bluuebunny/crossref_metadata_2025 huggingface dataset:
Cytokinin-Enhanced Regeneration of Plants From Microprojectile Bombarded Sugarcane Meristematic Tissue
Authors: Gambley RL , Bryant JD , Masel NP , Smith GR
Published: January 2006

Sugarcane plants expressing the firefly luciferase gene were regenerated from microprojectile-bombarded meristematic tissue. The concentration and type of cytokinin added to the basal media, both prior to bombardment and during regeneration, significantly affected the number of plantlets produced and the percentage of plantlets expressing luciferase. Maximum shoot initiation from meristematic tissue of sugarcane cultivar Q137 was achieved on media supplemented with N6-benzylaminopurine (BAP). There were differences in the numbers of shoots initiated from different sugarcane cultivars, and some optimisation for each cultivar was required. No luciferase activity was observed in plantlets regenerated in the absence of cytokinin. Sugarcane meristematic tissue preconditioned by exposure to cytokinin appears to be a suitable target tissue for the introduction of genes of agronomic importance into sugarcane. This approach should be readily adaptable to most commercial varieties of sugarcane with minor modification to the cytokinin treatment necessary for optimal regeneration.
Leaf Expansion Limits Dry Matter Accumulation of Salt-Stressed Maize
Authors: Cramer GR , Alberico GJ , Schmidt C
Published: January 2006

Two maize (Zea mays L.) hybrids, differing in their salt tolerance (percentage of control on a dry weight basis) and ability to accumulate Na+ in the shoot, were treated with 80 mol m-3 NaCl salinity or 80 mol m-3 NaCl plus 8.75 mol m-3 CaCl2. Multiple harvests were performed and the interactions of salinity with time were examined with growth analysis. Relative growth rate (RGR) and leaf area ratio (LAR) were significantly reduced by NaCl salinity, but net assimilation rate (NAR) was unaffected. Supplemental Ca2+ improved RGR by maintaining LAR closer to control values. LAR was inhibited in the early stages of salt stress, but was not limiting growth relative to controls in later stages. Salinity also reduced the specific leaf area and leaf weight ratio, which indicates that leaf expansion and carbon allocation were altered. Differences in salt tolerance between the hybrids were small, but significant throughout the lifecycle of the plants. These differences were associated with differences in leaf elongation rates and LAR within the first 9 days of salinity.
Effect of Temperature on Enzymes in the Pathway of Starch Biosynthesis in Developing Wheat and Maize Grain
Authors: Keeling PL , Banisadr R , Barone L , Wasserman BP , Singletary GW
Published: January 2006

Soluble starch synthase (SSS) is shown to be a major site of control of flux through the pathway of starch synthesis in developing wheat and maize grain. Temperatures above 25�C adversely affect flux, and therefore, limit yield. This process is linked to SSS which is heat sensitive. Two apparently different properties of SSS can be identified which differ in the period required before full activity is restored after heat treatment. First, enzyme rate is adversely affected by elevated temperature, an effect which is reversible on returning to a lower temperature. The effect on enzyme rate was quantified using enzyme Q10 which was found to begin to be sub-optimal above 20�C. Second, with a prolonged period of exposu;e to elevated temperature there is a loss of enzyme activity which is not freely reversible which we have termed thermal inactivation. Although this occurs at temperatures in excess of 20�C in wheat, higher temperatures of more than 30�C are needed in maize SSS. Elevated temperature did not affect the inherent stability or Q10 characteristics of other enzymes in the pathway of starch synthesis except for branching enzyme which we believe has minimal flux-control strength. SSS thermal inactivation may not be a major problem in field conditions for developing maize grain, because temperatures rarely are high enough. However, we suggest that the effect on enzyme Q10 is more physiologically relevant, since maize SSS is operating sub-optimally as temperatures exceed 20�C. Calculations of the reductions in maize US corn-belt yield showed that significant yield improvement might be obtained by a 5�C shift in the temperature optimum. Thus selections for a more temperature tolerant form of maize SSS were conducted using enzyme Q10 as a selection tool. Of several hundred maize specimens screened, two were found to be significantly different. However, attempts to use backcross breeding to transfer this trait from the tropical donor to another line have not yet succeeded. Transgenic approaches to altering relations of starch deposition are now underway.
Wheat Cultivars Vary Widely in Their Responses of Grain Yield and Quality to Short Periods of Post-Anthesis Heat Stress
Authors: Stone PJ , Nicolas ME
Published: January 2006

Short periods of high temperature (> 35�C) are common during the post-anthesis period in Australian wheat crops and have recently been shown to significantly reduce grain yield and quality. In view of this, 75 cultivars of wheat were screened for tolerance to 3 days of high temperature (max. 40�C). Detailed results for grain yield and quality are presented for five wheat cultivars in order to illustrate the wide range of responses to short periods of high temperature. Individual kernel mass decreased by up to 23%, depending on variety, and the gliadin : glutenin ratio altered in the range -9 to +18% in response to high temperature treatment, also depending on variety. Noodle swelling power was significantly affected by heat in two cultivars, but there was no significant change due to heat in the apparent amylose content in any variety. The marked response of several yield and quality components to a heat treatment lasting only ca 5% of the grain-filling period suggests that starch and protein synthesis do not immediately andlor fully recover from short, severe heat stress. In addition, we conclude that wheat shows considerable genetic variability in tolerance to short periods of high temperature for both grain yield and quality.
Growth-Inhibiting Properties of Xylem Exudate From Vitis vinifera
Authors: Campbell JA , Loveys BR , Lee VWK , Strother S
Published: January 2006

