Showing papers in "Australian Journal of Plant Physiology in 1999"
TL;DR: In this article, a globally-derived data set for site-averaged foliar delta(15)N, the delta of whole surface mineral soil and corresponding site factors (mean annual rainfall and temperature, latitude, altitude and soil pH) was assembled.
Abstract: We assembled a globally-derived data set for site-averaged foliar delta(15)N, the delta(15)N of whole surface mineral soil and corresponding site factors (mean annual rainfall and temperature, latitude, altitude and soil pH). The delta(15)N of whole soil was related to all of the site variables (including foliar delta(15)N) except altitude and, when regressed on latitude and rainfall, provided the best model of these data, accounting for 49% of the variation in whole soil delta(15)N. As single linear regressions, site-averaged foliar delta(15)N was more strongly related to rainfall than was whole soil delta(15)N. A smaller data set showed similar, negative correlations between whole soil delta(15)N, site-averaged foliar delta(15)N and soil moisture variations during a single growing season. The negative correlation between water availability (measured here by rainfall and temperature) and soil or plant delta(15)N fails at the landscape scale, where wet spots are delta(15)N-enriched relative to their drier surroundings. Here we present global and seasonal data, postulate a proximate mechanism for the overall relationship between water availability and ecosystem delta(15)N and, newly, a mechanism accounting for the highly delta(15)N-depleted values found in the foliage and soils of many wet/cold ecosystems. These hypotheses are complemented by documentation of the present gaps in knowledge, suggesting lines of research which will provide new insights into terrestrial N-cycling. Our conclusions are consistent with those of Austin and Vitousek (1998) that foliar (and soil) delta(15)N appear to be related to the residence time of whole ecosystem N.
439 citations
TL;DR: Phytase and acid phosphatase activities were measured in extracts from roots of 14- to 22- day old seedlings of a range of temperate pasture species that were grown aseptically in sand culture and under P-deficient relative to P-sufficient growth conditions in all of the pasture species.
Abstract: Phytase and acid phosphatase activities were measured in extracts from roots of 14- to 22- day old seedlings of a range of temperate pasture species that were grown aseptically in sand culture. Phytase activity from roots of phosphorus- (P-)-deficient Trifolium subterraneum L. was characterised. Activity was enhanced by 40% when extracts were passed through Sephadex G-25, and increased by a further 20–30% with the addition of either 1 mМ EDTA or 5 mМ cysteine to assay solutions. The optimum temperature for phytase activity was 50°C and the optimum pH was 5.3. When compared with phosphatase activity measured in the roots of T. subterraneum, phytase activity exhibited narrower pH and temperature optima, and was also more strongly inhibited by Co2+, Zn2+ and AsO42− ions. Significantly, for the five pasture species examined, phytase activity was less than 5% of the total acid phosphatase activity in extracts of plant roots. Measured phytase activity ranged between 0.13 and 1.7 nkat g–1 root fresh wt and was enhanced under P-deficient relative to P-sufficient growth conditions in all of the pasture species with the exception of Trifolium repens L., for which the Km constant for activity was 50% lower in P-deficient plants. When expressed on a root fresh wt basis, increases in phytase activity of ~1.25-fold were observed for extracts from T. subterraneum and Medicago polymorpha L., and of up to 3.3-fold for Danthonia richardsonii A.B. Cashmore and Phalaris aquatica L. Increases in acid phosphatase activity with P deficiency were less evident. Between 3.1% and 4.3% only of the total phytase activity measured in root extracts was eluted from intact roots into 0.1 М NaCl.
163 citations
TL;DR: It is suggested that nematodes control auxin distribution in the root and that flavonoids could be responsible for controlling auxin accumulation.
Abstract: We studied the expression of the auxin responsive promoter (GH3) fused to the gusA reporter gene in white clover (Trifolium repens cv. Haifa) during the initiation of root galls by root-knot nematodes (Meloidogyne javanica) to investigate whether nematode infection affects auxin distribution in developing galls. In search for a plant signal that would mediate changes in auxin location we studied the induction of the flavonoid pathway because flavonoids can act as auxin transport regulators. Three chalcone synthase (CHS1, CHS2 and CHS3) promoter:gusA fusions were examined in transgenic plants and flavonoids were detected using fluorescence microscopy. Within 24 h post inoculation CHS:gusA expression occurred around the invading nematode. At 48 h post inoculation CHS:gusA expression and flavonoids were detected throughout the infection site, followed by high GH3:gusA expression in the gall 48–72 h post inoculation. Initially (48–72 h post inoculation) high GH3:gusA expression in giant cell precursors was followed by low expression in the enlarging giant cells (96–120 h post inoculation), suggesting that auxin is needed as a trigger for giant cell initiation but not for later enlargement. We suggest that nematodes control auxin distribution in the root and that flavonoids could be responsible for controlling auxin accumulation.
141 citations
TL;DR: It is concluded that the potential to utilise amino acids as N sources is of widespread occurrence in plant communities and is not restricted to those from low temperature regimes or where N mineralisation is limited.
