of individual kernels from plants growing at 15/10�C daylnight was greatest 30-42 days after anthesis, with a considerable fall in both the accumulation of [14C]sucrose and the incorporation of label into starch 50 days after anthesis. Differences between cultivars in the accumulation of [14C]sucrose and starch synthesis by the starchy endosperm of isolated kernels followed differences in the growth rate of kernels on the plant, although the absolute rates of sucrose accumulation and incorporation into starch were only about 65% of those observed in the intact plant system. Differences between cultivars in their response to high temperature were, however, not evident in the [14C]sucrose uptake studies. The optimum temperature for kernel respiration, in the short term, was greater than 30�C. However, above 25�C the Q10 for kernel respiration was below 2, which is an indication that physical factors may influence cell metabolism at this temperature. We suggest that greater consideration should be given to the effect of temperature on sucrose transfer from the vascular system in the crease region of the kernel, through the endosperm cavity to the starchy endosperm.
Hormonal Control of Root Primordia Differentiation and Root Formation in Cultured Explants of Eucalyptus Globulus Seedlings.
Authors: Pelosi A , Lee MCS , Chandler SF , Hamill JD
Published: January 2006

Auxin-induced differentiation of root primordia, and their subsequent development into growing roots, was studied in explants of 8-day-old seedlings of Eucalyptus globulus subspp. globulus. Aseptic hypocotyl or root explants were cultured at 20 � 2�C on modified Murashige and Skoog medium containing 2% sucrose and various combinations of auxin andfor cytokinin. Of the auxins examined, indole-butyric acid (IBA) at concentrations of 10*-4 - 10*-5 M was most effective at inducing root primordia and led to a significant (P d 0.001) increase in the density of root primordia when compared with non-auxin treated control tissues. In root explants, maximal numbers of lateral root primordia (LRP) resulted after 16-18 h contact with auxin although they were not all visible until 42-48 h after initial exposure to auxin. Efficient conversion of LRP to lateral roots (LR) required tissues to be removed from auxin-induction medium within approximately 48 h and was inhibited by 6-benzylaminopurine (BAP) concentrations of 10*-5 M or greater. Treatment of root explants with IBA only for 16 h, followed by 96 h culture on medium containing a range of BAP concentrations showed that LRP induction was not inhibited by cytokinin concentrations as high as 10*-3 M. However, pretreatment of root explants for 6 h with BAP and other cytokinins, at 10*-5 M or higher concentrations, prevented the subsequent induction of LRP by IBA. Pretreatment of root explants with either 10*-3 M actinomycin D or 10*-3 M cycloheximide for 6 h prior to exposure to M IBA, or at any time up to 42 h after initial exposure to IBA, was also inhibitory to the subsequent induction of LRP suggesting that de novo transcription and translation of an essential gene(s) islare required for LRP formation following treatment with IBA. This study serves as a valuable preliminary to a more detailed investigation into the molecular mechanisms associated with root primordia formation in eucalypts.
Intra-Leaf Gradient of Assimilation Rate and Optimal Allocation of Canopy Nitrogen: a Model on the Implications of the Use of Homogeneous Assimilation Functions.
Authors: Badeck FW
Published: January 2006

A model on the allocation of nitrogen available for the construction of photosynthetic apparatus in leaves in different morphological positions is presented. The allocation pattern is calculated under the assumption that nitrogen distribution is optimised in order to maximise daily whole-plant assimilation. The solution is fairly sensitive to the assimilation function applied. It is shown that assimilation functions homogeneous in irradiance and nitrogen imply assimilation gradients and light-saturation characteristics of the whole canopy which contradict experimental findings. An equation for the calculation of electron transport rates as a function of the intra-leaf gradient of the photosynthetic photon flux density is presented. This inhomogeneous assimilation function leads to substantially different predictions of nitrogen allocation which reproduce a wider array of observed allocation patterns. The results presented in this paper support the thesis that the intra-leaf gradient of photosynthetic photon flux density and self-shading of the thylakoids need to be considered if the assimilation flux is to be modelled as a function of light as well as nitrogen availability on a mechanistic basis.
Sequence and Expression of a Caffeic Acid O-Methyl Transferase Cdna Homologue in the Tropical Forage Legume Stylosanthes Humilis.
Authors: Mcintyre CL , Rae AL , Curtis MD , Manners JM
Published: January 2006