An inhibitory effect on the growth of Lemna minor L. cultures has been demonstrated in xylem exudate from Vitis vinifera L. var. Waltham Cross bled from canes cut near the time of budburst. Most inhibitory activity was detected up to the time of maximal daily exudation, which corresponded closely with budburst. After this time the inhibitory activity rapidly disappeared. A similar pattern occurred in each of the 3 years of the study, 1988-1990. Using ultrafiltration, it was shown that most of the growth inhibitor activity of the crude exudate was located in the 0.5-10 kDa fraction. This fraction exhibited a seasonal variation in its bioactivity similar to that ofthe crude exudate samples. The 0.5-10 kDa fraction was found to contain abscisic acid but not in a sufficient quantity to account for the inhibitory effects. When chromatographically separated fractions corresponding to oligosaccharides were pooled, biological activity equivalent to that of the crude exudate was retained, which provides evidence that the inhibitor is possibly an oligosaccharide.
Yield Responses of Two Wheat Genotypes to Carbon Dioxide and Temperature in Field Studies Using Temperature Gradient Tunnels
Authors: Rawson HM
Published: January 2006

Clear, plastic-coated, temperature gradient tunnels (TGTs), 8 × 1.25 × 1-25 m were designed and built to examine how temperature and CO2 affect the yield of wheat in the field. Each of the three modules of each TGT was maintained at a different temperature above the ambient temperature using solar heating during the day and electric heating at night. The maximum day-time increment above ambient for the warmest module was 5�C and full-season averages were close to 2�C. TGTs were paired, with air in one being enriched to 700 μL L-1 CO2, and in the other being maintained at ambient CO2. Crops were planted in the TGTs at two sites in either summer (December) or winter (April and July) and they remained there until maturity. CO2 enrichment increased the yield in summer plantings by up to 36%. In winter plantings, with mean temperatures between sowing and anthesis of around lVC, the responses to CO2 were small averaging only 7% (range 1-12%). Though yield declined with increasing temperature in the TGTs in summer, there was a clear trend for an increasing response to CO2 at these higher temperatures, i.e. yield declined less. In summer, there was no convincing evidence for a different relative response to CO2 in two isolines which differed in maturity date, though the later line yielded more under the highest temperature regime (mean of 22-24�C between sowing and anthesis). In winter there was a strong trend for the isoline requiring less vernalisation to respond more to CO2. It is suggested that early progress towards flowering might predispose wheat to a greater CO2 response. Overall, the data indicated that the positive response to CO2 in grain yield is likely to increase at approximately 1.8% per 1°C in wheat crops that are not limited by water. Extrapolation indicated that the temperature at which there was no response to CO2 was 5�C. All yield responses reflected biomass responses as harvest index was unchanged by CO2
Sixty-Three Years Since Kautsky: Chlorophyll a Fluorescence
Authors: Govindje e
Published: January 2006

In 1931, using their eyes as instruments, H . Kautsky and A . Hirsch related the time course of chlorophyll a fluorescence with photosynthesis in a less-than-one-page article in Naturwissenschaften (see Kautsky's photograph). Chlorophyll a fluorescence is now being used by hundreds of investigators as a probe for various aspects of photosynthesis-from excitation energy transfer in picosecond time scale to CO2 fixation in minutes . It is not only a much used, but also a much abused, tool. It is used because of it being a non-invasive, rapid and a highly sensitive probe, and misused because it is sometimes not recognised that it is affected by various photosynthetic and other reactions. I submit that, like any other technique, if it is used with care and with due regard for its time dependence and competing parameters. it will remain as the one-most powerful tool for probing excitation energy transfer, primary photochemistry, electron flow on both the donor and the acceptor side of photosystem II (PSII) of oxygenic PSII. Further, it is very useful in the quick assay of PSII mutations, and down- regulation and other adjustments to stress (excess light, heat, heavy metal, nutrients and certain herbicides). In this paper, I will present my viewpoint, not a review, on the conceptual and experimental developments in this field. Whenever appropriate, and without any shame and humility, I will include some of my involvement in the excitement surrounding this field. I hope that this paper will serve as a starting point for further discussion of not only the history, but the practical use of chlorophyll a fluorescence as an intrinsic probe of stresses to plants, as well as individual reactions of oxygenic photosynthesis, when combined with other parallel measurements.
Reflections on the Nature and Function of the Photosystem II Reaction Centre
Authors: Seibert M
Published: January 2006

The reaction centre of photosystem II (PSII) converts light energy into chemical potential used by higher photosynthetic organisms to produce atmospheric oxygen. Water is the source of the oxygen and also the reductant required to fix atmospheric carbon dioxide. The purpose of this paper is to identify the physical location of the reaction centre complex in the thylakoid membrane, quantitate the core chlorophyll pigment stoichiometry in the isolated complex, discuss the primary charge-separation process, and examine current information on the nature of the primary electron donor of PSII.
Mechanism of Non-Photochemical Chlorophyll Fluorescence Quenching. I. The Role of De-Epoxidised Xanthophylls and Sequestered Thylakoid Membrane Protons as Probed by Dibucaine
Authors: Mohanty N , Yamamoto HY
Published: January 2006

