bluuebunny/crossref_metadata_2025 huggingface dataset:
Regulation of tomato fruit growth by epidermal cell wall enzymes
Authors: Thompson D. S. , Davies W. J. , Ho L. C.
Published: March 2003

Water relations of tomato fruit and the epidermal and pericarp activities of the putative cell wall loosening and tightening enzymes Xyloglucan endotransglycosylase (XET) and peroxidase were investigated, to determine whether tomato fruit growth is principally regulated in the epidermis or pericarp. Analysis of the fruit water relations and observation of the pattern of expansion of tomato fruit slices in vitro, has shown that the pericarp exerts tissue pressure on the epidermis in tomato fruit, suggesting that the rate of growth of tomato fruit is determined by the physical properties of the epidermal cell walls. The epidermal activities of XET and peroxidase were assayed throughout fruit development. Temporal changes in these enzyme activities were found to correspond well with putative cell wall loosening and stiffening during fruit development. XET activity was found to be proportional to the relative expansion rate of the fruit until growth ceased, and a peroxidase activity weakly bound to the epidermal cell wall appeared shortly before cessation of fruit expansion. No equivalent peroxidase activity was detected in pericarp tissue of any age. It is therefore plausible that the expansion of tomato fruit is regulated by the combined action of these enzyme activities in the fruit epidermis.
Translocation of amino acids from stem nodules of Sesbania rostrata demonstrated by GC‐MS in planta 15N isotope dilution
Authors: Godber I. M. , Parsons R.
Published: March 2003

We report a novel use of the 15N dilution technique to detail the translocation of amino compounds in the legume Sesbania rostrata. The conventional 15N dilution technique follows the dilution of 15N within a labelled plant, as 14N2 is fixed by symbiotic bacteria. In our experiments, stem‐nodulated Sesbania rostrata were enriched by feeding with 15N ammonium nitrate for 2 weeks, followed by a 1 week period where the only N available to the plants was via nitrogen fixation of atmospheric N2. We measured the composition, concentration and 15N enrichment of amino compounds in various plant tissues, both above and below the stem nodules, using GC‐MS and isotopic abundance mass spectrometry techniques. Approximately 28% of the total N in the stem nodules was derived from internal plant sources. The ureides allantoic acid and allantoin were not abundant in xylem, leaf or nodule tissues. The amides asparagine and glutamine were the major export products from stem nodules although a wide range of other amino compounds are also synthesized. Amino acids within the nodules had a low level of enrichment, demonstrating that a small fraction (≈ 11%) was derived from outside the nodules, and significant cycling of N (28% of xylem N) through the root system was revealed by measurements of 15N distribution and amino acid concentrations.
Modelling temperature effects on growth–respiration relations of maize
Authors: Taylor D. K. , Rank D. R. , Keiser D. R. , Smith B. N. , Criddle R. S. , Hansen L. D.
Published: March 2003

The temperature dependence of plant growth rate is related to the temperature dependence of respiratory metabolism. To determine how the effects of temperature on respiration rate and efficiency are transmitted to growth, this study measured the dark metabolic heat rate (q) and CO2 production rate (RCO2) in excised shoots of seedlings of 14 maize cultivars (Zea mays L.) at several temperatures. The temperature coefficients of q and RCO2 differ within a given cultivar and also differ among the cultivars. Both q and RCO2 exhibit an isokinetic temperature of 20 ± 3 °C. The measured temperature dependences of q and RCO2 were used to model the temperature dependences of both growth and substrate carbon conversion efficiency. This procedure may be useful in determining the suitability of cultivars for growth in a given climate and in understanding metabolic adaptation to climate.
Australian mulga ecosystems –13C and 15N natural abundances of biota components and their ecophysiological significance
Authors: Pate J. S. , Unkovich M. J. , Erskine P. D. , Stewart G. R.
Published: March 2003

Samples of recently produced shoot material collected in winter/spring from common plant species of mulga vegetation in eastern and Western Australia were assayed for 13C and 15N natural abundance. 13C analyses showed only three of the 88 test species to exhibit C4 metabolism and only one of seven succulent species to be in CAM mode. Non‐succulent winter ephemeral C3 species showed significantly lower mean δ13C values (– 28·0‰) than corresponding C3‐type herbaceous perennials, woody shrubs or trees (– 26·9, – 25·7 and – 26·2‰, respectively), suggesting lower water stress and poorer water use efficiency in carbon acquisition by the former than latter groups of taxa. Corresponding values for δ15N of the above growth and life forms lay within the range 7·5–15·5‰. δ15N of soil NH4+ (mean 19·6‰) at a soft mulga site in Western Australia was considerably higher than that of NO3– (4·3‰). Shoot dry matter of Acacia spp. exhibited mean δ15N values (9·10 ± 0·6‰) identical to those of 37 companion non‐N2‐fixing woody shrubs and trees (9·06 ± 0·5‰). These data, with no evidence of nodulation, suggested little or no input of fixed N2 by the legumes in question. However, two acacias and two papilionoid legumes from a dune of wind‐blown, heavily leached sand bordering a lake in mulga in Western Australia recorded δ15N values in the range 2·0–3·0‰ versus 6·4–10·7‰ for associated non‐N2‐fixing taxa. These differences in δ15N, and prolific nodulation of the legumes, indicated symbiotic inputs of fixed N in this unusual situation. δ15N signals of lichens, termites, ants and grasshoppers from mulga of Western Australia provided evidence of N2 fixation in certain termite colonies and by a cyanobacteria‐containing species of lichen. Data are discussed in relation to earlier evidence of nitrophily and water availability constraints on nitrate utilization by mulga vegetation.
Incorporation of [35S]methionine in higher plants reveals that stimulation of the D1 reaction centre II protein turnover accompanies tolerance to heavy metal stress
Authors: Geiken B. , MasojÍdek J. , Rizzuto M. , Pompili M. L. , Giardi M. T.
Published: March 2003

