bluuebunny/crossref_metadata_2025 huggingface dataset:
Osmotic Adjustment, Induced by Drought, in Seedlings of Three Eucalyptus Species
Authors: Myers BA , Neales TF
Published: January 2006

Osmotic adjustment was observed in pot-grown seedlings of Eucalyptus behriana, E. microcarpa and E. polyanthemos that had been subjected to one and two periods of drought. The osmotic potential of sap expressed from rehydrated leaves was significantly lower in seedlings which had wilted twice
(-2.02 � 0.05 MPa) compared with those which had wilted once (-1.86 � 0.05 MPa) and those which had been watered daily (-1.66 � 0.05 MPa). After two drought cycles, seedlings began to wilt at lower mean values of plant water potential (- 3.51 � 0.22 MPa) than those which had not wilted previously
(-3.14 � 0.22 MPa). Thus drought-induced osmotic adjustment apparently enhanced turgor maintenance. The ratio of turgid weight to dry weight was slightly, but significantly, smaller in the seedlings subjected to two drought cycles (3.83 � 0.04 MPa) compared with those subjected to one drought cycle (4.05 � 0.04). The osmotic adjustment that was induced by two drought cycles in these seedlings was about one third of the observed seasonal osmotic adjustment in mature trees of E. behriana and E. microcarpa in the field.
Increased Kernel Number in Norin 10-Derived Dwarf Wheat: Evaluation of the Cause
Authors: Fischer RA , Stockman YM
Published: January 2006

This study of the effect of major Norin 10 dwarfing genes (Rht1, Rht2) on kernel number in spring wheat emphasised three near-isogenic pairs of spring wheat, differing in the presence or absence of both genes. Plants were grown under controlled environment conditions with equal light intensities at their tops and were restricted to main stems by repeated tiller trimming. The dwarf wheats had a higher proportion of shoot dry weight in the spike at anthesis, an effect which field studies suggest is closely associated with their production of more kernels per unit land area.
Stems of dwarf genotypes were somewhat shorter as early as 50 days before anthesis (i.e. before floral initiation) but relative differences became especially great commencing at 35 days before anthesis. Apex and spike lengths were never very different. A greater proportion of dry matter was distributed to leaf lamina from 50 days before anthesis until the end of lamina growth in dwarf wheats, while partitioning to stems was lower from 50 days before anthesis onwards. Partitioning to spikes was only higher in the 15 days preceding anthesis.
Stem water-soluble carbohydrate (WSC) contents at anthesis were greater in dwarf wheats but maximum spike WSC contents reached at about 15 days before anthesis were lower. Spike morphogenesis including floret production and grain setting did not differ generally, except for a tendency with dwarf wheats for a longer floral initiation to anthesis interval and for more kernels per unit of spike weight at anthesis. The major unique feature of dwarf genotypes, the higher proportion of dry matter partitioned to the spike, appeared to be due to reduced competition from growing stems in dwarf wheats rather than intrinsic differences in the growth potential of their spikes.
In vivo Nitrate Reductase Activity in Leaves of Pearl Millet, Pennisetum americanum (L.) Leeke
Authors: Chanda SV , Joshi AK , Krishnan PN , Singh YD
Published: January 2006

In the in vivo assay of nitrate reductase (NR) in P. americanum leaves, addition of 1% (v/v) Triton X-100, potassium phosphate buffer (80 mM, pH 7.4) and 1.13 mM NADH to the assay medium resulted in maximum activity. With increasing concentration of NADH, saturation-type kinetics were observed. Based on this data metabolic pool concentration for NADH and apparent Km for nitrate reductase were determined.
In field studies with cultivars BJ-104, J-104 and 5141-A of P. americanum, the relative limitation of NO3-, NADH and nitrate reductase in NO3- assimilation was determined. NR activity was measured by four modifications of the in vivo assay technique (with NO3-, with NADH, without NO3- and NADH and with both NO3- and NADH additions to the reaction mixture) and with one in vitro technique. For all the cultivars, NADH was the major rate-limiting factor for in vivo assay during early growth stages, while at later stages, NO3- was limiting. At no stage was NR rate-limiting. It is concluded that NR activity alone may not serve as biochemical marker for improved efficiency of utilisation of nitrogen in P. americanum.
Growth Correlations and Assimilate Partitioning in the Arillate Fruit of Litchi chinensis Sonn
Authors: Huang HB , Qiu YX
Published: January 2006

Path analysis and partial correlation methods revealed a direct positive effect of the peel and seedcoat on the size of the aril, and also the positive partial correlations among these three variables. A negative partial correlation between embryo and aril, and a direct repressive effect of the former on the latter were confirmed. The embryo does not by itself stimulate peel growth but, by extending seed life, it results in a higher frequency of large-sized peels. The existence of the three types of fruits with or without an aril and/or embryo results from the disparity in the life-span of the liquid endosperm. The aril is inferior to the embryo in competing for assimilates. Aril growth is mediated by both the concentration of solutes accumulated in the vacuoles creating a more negative osmotic potential, and the restricting force coming from the peel which increases the pressure potential.
Lateral Stresses in Membranes at Low Water Potential
Authors: Wolfe J
Published: January 2006