Abstract: Soluble organic nitrogen, including protein and amino acids, was found to be a ubiquitous form of soil N in diverse Australian environments. Fine roots of species representative of these environments were found to be active in the metabolism of glycine. The ability to incorporate [N-15]glycine was widespread among plant species from subantarctic to tropical communities. In species from subantarctic herbfield, subtropical coral cay, subtropical rainforest and wet heathland, [N-15]glycine incorporation ranged from 26 to 45% of (NH4+)-N-15 incorporation and was 2- to 3-fold greater than (NO3-)-N-15 incorporation. Most semiarid mulga and tropical savanna woodland species incorporated [N-15]glycine and (NO3-)-N-15 in similar amounts, 18-26% of (NH4+)-N-15 incorporation. We conclude that the potential to utilise amino acids as N sources is of widespread occurrence in plant communities and is not restricted to those from low temperature regimes or where N mineralisation is limited. Seedlings of Hakea (Proteaceae) were shown to metabolise glycine, with a rapid transfer of N-15 from glycine to serine and other amino compounds. The ability to take up and metabolise glycine was unaffected by the presence of equimolar concentrations of NO3- and NH4+. Isonicotinic acid hydrazide (INH) did not inhibit the transfer of N-15-label from glycine to serine indicating that serine hydroxymethyltransferase was not active in glycine catabolism. In contrast aminooxyacetate (AOA) strongly inhibited transfer of N-15 from glycine to serine and labelling of other amino compounds, suggesting that glycine is metabolised in roots and cluster roots of Hakea via an aminotransferase.
136 citations
TL;DR: Salinity greatly advanced the time of floral initiation in the wheat cultivars, indicating that modification of apical development by salinity was not determined by salt in the apex itself, but mediated by signals or substrates derived from elsewhere in the plant.
Abstract: Two cultivars of wheat and two of barley differing in salt tolerance were grown at 0, 100 and 175 mM NaCl, and the development of the apex was followed as it turned from vegetative to reproduc- tive. Apices were taken for ion analysis when most of the spikelet primordia had been produced and the process of differentiation into floral organs was starting. K + concentrations in apices were unaffected by the salinity treatment, Na + concentrations were generally low in all treatments, below 50 mM, while Cl - levels were very low, below 30 mM. These levels of Na + and Cl - are too low to affect metabolism. In spite of these negligible effects on ion levels in the apex, salinity affected reproductive development: fewer spikelet primordia formed, and final spikelet numbers at ear emergence were reduced. Salinity greatly advanced the time of floral initiation in the wheat cultivars. This indicates that modification of apical development by salinity was not determined by salt in the apex itself, but mediated by signals or substrates derived from elsewhere in the plant.
135 citations
TL;DR: This overview deals with changing concepts regarding the distribution of the photosystems between stacked and unstacked thylakoid domains from a personal historical perspective and some functional implications of the lateral separation of most PSII complexes in appressed membrane regions of grana stacks from PSI complexes, ATP synthase and auxiliary proteins located in non-appressed membrane domains.
Abstract: The striking structural architecture of thylakoid membranes of higher plant and some green algal chloroplasts that house the light harvesting and energy transducing functions of chloroplasts have evoked many hypotheses concerning the significance of grana. The differentiation of the thylakoids into grana and stroma membrane regions is a morphological reflection of the non-random distribution of the photosystems II and I between appressed and non-appressed membrane domains, which became known as lateral heterogeneity. In this overview, the first section deals with changing concepts regarding the distribution of the photosystems between stacked and unstacked thylakoid domains from a personal historical perspective. The remaining section describes some functional implications of the lateral separation of most PSII complexes in appressed membrane regions of grana stacks from PSI complexes, ATP synthase and auxiliary proteins located in non-appressed membrane domains.
134 citations
TL;DR: A better understanding of final grain weight was reached when temperatures from the pre-anthesis period were included in the analysis of grain weight response to temperature.
Abstract: Individual grain weight is an important source of variation for grain yield in wheat. The aim of this study was to investigate the effect of short periods of high temperature immediately pre-anthesis, or during post-anthesis, on grain weight under field conditions. Thus, two wheat cultivars of different grain weight potential were sown on four different sowing dates to provide different temperature conditions during the pre- and post-anthesis periods. In addition, for two sowings, acrylic boxes were installed to increase spike temperature either immediately before anthesis, or during the lineal phase of the grain-filling period. Final grain weight was significantly affected by sowing date, genotype and grain position on the spike. Grain weight showed a clear relationship with the average temperature of the grain filling period, but this relationship was either linear or curvilinear, depending on the cultivar. Both high temperature treatments, i.e. at pre- or post-anthesis, significantly diminished final grain weight, and their effect was similar with the exception of heavier grains, which were unresponsive to the high temperature treatment at pre-anthesis. Finally, a better understanding of final grain weight was reached when temperatures from the pre-anthesis period were included in the analysis of grain weight response to temperature.