Dibucaine reportedly inhibits the light-induced transthylakoid proton gradient of chloroplasts without inhibiting energy-dependent non-photochemical chlorophyll fluorescence quenching (Laasch, H. and Weis, E. (1989). Photosynthesis Research 22, 137-146). We show that dibucaine can inhibit fluorescence quenching, depending on the de-epoxidation state of the xanthophyll cycle. Whereas dibucaine (20-40 μM) had little effect on fluorescence quenching in pre-illuminated-type thylakoids (loaded with zeaxanthin and antheraxanthin), it strongly inhibited quenching in dark-adapted-type thylakoids (no preinduction of de-epoxidation). Dibucaine inhibited lumen acidification similarly in both types of thylakoids and also the induction of violaxanthin de-epoxidation in dark-adapted thylakoids. Thus dark-adapted and pre-illuminated thylakoids differed in de-epoxidation states and their suspectibility to dibucaine inhibition of fluorescence quenching corresponded to this difference. The mechanism of inhibition of de-epoxidation by dibucaine is unclear. It could be due to the inhibition of lumen acidification but an inhibition of the violaxanthin available for de-epoxidation is not excluded. High dibucaine concentrations inhibited de-epoxidase activity directly. Dibucaine inhibition of fluorescence quenching, however, is not limited to the inhibition of de-epoxidation. Small but clear effects on fluorescence quenching were present in thylakoids even with de-epoxidation preinduced. Moreover, thylakoids with preinduced de-epoxidation were more resistant to dibucaine inhibition of fluorescene quenching when poised by salt treatments for proton partitioning into membrane-sequestered domains than when poised for proton partitioning into delocalised domains. We conclude that non-photochemical quenching of chlorophyll fluorescence depends on both de-epoxidised xanthophylls and sequestered proton domains in the thylakoid membranes
'Photoinhibition' During Winter Stress: Involvement of Sustained Xanthophyll Cycle-Dependent Energy Dissipation
Authors: Adams WW Iii , Demmig-Adams B , Verhoeven AS , Barker DH
Published: January 2006

Sustained decreases in intrinsic photosystem II efficiency (i.e. Fv/Fm) in response to high light and chilling temperatures were examined in eight species, and were found to be accompanied by the retention of zeaxanthin (Z) and antheraxanthin (A) overnight. The quantitative relationship between changes in Fv/Fm and the A + Z level during these sustained changes on cold days was similar to that obtained for rapidly reversible changes on warm days. Furthermore, upon removal of leaves from the field, recovery from 'photoinhibition' (the reversal of the depression of Fv/Fm) matched the timecourse of the epoxidation of Z and A to violaxanthin (V). These findings suggest that the 'photoinhibition' occuring in these species might be due to the sustained engagement of these de-epoxidised components of the xanthophyll cycle in photoprotective energy dissipation. When examined over the course of several days during the winter, the predawn conversion state of the xanthophyll cycle responded to the daily changes in minimum air (and leaf) temperature, such that the xanthophyll cycle was largely de-epoxidised prior to sunrise on cold nights and was present predominantly as V after nights when the nocturnal temperatures were above freezing. In addition, in some of the species examined, there was a large acclimation of the xanthophyll cycle pool size to the level of excessive light, with a much larger pool present in the leaves examined during the winter and that pool being de-epoxidised to Z and A to a much greater degree at midday than from similar leaves examined during the summer. The xanthophyll cycle, and the photoprotective energy dissipation process associated with it, would thus appear to provide plants the flexibility required to deal with the excessive levels of light absorbed by chlorophyll under a wide range of climatic conditions, and can quite possibly account for the 'photoinhibition' observed during winter stress.
Applications of Chlorophyll Fluorescence to Forest Ecology
Authors: Ball MC , Butterworth JA , Roden JS , Christian R , Egerton JJG , Wydrzynski TJ , Chow WS , Badger MR
Published: January 2006

Functioning of photosystem II (PSII) is the most sensitive indicator of environmental stress in plants. Changes in PSII activity can be assayed rapidly and non-destructively by measurement of chlorophyll fluorescence. While there have been many laboratory studies of chlorophyll fluorescence, fluorescence techniques have seldom been applied to questions in forest ecology. Most studies have emphasised the fluorescence parameter, Fv/Fm, which is well correlated with the quantum efficiency of photosynthetic carbon dioxide assimilation or oxygen evolution. This parameter reveals information which can be related to diurnal and seasonal variation in photosynthesis, plant growth and community dynamics. Thus, fluorescence techniques provide a powerful means of linking photosynthesis with higher levels of plant functioning and have great potential for research in forest ecology.
Utility of Modulated Fluorescence in Measuring Photosynthetic Activity of Antarctic Plants: Field and Laboratory Studies
Authors: Hovenden MJ , Seppelt RD
Published: January 2006