Severe dehydration of biological cells can produce large stresses and substantial strains in the membranes of some organelles. Water contents and water potentials which are critical for cellular damage in rather dry tissues may reflect a limiting stress characteristic of cell membranes. Different levels of tolerance of extreme dehydration may therefore be a result of different cellular osmotic pressures or of different stress-strain characteristics of the cellular membranes. In this paper, data from phospholipid osmotic stress measurements are used to model the stresses generated in membranes in dehydrated phases.
Effects of Photoinhibition on Photosynthetic Carbon Metabolism in Intact Isolated Spinach Chloroplasts.
Authors: Giersch C , Robinson SP
Published: January 2006

Pools of intermediates of the Calvin cycle were measured during photosynthetic 14CO2 fixation by intact isolated spinach chloroplasts. Photoinhibitibn (illumination for 8 min with 4000 �mol m-2 s-1 light in the absence of bicarbonate) decreased the subsequently measured rate of CO2 fixation. Individual compounds were differently affected: the ribulose-1,5-bisphosphate (RuBP) pool was drastically lowered, while that of fructose-1,6-bisphosphate (FBP) was increased, suggesting that photoinhibition causes a limitation in RuBP regeneration. An increase in FBP and decrease in RuBP were not observed during photosynthesis in low light at rates of CO2 fixation comparable to those in photoinhibited chloroplasts. This indicates that changes of the metabolite pools induced by photoinhibition were not due solely to decreased rates of electron transport. Activities of RuBP carboxylase and fructose-1,6-bisphosphatase (FBPase) were decreased by the photoinhibitory treatment. However, the activity of both enzymes in photoinhibited chloroplasts was still well in excess of that required to sustain the measured rates of carbon flux. Photoinhibition largely abolished the light-induced proton gradient across the chloroplast envelope. The concomitant acidification of the chloroplast stroma could inhibit FBPase activity. It is concluded that photoinhibition does not result in irreversible modification of the FBPase protein but that its activity may be decreased by changes in pH and possibly other factors in the chloroplast stroma.
Water Relations of Tomato Seed Germination
Authors: Haigh AM , Barlow EWR
Published: January 2006

Water uptake during the germination of UC 82B tomato seeds was triphasic. Seed Ψ measurements indicated that phase I imbibition occurred because of the large Ψ gradient between the seed and the imbibition solution (water). During phase II the seed Ψ was in equilibrium with the water. Phase III water uptake recommenced with the onset of radicle emergence without changes in Ψ or Ψπ.
Changes in embryo water content were also triphasic. During phase II the embryo Ψ remained at - 1.5 MPa, not in equilibrium with the imbibing solution. At radicle emergence it was - 0.8 MPa and rose to -0.3 MPa as the radicle elongated. There was no evidence of a lowering of embryo Ψπ nor of a build up of Ψp prior to radicle emergence. Water uptake studies with excised embryos indicated that, within the seed, the enclosing tissues prevented the embryo from taking up water.
It is suggested that embryo water content is restricted by the constraint on embryo expansion caused by the enclosing endosperm tissue. Lowering of embryo Ψπ to build up Ψp is not necessary for radicle emergence. The control of germination may lie in the mechanism which leads to weakening of this mechanical restraint of the endosperm.
Assimilate Movement in Lolium and Sorghum Leaves. IV. Photosynthetic Responses to Reduced Translocation and Leaf Storage
Authors: Wardlaw IF , Eckhardt L
Published: January 2006

The net CO2 exchange (NCE) rates of the last fully expanded leaves of Lolium temulentum L. and Sorghum bicolor Moench were examined in relation to the accumulation of photosynthate that occurred with time in continuous light (1100 μmol m-2 s-1 PAR). On a leaf area basis Lolium, although having a lower NCE, accumulated current photosynthate at a faster rate than Sorghum and had a greater capacity for storage. In Lolium there was an initial rapid rise in leaf solute, followed by a steady increase in the accumulation of starch, while in contrast in Sorghum there was an initial rapid rise in starch followed by a change to the accumulation of soluble photosynthate.
After 4-6 h in continuous light there was a fall in NCE in both species, with the response depending on the balance between source and sink tissue. The decrease' in NCE coincided with an increase in specific leaf weight (SLW) of about 40% and SLW showed a closer negative correlation with NCE than leaf solute potential (Ψs). The fall in NCE, associated with an accumulation of photosynthate, was primarily related to factors within the leaf and not to stomatal conductance. The fall in NCE could not be accounted for by a rise in photorespiration and did not appear to be associated directly with an increase in the partitioning of photosynthate into starch. In both Lolium and Sorghum a fall in apparent quantum yield coincided with the fall in NCE.
Extracellular Polysaccharides From Shaken Liquid Cultures of Zea mays
Authors: Bacic A , Moody SF , Mccomb JA , Hinch JM , Clarke AE
Published: January 2006

Callus initiated from root and embryo of Zea mays proliferated largely through aberrant root growth. In shaken liquid cultures (liquid cultures), the callus remained as clumps of roots but some cells were released into the medium. The extracellular polysaccharides (ECPS) from these culture supernatants are similar with respect to monosaccharide and glycosyl linkage composition to the slime secreted by roots of young Zea mays seedlings but are distinguished from ECPS produced by suspension cultured cell lines of other species by their high fucose content. Callus initiated from root and embryo therefore provides a model for studies of fucosyl-containing polysaccharides secreted by seedling roots.
Mangrove Photosynthesis: Response to High-Irradiance Stress
Authors: Bjorkman O , Demmig B , Andrews TJ
Published: January 2006