The isolation and characterisation of a cDNA encoding a caffeic acid 0-methyl transferase cDNA homologue (COMT) from Stylosanthes humilis are described. The clone is 1391 nucleotides in length, with an open reading frame encoding a predicted protein of 366 amino acids. Cluster analysis of the deduced amino acid sequence revealed extensive homology to other published O-methyl transferase sequences. Maximum levels of homology were seen with COMTs from alfalfa (87%) and aspen (84%). Southern analysis suggested that this enzyme is encoded by two genes in S. humilis. The mRNA is most strongly expressed in stem tissue, with intermediate levels of expression in young leaves and roots, and does not appear to be induced upon fungal infection or wounding.
Towards an Understanding of the Chemistry of Oxygen Reduction
Authors: Rich PR
Published: January 2006

Advances in understanding of the structure and the catalytic cycle of oxygen reduction of the protonmotive haem-copper terminal oxidases are reviewed. This information has been combined with our recent recognition of the need for electroneutrality of stable catalytic intermediates to produce a new working model of the essential elements of the chemiosmotic mechanism of coupling of oxygen reduction chemistry to vectorial proton translocation.
The Significance of a Two-Phase Growth Response to Salinity in Wheat and Barley
Authors: Munns R , Schachtman DP , Condon AG
Published: January 2006

Wheat and barley genotypes that differed in salt tolerance were used to test a hypothesis that the growth response to salinity has two phases. In the first phase there would be a large decrease in growth rate caused by the salt outside the roots, i.e. an 'osmotic' response. In the second phase there would be an additional decline in growth caused by salt having built up to toxic levels within plants, i.e. a 'salt-specific' response. If this two-phase model is correct, genotypes that differ in their ability to exclude salt or tolerate high internal salt concentrations would not differ in growth rate for some period of time in saline conditions. This hypothesis was tested using many genotypes that had previously been found to differ greatly in salt tolerance, as defined by differences in percent reduction in yield or biomass after prolonged exposure to NaCl. Leaf extension of 15 wheat and barley genotypes was measured daily while the NaCl in the nutrient solution was increased over 10 days to 250 mM. All 15 genotypes showed a similar percentage reduction in leaf extension rate. Dry matter production of four selected wheat genotypes that again differed greatly in salt tolerance was measured for up to 6 weeks in 150 mM NaCl. All genotypes showed the same growth reduction for 4 weeks. After this initial period the more salt- sensitive genotype showed a greater decline in growth. This occurred after 60% of the leaves were dead. These data strongly support the hypothesis that the growth response to salinity has two phases, and indicate that most changes in metabolism or gene expression leading to growth reductions during the first phase relate to the osmotic effect of salinity, not to any salt-specific effect. They also indicate that the salt within the plant reduces growth by causing premature senescence of old leaves and hence a reduced supply of assimilates to the growing regions.
Modelling Canopy Production. I. Optimal Distribution of Photosynthetic Resources
Authors: Sands PJ
Published: January 2006

On the basis of detailed numerical simulations, Field (1983. Oecologia 56, 341-347) stated that total canopy photosynthesis will be a maximum for a fixed total canopy leaf nitrogen provided the derivative δA/δN, where A is photosynthetic rate and N is leaf nitrogen concentration, has the same value throughout the canopy. This paper uses the calculus of variations to formally prove Field's assertion. It shows that if the single-leaf light response is a first-degree homogeneous function of both light-saturated photosynthetic rate Amax and intensity I of photosynthetically active radiation and if Amax is linearly related to N, then the optimal distribution of leaf nitrogen is linearly related to the decline in I with canopy depth, and Amax is proportional to this decline. The nature of photosynthetic gains due to optimisation of canopy nitrogen distribution is illustrated numerically for a simple model canopy. It is found that, for canopies with fixed mean leaf nitrogen, canopy photosynthesis is approximately proportional to canopy leaf area index (LAI), and the gain due to canopy optimisation compared with a uniform canopy is small for shallow canopies but pronounced for deep canopies. It is also found that, for canopies with fixed total leaf nitrogen, there is a canopy LAI which maximises canopy photosynthesis, and that this LAI and the corresponding canopy photosynthesis are approximately proportional to total canopy nitrogen.