Antarctic terrestrial plants face severe physiological stress conditions daily. In winter there is extreme cold and darkness. During summer there is continuous light for some months and plant surface temperatures may fluctuate through more than 50�C diurnally. Conditions are almost uniformly desiccative, except during spring when snow melt saturates thalli and plants may be completely inundated. The study of photosynthesis in these conditions is fundamental to a proper understanding of plant persistence, productivity and growth in polar regions. Lichens dominate the terrestrial vegetation of Antarctica and are therefore of great interest. Lichen photosynthesis has been shown to be dependent upon thallus water content and temperature as well as the irradiance incident on the thallus. Conventional infrared gas analysis (IRGA) techniques have been used most widely to study lichen photosynthesis in both the field and laboratory. Recent progress in the development of pulse amplitude modulated fluorescence systems has allowed the investigation of chlorophyll fluorescence in the field under naturally illuminated conditions. This paper highlights the utility of modulated fluorescence systems for the study of antarctic lichen photosynthesis, relates fluorescence results to those obtained by IRGA techniques, identifies problems inherent in fluorescence analysis of lichens and discusses possible areas of future research.
Catabolism of the Seed Storage Proteins From Lupinus Albus: Fate of Globulins During Germination and Seedling Growth.
Authors: Ferreira RB , Melo TS , Teixeira AN
Published: January 2006

The degradation of the major seed storage globulins of Lupinus albus L. was studied during the first 11 days after the onset of germination. The collagenolytic activity present in cotyledonary extracts was found to closely follow the rate and pattern of total globulin degradation. Little or no degradation of the three main globulins was observed during the first days of germination and seedling growth. The rate of proteolysis of the �-conglutin main subunit, composed of one heavy glycopolypeptide chain and one light non-glycosylated polypeptide chain, occurs gradually between days 3 and 11, with no apparent change in the globulin structure being detected during this time. Between days 3 and 5, �-conglutin, with at least 14 of its heavier polypeptide chains glycosylated, suffers a dramatic change in its structure and concentration, involving the appearance of a new set of polypeptides including a higher molecular mass group, whose concentration steadily declines until complete disappearance after 11 days, and a lighter molecular mass group, whose concentration surprisingly increases from 5 to 11 days. Finally, �-conglutin, with each of its four main subunits composed of a heavy glycopolypeptide chain and a 19 kDa non-glycosylated polypeptide chain, undergoes a pattern of degradation intermediate to those described for �- and �-conglutins in the sense that the new set of polypeptides coexist with polypeptides that were already present in the dry seed. The degradation of a-conglutin subunits starts at the level of their heavier polypeptide chains, releasing the intact 19 kDa polypeptide which gradually disappears after day 7. Our results indicate that �-, �- and �-conglutins are subjected to differential proteolysis during germination and seedling growth. Furthermore, the different polypeptide chains of each conglutin are also degraded at significantly different rates.
Phosphate Deficiency Increases the Rate Constant of Thermal Dissipation of Excitation Energy by Photosystem II in Intact Leaves of Sunflower and Maize.
Authors: Jacob J
Published: January 2006

Sunflower (Helianthus annuus L.) and maize (Zea mays L.) plants were grown in controlled environment chambers either with adequate supply or no external supply of inorganic phosphate. On the third fully-expanded leaves, chlorophyll fluorescence from photosystem II (PSII) was measured using a modulated fluorescence measuring system at various photon flux densities at room temperature. Phosphate deficiency resulted in an increase in the coefficient of non-photochemical quenching and a decrease in the coefficient of photochemical quenching of variable fluorescence. The efficiency of excitation energy capture by open PSII reaction centres and quantum yield of PSII photochemistry were decreased with phosphate deficiency. There was a significant effect of phosphate deficiency on in vivo PSII photochemistry which was independent of changes in thylakoid membrane energisation induced by the actinic light. An increase in the non-photochemical quenching of variable fluorescence with phosphate deficiency was due to an increased rate constant of thermal dissipation of excitation energy by PSII. Analyses of fluorescence signals suggest that phosphate deficiency decreased the rate constant of PSII photochemistry as well as the probability of excitation energy transfer from PSII antenna to PSII reaction centre. These effects were more apparent at low photon flux densities than at high photon flux densities. Regulation of energy transduction in the thylakoid and in vivo PSII activity in response to the physical environment of the plant are important aspects of environmental regulation of photosynthesis.
Relationship Between Climate and Provenance Variation in Eucalyptus Camaldulensis Dehnh.
Authors: Gibson A , Bachelard EP , Hubick KT
Published: January 2006

The morphology and physiology of Eucalyptus camaldulensis seedlings grown from seed collected from two locations in the dry tropics, two in the humid tropics and two in semi-arid Northern Australia were compared in phytotron growth cabinets under well-watered and water-limited conditions and diurnal temperature ranges of 30-25 and 22-15�C. Seedlings from the two locations in each climate resembled each other more closely than they resembled seedlings from the other climates across the range of conditions tested. When water-limited, seedlings from the dry tropical and semi-arid climates had a higher allocation of dry matter to roots than seedlings from the humid tropics. However, those from the dry tropics shed their lower leaves and initiated small-leaved axillary shoots while those from the semi-arid climate retained their leaves and did not produce axillary shoots. In contrast, seedlings from the humid tropics responded by reduced gas (CO2 and H2O) exchange without changes in morphology or allocation of dry matter. These responses are appropriate for growth in each climate and are consistent with observations made on wild trees in the field, on trees in plantations overseas and also with data from seedlings grown in earlier glasshouse experiments.
Photoinhibition of Photosynthesis Causes a Reduction in Vegetative Growth Rates of Dwarf Bean (Phaseolus vulgaris) Plants
Authors: Laing WA , Greer DH , Schnell TA
Published: January 2006