Efficiencies of photosynthetic energy conversion were determined in sun and shade leaves of several mangrove species, growing in an open intertidal habitat in North Queensland, by measuring the maximum photon yield of O2 evolution and 77K chlorophyll fluorescence characteristics. Preliminary meas- urements confirmed that mangrove leaves have low water potentials, low stomatal conductances and low light-saturated CO2 exchange rates. Mangrove sun leaves therefore received a very large excess of excitation energy.
Mangrove shade leaves had as high a photon yield of O2 evolution as non-mangrove leaves and their fluorescence characteristics were normal, showing that the energy conversion efficiency was unaffected by the high salinity. Mangrove sun leaves had markedly depressed photon yields and fluorescence was severely quenched showing that the efficiency of the photochemistry of photosystem II was reduced. The efficiency of energy conversion decreased with an increased radiation receipt. No such depression was detected in sun leaves of non-mangrove species growing in adjacent non-saline sites. Shading of man- grove sun leaves resulted in an increase in the efficiency of energy conversion but, in most species, more than 1 week was required for these leaves to reach the efficiency of shade leaves. Leaves exposed to direct sunlight had somewhat higher efficiencies in mangrove plants cultivated in 10% seawater as compared with full-strength seawater but the salinity of the culture solution had little effect on the increase in the efficiency upon shading.
Field and laboratory fluorescence measurements indicated that the reduced efficiency of energy conversion in mangrove sun leaves resulted from a large increase in the rate constant for radiationless energy dissipation in the antenna chlorophyll rather than from damage to the photosystem II reaction centres. We propose that this increase in radiationless energy dissipation serves to protect the reaction centres against damage by excessive excitation.
Characterisation of Three Components of Non-photochemical Fluorescence Quenching and Their Response to Photoinhibition
Authors: Demmig B , Winter K
Published: January 2006

Three components of non-photochemical fluorescence quenching were distinguished according to their response to irradiance and to their relaxation kinetics upon darkening. Two components of quenching were restricted to excessive irradiance and were interpreted to reflect radiationless dissipation. One relaxed rapidly upon darkening, and increased sharply when irradiance became excessive, i.e. as soon as net CO2 assimilation rate was no longer linearly related to irradiance, and attained a maximum value with only small further increases in irradiance. The second component relaxed slowly, increased mark- edly when the rapidly relaxing component had reached its maximum, and continued to increase linearly with increasing irradiance. The third component was already present at low irradiances, relaxed very slowly, and may be related to an altered distribution of excitation energy between PS II and PS I.
Following exposure to weak illumination under conditions preventing photosynthetic electron transport (20 mbar O2, zero CO2), the reduction state of Q was initially high and decreased as non- photochemical fluorescence quenching indicative of radiationless dissipation developed.
Subsequent to photoinhibitory treatments in high light and 20 mbar O2, zero CO2, an increased reduction state of Q as well as increased non-photochemical quenching of the two types indicative of increased heat dissipation was observed. In sunflower a lasting increase in the reduction state of Q was observed and fluorescence characteristics reflected photoinhibitory damage. In Nerium oleander, increased radiationless dissipation of the slowly relaxing type was the predominant response and the reduction state of Q was increased only transiently.
Regulation of Carboxylation and Photosynthetic Oscillations During Sun-Shade Acclimation in Helianthus annuus Measured With a Rapid-Response Gas Exchange System
Authors: Osmond CB , Oja V , Laisk A
Published: January 2006

The consequences of acclimation from shade to sun and vice versa for regulated photosynthetic metabolism were examined in H. annuus. A rapid-response gas exchange system was used to assess changes in carboxylation-related parameters (mesophyll conductance, assimilatory charge and CO2 capacity) and to analyse oscillations in CO2 fixation following transfer to high CO2 concentration as a function of intercellular CO2 concentration and light intensity. Data showed a two- to threefold change in all carboxylation-related parameters during acclimation in either direction. Dynamic regulation of carboxylation, indicated by changes in oscillatory response as a function of CO2 concentration at light saturation, remained unchanged, consistent with concerted regulation of ribulose-1,5-bisphosphate carboxylase-oxygenase during acclimation. However, the light dependency of oscillations changed during acclimation from shade to sun, and the range of oscillation was closely tied to the maximum rate of steady-state photosynthesis at CO2 saturation. These data imply that changes in the light-absorbing and electron transport components of the photosynthetic apparatus underlie the shift in regulatory behaviour during acclimation.
Compensating Effects to Growth of Changes in Dry Matter Allocation in Response to Variation in Photosynthetic Characteristics Induced by Photoperiod, Light and Nitrogen
Authors: Kuppers M , Koch G , Mooney H
Published: January 2006

High N-nutrition, high light and long photoperiod increased photosynthetic capacity (Amax) per leaf area of Raphanus sativus × raphanistrum. The effect of Amax on growth was enhanced by an increased fraction of dry matter partitioned into the shoot, resulting in a larger canopy. Low N-nutrition, low light and a short photoperiod reversed these responses. At intermediate combinations, such as low light and long photoperiod or high light and short photoperiod, Amax and the actual net photosynthesis per leaf area (A) neither correlated with the carbon uptake rate of the canopy as a whole nor with whole plant growth. A compensatory effect to growth of changes in dry matter partitioning in response to variation in photosynthetic characteristics induced by light and N-nutrition is discussed in terms of two competitive feedbacks. For growth a long photoperiod compensated for low light or low N-nutrition. The results indicate that extrapolations from rates to gains are only valid under defined environmental conditions, especially when different photoperiods and effects on leaf ontogeny are involved.
Isotopic Discriminations During the Accumulation of Nitrogen by Soybeans
Authors: Bergersen FJ , Peoples MB , Turner GL
Published: January 2006