The effects of cadmium stress (CdCl2) on photochemical activity and protein behaviour of photosystem II (PSII) were studied in vivo and in vitro. Treatments of pea (Pisum sativum) and broad bean (Vicia faba) plants with 0·05–5 mM cadmium (CdCl2) modified PSII activity with a resulting increase in electron transfer followed by an inhibition and damage to the oxygen‐evolving complex. Pulse‐chase experiments with [35S]methionine in vivo followed by the separation of the radiolabelled thylakoids into grana and stroma exposed regions indicated that the synthesis, degradation and assembly of the D1 protein were greatly affected by cadmium. Initially D1 synthesis increased, later slowing down when the stress became advanced; at the same time the D1 degradation was increased. Binding studies with radiolabelled [14C]herbicide revealed that the QB pocket activity was also altered. However, the primary consequence of cadmium stress was the disassembly of the stacked regions. The measurements indicated differential tolerance to cadmium stress between the two plant species, which was not caused by either differential metal uptake or binding to the PSII complex. This suggests that the resulting changes in D1 turnover are a consequence of an unknown primary effect of cadmium on the PSII apparatus. However, we show that the higher tolerance to heavy metal stress found in broad bean plants relative to pea is accompanied by stimulation of D1 turnover. These experiments supported by previous data suggest that modulation of D1 turnover under stress is a commonly occurring process.
Photosynthetic acclimation to long‐term exposure to elevated CO2 concentration in Pinus radiata D. Don. is related to age of needles
Authors: Turnbull M. H. , Tissue D. T. , Griffin K. L. , Rogers G. N. D. , Whitehead D.
Published: March 2003

The effects of CO2 enrichment on photosynthesis and ribulose‐1,5‐bisphosphate carboxylase/ oxygenase (Rubisco) in current year and 1‐year‐old needles on the same branch were studied on Pinus radiata D. Don. trees growing for 4 years in large, open‐top chambers at ambient (36 Pa) and elevated (65 Pa) CO2 partial pressures. At this age trees were 3·5–4 m tall. Measurements made late in the growing cycle (March) showed that photosynthetic rates at the growth CO2 concentration [(pCO2)a] were lower in 1‐year‐old needles of trees grown at elevated CO2 concentrations than in those of trees grown at ambient (pCO2)a. At elevated CO2 concentrations Vcmax (maximum carboxylation rate) was reduced by 13% and Jmax (RuBP regeneration capacity mediated by maximum electron transport rate) by 17%. This corresponded with photosynthetic rates at the growth (pCO2)a of 4·68 ± 0·41 μmol m–2 s–1 and 6·15 ± 0·46 μmol m–2 s–1 at 36 and 65 Pa, respectively (an enhancement of 31%). In current year needles photosynthetic rates at the growth (pCO2)a were 6·2 ± 0·72 μmol m–2 s–1 at 36 Pa and 10·15 ± 0·64 μmol m–2 s–1 at 65 Pa (an enhancement of 63%). The smaller enhancement of photosynthesis in 1‐year‐old needles at 65 Pa was accompanied by a reduction in Rubisco activity (39%) and content (40%) compared with that at 36 Pa. Starch and sugar concentrations in 1‐year‐old needles were not significantly different in the CO2 treatments. There was no evidence in biochemical parameters for down‐regulation at elevated (pCO2)a in fully fexpanded needles of the current year cohort. These data show that enhancement of photosynthesis continues to occur in needles after 4 years’ exposure to elevated CO2 concentrations. Photosynthetic acclimation reduces the degree of this enhancement, but only in needles after 1 year of growth. Thus, responses to elevated CO2 concentration change during the lifetime of needles, and acclimation may not be apparent in current year needles. This transitory effect is most probably attributable to the effects of developmental stage and proximity to actively growing shoots on sink strength for carbohydrates. The implications of such age‐dependent responses are that older trees, in which the contribution of older needles to the photosynthetic biomass is greater than in younger trees, may become progressively more acclimated to elevated CO2 concentration.
Seasonal accumulation of ultraviolet‐B screening pigments in needles of Norway spruce (Picea abies (L.) Karst.)
Authors: FISCHBACH R. J. , KOSSMANN B. , PANTEN H. , STEINBRECHER R. , HELLER W. , SEIDLITZ H. K. , SANDERMANN H. , HERTKORN N. , SCHNITZLER J.‐P.
Published: March 2003