125 citations
TL;DR: Results support the hypothesis that both ion channels and pumps are involved in variation potential depolarisation and demonstrate that action and variation potentials differ both in their mechanism of propagation and electrogenic nature.
Abstract: The nature and characteristics of the electrical response to wounding in the woody plant Vitis vinifera L. were examined. Following burning of a small area of a leaf, bioelectrical events spread throughout the shoot. The heat wound triggered stem deformations (widening–contraction), which preceded changes in biopotentials and that are interpreted as reflecting wound-induced hydraulic signals. It also caused marked decrease of extracellular resistance in stem tissues, starting about 15 s after stimulation, while intracellular resistance did not show any modification. Under an N2 atmosphere (hypoxic conditions), action potentials disappeared and the amplitude of variation potentials decreased approximately 50%. At saturating humidity variation potentials were completely eliminated, while action potentials were evident. Taken together, the results demonstrate that action and variation potentials differ both in their mechanism of propagation and electrogenic nature. Action potentials are ‘genuine’ self-propagating electrical signals travelling at a velocity of about 10 cm s-1, with a metabolic nature involving active components (electrogenic pumps). Variation potentials are a ‘local’ response to the passage of an hydraulic wave. Results support the hypothesis that both ion channels and pumps are involved in variation potential depolarisation.
98 citations
TL;DR: The results suggest that the ‘low chlorophyll’ feature of Syrian barley landraces is related to their drought adaptation which is manifested by a low g S which reduces leaf absorbance which, in turn, reduces the potentially damaging heating effect of high solar radiation in droughted plants whose stomata are closed.
Abstract: The Syrian barley landrace Tadmor is adapted to semi-arid environments and characterized by a reduced chlorophyll content (ca −25% on a leaf area basis) compared to improved barley genotypes, such as the European variety Plaisant. Tadmor leaves had reduced stomatal conductance (gS ) compared to Plaisant leaves both under well-watered conditions and during water stress. Both Tadmor and Plaisant barley seedlings were progressively acclimated to high temperature (39°C) and high photon flux density (1600 µmol photons m −2 s −1 ). During acclimation, the chlorophyll content of Tadmor leaves further decreased whereas the carotenoid concentration remained virtually unchanged, leading to a marked increase in the carotenoid:chlorophyll ratio. The chlorophyll content of acclimated Tadmor leaves was reduced to approximately half of the chlorophyll content of Plaisant leaves grown under the same conditions. Loss of chlorophyll in Tadmor leaves was not observed when only one environmental factor was increased (temperature or photon flux density). In the improved variety, both chlorophylls and carotenoids accumulated during acclimation to heat and strong light, leading to an almost constant carotenoid:chlorophyll ratio. The loss of chlorophyll in the Syrian landrace was associated with limited changes in the photosynthetic characteristics of the leaves (oxygen evolution, electron transport quantum yield, chlorophyll antenna size of photosystem II). Plaisant leaves, but not Tadmor leaves, exhibited symptoms of oxidative damage during growth in strong light at high temperature. When the stomata were closed, sudden exposure to bright light caused a smaller increase in leaf temperature in Tadmor than in Plaisant. Taken together, our results suggest that the ‘low chlorophyll’ feature of Syrian barley landraces is related to their drought adaptation which is manifested by a low g S : the very low chlorophyll content decreases leaf absorbance which, in turn, reduces the potentially damaging heating effect of high solar radiation in droughted plants whose stomata are closed.
93 citations
TL;DR: This review summarises recent progress in understanding the structure, function and dynamics of cyanobacterial thylakoid membranes, highlights the questions that remain to be answered and suggests some possible approaches towards solving those questions.
Abstract: In recent years there has been remarkable progress in determining the three-dimensional structures of photosynthetic complexes. A new challenge is emerging: can we understand the organisation and interaction of those complexes in the intact photosynthetic membrane? Intact membranes are complex, dynamic systems. If we are to understand the function of the intact membrane, we will need to understand the organisation of the complexes, how they can diffuse and interact in the membrane, how they are assembled, repaired and broken down, and how their function is regulated. Cyanobacteria have some crucial advantages as model systems. The complete sequencing of the Synechocystis 6803 genome, coupled with the ease of genetic manipulation of Synechocystis (and certain other cyanobacteria) have given us a unique tool for studying a photosynthetic organism. Furthermore, some cyanobacteria have a very simple, regular thylakoid membrane structure. The unique geometry of photosynthetic membranes of these cyanobacteria will greatly facilitate biophysical studies of membrane function. This review summarises recent progress in understanding the structure, function and dynamics of cyanobacterial thylakoid membranes, highlights the questions that remain to be answered and suggests some possible approaches towards solving those questions.
93 citations
TL;DR: The Agrobacterium-mediated transformation system was extended to two indica cultivars: a widely cultivated breeding line IR-64 and an elite basmati cultivar Karnal Local, and absence of gene sequences outside the transferred DNA (T-DNA) region confirmed absence of any long T-DNA transfer.