Effects of photoinhibition of photosynthesis were assessed on growth rates of dwarf bean (Phaseolus vulgaris L.). Plants were grown at four photon flux densities (PFDs) ranging from 300 to 1000 pmol m-2 s-1 and at 25/20�C and 17.5/17.5�C. Plants were also grown and transferred from 25 to 17.5�C either at 500 and 1300 μmol m-2 s-1 or from 500 to 1300 μmol m-2 s-1. Relative growth rates (RGRs) were determined by destructive harvests and photoinhibition was assessed by fluorescence and photon yields of O2 evolution. Carotenoid pigments were analysed by HPLC. The RGRs were PFD-dependent, increasing to a maximum at 430 μmol mm-2 s-1 at 17.5�C and at 650 μmol m-2 s-1 at 25�C. At 25�C, growth rates at 1300 μmol m-2 s-1 were inhibited about 25% over that at light saturation. Transferring the plants from 25 to 17.5�C at constant PFD reduced RGR by a similar amount. Concurrently increasing the PFD resulted in a further 9% reduction in RGR. These changes were consistent with photoinhibition-induced limitations on photosynthesis. Correlated with growth rate reductions were marked changes in both photon yield and FvFm, revealing a causal link between photoinhibition of photosynthesis and inhibition of growth rates. However, acclimation occurred via zeaxanthin-mediated photoprotection during exposure to the photoinhibitory conditions, ameliorating the longer term effects of photoinhibition on growth.
Photosynthetic Pathway-Related Ultrastructure of C3, C4 and C3-Like C3-C4 Intermediate Sedges (Cyperaceae), With Special Reference to Eleocharis
Authors: Bruhl JJ , Perry S
Published: January 2006

The ultrastructure of photosynthetic organs (leaf blades and culms) was investigated in eight species from four genera of sedges: Fimbristylis (C, fimbristyloid anatomy), Pycreus (C4 chlorocyperoid anatomy), Rhynchospora (C4 rhynchosporoid anatomy) - all NADP-ME (malic enzyme) type, and uninvestigated C3, C4 (eleocharoid anatomy, NAD-ME type) and C3-like C3-C4 intermediate species of Eleocharis. Ultrastructural characteristics previously reported for the former anatomical types are largely confirmed, though some evidence of poorly developed peripheral reticulum in C4 rhynchosporoid sedges is presented. Sedges, regardless of anatomical and biochemical type, possess a suberised lamella in photosynthetic organs which is invariably present in and confined to the mestome sheath cell walls, though it is often incomplete in the radial walls. By contrast with other C4 sedges, NAD-ME Eleocharis species and the C3-like C3-C4 intermediate E. pusilla possess abundant mitochondria and chloroplasts with well-stacked grana in the photosynthetic carbon reduction (PCR) (Kranz)/bundle sheath cells. Peripheral reticulum is well developed in NAD-ME species in both PCR and photosynthetic carbon assimilation (PCA) (C4 mesophyll) chloroplasts, but differs from that seen in chlorocyperoid and fimbristyloid type sedges. The suberised lamella and starch grains (well preserved), and granal stacks (poorly preserved) are identifiable in dried herbarium material (Eleocharis). Prediction of C4 biochemical type of sedges should be possible by combining anatomical, ultrastructural and δ13C value data. The significance of the ultrastructural similarities between the C4 NAD-ME and C3-C4 intermediate Eleocharis species is discussed.
Effect of Fusicoccin on Sucrose Exchanges Between a Host, Pelargonium zonale, and a Higher Parasitic Plant, Cuscuta reflexa
Authors: Bock FD , Fer A , Marigo G
Published: January 2006

The sucrose exchange characteristics at the level of contact between a host, Pelargonium zonale (L.) Aiton, and a parasite, Cuscuta reflexa Roxb., were investigated by studying the effects of fusicoccin (FC). With isolated haustoria of Cuscuta, FC stimulated the activity of the plasma membrane ATPase. This action was established by FC-increased proton extrusion. FC-increased O2 consumption and FC-increased 86Rb uptake. Nevertheless, FC strongly inhibited (by 30-50%) sucrose uptake and strongly enhanced (by 42-47%) sucrose release. These results, which are discussed in relation to the physiology of the association between host and parasite, outline the very special properties of the haustorium of Cuscuta.
Cellular Localisation of Polyphenol Oxidase and Peroxidase Activity in Litchi chinensis Sonn. Pericarp
Authors: Underhill SJR , Critchley C
Published: January 2006

Cellular localisation of visual browning and oxidative activity studies were conducted to determine the relative significance of polyphenol oxidase (PPO) and peroxidase (POD) activities during pericarp browning. Pericarp browning was first observed on the protuberance apices and subsequently extended uniformly over the entire pericarp surface. Anatomically, browning was highly localised and restricted to the epicarp and the upper mesocarp. PPO and POD activities were highest in the epicarp, with progressively less activity in both the mesocarp and endocarp. In situ localisation of oxidative activity using tissue blots confirmed high epicarp PPO activity. POD activity, although primarily restricted to mesocarp vascular tissue, was also detected in the epicarp. We believe that litchi pericarp browning is due to highly localised oxidative activity in the epicarp and upper mesocarp. As PPO and POD activities were significantly higher in this tissue and browning was not observed when both enzymes were selectively inhibited, it is postulated that both PPO and POD activities are associated with litchi pericarp browning. The current theory that litchi pericarp browning is only caused by PPO activity needs to be re-appraised to determine the relative role of POD activity.
Post-Sieve Element Transport of Sucrose in Developing Seeds
Authors: Patrick JW , Offler CE
Published: January 2006