Conifer needles are highly effective in screening ultraviolet‐B radiation (280–320 nm). This ability is mainly attributed to the presence of flavonoids and hydroxycinnamic acids in the epidermal tissue. In two field cabinet experiments with two different clones of Norway spruce we assessed the seasonal accumulation of UV‐B screening pigments under near‐ambient, and close‐to‐zero UV‐B irradiation. At the beginning of needle development, i.e. in June, kaempferol 3‐O‐glucoside was the dominant UV‐B screening pigment. It was replaced during needle differentiation by the more effective diacylated flavonol glucosides, particulary kaempferol 3‐O‐(3",6"‐O‐di‐p‐coumaroyl)‐glucoside, which reached highest concentrations in July. In addition to the soluble pool of diacylated flavonol glucoside derivatives, a cell wall‐bound UV‐B screen in the epidermal cell walls was formed during needle differentiation, consisting mainly of p‐coumaric acid and kaempferol 3‐O‐glucoside. An effect of UV‐B radiation on the accumulation of diacylated flavonol glucosides was only observed in 1996 with clone 2, when the concentrations of kaempferol 3‐O‐(3",6"‐O‐di‐p‐coumaroyl)‐glucoside were significantly higher in July and August under field, and near‐ambient than under close‐to‐zero UV‐B irradiance. For wall‐bound p‐coumaric acid and kaempferol 3‐O‐glucoside UV‐B radiation enhanced the concentrations of these compounds by approximately 20% in relation to the concentrations in close‐to‐zero UV‐B‐treated plants in both field cabinet experiments.
Structural and functional features of the leaves of Ranunculus trichophyllus Chaix., a freshwater submerged macrophophyte
Authors: RASCIO N. , CUCCATO F. , DALLA VECCHIA F. , LA ROCCA N. , LARCHER W.
Published: March 2003

AZA, 5‐Acetamido‐1,3,4‐thiadiazole‐2‐sulphonamide CA, carbonic anhydrase DIC, dissolved inorganic carbon Hepes, A‐(2‐hydroxyethyl)‐1 piperazine‐ethane sulfonic acid IC, inorganic carbon PAR, photosynthetic active radiation PATAg, periodic acid‐thiosemicarbazide‐silver proteinate Tris, tris (hydroxymethyl)‐aminomethane  The structural and physiological strategies developed by the leaves of the freshwater macrophyte Ranunculus trichophyllus to adapt to submersed life were studied. Photosynthesis is carried out mainly by the epidermis cells of the numerous segments into which the leaf is finely dissected. In these cells, containing most of the chloroplasts, a peculiar organization of the wall has been identified by cytochemical tests. A thin compact outer region covers the cell surface and splits up forming large lacunae between adjacent cells. Below it, a thick and loose inner region rich in hydrophilic pectic acids occurs, which grows in along the cell sides giving rise to wide transfer areas. In this latter cell wall region, in which the cell/environment contact and exchanges are amplified, the systems for inorganic carbon supply to photosynthetic cells operate. The leaves of R. trichophyllus can rely on environmental CO2 and HCO3– as sources of inorganic carbon for photosynthesis. A mechanism for bicarbonate utilization seems to involve its conversion to CO2 by an apoplastic carbonic anhydrase, whose activity gains importance as the availability of environmental CO2 decreases. Interestingly, it has been demonstrated that in this species CO2 can also be obtained from HCO3– by a photodependent increase in plasmamembrane H+‐ATPase activity in the transfer areas of the epidermis cells. This is the first time that such a mechanism has been noted in a nonpolar leaf of a submerged macrophyte.
Quantification of the rate of CO2 formation in the periplasmic space of microalgae during photosynthesis. A comparison of whole‐cell rate constants for CO2 and HCO3– uptake among three species of the green alga Chlorella
Authors: MATSUDA Y. , WILLIAMS T. G. , COLMAN B.
Published: March 2003

As previously described, the absolute rate of photosynthesis due to a limited concentration of dissolved inorganic carbon at alkaline pH, where the rate of CO2 formation is strictly limited, plotted as a function of chlorophyll (Chl) concentration, will take the form of a rectangular hyperbola combined with a linear rate directly proportional to [Chl], which are, respectively, due to the contribution of CO2 and HCO3– to photosynthesis. This model represents that the mathematical asymptote of absolute rate of photosynthesis versus cell density is described by the whole‐cell rate constant for HCO3– uptake and the maximum rate of CO2 formation in the extracellular space. This means that any trace modification of the CO2 formation rate outside the cell will alter the photosynthetic rate and should be detectable experimentally. In air‐grown Chlorella ellipsoidea and C. kessleri and in high CO2‐grown C. saccharophila, the graph of the absolute rate of photosynthesis against [Chl] clearly followed the mathematical model described above and the actual CO2 formation rates outside the cells were not significantly different from the calculated rates. It also indicated that the whole‐cell rate constants for CO2 and HCO3– uptake in air‐grown C. ellipsoidea and C. saccharophila were similar at ≈ 300 and 2·0 mm3μg–1 Chl min–1, respectively, whereas those in air‐grown C. kessleri were ≈ 550 and 15 mm3μg–1 Chl min–1. These results indicate that no acidification of the periplasmic space occurs, and there is no trace activity of external carbonic anhydrase in these microalgae.
Acid phosphatase activity in phosphorus‐deficient white lupin roots
Authors: Gilbert G. A. , Knight J. D. , Vance C. P. , Allan D. L.
Published: March 2003

