Showing papers in "Plant Cell and Environment in 1993"
TL;DR: It is argued that salts taken up by the plant do not directly control plant growth by affecting turgor, photosynthesis or the activity of any one enzyme, and rather, the build-up of salt in old leaves hasten their death, and the loss of these leaves affects the supply of assimilates or hormones to the growing regions and thereby affects growth.
Abstract: Recent progress in improving the salt tolerance of cultivated plants has been slow. Physiologists have been unable to define single genes or even specific metabolic processes that molecular biologists could target, or pinpoint the part of the plant in which such genes for salt tolerance might be expressed. While the physiological might be expressed. While the physiological processes are undoubtedly complex, faster progress on unraveling mechanisms of salt tolerance might be made if there were more effort to test hypotheses rather than to accumulate data, and to integrate cellular and whole plant responses. This article argues that salts taken up by the plant do not directly control plant growth by affecting turgor, photosynthesis or the activity of any one enzyme. Rather, the build-up of salt in old leaves hasten their death, and the loss of these leaves affects the supply of assimilates or hormones to the growing regions and thereby affects growth.
1,500 citations
TL;DR: In this article, an integration of hydraulic and chemical signals which control stomatal conductance of plants in drying soil, and suggest that such a system is more likely than control based on chemical signals or water relations alone.
Abstract: We describe here an integration of hydraulic and chemical signals which control stomatal conductance of plants in drying soil, and suggest that such a system is more likely than control based on chemical signals or water relations alone. The determination of xylem [ABA] and the stomatal response to xylem [ABA] are likely to involve the water flux through the plant. (1) If, as seems likely, the production of a chemical message depends on the root water status (Ψr), it will not depend solely on the soil water potential (Ψs) but also on the flux of water through the soil-plant-atmosphere continuum, to which are linked the difference between Ψr and Ψs. (2) The water flux will also dilute the concentration of the message in the xylem sap. (3) The stomatal sensitivity to the message is increased as leaf water potential falls. Stomatal conductance, which controls the water flux, therefore would be controlled by a water-flux-dependent message, with a water-flux-dependent sensitivity. In such a system, we have to consider a common regulation for stomatal conductance, leaf and root water potentials, water flux and concentration of ABA in the xylem. In order to test this possibility, we have combined equations which describe the generation and effects of chemical signals and classical equations of water flux. When the simulation was run for a variety of conditions, the solution suggested that such common regulation can operate. Simulations suggest that, as well as providing control of stomatal conductance, integration of chemical and hydraulic signalling may also provide a control of leaf water potential and of xylem [ABA], features which are apparent from our experimental data. We conclude that the root message would provide the plant with a means to sense the conditions of water extraction (soil water status and resisance to water flux) on a daily timescale, while the short-term plant response to this message would depend on the evaporative demand.
480 citations
TL;DR: The extent to which stomatal conductance was capable of responding to reduced hydraulic conductance (k) and preventing cavitation-inducing xylem pressures was evaluated in the small riparian tree, Betula occidentalis Hook.
Abstract: The extent to which stomatal conductance (gs) was capable of responding to reduced hydraulic conductance (k)and preventing cavitation-inducing xylem pressures was evaluated in the small riparian tree, Betula occidentalis Hook. We decreased k by inducing xylem cavitation in shoots using an air-injection technique. From 1 to 18 d after shoot injection we measured midday transpiration rate (E), gs, and xylem pressure (Ψp-xylem) on individual leaves of the crown. We then harvested the shoot and made direct measurements of k from the trunk (2–3 cm diameter) to the distal tip of the petioles of the same leaves measured for E and gs. The k measurement was expressed per unit leaf area (kl, leaf-specific conductance). Leaves measured within 2 d of shoot injection showed reduced gs and E relative to non-injected controls, and both parameters were strongly correlated with kl At this time, there was no difference in leaf Ψp-xylem between injected shoots and controls, and leaf Ψp-xylem was not significantly different from the highest cavitation-inducing pressure (Ψp-cav) in the branch xylem (-1.43 ± 0.029 MPa, n=8). Leaves measured 7–18 d after shoots were injected exhibited a partial return of gs and E values to the control range. This was associated with a decrease in leaf Ψp-xylem below Ψp-cav and loss of foliage. The results suggest the stomata were incapable of long-term regulation of E below control values and that reversion to higher E caused dieback via cavitation.
337 citations
TL;DR: A method is described whereby the saturated capacity for rapidly reversible non-photochemical quenching can be compared between plant species and it was found that, whilst this capacity was species dependent, it did not depend upon the conditions under which the plant was grown.