Soybeans were grown in a glasshouse in sand-vermiculite medium supplied daily with a mineral nutrient solution essentially free of combined N or containing 5 mM nitrate of known 15N abundance. The natural abundance of 15N in parts of plants and in nitrogen remaining in the medium was determined from 15 days after planting until fruiting.
In nodulated plants completely dependent on N2 fixation for growth, the δ15N of plant nitrogen was uniformly negative at 56 days (overall mean: -0.90� 0.17) after adjustment for the effect of seed nitrogen. The δ15N of root nodules increased with time (max. 9.6‰), as that of shoots declined (min.
- 1.3 ‰). The δ15N of every mainstem trifoliolate leaf and of the first (unifoliolate) leaf declined from initially positive values (0.5 to 2 ‰) to about - 2‰ with similar time courses, irrespective of the time of initiation. There were no significant losses of N from the plants during growth. There were differences between the δ15N of the total N of root-bleeding xylem sap and of sap extracted by vacuum treatment of stems. These were due to differences between the proportions of ureide-N and amino-N and between the δ15N values of these components.
When nodulated plants were supplied daily with 5 mM nitrate (δ15N = 7.68‰) between 21 and 35 days, N2 fixation was reduced to 63% of N assimilated but growth and accumulation of nitrogen were affected little. Following removal of nitrate, there were changes in growth which led to enhanced nodulation and N2 fixation. The δ15N of the total N of trifoliolate leaves which were initiated or expanded before or during the period of nitrate treatment remained positive; those expanded or initiated after the treatment became negative in δ15N, as in the corresponding leaves of untreated nodulated plants. The δ15N of nodules was unaffected by the nitrate treatment. In plants (non-nod. Clark '63) supplied continuously with nitrate, the δ15N of the total N of entire plants rose quickly from values for seeds, but to values significantly higher than in the nitrate.
These results are discussed in relation to the effects on the use of 15N natural abundance data for estimating utilisation of atmospheric N2 by nodulated plants.
Water Pathways in Wheat Leaves. IV. The Interpretation of Images of a Fluorescent Apoplastic Tracer
Authors: Canny MJ
Published: January 2006

Sections of wheat leaves fed with the fluorescent apoplastic tracer sulforhodamine G (SR) through the xylem were prepared by freeze-substitution and resin embedding. The distribution of fluorescence intensity (FI) of the tracer was measured by microspectrofluorometry at a resolution of 0.4 �m. SR was found to move within cell walls in restricted paths less than 200 nm wide. The name 'nanopaths' is suggested for these. The highest FI was found around the mestome-sheath / parenchyma-sheath border on the xylem side, and was shown to be due, not to binding of the tracer to wall components, but to the generation of a very high concentration of SR there by the separation of water from the solute. This separation cannot be evaporative but must be osmotic, and is presented as evidence of a major symplastic water movement starting at the parenchyma sheath cell membrane. The main resistance to water loss from the veins is at the mestome sheath and appears to be controlled by the suberised lamellae.
Electron and Proton Transfers around the b/f Complex in Chloroplasts: Modelling the Constraints on Q-cycle Activity
Authors: Hope AB , Matthews DB
Published: January 2006

The requirements for the operation of a Q-cycle in thylakoids are discussed. A computer model is described in which the state of reduction of components of the b/f complex is followed, single turnover at a time. Standard reduction potentials from the literature were assigned to the cytochrome b563 molecules; those for the second electron oxidation of plastoquinol at p-sites, and for the reduction of plastoquone at n-sites, were found by optimising the predicted stimulation of proton uptake by valinomycin. The stimulation has been attributed to uptake by doubly reduced plastoquinone at b/f complexes, a process thought to continue only if the membrane potential (ΔV) is kept low by ionophores. ΔV was simulated in the model via the electrochromic signal; its effect on electron transfers in the b/f complex was incorporated by modifying the reduction potentials.
The extent of valinomycin stimulation of proton uptake, its dependence on [valinomycin] and flash frequency, the slow phase of the electrochromic signal and the extent of cytochrome b reduction were predicted by the model when the standard reduction potential for the p-site was set at -0.05 to -0.08 V. with that for the n-site at about 0 V.
Fruit Growth and Dry Matter Accumulation in Grapefruit During Periods of Water Withholding and After Reirrigation
Authors: Cohen A , Goell A
Published: January 2006

Changes in volume, fresh weight and dry matter (DM) contents were followed in fruits from girdled and non-girdled branches borne on regularly irrigated (RI) as well as water-stressed (S) trees. Water stress was imposed by withholding irrigation for various periods. The results indicate that, even during prolonged periods of drought, DM accumulation in fruits on S trees was only slightly impaired, even when fruit volume growth was reduced to zero or even to shrinkage. After irrigation was resumed, fruits from S trees grew faster than those from RI trees, indicating that some of the DM which had accumulated was available for volume growth. The possibility of using the rate of DM accumulation in the fruit as an indicator for the timing of irrigation is discussed.
Vegetative Growth Distribution During Water Deficits in Vitis vinifera L
Authors: Schultz HR , Matthews MA
Published: January 2006