White lupin (Lupinus albus L.) develops proteoid roots when grown in phosphorus (P)‐deficient conditions. These short, lateral, densely clustered roots are adapted to increase P availability. Previous studies from our laboratory have shown proteoid roots have higher rates of non‐photosynthetic carbon fixation than normal roots and altered metabolism to support organic acid exudation, which serves to solubilize P in the rhizosphere. The present work indicates that proteoid roots possess additional adaptations for increasing P availability and possibly for conserving P in the plant. Roots from P‐deficient (–P) plants had significantly greater acid phosphatase activity in both root extracts and root exudates than comparable samples from P‐sufficient (+P) plants beginning 10 d after emergence. The increase in activity in –P plants was most pronounced in the proteoid regions. In contrast, no induction of phytase activity was found in –P plants compared to +P plants. The number of proteoid roots present was not affected by the source of phosphorus supplied, whether organic or inorganic forms. Adding molybdate to the roots increased the number of proteoid roots in plants supplied with organic P, but not inorganic P. Increased acid phosphatase activity was detected in root exudates in the presence of organic P sources. Native‐polyacrylamide gel electrophoresis demonstrated that under P‐deficient conditions, a unique isoform of acid phosphatase was induced between 10 and 12 d after emergence. This isoform was found not only within the root, but it comprised the major form exuded from proteoid roots of –P plants. The fact that exudation of proteoid‐root‐specific acid phosphatase coincides with proteoid root development and increased exudation of organic acids indicates that white lupin has several coordinated adaptive strategies to P‐deficient conditions.
Why plants bother: root proliferation results in increased nitrogen capture from an organic patch when two grasses compete
Authors: Hodge A. , Robinson D. , Griffiths B. S. , Fitter A. H.
Published: March 2003

We investigated whether the capacities of Lolium perenne L. and Poa pratensis L. roots to proliferate locally and to alter local nitrogen (N) inflows in a decomposing organic matter patch were important in their capture of N when grown together. In the presence of a patch, plants of both species were significantly heavier and contained more N. Root length and weight densities increased in the patch, but specific root length was unaltered. Although both species proliferated roots in the patch, L. perenne produced greater root length densities than P. pratensis, and also captured more N from the patch. Indeed, total N uptake from the patch was related to root length density within the patch. N inflows (rate of N uptake per unit root length) in the patch were no faster than in the whole root system for both species. Under the conditions of this study, root proliferation in an organic patch was more important for N capture from the patch than alterations in N inflows. Local proliferation of roots may be a key factor in interspecific competition for non‐uniformly distributed supplies of N in natural habitats, so resolving the previous uncertainty as to the ‘adaptive’ nature of root proliferation.
Partitioning of reserve and newly assimilated carbon in roots and leaf tissues of Lolium perenne during regrowth after defoliation: assessment by 13C steady‐state labelling and carbohydrate analysis
Authors: Morvan‐Bertrand A. , Pavis N. , Boucaud J. , Prud’Homme M.‐P.
Published: March 2003

The relative significance of the use of stored or currently absorbed C for the growth of leaves or roots of Lolium perenne L. after defoliation was assessed by steady‐state labelling of atmospheric CO2. Leaf growth for the first two days after defoliation was to a large extent dependent on the use of C reserves. The basal part of the elongating leaves was mainly new tissue and 91% of the C in this part of the leaf was derived from reserves assimilated prior to defoliation. However, half of the sucrose in the growth zone was produced from photosynthesis by the emerged leaves. Fructans that were initially present in elongating leaf bases were hydrolysed (loss of 93 to 100%) and the resulting fructose was found in the new leaf bases, suggesting that this pool may be used to support cell division and elongation. Despite a negative C balance at the whole‐plant level, fructans were synthesized from sucrose that was translocated to the new leaf bases. After a regrowth period of 28 d, 45% of the C fixed before defoliation was still present in the root and leaf tissue and only 1% was incorporated in entirely new tissue.
Acclimation of snow gum (Eucalyptus pauciflora) leaf respiration to seasonal and diurnal variations in temperature: the importance of changes in the capacity and temperature sensitivity of respiration
Authors: Atkin O. K. , Holly C. , Ball M. C.
Published: March 2003

We investigated the relationship between daily and seasonal temperature variation and dark respiratory CO2 release by leaves of snow gum (Eucalyptus pauciflora Sieb. ex Spreng) that were grown in their natural habitat or under controlled‐environment conditions. The open grassland field site in SE Australia was characterized by large seasonal and diurnal changes in air temperature. On each measurement day, leaf respiration rates in darkness were measured in situ at 2–3 h intervals over a 24 h period, with measurements being conducted at the ambient leaf temperature. The rate of respiration at a set measuring temperature (i.e. apparent ‘respiratory capacity’) was greater in seedlings grown under low average daily temperatures (i.e. acclimation occurred), both in the field and under controlled‐environment conditions. The sensitivity of leaf respiration to diurnal changes in temperature (i.e. the Q10 of leaf respiration) exhibited little seasonal variation over much of the year. However, Q10 values were significantly greater on cold winter days (i.e. when daily average and minimum air temperatures were below 6° and –1 °C, respectively). These differences in Q10 values were not due to bias arizing from the contrasting daily temperature amplitudes in winter and summer, as the Q10 of leaf respiration was constant over a wide temperature range in short‐term experiments. Due to the higher Q10 values in winter, there was less difference between winter and summer leaf respiration rates measured at 5 °C than at 25 °C. The net result of these changes was that there was relatively little difference in total daily leaf respiratory CO2 release per unit leaf dry mass in winter and summer. Under controlled‐environment conditions, acclimation of respiration to growth temperature occurred in as little as 1–3 d. Acclimation was associated with a change in the concentration of soluble sugars under controlled conditions, but not in the field. Our data suggest that acclimation in the field may be associated with the onset of cold‐induced photo‐inhibition. We conclude that cold‐acclimation of dark respiration in snow gum leaves is characterized by changes in both the temperature sensitivity and apparent ‘capacity’ of the respiratory apparatus, and that such changes will have an important impact on the carbon economy of snow gum plants.
Carbohydrate metabolism in a heat‐girdled maize source leaf
Authors: Jeannette E. , Reyss A. , Grégory N. , Gantet P. , Prioul J.‐L.
Published: March 2003