Abstract: The reversible dissipation of excitation energy in higher plants is believed to protect against light-induced damage to the photosynthetic apparatus. This dissipation is measured as the non-photochemical quenching of chlorophyll fluorescence. A method is described whereby the saturated capacity for rapidly reversible non-photochemical quenching can be compared between plant species. This method was applied to 22 common British plant species whose habitat was quantified using an index that describes shade tolerance. An association was found between occurrence in open habitats and a high capacity for non-photochemical quenching. It was found that, whilst this capacity was species dependent, it did not depend upon the conditions under which the plant was grown. The possible role of zeaxanthin as a determinant of quenching capacity was examined by measuring the contents of xanthophyll cycle carotenoids for each species. Comparing species, no correlation was seen between the saturated level of non-photochemical quenching and zeaxanthin content expressed relative to either total carotenoid or to chlorophyll. When zeaxanthin was expressed relative to the amount of xanthophyll cycle intermediates (zeaxanthin, antheraxanthin and violaxanthin), a weak correlation was seen.
321 citations
TL;DR: The effects of long-term CO2 enhancement and varying nutrient availability on photosynthesis and ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco) were studied on loblolly pine seedlings and demonstrated acclimation of photosynthetic processes to elevated CO2 through reallocation of N.
Abstract: The effects of long-term CO2 enhancement and varying nutrient availability on photosynthesis and ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco) were studied on loblolly pine (Pinus taeda L.) seedlings grown in two atmospheric CO2 partial pressures (35 and 65 Pa) and three nutrient treatments (low N, low P, and high N and P). Measurements taken in late autumn (November) after 2 years of CO2 enrichment and nutrient addition showed that photosynthetic rates were higher for plants grown at elevated CO2 only when they received supplemental N. Total rubisco activity and rubisco content decreased at elevated CO2, but there was an increase in activation state. At elevated CO2, proportionately less N was found in rubisco and more N was found in the light reaction components. These results demonstrate acclimation of photosynthetic processes to elevated CO2 through reallocation of N. Loblolly pine grown in nutrient conditions similar to native soils (low N availability) had lower needle N and chlorophyll content, lower total rubisco activity and content, and lower photosynthetic rates than plants grown at high N and P. This suggests that the magnitude of the photosynthetic response to a future, high-CO2 environment will be dependent on soil fertility in the system.
287 citations
TL;DR: In this paper, Triticum aestivum L., cv. Mercia was grown in chambers under light and temperature conditions similar to the UK field environment for the 1990/1991 growing season at two levels each of atmospheric CO2 concentration (seasonal means: 361 and 692 μmol mol−1), temperature (tracking ambient and ambient +4°C) and nitrogen application (equivalent to 87 and 489 kg ha−1 total N applied).
Abstract: Winter wheat (Triticum aestivum L., cv. Mercia) was grown in chambers under light and temperature conditions similar to the UK field environment for the 1990/1991 growing season at two levels each of atmospheric CO2 concentration (seasonal means: 361 and 692 μmol mol−1), temperature (tracking ambient and ambient +4°C) and nitrogen application (equivalent to 87 and 489 kg ha−1 total N applied). Total dry matter productivity through the season, the maximum number of shoots and final ear number were stimulated by CO2 enrichment at both levels of the temperature and N treatments. At high N, there was a CO2-induced stimulation of grain yield (+15%) similar to that for total crop dry mass (+12%), and there was no significant interaction with temperature. This contrasts with other studies, where positive interactions between the effects of increases in temperature and CO2 have been found. Temperature had a direct, negative effect on yield at both levels of the N and CO2 treatments. This could be explained by the temperature-dependent shortening of the phenological stages, and therefore, the time available for accumulating resources for grain formation. At high N, there was also a reduction in grain set at ambient +4°C temperature, but the overall negative effect of warmer temperature was greater on the number of grains (-37%) than on yield (-18%), due to a compensating increase in average grain mass. At low N, despite increasing total crop dry mass and the number of ears, elevated CO2 did not increase grain yield and caused a significant decrease under ambient temperature conditions. This can be explained in terms of a stimulation of early vegetative growth by CO2 enrichment leading to a reduction in the amount of N available later for the formation and filling of grain.
277 citations
TL;DR: Trees of Juglans regia L. shed leaves when subjected to drought because of the higher vulnerability of petioles than stems to water-stress induced cavitation, which is discussed in the context of the plant segmentation hypothesis.
Abstract: Trees of Juglans regia L. shed leaves when subjected to drought. Before shedding (when leaves are yellow), the petioles have lost 87% of their maximum hydraulic conductivity, but stems have lost only 14% of their conductivity. This is caused by the higher vulnerability of petioles than stems to water-stress induced cavitation. These data are discussed in the context of the plant segmentation hypothesis.
272 citations
TL;DR: Light gradients were measured in leaves that had different types of anatomical development of the mesophyll but similar pigment content, suggesting that palisade tissue may help distribute light more uniformly to chloroplasts within the leaf.