Abstract: The Agrobacterium-mediated transformation system was extended to two indica cultivars: a widely cultivated breeding line IR-64 and an elite basmati cultivar Karnal Local. Root tips and shoot tips of seedlings, and scutellar-calli derived from mature seeds showed high-efficiency Agrobacterium tumefaciens infection and stable transformation. In addition to the superbinary vector pTOK233 in Agrobacterium strain LBA4404, almost equally high levels of transformation were achieved with a relatively much smaller (13.1 kb) binary vector (pCAMBIA1301) in a supervirulent host strain AGL1. In both cases, as well as in both cultivars, while 60–90% of the infected explants produced calli resistant to the selectable agent hygromycin, 59–75% of such calli tested positive for GUS. A high level (400 µM) of acetosyringone in the preinduction medium for Agrobacterium and a higher level (500 µM) in the cocultivation medium was necessary for an enhancement in transformation frequency of the binary vector to levels comparable to a superbinary. Hygromycin-resistant calli could be produced from all the explants used. Transformants could be regenerated for both cultivars using the superbinary and binary vector, but only for calli of scutellar origin. In addition to the molecular confirmation of hpt and gus gene transfer and transcription, absence of gene sequences outside the transferred DNA (T-DNA) region confirmed absence of any long T-DNA transfer.
TL;DR: Findings provide direct evidence to support the hypothesis that Al interactions with cytoplasmic Ca 2+ are involved in the Al toxicity syndrome in plants.
Abstract: Disturbance of cytoplasmic Ca 2+ homeostasis, leading to breakdown in Ca 2+ -mediated signal transduction processes, has been suggested as a primary mechanism of aluminium (Al) rhizotoxicity in plants. To test this hypothesis, Al-related changes in cytoplasmic free activity of Ca 2+ ions ([Ca 2+ ] c ) in root apical cells of near-isogenic wheat (Triticum aestivum L.) lines differing in Al tolerance at a single locus were examined by visualising the Ca 2+ -sensitive probe Fluo-3 with confocal laser scanning microscopy. Exposure of roots to 50 µM AlCl 3 (pH 4.2) led to an increase in [Ca 2+ ] c of root apical cells in both Al-sensitive (ES8) and Al-tolerant (ET8) wheat lines. An increase in [Ca 2+ ] c was greater in ES8 than in ET8; after 1-h treatment with 50 µM AlCl 3 an increase in [Ca 2+ ] c was 48 and 27 % in ET8 and ES8, respectively. An increase in [Ca 2+ ] c of ES8 roots, but not ET8 roots, was observed upon treatment with 2.6 µM AlCl 3 (pH 4.5). Al-related increases in [Ca 2+ ] c were correlated with inhibition of root growth. The Al-induced increase in the [Ca 2+ ] c was reversible upon removing AlCl 3 . These findings provide direct evidence to support the hypothesis that Al interactions with cytoplasmic Ca 2+ are involved in the Al toxicity syndrome in plants.
TL;DR: The cumulative effect of water stress is incorporated, in a phenomenological model of stomatal conductance of upper canopy foliage (gsu) for Eucalyptus globulus and Eucallyptus nitens, to explain 70% of the variation in gsu of irrigated trees using the same parameter values for both species.
Abstract: This paper incorporates the cumulative effect of water stress, in a phenomenological model of stomatal conductance of upper canopy foliage (gsu) for Eucalyptus globulus and Eucalyptus nitens. Maximum observed stomatal conductance of irrigated trees was moderated by three functions describing the response of gsu to light, air temperature and vapour pressure deficit. This model explained 70% of the variation in gsu of irrigated trees using the same parameter values for both species. Daily total conductance was calculated for rainfed (Gsu,R) and irrigated (Gsu,I) trees. Linear relationships between the ratio of Gsu,R/G su,I (f(W)) and pre-dawn water potential explained only 46 and 30%, respectively, of variation in f(W) for E. globulus and E. nitens. The average value of pre-dawn water potential for the preceding x days (Sψx) was calculated for a range of values of x. When x was 8 for E. globulus and 20 for E. nitens a single exponential decay function accounted for 70% of variation in f(W) for both species. This result and the number of days over which it was necessary to calculate (Sψx) were consistent with previous studies of the leaf water relations, canopy and sapwood development, and stomatal behaviour of the two species.
TL;DR: It is suggested that some of these rice-associating bacteria possess important genes that enhance their ability to intimately colonise niches on and within rice tissues, and promote rice plant growth.