The expansion of plant organs is inhibited by water deficits but the effect of ontogeny on growth sensitivity is not known. Therefore, growth responses of shoot organs (internodes, leaves, tendrils) of Vitis vinifera L. cv. White Riesling to developing water deficit were investigated under controlled environmental conditions. Growth of organs at a node position was asynchronous, with internode growth being more restricted than leaf and tendril growth in time (duration of growth) and space (number of node positions at which growth occurred).
The timing of initial growth inhibition and final growth cessation caused by soil water deficits was identical in internodes, leaves, and tendrils. The degree of growth inhibition at low water potential was similar among organs, whether linear or volume changes were considered and whether the most rapidly expanding organs or organs of similar developmental stage were compared. In addition, the relative partitioning of growth among internodes, leaves, and tendrils along single shoots was unaltered when growth was inhibited by water deficits. Growth of each organ was inhibited initially at soil water potential of -0.065 MPa and ceased completely at -0.54 MPa. Therefore, it was concluded that the sensitivity of growth to water deficits did not differ among shoot organs.
The region along the shoot in which organs expanded was reduced during water deficits. Similarly, the regions within organs in which growth occurred diminished as water deficits developed. Although growth was inhibited in all tissues, inhibition was complete in older tissues when some growth was maintained in younger tissues. Therefore, it was concluded that sensitivity of growth to water deficits increased with ontogeny.
Nitrogenase Activity by Subterranean Clover and Other Temperate Pasture Legumes
Authors: Davey AG , Simpson RJ
Published: January 2006

Nitrogenase (C2H2-reduction) activity and nodulated root respiration of intact plants of subterranean clover (Trifolium subterraneum L.) cv. Seaton Park nodulated by Rhizobium trifolii WU95 were measured in a flow-through system. Simultaneous declines in nitrogenase activity and respiration were exhibited 2 min after 10% C2H2 had been introduced into the gas stream. Declines in nitrogenase activity and nodulated root respiration provided an estimate of the efficiency of nitrogenase activity (mol CO2 evolved/mol C2H4 produced). The pre-decline rate of nitrogenase activity at time zero was thus calculated as the product of the respiration associated with nitrogenase activity and the reciprocal of the efficiency of nitrogenase activity. Pre-decline rates of nitrogenase activity were similar to peak rates for several pasture legumes. However, post-decline rates of activity were as much as 70% lower than the pre-decline rate. The age of subterranean clover plants had an important influence on the magnitude of the C2H2-induced decline; young plants exhibited the largest C2H2-induced inhibition of nitrogenase activity. Neither sainfoin (Onobrychis viciifolia Scop.) cv. Othello nodulated by Rhizobium sp. CC1108 nor yellow serradella (Ornithopus compressus L.) cv. Pitman nodulated by R. lupini WU425 exhibited C2H2-induced declines in nitrogenase activity. Nitrogenase-linked respiration of subterranean clover at the 14-leaf stage accounted for 50% of total nodulated root respiration. The oxygen diffusion resistance of the nodules increased in the presence of C2H2 but the effect was reversible once C2H2 was removed from the gas atmosphere. The pre-decline rate of acetylene reduction activity of subterranean clover reached a maximum at 10% C2H2. The C2H2-induced decline in nitrogenase activity was lower at subsaturating pC2H2 and was not detected at 0.4% C2H2.
Influence of Light Regime and Leaf Nitrogen Concentration on 77k Fluorescence in Leaves of Four Tropical Grasses: No Evidence for Photoinhibition
Authors: Ludlow MM , Samarakoon SP , Wilson JR
Published: January 2006

This work was undertaken to determine if the stimulation of growth associated with shading of some tropical C4 grasses growing on soils with low to moderate nitrogen availability is partly due to overcoming photoinhibition (i. e. damage caused by excessive light). Four grasses (green panic, carpet grass, buffalo grass and kikuyu) were grown in full sunlight and at 37% of full sunlight, and given a low or high nitrogen supply. Despite differences of up to twofold in leaf nitrogen and chlorophyll concentration between high and low nitrogen treatments in all four grasses, photoinhibition measured by reduction in chlorophyll fluorescence was less than 3% in leaves of low nitrogen content that developed in full sunlight. Therefore, photoinhibition is not a contributor to the poor growth of nitrogen-limited grasses in full sunlight.
A second objective was to determine if low nitrogen content predisposed shade-grown leaves to photoinhibition when they were subsequently exposed to full sunlight. Green panic plants that had been given either high or low nitrogen supply and grown in 37% of full sunlight were transferred to full sunlight and the extent of photoinhibition was followed over 8 days. The amount of photoinhibition that occurred was small (<6%) compared with plants grown at either nitrogen level in full sunlight. Therefore, shade and low nitrogen content separately or in combination did not cause any appreciable photoinhibition in green panic.
Adaptation of Elodea and Potamogeton to Different Inorganic Carbon Levels and the Mechanism for Photosynthetic Bicarbonate Utilisation
Authors: Elzenga JTM , Prins HBA
Published: January 2006