Abstract: Light gradients were measured in leaves that had different types of anatomical development of the mesophyll but similar pigment content. Leaves of the legume, Thermopsis montana, had columnar palisade and spongy mesophyll whereas leaves of the monocot, Smilacina stellata, had spongy mesophyll only. Light gradients were measured at 550 nm in both types of leaves when they were irradiated with collimated or diffuse light. When irradiated with collimated light, light gradients were steeper in leaves with spongy mesophyll in comparison to those that had palisade tissue. On the other hand, light gradients were similar between both leaf types when they were irradiated with diffuse light. Thus, columnar palisade cells facilitated the penetration of collimated light over diffuse light. These results suggest that palisade tissue may help distribute light more uniformly to chloroplasts within the leaf. Moreover, the functional significance of palisade tissue may be related to the amount of collimated light within the natural environment.
256 citations
TL;DR: The present study demonstrates the existence of adaptive processes which rapidly adjust the in vivo thermal stability of PSII in response to temperature increase and suggests that the above-described PSII changes could be of prime importance for the plant's behaviour in the field.
Abstract: Photosystem II (PSII) is considered to be one of the most thermolabile aspects of photosynthesis. In vivo measurements of chlorophyll fluorescence and photosynthetic oxygen evolution in 25°C-grown potato leaves (cv. Haig) indicated that the threshold temperature Tc above which PSII denatures was indeed rather low–about 38°C–with temperatures higher than Tc causing a rapid and irreversible loss of PSII activity. The present study demonstrates the existence of adaptive processes which rapidly adjust the in vivo thermal stability of PSII in response to temperature increase. Transfer of potato leaves from 25°C to temperatures slightly lower than Tc (between 30 and 35°C) was observed to cause an upward shift of the Tc value without any appreciable loss of PSII activity. This increase in PSII thermotolerance was substantial (around +5°C in the Haig cultivar), rapid (with a half-time of ∼20 min) and slowly reversible at 25°C (>24h). As a consequence, high temperatures (e.g. 40°C) which caused a complete and irreversible inhibition of the PSII function had very little effect in 35°C-treated leaves, thus suggesting that the above-described PSII changes could be of prime importance for the plant's behaviour in the field. Accordingly, the rise in Tc at 35°C was much larger (+8°C) in Sahel, a stress-resistant potato variety, than in the heat-sensitive Haig cultivar.
237 citations
TL;DR: In plants with an adequate nitrogen supply (from nodules or combined nitrogen in soil), a high concentration of nitrogen containing compounds in the phloem from the lower leaves may inhibit nodule growth as well as activity.
Abstract: We present a mechanism of regulation of growth and activity of legume root nodules which is consistent with published experimental observations. The concentration of reduced nitrogen compounds, probably amino acids, flowing into the nodules from the phloem, is sensed by the nodules; growth and activity of the nodules is adjusted accordingly. In many legumes this response may involve changes in the oxygen diffusion resistance of the nodule cortex. A straightforward feedback mechanism in which nodule activity is lowered when reduced N in the phloem is high and increased when it is low is envisaged. Almost all import into nodules is via the phloem sap originating in the lower leaves. As a plant develops, these mature leaves no longer utilize nitrogen delivered in the xylem and so export it in the phloem. In plants with an adequate nitrogen supply (from nodules or combined nitrogen in soil), a high concentration of nitrogen containing compounds in the phloem from the lower leaves may inhibit nodule growth as well as activity. This suggestion is an alternative to the hypotheses of carbohydrate deprivation or nitrate inhibition which are commonly used to explain the effects of combined nitrogen on nodule growth and activity.
232 citations
TL;DR: In this article, two published models of canopy photosynthesis, MAESTRO and BIOMASS, are simulated to examine the response of tree stands to increasing ambient concentrations of carbon dioxide (Ca) and temperatures.
Abstract: Two published models of canopy photosynthesis, MAESTRO and BIOMASS, are simulated to examine the response of tree stands to increasing ambient concentrations of carbon dioxide (Ca) and temperatures. The models employ the same equations to described leaf gas exchange, but differ considerably in the level of detail employed to represent canopy structure and radiation environment. Daily rates of canopy photosynthesis simulated by the two models agree to within 10% across a range of CO2 concentrations and temperatures. A doubling of Ca leads to modest increases of simulated daily canopy photosynthesis at low temperatures (10% increase at 10°C), but larger increases at higher temperatures (60% increase at 30°C). The temperature and CO2 dependencies of canopy photosynthesis are interpreted in terms of simulated contributions by quantum-saturated and non-saturated foliage. Simulations are presented for periods ranging from a diurnal cycle to several years. Annual canopy photosynthesis simulated by BIOMASS for trees experiencing no water stress is linearly related to simulated annual absorbed photosynthetically active radiation, with light utilization coefficients for carbon of ɛ= 1.66 and 2.07g MJ−1 derived for Ca of 350 and 700 μmol mol−1, respectively.