Abstract: The interactions between two groups of rice endophytic bacterial strains and several rice cultivars were investigated. Various strains of Rhizobium leguminosarum bv. trifolii, originally isolated from rice plants grown in Egypt, comprise one group. The second group of bacterial strains was isolated from rice cultivars grown in the Philippines. Inoculation experiments with rice seedlings showed that specific isolates of these rice-associating bacteria could either promote, inhibit, or have no influence on rice plant growth. Furthermore, these growth effects were greatly influenced by the environmental growth conditions used. Studies to examine root colonisation patterns, using Rhizobium strains into which a plasmid expressing the green fluorescent protein has been placed, showed that the bacteria preferentially colonise rice seedling surfaces mainly in clumps. This occurs along grooves on the rice root surface, or at the emerging lateral root zones and at the root tips. However, rhizobia could also colonise intercellularly in lateral roots formed on the main roots near the culm region of the seedling. Under the growth conditions used, this occurred most frequently with strain R4 which multiplied and migrated to form long lines of individual bacterial cells along the inside of growing lateral roots. A bioassay to measure bacterial multiplication in rice leaves showed that the rice-associating strains can multiply and survive at different rates within these tissues. They were not, however, detected migrating into other parts of the leaf from the original site of pressure-infiltration, indicating that the bacterial ability to migrate within the lateral roots is not matched by a similar capacity in rice leaves. We suggest that some of these rice-associating bacteria possess important genes that enhance their ability to intimately colonise niches on and within rice tissues, and promote rice plant growth.
TL;DR: The results demonstrate that Mn compartmentation occurs at both the tissue and the organelle level and that Mn accumulation in the epidermis-enriched fraction could contribute to Mn tolerance in common bean.
Abstract: Non-aqueous fractionation was used to characterize subcellular and tissue Mn compartmentation of mature and immature leaves of two common bean (Phaseolus vulgaris L.) cultivars contrasting in their response to Mn toxicity. Excess Mn decreases leaf CO2 assimilation through a reduction of chlorophyll content in immature leaves with no effect detected on mature leaves. We hypothesized that differential accumulation of Mn in chloroplasts occurs at different leaf developmental stages. Chloroplasts of immature leaves accumulated at least three times as much Mn as those of mature leaves at equivalent total foliar Mn. Chlorosis was positively correlated with Mn concentration in chloroplasts from high-Mn plants (r2 = 0.96; P = 0.003) but was not correlated with Mn in unfractionated tissue (r2 = 0.026; P = 0.793) nor with Mn in the epidermis-enriched fraction (r2 = 0.33; P = 0.314). Both cultivars showed high accumulation of Mn in the vacuoles as determined by the co-localization of α-mannosidase and Mn content on a continuous density gradient. Cultivars differed significantly in Mn concentration in an epidermis-enriched fraction, with the tolerant cultivar Calima accumulating more Mn in this fraction than the sensitive cultivar ZPV-292. In both cultivars, Mn was accumulated up to 2400 µg g–1 dry weight in crystal-type structures whereas the unfractionated leaf tissue contained about 500 µg g–1 dry weight. The results demonstrate that Mn compartmentation occurs at both the tissue and the organelle level and that Mn accumulation in the epidermis-enriched fraction could contribute to Mn tolerance in common bean. The role of Mn accumulation in structures resembling oxalate crystals is discussed.
TL;DR: Salinity stress diminished the inherent efficiency of the C4 CO2 concentrating mechanism in addition to reducing net CO2 uptake in plants grown in a glasshouse at five levels of salinity.
Abstract: Photosynthetic gas exchange, carboxylase activities, and leaf tissue carbon isotope discrimination (Δ) were measured in Atriplex lentiformis (Torr.) Wats. (saltbush) plants grown in a glasshouse at five levels of salinity ranging from 0 to 600 mM NaCl. The net CO2 assimilation rate decreased by 64% from the lowest to the highest level of salinity imposed. The quantum yield for CO2 uptake was maximal in plants grown at 50 mM NaCl, and decreased sharply above and below this salinity level. The ratio of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activity to that of phosphoenolpyruvate carboxylase (PEPC) decreased from 0.96 in plants grown at 0 mM NaCl to 0.37 in plants grown at 600 mM NaCl because PEPC activity on a leaf area basis increased linearly with increasing salinity, while Rubisco activity remained relatively constant. Compensatory changes in the leaf area/dry weight ratio and area-based leaf N content with increasing salinity suggested that the linear increase in PEPC activity was a passive response to increasing area-based leaf N content, whereas Rubisco activity on a leaf N basis actually dropped sharply. Relative leakage of CO2 from the bundle sheath, calculated from measurements of gas exchange and foliar Δ values, increased with increasing salinity in parallel with the decrease in the ratio of C3 cycle activity relative to C4 cycle activity. These results suggest that salinity stress diminished the inherent efficiency of the C4 CO2 concentrating mechanism in addition to reducing net CO2 uptake.
TL;DR: A model for the topological organization of the chloroplast thylakoid membrane is presented, which accounts both for the folding of one continuous membrane into multiple grana stacks as seen in cross-sectional electron micrographs and for the rapid reversible unfolding (destacking) of thegrana stacks into lamellar sheets.