The different abilities of three submerged aquatic macrophytes, Elodea canadensis, E. nutallii and Potamogeton lucens, cultured under high and low CO2 concentrations, to utilise bicarbonate was found to correlate with the ability to exhibit a polar leaf pH reaction (i. e. acidification of the medium on one side of the leaf). The utilisation of bicarbonate did not depend on the induction of extracellular carbonic anhydrase activity as found with unicellular algae. Although the bicarbonate utilisation was inhibited by acetazolamide, an inhibitor of carbonic anhydrase activity, there was no difference in the concentration of extracellular carbonic anhydrase between leaves with high and with low bicarbonate utilisation. In experiments using the isotopic disequilibrium method we found only a small contribution of bicarbonate to the carbon fixation of protoplast. The percentage of bicarbonate contribution to the fixation did not differ between protoplasts isolated from Potamogeton leaves with high bicarbonate utilisation (from a low CO2 culture) and from leaves with low bicarbonate utilisation (from a high CO2 culture). We conclude that bicarbonate utilisation depends on the polar leaf pH reaction and that CO2 is the carbon species that is taken up by the leaf.
The Effect of Sodium Nutrition on the Pool Sizes of Intermediates of the C Photosynthetic Pathway
Authors: Johnston M , Grof CPL , Brownell PF
Published: January 2006

Changes in C4 pathway intermediates in response to sodium nutrition, consistent with the hypothesis that there is a limitation in the conversion of pyruvate to phosphoenolpyruvate (PEP) in the mesophyll cells in sodium-deficiency, were observed in the C4 species, Kochia childsii, Chloris gayana, Amaranthus edulis, Amaranthus tricolor and Atriplex spongiosa. In the C3 species Lycopersicon esculentum (tomato), no differences were observed when grown with or without added sodium.
Of the group 1 elements, only sodium, irrespective of the salt supplied to deficient cultures, effected these changes in the C4 species.
In the light, concentrations of aspartate, PEP and 3-phosphoglycerate (3-PGA) were lower and those of pyruvate and alanine were greater in sodium-deficient than normal plants. In the dark, concentrations of aspartate, pyruvate, alanine, PEP and 3-PGA were similar in sodium-deficient and normal plants.
In the C4 species Atriplex spongiosa, the concentration of sodium required to bring about these changes corresponded to that required for growth responses.
Rapid increases in the concentrations of malate, PEP and 3-PGA and decreases in pyruvate and alanine were observed following the direct application of sodium to leaves.
Salinity Results in Polyamine Accumulation in Early Rice (Oryza sativa L.). Seedlings
Authors: Basu R , Maitra N , Ghosh B
Published: January 2006

Saline (NaCl) stress in germinated seedlings of rice cv. Rupsail causes an increase in polyamine content and concomitantly an enhancement of arginine decarboxylase (ADC) activity; in the case of putrescine, the increase is 100%. The responsiveness of coleoptiles and roots is different; ADC activity is maximum in roots and coleoptiles at 50 mM and 100 mM respectively. Difluoromethyl arginine, a potent inhibitor of ADC, reduces both polyamine accumulation and ADC activity. NaCl can enhance the activity of ADC by 1.6-fold in vitro. [14C]leucine incorporation into protein at 400 mM NaCl is decreased 4- and 10-fold in coleoptiles and roots, respectively. Efflux of sugar, amino acid, polyamines and total electrolytes increased gradually with the increase in concentration of salt. Influx of Na+ and Cl- and efflux of K+ in coleoptiles and roots are directly proportional to the concentration of NaCl applied.
Photosynthetic Rates and Solute Partitioning in Relation to Growth of Salt-Treated Pistachio Plants
Authors: Walker RR , Torokfalvy E , Behboudian MH
Published: January 2006

Seedlings of Pistacia vera cv. Kerman were grown in a porous medium under glasshouse conditions and treated for up to 5 weeks with 0.5 (control), 100 or 175 mol m-3 Cl- in irrigation water, with accompanying cations Na+ , Ca2+ and Mg2+ in the molar ratio 6 : 1 : 1. Salinity treatments were raised to the respective levels by daily increments of 25 mol m-3 Cl-.
Shoot growth was decreased by 100 mol m-3 CI- and ceased during treatment with 175 mol m-3 CI- . Turgor was maintained in expanding leaves during and after the period of incremental increase in salinity up to the maximum of 175 mol m-3 CI-. There was an increase in sucrose and reducing sugar concentrations on a tissue water basis in expanding leaves of plants after 2 days of treatment with 175 mol m-3 CI- but only a marginal increase in Cl- and Na+. Extension rates and final lengths of successively emerging leaves decreased with duration of treatment with 175 mol m-3 CI-. Emergence of new leaves eventually ceased on these plants.
Bulk leaf Cl- and Na+ concentrations of expanded 42-day-old leaves after treatment of plants for 35 days with 175 mol m-3 CI- were 255 and 142 mol m-3, respectively, but despite these concentrations photosynthetic rates were not reduced. There was an increase in proline and a decrease in shikimic acid in these leaves. Rates of water vapour loss per unit leaf area of whole plants did not differ markedly between control and salt (175 mol m-3 CI-)-treated plants after 40 days of salt treatment. The salt- stressed plants accumulated higher sucrose and starch concentrations in the stem and higher concentrations of sucrose, reducing sugars and starch in the main root. Total carbohydrate concentrations were also marginally higher in the root tips of salt-treated plants.
These data indicate that photosynthetic rate of expanded leaves, photoassimilate supply to, water relations of, and Cl- and Na+ concentrations of expanding leaves are unlikely to be responsible for the growth reduction of pistachio plants observed at relatively high salinities.
Membrane Transport at the Biotrophic Interface: an Overview
Authors: Smith FA , Smith SE
Published: January 2006