TL;DR: It was concluded that the FOB and iron were taken up by the axenic roots of cucumber in a highly efficient manner, most likely as the iron-siderophore complex, and at rates that could be significant to dicot nutrition.
Abstract: To eliminate the confounding effects of microorganisms and to examine the direct utilization of microbial siderophores as iron sources by higher plants, a hydroponic cultural system and methodology was developed to grow plants with axenic roots. This report presents a description of this system, and also its use to determine the efficacy of the microbial siderophore ferrioxamine B (FOB), compared to the synthetic iron chelate FeEDTA, and the phytosiderophores (PS) of barley as an iron source for alleviating iron stress in the model dicot cucumber. It was observed that FOB gave superior plant biomass and was preferentially utilized to restore chlorophyll synthesis in long-term experiments when chelates were supplied at 5mmol m−3 concentrations and nutrient solution was buffered against pH change at 7.4 with solid phase CaCO3. In addition, autoradiograms indicated that 59Fe from FOB was rapidly translocated to shoots through vascular tissues and was specifically distributed to regions of rapid growth and to iron-stressed, but still expanding young leaves. The siderophore itself could be detected within 2h in xylem exudates, regardless of whether or not plants were exposed to metabolic inhibitors. It was concluded that the FOB and iron were taken up by the axenic roots of cucumber in a highly efficient manner, most likely as the iron-siderophore complex, and at rates that could be significant to dicot nutrition. The results also suggested that cucumber may transport FOB through the transpiration stream to upper parts of plants, where the iron would be reductively released from the siderophore for shoot nutrition.
TL;DR: Results of 13C discrimination and gas exchange show that genotypes from North Africa have higher WUE than those from the Middle East and that CO2 from respiration may be used as source of carbon for ear photosynthesis.
Abstract: The role of ear photosynthesis in grain filling was studied in a number of durum wheat (Triticum turgidum var durum L.) landraces and varieties from the Middle East, North Africa, and from the collections of ‘Institut National de la Recherche Agronomique’ (INRA, France) and ‘Centro International de Mejora de Maiz y Trigo’ (CIMMYT, Mexico). Plants were grown in the field in a Mediterranean climate. Flag leaves (blade plus sheath) and ears were kept in the dark from 1 week after anthesis to maturity which reduced grain weight by 22.4% and 59.0%, respectively. In a further experiment, the carbon isotope discrimination ratio (Δ) of ear bracts, awns and flag leaves was measured on samples taken at anthesis and on mature kernels. The mean value of Δ for the water soluble fraction of bracts (17.0‰) and awns (17.7‰) were lower than those of leaves (19.5‰) and fairly similar to those of kernels (17.4‰) averaged across all genotypes. Data indicate that most of the photosynthates in the grain come from ear parts and not from flag leaves. In addition, a higher water use efficiency (WUE) of ear parts than of the flag leaf is suggested by their lower Δ values. Gas exchange in ears and flag leaves was measured during grain filling. Averaged over all genotypes, CO2 diffusive conductance was about five times higher in the flag leaf than in the spike (with distal portions of awns outside the photosynthetic chamber) 2 weeks after anthesis. In absolute terms, the dark respiration rate (Rd) was greater than the net photosynthesis rate (Pn) by a factor of 1.74 in the spike, whereas Rd was much smaller, only 22.1, 65.7 and 24.8% of Pn in blade, sheath and awns, respectively. Data indicate that photosynthesis, and hence the water use efficiency (photosynthesis/transpiration), is greatly underestimated in ears because of the high rates of respiration which diminish the measured rates of net CO2 exchange. Results of 13C discrimination and gas exchange show that genotypes from North Africa have higher WUE than those from the Middle East. The high Rd values of ears as well as their low diffusive conductance suggest that CO2 from respiration may be used as source of carbon for ear photosynthesis. In the same way, the anatomy of glumes, for example, supports the role of bracts using internal CO2 as source of photosynthesis. In the first experiment, the Δ in mature grains from culms with darkened ears compared with control culms provided further evidence in support of this hypothesis. Thus, the Δ from kernels of control plants was 0.40 higher than that from ear-darkened plants, probably because of some degree of refixation (recycling) of respired CO2 in the grains.
TL;DR: In this paper, a construction crane equipped with a gondola was used to gain access to the uppermost level in the crown of a 35m-tall individual, where stomatal conductance at the single leaf scale and transpiration and total vapour phase conductance (stomatal and boundary layer) at the branch scale were measured simultaneously using the independent techniques of porometry and stem heat balance, respectively.