Abstract: A model for the topological organization of the chloroplast thylakoid membrane is presented. A series of illustrations is provided, which outline a suggested 3-dimensional structure in cross-section and in full shape, which accounts both for the folding of one continuous membrane into multiple grana stacks as seen in cross-sectional electron micrographs and for the rapid reversible unfolding (destacking) of the grana stacks into lamellar sheets.
TL;DR: In the light, mitochondrial respiration estimated from the emission of 12 CO2 after complete labeling of photorespiration was lower than in the dark, suggesting either light-inhibition or recycling of respiratory carbon.
Abstract: Photorespiration rate can be estimated in vivo but the actual amount of photorespiratory CO2 emitted or recycled by the leaf is largely unknown. We exploited the insensitivity of infrared gas analy- zers to 13 CO2 to detect the photorespiratory CO2 emission of leaves exposed to air containing only 13 CO2. Photorespiratory CO2 emission was calculated by subtracting the 12 CO2 emitted under non-photorespira- tory (2% O2) conditions from that emitted under photorespiratory (21% O2) conditions. Illuminated leaves of herbaceous and tree species emitted a constant amount of 12 CO2 within 30-60 s after switching to 13 CO2. Photorespiratory CO2 emission was less than 20% of the photorespiratory rate simultaneously calculated by combining fluorescence and gas exchange. Thus, the leaves recycled more than 80% of photorespiratory CO2. The highest recycling was associated with high rates of photosynthesis in tomato and spinach. In the dark, mitochondrial respiration measured by the emission of 12 CO2 in air with 13 CO2 was similar to that measured by conventional gas exchange in ambient air, thus confirming the accuracy of our method. In the light, mitochondrial respiration estimated from the emission of 12 CO2 after complete labeling of photorespiration was lower than in the dark, suggesting either light-inhibition or recycling of respiratory carbon.
TL;DR: It is suggested that CAM may be more prevalent in tropical epiphytic and lithophytic ferns than currently envisaged and carbon isotope ratios may not, by themselves, be sufficient for the identification of weak CAM.
Abstract: Crassulacean acid metabolism (CAM) was observed in three species of tropical ferns, the epiphytes Microsorium punctatum and Polypodium crassifolium and the lithophyte Platycerium veitchii. Polypodium crassifolium and P. veitchii exhibited characteristics of weak CAM. Although no net nocturnal CO2 uptake was observed, the presence of CAM was inferred from nocturnal increases in titratable acidity of 4.7 and 4.1 µequiv (g fr wt)–1 respectively, a reduction in the rates of net CO2 evolution during the first half of the dark period, and the presence of a CAM-like decrease in net CO2 uptake during the early light period. In M. punctatum net CO2 uptake during the first half of the dark period was accompanied by an increase in titratable acidity of 39.2 µequiv (g fr wt)–1 and a pronounced reduction in net CO2 uptake during the early light period. When water was withheld from P. crassifolium and M. punctatum, net CO2 uptake during the light was reduced markedly but there was no change in the extent or patterns of CO2 exhange in the dark. As a consequence, the proportion of carbon gained due to CO2 fixation in the dark increased from 2.8 and 10% to 63.5 and 49.3%, respectively (100% being net CO2 uptake during the light plus the estimated CO2 uptake during the dark). After 9 days without added water, dark CO2 uptake was responsible for the maintenance of a net 24 h carbon gain in P. crassifolium. Platycerium veitchii, P. crassifolium and M. punctatum exhibited carbon isotope ratios of between –25.9 and –22.6‰ indicating that carbon isotope ratios may not, by themselves, be sufficient for the identification of weak CAM. We suggest that CAM may be more prevalent in tropical epiphytic and lithophytic ferns than currently envisaged.
TL;DR: Northern blot analysis indicated that the ghFAD2-1 transcript was specifically induced during embryo development and expression of the transcript could not be detected in leaves and Southern blot analysis revealed that microsomal ω-6 desaturase in cotton is encoded by a small multigene family.
Abstract: A cDNA (ghFAD2-1) encoding a seed-specific microsomal ω-6 desaturase was isolated from a cotton (Gossypium hirsutum L. cv. Deltapine-16) embryo cDNA library. The deduced amino acid sequence showed substantial similarity to other plant microsomal ω-6 desaturases. Northern blot analysis indicated that the ghFAD2-1 transcript was specifically induced during embryo development and expression of the transcript could not be detected in leaves. Southern blot analysis using the coding region and 3′ untranslated region of ghFAD2-1 revealed that microsomal ω-6 desaturase in cotton is encoded by a small multigene family. There are at least two copies of ghFAD2-1 in two tetraploid cotton species (G. hirsutum L. and G. barbadense L.) and at least one copy in diploid cotton species (G. herbaceum L., G. raimondii Ulbrich and G. robinsonii Mueller).
TL;DR: This form of the model was tested against field measurements of photosynthesis, transpiration and stomatal conductance in Scots pine shoots during three clear summer days and explained 84–98% of the variance in the data.