Heat‐girdling in maize leaves was used for blocking assimilate export and producing alterations in carbohydrate metabolism. A rapid drop (< 24 h) in CO2 fixation and a large increase of sucrose and starch content were observed. Reduction in photosynthetic assimilation was not due to stomatal limitation since the girdled leaves remained turgid and their photosynthetic rate was not restored by 4% CO2. After 24 h, the girdled leaves became red through accumulation of anthocyanins and reduction of chlorophylls. A possible relationship between carbohydrate accumulation in girdled leaves and gene expression was examined through transcript levels of the ribulose‐1,5‐bisphosphate carboxylase oxygenase small‐subunit (rbcS), the leaf‐specific ADPglucose pyrophosphorylase small‐subunit (Agp‐l2), and Bz1, an anthocyanin pathway‐specific gene. The mRNA levels in girdled leaves were also compared with those in excised leaves placed on sucrose or sorbitol solutions. rbcS mRNA disappeared 8 h after the girdling treatment and after 1 day on sucrose. Agp‐l2 expression was reduced by two‐thirds with girdling or sucrose feeding while Bz1 expression was strikingly enhanced by both treatments. Thus, carbohydrate accumulation produced a short‐term feedback inhibition of photosynthesis and some long‐term modifications in gene expression.
Peroxidase isozyme patterns in the skin of maturing tomato fruit
Authors: Andrews J. , Malone M. , Thompson D. S. , Ho L. C. , Burton K. S.
Published: March 2003

The cessation of tomato fruit growth is thought to be induced by an increase in the activity of enzymes which rigidify cell walls in the fruit skin. Peroxidase could catalyse such wall‐stiffening reactions, and marked rises in peroxidase activity were recently reported in skin cell walls towards fruit maturity. Peroxidase isoforms in the fruit are here analysed using native gel electrophoresis. New isoforms of apparent Mr 44, 48 and 53 kDa are shown to appear in cell walls of the fruit skin at around the time of cessation of growth. It is inferred that these isozymes are present in the cell wall in vivo. Fruit from a range of non‐ripening mutants were also examined. Some of these do not soften or ripen for many weeks after achieving their final size. The new isozymes were found in skin cell walls of mature fruit in each of these mutants, as in the wild‐type and commercial varieties. It is concluded that the late‐appearing isozymes are not associated with fruit ripening or softening, and are probably not ethylene‐induced. They may act to control fruit growth by cross‐linking wall polymers within the fruit skin, thus mechanically stiffening the walls and terminating growth.
Turgor regulation in the salt‐tolerant alga Chara longifolia
Authors: Stento N. A. , Ryba N. Gerber , Kiegle E. A. , Bisson M. A.
Published: March 2003

Chara longifolia is a salt‐tolerant Charophyte which regulates its turgor inresponse to osmotic stress. Membrane depolarization, in creased membrane conductance, and cessation of cytoplasmic streaming (due to increase in cytoplasmic Ca2+) precede regulation in response to hypotonic stress. Measurements of these three parameters are presented here with simultaneous turgor measurements. Variability in the occurrence, rate and extent of turgor regulation in individual cells was correlated with magnitude of the stress. Hypertonic stress showed the same slow time course as was found previously, requiring several days for complete regulation. Fifty μM nifedipine, a Ca2+ channel blocker, inhibited turgor regulation. In the presence of 5 μM nifedipine, turgor regulation was delayed. An increase in conductance preceded regulation, but membrane depolarization was less and no detectable change in cytoplasmic streaming was observed, requiring modifications to a previously presented model for turgor regulation. There was no significant difference in 45Ca2+ influx under control and stress conditions. However, the control flux was insensitive to nifedipine, whereas under stress the flux is inhibited 54% by nifedipine. We suggest that osmotic stress results in a rapid increase in a nifedipine‐sensitive Ca2+ entry mechanism, followed very quickly by a decrease in the control entry mechanism.
Ion channels and the transduction of light signals
Authors: Spalding E. P.
Published: March 2003

Studies of biological light‐sensing mechanisms are revealing important roles for ion channels. Photosensory transduction in plants is no exception. In this article, the evidence that ion channels perform such signal‐transducing functions in the complex array of mechanisms that bring about plant photomorphogenesis will be reviewed and discussed. The examples selected for discussion range from light‐gradient detection in unicellular algae to the photocontrol of stem growth in Arabidopsis. Also included is some discussion of the technical aspects of studies that combine electrophysiology and photobiology.
Turgor pressure, membrane tension and the control of exocytosis in higher plants
Authors: Fricke Wieland , Jarvis Michael C. , Brett Christopher T.
Published: March 2003