Abstract: Stomatal control of crown transpiration was studied in Anacardium excelsum, a large-leaved, emergent canopy species common in the moist forests of Central and northern South America. A construction crane equipped with a gondola was used to gain access to the uppermost level in the crown of a 35-m-tall individual. Stomatal conductance at the single leaf scale, and transpiration and total vapour phase conductance (stomatal and boundary layer) at the branch scale were measured simultaneously using the independent techniques of porometry and stem heat balance, respectively. This permitted the sensitivity of transpiration to a marginal change in stomatal conductance to be evaluated using a dimensionless coupling coefficient (1-ω) ranging from zero to 1, with 1 representing maximal stomatal control of transpiration. Average stomatal conductance varied from 0.09 mol m−2 s−1 during the dry season to 0.3 mol m−2 s−1 during the wet season. Since boundary layer conductance was relatively low (0.4 mol m−2 s−1), 1-ω ranged from 0.46 during the dry season to only 0.25 during the wet season. A pronounced stomatal response to humidity was observed, which strongly limited transpiration as evaporative demand increased. The stomatal response to humidity was apparent only when the leaf surface was used as the reference point for measurement of external vapour pressure. Average transpiration was predicted to be nearly the same during the dry and wet seasons despite a 1 kPa difference in the prevailing leaf-to-air vapour pressure difference. The patterns of stomatal behaviour and transpiration observed were consistent with recent proposals that stomatal responses to humidity are based on sensing the transpiration rate itself.
TL;DR: In this article, the authors fed 13CO2 to leaves of Quercus rubra and monitored the incorporation of 13C into isoprene by mass spectrometry, and found all possible labeling patterns from completely unlabeled to fully labelled isoprenes.
Abstract: Biogenic isoprene substantially affects atmospheric chemistry, but it is not known how or why many plants, especially trees, make isoprene. We fed 13CO2 to leaves of Quercus rubra and monitored the incorporation of 13C into isoprene by mass spectrometry. After feeding 13CO2 for 9 min we found all possible labelling patterns from completely unlabelled to fully labelled isoprene. By 18 min, 84% of the carbon atoms in isoprene were 13C. Labelling of the last 20% of the carbon atoms was much slower than labelling of the first 80%. The rate of labelling of isoprene was similar to that reported for phosphoglyceric acid indicating that there is a close linkage between the carbon source for isoprene synthesis and the photosynthetic carbon reduction pathway.
TL;DR: In this article, the authors studied the effect of xylem embolization induced by both winter freezing and summer drought on Quercus ilex L. growing in the southern Mediterranean Basin region.
Abstract: Quercus ilex L. growing in the southern Mediterranean Basin region is exposed to xylem embolism induced by both winter freezing and summer drought. The distribution of the species in Sicily could be explained in terms of the different vulnerability to embolism of its xylem conduits. Naturally occurring climatic conditions were simulated by: (1) maintaining plants for 3h at ambient temperatures of 0, - 1-5, - 2 5, - 5 0 and - 11 °C; and (2) allowing plants to dry out to ratios of their minimum diurnal leaf water potentials {\\>\) to that at the turgor loss point (vj/up) of 0 6, 0 9,1 05, 1 20 and 1-33. The loss of hydraulic conductivity of one-year-old twigs reached 40% at - 1 5°C and at v|j,/vt
TL;DR: In this paper, a bioclimatic model of water balance was used to give the actual evapotranspiration as well as the soil water content on a daily basis, and δ13C values in tree-ring cellulose of beech (Fagus sylvatica L.) were analyzed for the period from 1950 to 1990.
Abstract: Environmental influences on carbon isotope fractionation in tree rings require further elucidation in order to use this parameter as a biological marker of climatic variations. δ13C values in tree-ring cellulose of beech (Fagus sylvatica L.) were analysed for the period from 1950 to 1990. A bioclimatic model of water balance was used to give the actual evapotranspiration as well as the soil water content on a daily basis. δ13C shows a significant decrease from –24·5‰ to –25‰ over this period. Internal CO2 concentration changes from 200 to 220 ppm in relation with the rise of atmospheric CO2. Beside a slight non-significant inter-individual variation, a large year-to-year variation exists. The relative extractable soil water of July, combined with the value of δ13C for the previous year, predicts as much as 70% of this variance. Air temperature or precipitation accounted for less variation. δ13C is strongly correlated with basal area increment, but appeared a more reliable indicator of water status at the stand level.
TL;DR: In N-deficient plants, exogenous sucrose led to decreased protein, Rubisco and chlorophyll content in shoots, in contrast to the other conditions, and a higher protein content and a general increase of catabolic enzyme activities and growth in the roots.