Abstract: The ‘optimality hypothesis’ of gas exchange in plants has been studied since the 1970s, but testing it in the field has proven difficult. A recent reformulation of the hypothesis with detailing assumptions on leaf structure makes it possible to solve the optimisation problem explicitly, such that the predictions of gas exchange are readily testable against field data. This form of the model was tested against field measurements of photosynthesis, transpiration and stomatal conductance in Scots pine (Pinus sylvestris) shoots during three clear summer days. Model parameters were estimated independently from photosynthesis measurements on preceding days. The measurements were carried out at a new field measurement station with a very low level of noise. The predictions of photosynthesis, transpiration andstomatal conductance explained 84–98% of the variance in the data.
Journal Article•
TL;DR: Snow gum (Eucalyptus pauciflora Sieb. ex Spreng.) seedlings were grown from autumn through spring in open top chambers located in a pasture naturally subject to freezing temperatures in either ambient or elevated CO2 concentrations, with seasonal differences in photoinhibition consistent with seasonal Differences in the capacity for growth.
Abstract: Snow gum (Eucalyptus pauciflora Sieb. ex Spreng.) seedlings were grown from autumn through spring in open top chambers located in a pasture naturally subject to freezing temperatures in either ambient or elevated (350 µL L-1 above ambient) CO2 concentrations. Sustained reduction in quantum efficiency, as measured by chlorophyll fluorescence (Fv/Fm), in over-wintering leaves may be related to seasonal down-regulation of photosynthesis, combined with cumulative effects of freeze- induced damage to the photosynthetic apparatus, with the effect being greater in leaves grown under elevated [CO2]. Down-regulation of photosynthesis apparently occurred in response to seasonal limitations to growth which were not overcome by elevation of [CO2] despite temperatures being favorable for photosynthesis during most of the photoperiod. Elevated [CO2] had no effect on growth of over-wintering seedlings, but enhanced growth in spring when minimum temperatures rose consistently above freezing. As there were no effects of elevated [CO2] on allocation, the stimulation of growth in spring was attributable to increase in net assimilation rates. Thus seasonal differences in photoinhibition were consistent with seasonal differences in the capacity for growth, with plants grown under elevated [CO2] having to dissipate more excess excitation energy over-winter.
TL;DR: The interaction between proton and aluminium toxicities was examined in soybean, pea and bean species, and the characteristic genotype response for the three species examined were consistent with the hypothesis that responses of genotypes may be predicated upon conditions emphasizing irreversible (cell division) effects.
Abstract: The interaction between proton and aluminium toxicities was examined in soybean (Glycine max L.), pea (Pisum sativum L.) and bean (Phaseolus vulgaris L.). Accurate characterization of genotype Al-sensitivity in solution culture experiments, employing a continuous Al-exposure, required response definition at both growth-enhancing and growth inhibiting activities. Essentially, this involved multiple controls to account for the interactive effects. Experiments measuring growth during a recovery period in Al-free solution were more successful in distinguishing genotypic response to Al, especially in species demonstrating high H+ -sensitivity. The time, concentration and basal solution dependencies of the characteristic genotype response for the three species examined were consistent with the hypothesis that responses of genotypes may be predicated upon conditions emphasizing irreversible (cell division) effects. Cell extension effects, however, appear to be rapid, largely-reversible and often not different between differentially Al-sensitive genotypes growing in a complete nutrient solution.
TL;DR: In this article, the potential, differential effect of a trichome layer or a pigmented epidermis on the light microenvironment within the mesophyll was investigated using a quartz fibre-optic microprobe.
Abstract: Young leaves of three grape cultivars having different surface characteristics (cv. Athiri, pubescent; cv. Soultanina, glabrous green; and cv. Fraoula, glabrous red-brown) only during the early stages of their development, were used to investigate the potential, differential effect of a trichome layer or a pigmented epidermis on the light microenvironment within the mesophyll. The penetration of forward propagated 310, 360 and 430 nm radiation into the leaf tissues was monitored using a quartz fibre-optic microprobe. The mesophyll of the young leaves of Athiri was much better protected from ultraviolet-A, ultraviolet-B and high visible radiation compared to the other two cultivars. Abaxial, as well as adaxial trichome layers attenuated almost all incident radiation at 310 nm and 360 nm and a considerable part of the blue light (430 nm). No significant differences in light attenuation from epidermal and mesophyll layers between the other two cultivars were observed. The adaxial epidermis of young and dehaired leaves of cv. Athiri was largely ineffective in absorbing the incident ultraviolet- B radiation. In addition, the dehaired lamina of such leaves exhibited 80% lower relative concentration of ultraviolet-absorbing compounds per leaf surface area, compared to leaves of similar ontogenetic stage in the cv. Soultanina. It is proposed that the occurrence of a dense trichome layer in young leaves, besides other proposed functions, may play a protective role against not only ultraviolet radiation damage, but also against high insolation. This protection could be advantageous under stress conditions during leaf development.
TL;DR: Photosynthesis was studied in relation to light use in the mangrove, Avicennia marina (Forsk.) Vierh.