Developing seeds of cereals and grain legumes have proven to be useful experimental models to examine post-sieve element assimilate transport in sink tissues. Morphologically, these seeds offer well-defined sinks in which the processes of sucrose import plus efflux and influx plus metabolism may be examined independently. In all cases, sucrose is delivered through the phloem to the maternal seed tissues. Unloading from the sieve element-companion cell complexes is symplastic. Subsequently, sucrose moves through a symplastic route to cells responsible for sucrose efflux to the seed apoplast. The efflux cells are located at, or near, the maternal/filial interface. Sucrose is retrieved from the seed apoplast by the outermost cell layers of the filial tissues. Subsequent transfer of sucrose to the sites of storage in the filial tissues is confined principally to a symplastic route. Sucrose efflux from the maternal tissues appears to be passive in cereals and energy dependent in grain legumes, possibly through a sucrose/proton antiport system. Sucrose influx across the plasma membranes of the filial cells is energy dependent and, for grain legumes, is energy coupled through a sucrose/proton symporter. Studies on the control of post-sieve element transport of sucrose have focused largely on the membrane transport steps. The role of phytohormones as modulators of sucrose transport is uncertain in grain legumes, efflux from the maternal cells could be regulated by rates of sucrose utilisation in the filial tissues through a turgor homeostat mechanism located in the efflux cells.
Thermal Characteristics of Soluble Starch Synthase From Wheat Endosperm
Authors: Jenner CF , Denyer K , Guerin J
Published: January 2006

The aim of the work reported in this paper was to characterise the thermal responses of soluble starch synthase (SSS) extracted from the endosperm of the developing wheat grain. Using partially purified preparations of SSS, the reaction obeyed Michaelis-Menten kinetics with both substrates amylopectin and ADPglucose. Both the Vmax and the Km varied with temperature. Values for Vmax were higher at 45�C compared with 25�C. However, the Km values for both substrates were also higher at 45�C than at 25�C indicating that the affinity of the enzyme for its substrates was reduced at the higher temperature. Over the temperature range 15-45�C, the Km for arnylopectin was minimal at 20�C, and rose exponentially between 25 and 45�C. Kinetic analyses indicated that the reaction was sequential and that the substrates could bind to the enzyme in either order. At 25�C the binding of one substrate to the enzyme increased the affinity of the complex for the second substrate but at 45�C these effects were abolished. These thermal characteristics of SSS could explain certain important features of the temperature responses of starch deposition in the wheat grain in vivo.
Partitioning of Carboxylase Activity in Nitrogen-Stressed Sugarcane and Its Relationship to Bundle Sheath Leakiness to CO2, Photosynthesis and Carbon Isotope Discrimination
Authors: Ranjith SA , Meinzer FC , Perry MH , Thom M
Published: January 2006

We studied the effects of external nitrogen (N) supply on ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activity, phosphoenolpyruvate carboxylase (PEPC) activity, leaf gas exchange, carbon isotope discrimination (Δ), and bundle sheath leakiness to CO2 (Φ) in two cultivars of the C4 grass, sugarcane (Saccharum spp. hybrid). In addition to reducing overall levels of carboxylase activity and therefore photosynthetic rates, reduced N supply altered the partitioning of carboxylase activity. Under long-term N stress (4 months) Rubisco activity decreased more than PEPC activity causing significant reductions in the Rubisco/PEPC activity ratio, a measure of the ratio of C3 to C4 cycle activity. Concurrent determinations of Δ for leaf dry matter and the prevailing ratio of intercellular to ambient partial pressure of CO2 (pI/pa) during leaf gas exchange suggested that the decreased partitioning of N to Rubisco activity under long-term N stress led to increased Φ and reduced photosynthetic efficiency. The two cultivars studied maintained similar PEPC activities but differed genetically in regard to investment of N in Rubisco. Greater investment of N in Rubisco was associated with higher rates of photosynthesis and growth at similar or slightly lower leaf N content, indicating that greater relative investment of N in Rubisco activity also led to higher N-use efficiency. The results suggest that regulation of the ratio of C3 to C4 pathway activity and its consequences for Φ may play a key role in the photosynthetic performance and growth of C4 grasses under both favourable and stressful conditions.
Effect of Timing of Heat Stress During Grain Filling on Two Wheat Varieties Differing in Heat Tolerance. I. Grain Growth
Authors: Stone PJ , Nicolas ME
Published: January 2006