Abstract: A simple method of growing plants in agar was exploited to investigate the effect of long-term nitrogen (N) and phosphorus (P) deficiencies on respiratory metabolism and growth in shoots and roots of Nicotiana tabacum seedlings, and their interaction with exogenously supplied sucrose. Levels of hexose phosphates and 3-phosphoglyceric acid (3-PGA) were low in P-deficient shoots and roots and high in N-deficient shoots and roots. The ratio of hexose phosphates to 3-PGA and levels of fructose-2,6-bisphosphate were high in P-deficient plants and low in N-deficient plants. These data reflect differences in the way metabolism was perturbed, yet both deficiencies were associated with increased root growth relative to shoot growth, starch accumulation in the shoots, and soluble carbohydrate accumulation, especially hexoses, in the roots. Enzymes for sucrose degradation (sucrose synthase, acid and alkaline invertase) and glycolysis (phosphofructokinase, pyrophosphate-dependent phospho-fructokinase and pyruvate kinase) remained unaltered or declined in the shoots and roots. The accumulation of hexoses in roots of N- and P-deficient plants may result from maintenance of high invertase activities relative to sucrose synthase and glycolytic enzymes in the roots. The possibility that hexose accumulation may drive preferential root growth osmotically in N and P deficiencies is discussed. The addition of sucrose to roots to further investigate the interaction of carbohydrates with growth and allocation in low N and low P produced clear effects even though endogenous levels of soluble carbohydrate were already high in the nutrient-deficient plants. In complete nutrition, growth was stimulated, protein content particularly of the roots was increased and there was a preferential increase in activity of sucrose synthase in roots. At low P, enzyme activities in roots were increased, including sucrose synthase, and protein content increased, particularly in the roots, but there was no increase in growth. In N-deficient plants, exogenous sucrose led to decreased protein, Rubisco and chlorophyll content in shoots, in contrast to the other conditions, and a higher protein content and a general increase of catabolic enzyme activities and growth in the roots.
Abstract: During one growing period, 5-year old spruce trees were exposed in environmental chambers to elevated concentrations of carbon dioxide (750 cm[sup 3] m[sup -3]) and ozone (0.08 cm[sup 3] m[sup -3]) as single variables or in combination. Control concentrations of the gases were 350 cm[sup 3] m[sup -3] CO[sub 2] and 0.02 cm[sup 3] m[sup -3] ozone. To investigate whether an elevated CO[sub 2] concentration can prevent adverse ozone effects by reducing oxidative stress, the activities of the protective enzymes superoxide dismutase, catalase and peroxidase were determined. Furthermore, shoot biomass, pigment and protein contents of two needle age classes were investigated. Ozone caused pigment reduction and visible injury in the previous year's needles and growth reduction in the current year's shoot. In the presence of elevated concentrations of ozone and CO[sub 2], growth reduction in the current year's shoots was prevented, but emergence of visible damage in the previous year's needles was only delayed and pigment reduction was still found. Elevated concentrations of ozone or CO[sub 2] as single variables caused a significant reduction in the activities of superoxide dismutase and catalase in the current year's needles. Minimum activities of superoxide dismutase and catalase and decreased peroxidase activities were found in both needle age classes from spruce trees grown at enhanced concentrations of both CO[sub 2] and ozone. These results suggest a reduced tolerance to oxidative stress in spruce trees under conditions of elevated concentrations of both CO[sub 2] and ozone.
TL;DR: A tallgrass prairie ecosystem was exposed to ambient and twice-ambient COz concentrations in open-top chambers and compared to unchambered ambient CO* during the entire growing season from 1989 through 1991 as mentioned in this paper.
Abstract: A tallgrass prairie ecosystem was exposed to ambient and twice-ambient COz concentrations in open-top chambers and compared to unchambered ambient CO* during the entire growing season from 1989 through 1991. Dominant species were Andropogon gerardii (Cd), A. scoparius (Cd), Sorghastrum nutans (CA) and Pea pratensis (CA). Nitrogen and phosphorus concentrations in A. gerardii, P. pratensis and dicotyledonous herbs above ground biomass were estimated by periodic sampling throughout the growing season in 1989 and 1990. In 1991, N and P concentrations in peak biomass were estimated by an early August harvest. N and P concentrations in root production as a function of treatment were estimated using root ingrowth bags that remained in place throughout the growing season. Total N and Pin above- and belowground biomass were calculated as products of concentration and peak biomass by species groups. N concentration in A. gerardii and dicotyledonous herb aboveground biomass was lower and total N higher in elevated CO2 plots than in ambient CO* plots. N concentration in P. pratensis aboveground biomass was lower in elevated CO2 plots than in ambient, but total N did not differ among treatments in 2 out of 3 years. In 1990, N concentration in root ingrowth bag biomass was lower and total N greater in elevated CO;? than in ambient CO* plots. Root ingrowth bag biomass N concentration did not differ among treatments in 1991, but total N was greater in elevated COz plots than in ambient CO2 plots. P concentration was lower under elevated CO2 compared to ambient in 1989, but did not differ substantially among treatments in 1990 or 1991. In all years, total P in aboveground A. gerardii and root ingrowth bag biomass was greater under elevated CO2 than ambient. P concentration and total P in P. pratensis was similar among treatments.