Both turgor pressure and differences in membrane tension are capable of providing an energy input into exocytosis, the process of fusion of Golgi vesicles with the cell membrane in plants. It is shown that the contribution of turgor pressure is much larger than that of membrane tension, so that the exocytotic process is not likely on thermodynamic grounds to be reversible unless another source of energy is made available. However, recycling of membrane material as flattened, empty vesicles is energetically possible and is likely to be favoured when the magnitude of membrane tension in the cell membrane is low. Thus the outward flows of membrane and cell wall material are in principle linked to turgor, whereas membrane tension influences the inward flow of membrane material.
The onset of photosynthetic acclimation to elevated CO2 partial pressure in field‐grown Pinus radiata D. Don. after 4 years
Authors: Griffin K. L. , Tissue D. T. , Turnbull M. H. , Whitehead D.
Published: March 2003

The effects of CO2 enrichment on photosynthesis and ribulose‐1,5‐bisphosphate carboxylase/oxygenase (rubisco) were studied in current year and 1‐year‐old needles of the same branch of field‐grown Pinus radiata D. Don trees. All measurements were made in the fourth year of growth in large, open‐top chambers continuously maintained at ambient (36 Pa) or elevated (65 Pa) CO2 partial pressures. Photosynthetic rates of the 1‐year‐old needles made at the growth CO2 partial pressure averaged 10·5 ± 0·5 μmol m−2 s−1 in the 36 Pa grown trees and 11·8 ± 0·4 μmol m−2 s−1 in the 65 Pa grown trees, and were not significantly different from each other. The photosynthetic capacity of 1‐year‐old needles was reduced by 25% from 23·0 ± 1·8 μmol m−2 s−1 in the 36 Pa CO2 grown trees to 17·3 ± 0·7 μmol m−2 s−1 in the 65 Pa grown trees. Growth in elevated CO2 also resulted in a 25% reduction in Vcmax (maximum carboxylation rate), a 23% reduction in Jmax (RuBP regeneration capacity mediated by maximum electron transport rate) and a 30% reduction in Rubisco activity and content. Total non‐structural carbohydrates (TNC) as a fraction of total dry mass increased from 12·8 ± 0·4% in 1‐year‐old needles from the 36 Pa grown trees to 14·2 ± 0·7% in 1‐year‐old needles from the 65 Pa grown trees and leaf nitrogen content decreased from 1·30 ± 0·02 to 1·09 ± 0·10 g m−2. The current‐year needles were not of sufficient size for gas exchange measurements, but none of the biochemical parameters measured (Rubisco, leaf chlorophyll, TNC and N), were effected by growth in elevated CO2. These results demonstrate that photosynthetic acclimation, which was not found in the first 2 years of this experiment, can develop over time in field‐grown trees and may be regulated by source‐sink balance, sugar feedback mechanisms and nitrogen allocation.
Ecophysiological consequences of differences in plant size: in situ carbon gain and water relations of the epiphytic bromeliad, Vriesea sanguinolenta
Authors: Schmidt Gerold , Zotz Gerhard
Published: March 2003

This field study with the C3 bromeliad Vriesea sanguinolenta (Cogn. & Marchal 1874) was initiated to explore the importance of size‐related ecophysiological changes in vascular epiphytes in a natural tropical setting. In this species, a step change from atmospheric to tank‐forming life form occurs during early ontogeny, followed by a continuous size increase of individuals with water‐impounding tanks. Although our study focused on the water‐impounding phase, this growth pattern also allowed us to compare ecophysiological consequences of a step change in life form with those associated with size increments among plants of identical life form. The shift in life form was accompanied by relatively minor changes, for example in leaf morphology (decrease in leaf thickness and trichome density) and leaf physiology (decrease in photosynthetic capacity), while there were more substantial changes during the tank‐forming phase. A major trend was a decreasing dependence of larger plants on internally stored water due to a more efficient tank. We suggest that the resulting, more reliable water supply in larger plants may be the proximate cause for the observed size‐related differences in leaf anatomy (relative reduction of water storage tissue, and relative and absolute increase in chlorenchyma thickness), leaf morphology (increase in stomatal density, decrease in trichome density), and leaf physiology (increase in net rates of CO2 uptake, more conservative stomatal behaviour, higher residual transpiration). The results are compared with previous studies on heteroblasty in bromeliads, but are also discussed in the context of a gradual shift from a drought‐tolerance to a drought‐avoidance strategy.
Ion transport in roots: measurement of fluxes using ion‐selective microelectrodes to characterize transporter function
Authors: Newman I. A.
Published: March 2003

The transport of mineral ions into and out of tissues and cells is central to the life of plants. Ion transport and the plasma membrane transporters themselves have been studied using a variety of techniques. In the last 15 years, measurement of specific ion fluxes has contributed to the characterization of transport systems. Progress in molecular genetics is allowing gene identification and controlled expression of transporter molecules. However the molecular expression of transporter gene products must be characterized at the functional level. The ion‐selective microelectrode technique to measure specific ion fluxes non‐invasively is ideally suited to this purpose. This technique, its theory, its links with others and its application and prospects in plant science, are discussed. Ions studied include hydrogen, potassium, sodium, ammonium, calcium, chloride and nitrate. Applications discussed include: solute ion uptake by roots; gravitropism and other processes in the root cap, meristematic and elongation zones; Nod factor effect on root hairs; osmotic and salt stresses; oscillations; the effects of light and temperature. Studies have included intact roots, leaf mesophyll and other tissues, protoplasts and bacterial biofilms. A multi‐ion capability of the technique will greatly assist functional genomics, particularly when coupled with imaging techniques, patch clamping and the use of suitable mutants.
The effect of nitrogen on growth and key thallus components in the two tripartite lichens, Nephroma arcticum and Peltigera aphthosa
Authors: Sundberg B. , Näsholm T. , Palmqvist K.
Published: March 2003