Short periods of very high temperature (> 35�C) are common in many of the world's wheat growing areas and can be a significant factor in reducing yield and quality of wheat. This study was designed to determine the stage at which grain growth is most sensitive to a short period of high temperature and to examine whether varietal differences in heat tolerance are expressed throughout the whole grain-filling period. Two varieties of wheat differing in heat tolerance (cvv. Egret and Oxley) were exposed to a short (5 days) period of very high temperature (40�C max. for 6 h each day) at 5-day intervals throughout grain filling, starting from 15 days after anthesis (DAA) and concluding at 50 DAA. Responses of grain dry matter accumulation and water content to high temperature were monitored throughout grain filling, and the results compared with controls maintained at 21/16�C day/night. Varietal differences in heat tolerance were expressed throughout the grain-filling period. Mature individual kernel mass was most sensitive to heat stress applied early in grain filling and became progressively less sensitive throughout grain filling, for both varieties. Reductions in mature kernel mass resulted primarily from reductions in duration rather than rate of grain filling.
Changes in in vivo Protein Complements in Drying Leaves of the Desiccation-Tolerant Grass Sporobolus stapfianus and the Desiccation-Sensitive Grass Sporobolus pyramidalis
Authors: Kuang J , Gaff DF , Gianello RD , Blomstedt CK , Neale AD , Hamill JD
Published: January 2006

Protein synthesis in vivo was studied with two-dimensional SDS-PAGE of extracts of leaves on intact drying plants of Sporobolus stapfianus (a desiccation-tolerant grass) and S. pyramidalis (a desiccation-sensitive species). Protein complements were also studied in dried, detached leaves of S. stapfianus (detached leaves are also desiccation-sensitive). Extensive changes in in vivo proteins were observed in S. stapfianus plants drying intact: 25 novel proteins, 10 proteins augmented, 13 proteins decreased and 7 proteins disappeared. Two main phases could be distinguished in leaves as plants dry. In the first phase (85-51% relative water content (RWC) range), 10 novel proteins appeared and 2 proteins increased in content. In the second phase (37-3.5% RWC), 15 novel proteins appeared and 2 proteins increased in content. Some changes in protein patterns were also observed in desiccation- sensitive leaves, i.e. in (a) S. stapfianus leaves drying detached and in (b) drying S. pyramidalis leaves attached. These changes were fewer than those in desiccation-tolerant leaves, i.e. in (c) S. stapfianus leaves drying on intact plants; that is, (a) and (b) differed from each other and from (c), which indicates that there is no common injury-related pattern of protein change. These results are consistent with the view that changes in protein complements accompany the induction of desiccation tolerance in drying plants of S. stapfianus.
Nitrate Use and Xylem Exudation in Detopped Wheat Seedlings: an Early Diagnosis for Predicting Varietal Differences in Nitrogen Utilisation?
Authors: Barthes L , Deleens E , Bousser A , Hoarau J , Prioul JL
Published: January 2006

Differences in nitrate use for two wheat varieties (Soissons and Th�s�e) were found from studying nitrogen (N) composition in stumps collected at dawn in field experiments-soluble extracts showed that Soissons shoots received less amino acids (mainly glutamine) compared with nitrate, whereas Th�s�e shoots received more amino acids compared with nitrate. Amino acid and nitrate supply to Soissons shoots were maintained at a more constant level compared with Th�s�e. An interpretation of these varietal characteristics was attempted by examining properties of young seedlings grown in controlled conditions. Xylem sap rates in response to KCl or KNO3 were determined in detopped seedlings: Soissons exudation was always greater than that of Th�s�e. Compared with KCl, the addition of KNO3 in the root medium induced an enhancement of the flux of xylem exudate for both varieties but the enhancement due to KNO3 was lower for Soissons (× 1.6) than for Th�s�e (× 2.1). The increase of root nitrate reductase (NR) activity in response to external nitrate concentration or to incubation time was also markedly higher in Th�s�e and was associated with higher concentrations of amino acids and NH4+ and lower nitrate in the xylem sap than for Soissons. The faster water flux and the greater nitrate concentration in the xylem for Soissons could explain its higher shoot NR activity. Moreover, a 15NO3- tracing showed that Soissons accumulated significantly more new N in the shoot than Th�s�e. The characteristics of xylemic sap of young seedlings in response to nitrate could be used as criteria for describing varietal differences in N use.
Assimilate Partitioning Within Floret Components of Contrasting Rice Spikelets Producing Qualitatively Different Types of Grains
Authors: Patel R , Mohapatra PK
Published: January 2006

Grain quality, soluble and insoluble carbohydrate contents, and activities of two key enzymes (sucrose synthase and invertase) involved in starch synthesis in the endosperm were measured in the components of the fertile florets of spikelets located at the tip of the uppermost primary branch (top spikelet), and the penultimate position of the lowermost primary branch (basal spikelet) of rice panicles during the period begining some days before anthesis to maturity. The basal spikelet, which reached anthesis a week after the top spikelet, produced a partially filled poor quality grain in contrast to the good quality high density grain produced by the latter. Sucrose was the major translocatable sugar in the organs of the fertile florets of both top and basal spikelets, and poor grain filling of the latter was not caused by deficiency of this material due to any resistance in supply in the external protective organs. The activities of the sucrose synthase and invertase were higher and lower respectively, in the endosperm cells of the top spikelet compared with the basal spikelet. We conclude that poor synthesis of the starch leading to partial grain filling in the basal spikelet is due to a lower activity of sucrose synthase.
Modelling Canopy Production. III. Canopy Light-Utilisation Efficiency and Its Sensitivity to Physiological and Environmental Variables
Authors: Sands PJ
Published: January 2006