TL;DR: In this article, small birch plants were grown for up to 80 d in a climate chamber at varied relative addition rates of nitrogen in culture solution, and at ambient (350 μmol mol-1) or elevated (700 μmol methylated carbon dioxide) concentrations of CO2.
Abstract: Small birch plants were grown for up to 80 d in a climate chamber at varied relative addition rates of nitrogen in culture solution, and at ambient (350 μmol mol-1) or elevated (700 μmol mol-1) concentrations of CO2. The relative addition rate of nitrogen controlled relative growth rate accurately and independently of CO2 concentration at sub-optimum levels. During free access to nutrients, relative growth rate was higher at elevated CO2. Higher values of relative growth rate and net assimilation rate were associated with higher values of plant N-concentration. At all N-supply rates, elevated CO2 resulted in higher values of net assimilation rate, whereas leaf weight ratio was independent of CO2. Specific leaf area (and leaf area ratio) was less at higher CO2 and at lower rates of N-supply. Lower values of specific leaf area were partly because of starch accumulation. Nitrogen productivity (growth rate per unit plant nitrogen) was higher at elevated CO2. At sub-optimal N-supply, the higher net assimilation rate at elevated CO2 was offset by a lower leaf area ratio. Carbon dioxide did not affect root/shoot ratio, but a higher fraction of plant dry weight was found in roots at lower N-supply. In the treatment with lowest N-supply, five times as much root length was produced per amount of plant nitrogen in comparison with optimum plants. The specific fine root length at all N-supplies was greater at elevated CO2. These responses of the root system to lower N-supply and elevated CO2 may have a considerable bearing on the acquisition of nutrients in depleted soils at elevated CO2. The advantage of maintaining steady-state nutrition in small plants while investigating the effects of elevated CO2 on growth is emphasized.
TL;DR: The importance of ABA-metabolism for the control of leaf conductance is discussed in the context of root-to-shoot communication by ABA in the xylem stream.
Abstract: Abscisic acid (ABA) fed in pulses to the petioles of detached cherry leaves in enclosed leaf chambers, caused a reduction in leaf conductance. The degree of inhibition was analysed with respect to the amount of ABA fed and to concentration of ABA in the feeding solution. Regression analysis of the data showed both variables to have a significant effect on leaf conductance. A hypothetical maximum ABA concentration occurring in the leaf apoplast was calculated for each pulse from a simple model. This variable explained more of the variance within the data than either the amount or the applied concentration variable. A value for the rate at which ABA is removed from the apoplast is derived from the experimental data using the model. A second experiment attempted to evaluate this rate directly, by measuring the rate of catabolism of labelled ABA within the leaf. The results suggested a half-life of 36 min for the initial rate of decay. This figure is similar to that derived from the model, the importance of ABA-metabolism for the control of leaf conductance is discussed in the context of root-to-shoot communication by ABA in the xylem stream.
TL;DR: Varying the level of S. hermonthica infection showed that the effect of N on host photosynthesis cannot be explained by differences in the mass or number of parasites supported by the host, and the negative effect of the parasite was reversed.