Relationships between growth, nitrogen and concentration of unique biont components were investigated for the tripartite lichens Nephroma arcticum (L.) Torss. and Peltigera aphthosa (L.) Willd. Nitrogen availability was manipulated during 4 summer months by removing cephalodia and their associated N2 fixation activity, or by weekly irrigation with NH4NO3. Chlorophyll and ribulose 1·5‐biphosphate carboxylase/oxygenase (Rubisco), and chitin and ergosterol were used as photobiont and mycobiont markers, respectively. Nitrogen concentrations were similar in older and newer parts of the same thallus, varying between 2 and 5 g m−2, with P. aphthosa having higher concentrations than N. arcticum. Both chlorophyll (Chl a) and chitin were linearly correlated with thallus N, but N. arcticum invested more in fungal biomass and had lower Chl a concentrations in comparison with P. aphthosa at equal thallus N. During the 4 months, control and N‐fertilized thalli of N. arcticum increased in area by 0·2 m2 m−2 and P. aphthosa by 0·4 m2 m−2. Thallus expansion was significantly inhibited in samples without cephalodia, but there was no effect on lichen weight gain. Mean relative growth rate (RGR; mg g−1 d−1) was 3·8 for N. arcticum and 8·4 for P. aphthosa, when time (d) reflected the lichen wet periods. RGR was 2–3 times lower when based on the whole time, i.e. when including dry periods. The efficiency (e) of converting incident irradiance into lichen biomass was positively and linearly correlated with thallus Chl a concentration to the same extent in both species. The slower growth rates of N. arcticum, in comparison with P. aphthosa, could then be explained by their lower nitrogen and Chl a concentrations and a subsequently lower light energy conversion efficiency. Functional and dynamic aspects of resource allocation patterns of the two lichens are discussed in relation to the above findings.
Abscisic acid regulation of gene expression during water‐deficit stress in the era of the Arabidopsis genome
Authors: Bray E. A.
Published: March 2003

Changes in gene expression may lead to cellular adaptation of water‐deficit stress, yet all of the induced mRNAs may not play this role. Changes in gene expression must be signalled by transduction mechanisms that first sense a water deficit. This first step triggers changes in gene expression that function to synthesize additional signals such as abscisic acid (ABA). The enzymes involved in ABA biosynthesis have been cloned and their regulation during water‐deficit stress is being characterized. Once ABA levels are increased, further signalling mechanisms are initiated to signal new gene expression patterns that are proposed to play a role in cellular adaptation to water‐deficit stress. As the genome of Arabidopsis is now completed, much more information can be exploited to characterize these responses.
A possible stress physiological role of abscisic acid conjugates in root‐to‐shoot signalling
Authors: Sauter A. , Dietz K.‐J. , Hartung W.
Published: March 2003

Abscisic acid (ABA) conjugates, predominantly their glucose esters, have recently been shown to occur in the xylem sap of different plants. Under stress conditions, their concentration can rise substantially to levels that are higher than the concentration of free ABA. External ABA conjugates cannot penetrate apoplastic barriers in the root. They have to be hydrolysed by apoplastic enzymes in the root cortex. Liberated free ABA can then be redistributed to the root symplast and dragged directly across the endodermis to the stele. Endogenous ABA conjugates are formed in the cytosol of root cells, transported symplastically to the xylem parenchyma cells and released to the xylem vessels. The mechanism of release is unknown; it may include the action of ABC‐transporters. Because of its extremely hydrophilic properties, ABA‐GE is translocated in the xylem of the stem without any loss to the surrounding parenchyma. After arrival in the leaf apoplast, transporters for ABA‐GE in the plasmalemma have to be postulated to redistribute the conjugates to the mesophyll cells. Additionally, apoplastic esterases can cleave the conjugate and release free ABA to the target cells and tissues. The activity of these esterases is increased when barley plants are subjected to salt stress.
Effects of pubescence and waxes on the reflectance of leaves in the ultraviolet and photosynthetic wavebands: a comparison of a range of species
Authors: Holmes M. G. , Keiller D. R.
Published: March 2003

Total reflectance of ultraviolet and photosynthetically effective wavelengths was measured for a range of different leaf types. Two approaches were employed. Firstly, reflectance of monochromatic wavebands at 330 and 680 nm was measured for a total of 45 different species covering a wide range of genera. In the second, specific leaf types that displayed different degrees of reflectance were treated to remove hairs and waxes that contributed to their reflectance. Selected waxy and non‐waxy leaves were also studied in more detail over the spectral range 270–500 nm. It was found that both pubescence (presence of hairs) and glaucousness (presence of a thick epicuticular wax layer) had marked effects on total reflectance. Pubescent leaves tended to be more effective in reflecting longer wavelengths than ultraviolet radiation. The extent of this effect depended on hair type. Glaucous leaves demonstrated that surface waxes were very effective reflectors of both UV and longer wavelength radiation.
Soil insects alter fine root demography in peach (Prunus persica)
Authors: Wells C. E. , Glenn D. M. , Eissenstat D. M.
Published: March 2003