A previously published model which predicts daily canopy photosynthesis from standard daily meteorological data and parameters of the single-leaf photosynthetic light-response curve is extended to predict annual canopy photosynthesis. The model is based on biologically plausible assumptions about canopy structure and assumes the single-leaf light response is a non-rectangular hyperbola of arbitrary shape. It uses simple algorithms for taking diurnal variation of temperature, and seasonal temperature acclimation of photosynthesis, into account. The model provides theoretical support for the observation that production by a canopy is proportional to irradiance intercepted or absorbed by the canopy. The model is used to explore the sensitivity of annual photosynthesis to parameters of the single-leaf light response. It is shown how the sensitivity of annual canopy photosynthesis to various factors can be determined from the sensitivity of parameters of the light response to the same factors. In particular, annual photosynthesis is shown to be sensitive to the effects of nutrition and temperature on light-saturated photosynthetic rate, and to seasonal temperature acclimation of photosynthesis. It is important that the variation with temperature, nutrition and season of parameters of the single-leaf light-response function be determined.
Seasonal Variation in pH, Carbohydrate and Nitrogen of Xylem Exudation of Vitis vinifera
Authors: Campbell JA , Strother S
Published: January 2006

Over two seasons, 1989 and 1990, pH and carbohydrate concentration of the xylem exudate of Vitis vinifera L. var. Waltham Cross were examined around the time of budburst. During this period in 1990, weekly determinations of NH4+ and NO3- concentrations in xylem exudate were also performed. Over the 2 years, exudate pH varied inversely with daily exudate flow, falling from plateau levels of approximately pH 6.0 to lower than pH 5.5 around the date of greatest daily exudation, then rising again to a plateau of about pH 7.5. Exudate carbohydrate concentration variations were also consistent over the 2 years of the study, falling from plateau values of approximately 120 mg glucose equivalents L-1 (approximately 660 μM) at or immediately prior to the date of maximal exudation flow, to zero values within 4 weeks. These data, as well as reflecting a mobilisation of stored carbohydrates to apical tissues prior to budburst, also concur with previously observed activities of an apparently carbohydrate plant growth inhibitor in grapevine exudate. Exudate NH4+ and NO3- concentrations both increased directly with exudate flow up to the date of maximal daily exudation, then fell again to lower levels. Unlike seasonal pH variation, NH4+ and NO3- concentrations were not significantly correlated to daily exudate volume. The seasonal variations in pH infer a flow-dependent mobilisation from storage tissues, the reasons for which at budburst are discussed.
Genetic Variation in Carbon Isotope Discrimination and Its Relation to Stomatal Conductance in Pima Cotton (Gossypium barbadense)
Authors: Lu Zhenmin , Chen Jiwei , Percy RG , Sharifi MR , Rundel PW , Zeiger E
Published: January 2006

Stable carbon isotope discrimination (Δ) was evaluated in primitive and cultivated Gossypium barbadense L. Significant differences among cultivated Pima lines were positively associated with the degree of selection for lint yield and heat resistance. A population mean study of a cross between B368, a primitive, uncultivated G. barbadense, and Pima S-6, an advanced line, showed that Δ is probably under genetic control, and could be a suitable selection trait in breeding programs. Eleven uncultivated accessions of primitive G. barbadense of varying origins grown in one environment showed a broad range of Δ values (18.8-20.50), pointing to substantial genetic variation of Δ in the G. barbadense germplasm. A was strongly correlated with stomatal conductance (gs) in the commercial lines, a segregating F2 population of the B368 × Pima S-6 cross, and the collection of uncultivated G. barbadense. This relationship indicates that variation in gs is the main source of variation for Δ in both uncultivated and commercial G. barbadense. The positive correlation between Δ, gs and yield in the commercial Pima lines provides further evidence for selection pressures on higher gs ensuing from selection for higher yield and heat resistance. Selection for higher Δ could increase yield in crops grown in hot, high irradiance, and well-irrigated environments.
Limitation of the Rate of Ribulosebisphosphate Carboxylase Activation by Carbamylation and Ribulosebisphosphate Carboxylase Activase Activity: Development and Tests of a Mechanistic Model
Authors: Woodrow IE , Kelly ME , Mott KA
Published: January 2006

A mechanistically-based model of light-mediated activation of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) is developed. The model describes the kinetics of Rubisco activation following a relatively rapid increase in photon flux density (PPFD) from an initially low level. Underlying the model is the assumption that there are two slow processes that could potentially limit the rate of light-mediated Rubisco activation. These processes are the addition of the activator CO2 to the large subunit of Rubisco, which is accompanied by a conformational change in the enzyme (carbamylation), and activase-mediated removal of ribulose 1,5-bisphosphate from the inactive form of the enzyme. The contribution of these slow processes to the overall activation kinetics of Rubisco was resolved by measuring Rubisco activation in whole spinach leaves using non-steady-state CO2 exchange. It was found that when the change in PPFD was relatively small and a correspondingly small proportion of the Rubisco pool was activated, the kinetics of activation were highly sensitive to the intercellular CO2 concentration (ci). The apparent rate constant for activation under these conditions was found to be similar to that for the carbamylation of purified spinach Rubisco. When the change in PPFD and the proportion of Rubisco activated was relatively large, however, the kinetics of Rubisco activation were almost completely CO2 insensitive and were consistent with those of an enzyme-catalysed reaction. It is suggested that (1) CO2-insensitive activation reflects the operation of Rubisco activase and (2) the increasing CO2 sensitivity seen as the change in PPFD decreases reflects a transition to limitation by carbamylation.