Abstract: The extent to which the parasitic angiosperm Striga hermonthica reduces the growth of its sorghum host is dependent on the concentration of nitrogen (as NH4NO3 in 40% Long Ashton Solution) supplied to the plants. The biomass of 0.5,1 and 2 mol m−3 N-grown infected plants was 22,30 and 66%, respectively, of uninfected plants after 140d growth. The biomass of 3 and 4 mol m−3 N-grown infected plants differed little from uninfected plants. No grain was set in 0.5 and 1 mol m−3 N-grown infected plants, grain yield reached 42 and 73% of controls in 2 and 3 mol m−3 N-grown plants, and was unaffected in 4 mol m−3 N-grown plants. Striga hermonthica also altered the allometry and architecture of the host, at all but the highest N concentration. Higher N concentration (3 and 4 mol m −3 N) reduced the growth of S. hermonthica. Foliar N concentrations in sorghum ranged from 11 mg g−1 dwt. in 0.5 mol m−3 N-grown plants, to 28 mg g−1 dwt. in 4 mol m−3 N-grown plants, and were not affected by S. hermonthica. Higher N concentrations were measured in S. hermonthica, and ranged from 18 to 45 mg g−1 dwt. in 0.5 and 3 mol m−3 N-grown plants, respectively. The relationship between photosynthesis (CO2 flux) and N concentration differed between uninfected and infected sorghum. This was most apparent in 0.5 mol m−3 N-grown plants, with rates of 16 and 11 μmol m−2 s−1 in uninfected and infected plants, respectively (at 1500–1800 μmol m−2 s−1 photosynthetic photon flux density). At higher N concentrations, this difference was smaller, with both sets of plants reaching 26 μmol m−2 s−1 at 4 mol m−3 N. Varying the level of S. hermonthica infection showed that the effect of N on host photosynthesis cannot be explained by differences in the mass or number of parasites supported by the host. At low levels of infection in 1 mol m−3 N-grown plants, the negative effect of the parasite was reversed, and photosynthesis in infected plants exceeded that in uninfected plants by 20%. Photosynthesis in S. hermonthica at 3 mol m−3 N (8 μmol m−2 s−1) was double that in 0.5 mol m−3 N-grown plants. Stable carbon isotope and gas exchange measurements data demonstrated that this higher level of autotrophic carbon fixation was accompanied by a lower dependency on hetero trophic carbon. The latter ranged from 27 to 6% in 0 5 mol m−3 and 3 mol m−3 N-grown plants, respectively.
TL;DR: 15N abundance values failed to distinguish N2 fixing from non-fixing woody species, and therefore, could not be used in the ecosystem to determine the dependence of putative nitrogen fixing species on N2 fixation.
Abstract: Studies of the variation in δ15N values for plants from a fire-prone Banksia woodland in South West Australia showed that pioneer herbaceous, non-mycorrhizal species which were active in nitrate reduction and storage, had the highest values (1.81%c). A detailed study of one such species Ptilotus polystachus demonstrated a close correspondence between the δ15N values of soil nitrate, xylem nitrate and leaf total nitrogen, suggesting an exclusive reliance on nitrate ions as nitrogen source. These pioneer species also showed a preponderance of the chloroplastic isoform of glutamine synthetase while woody species generally had higher activity associated with the cytosolic isoform. The group comprising monocotyledonous hemicryptophytes and geophytes contained species with slightly positive δ15N values and moderately active in nitrate reduction and storage. Nitrogen-fixing species had the lowest δ15N values (–0.36‰), irrespective of their apparent utilisation of nitrate. However, woody resprouter species which had low levels of nitrate reduction and storage had δ15N values which fell within the range of values obtained for the miscellaneous assemblage of N2-fixing species. Consequently, 15N abundance values failed to distinguish N2 fixing from non-fixing woody species, and therefore, could not be used in the ecosystem to determine the dependence of putative nitrogen fixing species on N2 fixation. The study demonstrated complex patterns of nitrogen utilization in the ecosystem in which exploitation of different nitrogen resources related to plant life form and the physiological attributes of nitrogen assimilation by component species.
TL;DR: It is hypothesized that the antigen on the surface of the protoplast may correspond to the antigen in the walls, and such antigen may be responsible for adhesion of membrane to wall at the linkage sites visualized by CaCl2 plasmolysis, and the linkage Sites may be transmembrane proteins to which cytoskeleton can attach at the inner surface.
Abstract: Wall-to-wall linkage may help maintain cell integrity and polarity, and focus mechanical stress from wall to mech-anotransductive ion channels within the plasm a lemma. When cells of onion bulb scale epidermis shrink during plasmolysis with CaCl2, the plasmalemma remains attached to the cell wall by Hechtian strands which we hypothesize might possibly be drawn out from linkages fulfilling the above functions. We show that at least many of the attachment loci are independent of the plasmodesmata. A priori, wall glycoproteins seem good candidates for the wall-to-membrane linkers; therefore, we investigated the distribution in wall and plasmalemma of antigen recognized by antibody to hydroxyproline-rich glycoprotein (HRGP). Using fluorescent secondary antibodies, we showed that polyclonal antibodies prepared against wall HRGP from soybean bind to the onion walls (following mild depectination), but also bind to the plasmalemma after the wall is enzymatically digested. The distribution of the antibodies is punctate. On the plasmalemma, the points tend to be scattered more or less uniformly, but can cluster at termini of large streaming strands (which rarely form in wall-constrained cells.) These streaming strands can be seen to exert tension on the membrane. We hypothesize that (1) the antigen on the surface of the protoplast may correspond to the antigen in the walls, (2) such antigen may be responsible for adhesion of membrane to wall at the linkage sites visualized by CaCl2 plasmolysis, and (3) the linkage sites may be transmembrane proteins to which cytoskeleton can attach at the inner surface.