Many studies of interactions between the organisms which form biotrophic symbioses have concentrated on extracellular events such as signals and responses that involve modifications to cell wall metabolism. It is clear, however, that formation of a biotrophic association must also involve signals and responses at the membrane level and modifications to membrane activity resulting in changes in the transfer of nutrients.
In parasitic biotrophs the modifications to membrane transport result in unidirectional transport (at least in the long term).
In mycorrhizal associations, lichens and N2-fixing symbioses, it appears that well regulated bidirectional transport of nutrients between symbionts must occur, thus allowing a persistent compatible (and mutualistic) symbiosis. Evidence for such membrane modifications comes from changes in ATPase activity. The overall question to be considered in mutualistic symbioses is whether nutrient transfer processes have analogies elsewhere in the physiology of plants or whether 'new' transport events are switched on as a result of interactions between the organisms.
Solute Efflux From the Host at Plant-Microorganism Interfaces
Authors: Patrick JW
Published: January 2006

The exchange of solutes between a plant and invading microorganism involves transport across both plant and microorganism membranes separated by a common apoplast. An empirical analysis of the interrelationship between these two membrane transport steps is undertaken with emphasis on transfer of reduced carbon from host to microorganism. The analysis leads to the conclusion that solute efflux from the plant partner has the potential to exert significant control over net transport of solutes from the plant to microorganism. The nature of solute efflux from the plant partner was evaluated for examples of pathological, mycorrhizal and Rhizobium associations. Estimates of solute efflux across the perceived interfacial membrane of the plant were greater than those of non-infected tissues. The principal factors contributing to the enhanced solute efflux in the presence of a microorganism were considered to be elevation of solute concentrations in the plant cytoplasm and modification of membrane transport. Possible modes of action by the invading microorganism on these factors are examined with particular attention being paid to the modification of membrane transport.
Diffusion of Oxygen in Relation to Structure and Function in Legume Root Nodules
Authors: Dakora FD , Atkins CA
Published: January 2006

Although substantial rates of oxidative phosphorylation by bacteroids of Rhizobium are required to support sustained N2 fixation in legume nodules, the enzyme nitrogenase is extremely sensitive to oxygen. This apparent paradox indicates that nodules must exercise effective control over internal oxygen concentration. Structural features including lenticel development, the thickness and arrangement of cells and air spaces in the inner and outer cortex, the presence or completeness of the common endodermis as well as the distribution of infected cells, uninfected cells and air spaces in the nodule medulla are important to gaseous ventilation of the organ. Among these, the organisation of cells and water / gas- containing extracellular voids in the inner cortex bounding the infected medulla zone are critical components to overall diffusive resistance. These elements also appear to provide the means for the oper- ation of a reversible variable diffusion barrier to control gas movement. Inhibition of nodule functioning by a widely diverse range of factors, e. g. restricted H2O or photosynthate supply, exposure to acetylene or combined N, appears to be mediated through increased diffusion resistance.
Ultrastructure of Mycorrhizas of Dracophyllum secundum R. Br. (Ericales:Epacridaceae)
Authors: Allen WK , Allaway WG , Cox GC , Valder PG
Published: January 2006

Dracophyllum secundum R. Br. (Epacridaceae) often possessed ericoid mycorrhizas; fungal endophytes formed coils within cells of the epidermis of hair-roots. The plant plasma membrane extended around the hyphae. In some epidermal cells of hair-roots, both plant and fungal cells retained their structural integrity, both partners showing mitochondrial, vacuolar and lipid droplet profiles, and with much of the plant cytoplasm associated with the hyphal coils. In other epidermal cells of hair-roots, fungal coils were present but cytoplasmic features of both symbionts appeared to have broken down. Some epidermal cells showed no evidence of fungal infection. These three arrangements could occur in root-cells of the same age, and are interpreted as resulting from different stages in the development and degeneration of the infection by the mycorrhizal fungus. Two structural types of fungal endophyte here found in ericoid mycorrhizas in D. secundum: one with simple septa, Woronin bodies and two-layered walls (presumed to be an Ascomycete), and another with dolipore septa with imperforate parenthesomes (presumed to be a Basidiomycete). The possibilities that the mycorrhizas may be seasonal, and that mycorrhizal status varies from place to place, are discussed.
Salinity, Stomatal Responses and Whole-Tree Hydraulic Conductivity of Orchard 'Washington Navel' Orange, Citrus sinensis (L.) Osbeck
Authors: Lloyd J , Howie H
Published: January 2006