Minirhizotrons were used to assess the effects of soil insect suppression on the demography of peach fine roots (<1 mm diameter) over two growing seasons. The experiment was conducted at the USDA–ARS Appalachian Fruit Research Station in Kearneysville, WV, USA using six 15‐year‐old peach trees. Clear butyrate minirhizotrons were installed beneath each tree in April 1996. Soil drench treatments were applied around individual minirhizotron tubes at monthly intervals and consisted of 1 L of water or 250 µL of a broad‐spectrum insecticide in 1 L of water. Roots were videotaped at 2‐ to 4‐week intervals during the 1996 and 1997 growing seasons. Insecticide application was associated with a significant increase in fine root longevity: the median lifespans of insecticide‐treated roots were 46–125 d longer than those of control roots. In addition, the development of brown pigmentation was significantly delayed in insecticide‐treated roots. Insecticide application did not appear to increase soil fertility, as accumulation of NO3–, NH4+, and PO42‐ on mixed bed ion‐exchange resin was similar in treated and untreated soil. These results suggest that interactions with below‐ground insects can significantly influence root longevity and may alter the rate at which roots undergo developmental changes in anatomy and physiology.
Carbon and nitrogen dynamics in acid detergent fibre lignins of beech (Fagus sylvatica L.) during the growth phase
Authors: Dyckmans J. , Flessa H. , Brinkmann K. , Mai C. , Polle A.
Published: March 2003

To study the incorporation of carbon and nitrogen in different plant fractions, 3‐year‐old‐beech (Fagus sylvatica L.) seedlings were exposed in microcosms to a dual‐labelling experiment employing 13C and 15N throughout one season. Leaves, stems, coarse and fine roots were harvested 6, 12 and 18 weeks after bud break (June to September) and used to isolate acid‐detergent fibre lignins (ADF lignin) for the determination of carbon and nitrogen and their isotope ratios. Lignin concentrations were also determined with the thioglycolic acid method. The highest lignin concentrations were found in fine roots. ADF lignins of all tissues analysed, especially those of leaves, also contained significant concentrations of nitrogen. This suggests that lignin‐bound proteins constitute an important cell wall fraction and shows that the ADF method is not suitable to determine genuine lignin. ADF lignin should be re‐named as ligno‐protein fraction. Whole‐leaf biomass was composed of 50 to 70% newly assimilated carbon and about 7% newly assimilated nitrogen; net changes in the isotope ratios were not observed during the experimental period. In the other tissues analysed, the fraction of new carbon and nitrogen was initially low and increased significantly during the time‐course of the experiment, whereas the total tissue concentrations of carbon remained almost unaffected and nitrogen declined. At the end of the experiment, the whole‐tissue biomass and ADF lignins of fine roots contained about 65 and 50% new carbon and about 50 and 40% new nitrogen, respectively. These results indicate that significant metabolic activity was related to the formation of structural biopolymers after leaf growth, especially below‐ground and that this activity also led to a substantial binding of nitrogen to structural compounds.
Phosphorus sources and availability modify growth and distribution of root clusters and nodules of native Australian legumes
Authors: Adams M. A. , Bell T. L. , Pate J. S.
Published: March 2003

A variety of native Western Australian legumes produced root clusters in sand culture confirming field and published observations. In general, these legumes grew equally well when supplied with organic or inorganic sources of phosphorus. The nitrogen content of shoots and roots varied little among treatments for all species, however, phosphorus content was always greater in plants supplied with inositol‐P. The plasticity of root growth in response to localized placement of organic and inorganic sources of phosphorus was demonstrated using a simple ‘split root’ technique. Total root dry weight was, on average, more than doubled in P‐amended sand when compared with non‐amended sand. Root clusters tended to be produced in areas of relatively high phosphorus concentration and nodules in areas of low phosphorus concentration. Levels of phosphorus in lateral roots grown in P‐amended sand were significantly different from lateral roots grown in the corresponding non‐amended sand. Growth increases averaging 70% for white sand to over 100% for yellow sand indicated a large degree of ‘plasticity’ in roots under conditions of heterogeneous supply of phosphorus. Spatially exclusive development of organs for the acquisition of nutrients is discussed in relation to requirements for carbon in organ production and maintenance.
Photosynthetic acclimation of Pinus taeda (loblolly pine) to long‐term growth in elevated pCO2 (FACE)
Authors: Rogers A. , Ellsworth D. S.
Published: March 2003

Growth in elevated pCO2 generally leads to a stimulation of net CO2 uptake rate. However, with long‐term growth the magnitude of this stimulation is often reduced. This phenomenon, termed acclimation, has been largely attributed to a loss of Rubisco (ribulose 1,5 bisphosphate carboxylase). The mechanism by which Rubisco content declines with long‐term growth is not certain. There is evidence for a sugar‐mediated, selective down‐regulation of Rubisco protein and also for a non‐selective loss of total leaf nitrogen, which impacts Rubisco levels indirectly. Over a season, and including needles at different developmental stages, we investigated these two potential mechanisms in well‐developed Pinus taeda grown for approximately 2·5 years in elevated (56 Pa) pCO2 using free air CO2 enrichment technology. Photosynthetic acclimation, as manifested by a decrease in the activity of Rubisco measured both in vivo (− 25%, via gas exchange) and in vitro (− 35%, via enzyme assays), was observed with growth in elevated pCO2. This acclimation was observed in one‐year‐old needles but not in current‐year needles. Needles exhibiting acclimation had reduced levels of Lsu Rubisco (− 25%) and an increased foliar carbohydrate content (+ 30%) but showed no evidence of a decrease in needle nitrogen or total protein content. These data support the concept that photosynthetic acclimation in elevated pCO2 is caused by a selective down‐regulation of Rubisco.