Diurnal patterns in stomatal conductance and bulk water potentials were measured over a 6-month period for spring flush leaves on 24-year-old Washington Navel orange trees that had been irrigated with water containing either 5 mol m-3 NaCl or 20 mol m-3 NaCl for 5 years prior to measurements.
During summer and autumn, at early morning measurement times, stomatal conductances of leaves on trees irrigated with 20 mol m-3 were significantly below those on trees irrigated with 5 mol m-3 NaCl. Lower values on high salinity trees were not attributable to more negative water potentials or lower turgor pressures but were apparently due to an inability of stomata on leaves from salinised trees to open in response to low vapour pressure deficits (VPDs). There was little effect of salinity on stomatal conductances during afternoon measurements when high vapour pressure deficits prevailed. Laboratory studies confirmed that stomata on salinised trees are less responsive to VPD than those from unsalinised trees.
When measurements were made during winter months there was no effect of salinity on diurnal patterns of stomatal conductances but leaf water potentials were less negative for leaves of salinised trees during daylight hours.
Hydraulic conductance (G) of trees to liquid water flow was greater for trees irrigated with 5 mol m-3 NaCl in summer, but seasonal reductions in G for trees irrigated with 5 mol m-3 NaCl occurred to a far greater extent than for trees irrigated with 20 mol m-3 NaCl. This may have been a consequence of a reduction in leaf areas of salinised trees during summer and autumn without concomitant decreases in root length.
Relationship Between Root/Soil Hydraulic Properties and Stomatal Behaviour in Sugarcane
Authors: Saliendra NZ , Meinzer FC
Published: January 2006

Stomatal conductance, leaf and soil water status, transpiration, and apparent root hydraulic conductance were measured during soil drying cycles for three sugarcane cultivars growing in containers in a greenhouse. At high soil moisture, transpiration and apparent root hydraulic conductance differed considerably among cultivars and were positively correlated, whereas leaf water potential was similar among cultivars. In drying soil, stomatal and apparent root hydraulic conductance approached zero over a narrow (0.1 MPa) range of soil water suction. Leaf water potential remained nearly constant during soil drying because the vapor phase conductance of the leaves and the apparent liquid phase conductance of the root system declined in parallel. The decline in apparent root hydraulic conductance with soil drying was manifested as a large increase in the hydrostatic pressure gradient between the soil and the root xylem. These results suggested that control of stomatal conductance in sugarcane plants exposed to drying soil was exerted primarily at the root rather than at the leaf level.
Influence of Water Deficits on the Water Relations and Growth of Echinochloa turneriana, Echinochloa crus-galli, and Pennisetum americanum
Authors: Conover DG , Sovonick-Dunford SA
Published: January 2006

Echinochloa turneriana (Domin) J. M. Black (Channel millet) is a wild grass native to the arid Channel Country of inland Australia, where it is used as forage and hay. In its native habitat flooding induces germination, and the plant can complete its life cycle with this single watering. The present study was conducted to gain a better understanding of the mechanisms which allow Channel millet to develop from germination to harvest with a single watering. Channel millet's drought resistance was compared with that of Pennisetum americanum (L.) Leeke (pearl millet), an important drought-resistant crop, and Echinochloa crus-galli (L.) Beauv. var. oryzicola (Vasinger) Ohwi (oryzicola), an obligate rice field weed. Plants of each species were grown under well-watered (control) and droughted conditions under long days in a greenhouse and under long and short days ifi a growth cabinet. Under long days, control and droughted Channel millet plants flowered before controls of pearl millet and oryzicola. Under long days and droughted conditions in the greenhouse flowering was delayed, and plant height, shoot dry weight, panicle number and tiller number were decreased in each species. Active osmotic adjustment under drought was detected in pearl millet by the pressure-volume and expressed sap methods, but was not observed in the other two species. The bulk modulus of elasticity decreased (cell wall elasticity increased) significantly under drought in Channel millet, but not in pearl millet or oryzicola. The apoplastic water volume was about 20% for all species and treatments. This study suggests that Channel millet's rapid development is important in helping the plant to complete its life cycle before the onset of a severe water deficit. Once severe water deficits develop, no osmotic adjustment occurs in Channel millet, and turgor is significantly decreased despite a decrease in the bulk modulus of elasticity.
Natural 15N Abundance of NH4+, Amide N, and Total N in Various Fractions of Nodules of Peas, Soybeans and Lupins
Authors: Shearer G , Kohl DH
Published: January 2006

Nodules of certain N2-fixing root nodules are substantially enriched in 15N compared with non-nodular tissues. This enrichment usually resides largely within bacteroids. Isotope discrimination associated with export of ammonia(um) from the bacteroid would result in 15N enrichment of NH4+ within bac- teroids. Bacteroid protein synthesis from this pool of 15N enriched NH4+ would then account for enrichment of the bacteroids. Measurements of 15N abundances of total N and free NH4+ in nodule fractions from lupins (Lupinus luteus), soybeans (Glycine max) and peas (Pisum sativum) showed this was not the case. With the inocula used in experiments reported here, lupin and soybean nodules were enriched in 15N, while pea nodules were not. There was no correlation between 15N abundances of NH4+ and total N in the nodule fractions (r= 0.445, P> 0.2). We conclude that isotope discrimination associated with ammonia(um) transport does not explain the 15N elevation of lupin and soybean nodules. We also conclude, on the basis of the large isotope effect for the equilibrium between NH4+ and NH3, that most of the ammonia(um) is exported from bacteroids as NH4+ rather than NH3.
We also measured the 15N abundance of free amide N. There was a strong correlation between 15N abundances of free amide N and total N in nodule fractions (r=0.924, P<0,001), suggesting that amide N is a significant source of N to the amino acid pools from which proteins are synthesised.