Abstract: Photosynthesis was studied in relation to light use in the mangrove, Avicennia marina (Forsk.) Vierh. var. australasica (Walp.) Moldenke, growing under soil salinities equivalent to one and two times seawater (i.e. 35 and 60‰). Midday CO2 assimilation rates averaged 7.6 0.7 and 4.3 0.3 µmol m–2 s–1 at the seawater and hypersaline sites, respectively. Despite this difference, xanthophyll pool sizes per Chl and epoxidation states were similar at both sites. Non-photochemical quenching also indicated comparable energy dissipation from pigment beds. Electron transport rates calculated from fluorescence characteristics were also similar and exceeded the requirements to sustain measured assimilation rates. However, cell wall conductance was low in seawater plants (75 mmol m2 s–1 ) and declined to 40 mmol m–2 s–1 in hypersaline plants. This would cause CO2 concentrations in chloroplasts (Cc ) to be lower than expected from measurements of intercellular CO2 concentrations (Ci ). In seawater plants, Cc was estimated to be 144 µmol mol–1 when Ci was 245 mmol mol–1, while values for Cc and Ci in hypersaline plants were 78 and 212 mmol mol–1, respectively. Reductions in Cc would enhance rates of photorespiration relative to assimilation, with the higher photorespiratory rates being sufficient to account for apparent excess electron transport rates.
TL;DR: These features indicate the involvement of at least three steps in development of resistance in Framida roots: the first is linked to a partial inhibition of development of the young haustorium; the second could play a role in the physiological events that decrease nutrient translocation towards the haus- toria; and the last seems to be associated with the accumulation of a coloured phenolic-like material.
Abstract: Germination, attachment to host root and growth of Striga hermonthica (Del.) Benth. seeds and seedlings were studied in in vitro co-culture w ith Sorghum bicolor (L.) Moench and in pot experiments. Two varieties, the resistant Framida and the susceptible CK-60B, were used. Histological, morphological and physiological studies revealed the key stages of resistance mechanisms involved. Resistance of Framida to Striga does not occur at the germination or the attachment stages, since its roots do not support fewer Striga than does CK-60B. As Framida roots support the lowest number of young Striga stems with scale leaves, its resistance appears to occur during the establishment of a functional haustorium. Metabolite uptake by the haustorium and growth rate of the young parasite were lower on Framida roots than on CK-60B roots, even when similarly developed haustoria were compared. Furthermore, at a later stage of infestation, significant accumulation of a coloured material likely to be rich in phenolic compounds was observed in and around Framida conductive tissues, but not CK-60B tissues. These features indicate the involvement of at least three steps in development of resistance in Framida roots: the first is linked to a partial inhibition of development of the young haustorium; the second could play a role in the physiological events that decrease nutrient translocation towards the haustoria; and the last seems to be associated with the accumulation of a coloured phenolic-like material.
TL;DR: Association of putative PSII fluorescence with pyrenoids was found in most taxa, although fluorescence of trans-pyrenoid thylakoids was seen in one case only, and it is speculated that this fluorescence may represent a subset of PSII that functions withThylakoid carbonic anhydrase to provide locally high concentrations of CO2 to the Rubisco in the pyrenoid core, as predicted by Raven.
Abstract: Confocal microscopy was used to examine heterogeneity of chlorophyll fluorescence in chloroplasts of selected green algae, in the light of evidence that the technique reveals the distribution of photosystem II (PSII). Three levels of complexity were seen: (1) uniform fluorescence (Codium) or intergrading zones of bright and less bright fluorescence in genera known from electron microscopy to have irregular areas of thylakoid appression (e.g. Chlamydomonas — in which Bertos and Gibbs (J. Phycol., 34, 1009, 1998) have found absence of segregation of photosystem I (PSI) and PSII, Ulothrix, Stigeoclonium, Draparnaldia); (2) a pattern of 1–2 µm patches of fluorescence on a less bright uniform background, in taxa where more organized thylakoid stacking (but not grana sensu higher plants) is seen by electron microscopy (Ulva, Oedogonium); and (3) Discrete 0.5–2 µm spots of fluorescence in a relatively fluorescence-free background, closely resembling higher plant grana (Cladophorophyceae, Zygnematales, Coleochaete, Nitella). Further investigation of these states may illuminate the evolution of higher plant thylakoid systems, where PSII is segregated into grana, and may provide clues concerning the adaptive advantages of grana. Association of putative PSII fluorescence with pyrenoids was found in most taxa, although fluorescence of trans-pyrenoid thylakoids was seen in one case only. The association prevails whether or not there is a starch sheath around the pyrenoids, and is persistent in pyrenoids isolated from Spirogyra. We speculate that this fluorescence may represent a subset of PSII that functions with thylakoid carbonic anhydrase to provide locally high concentrations of CO2 to the Rubisco in the pyrenoid core, as predicted by Raven (Plant, Cell and Environment, 20, 147, 1997).
TL;DR: Despite the continuum of intercellular air spaces in the root cortical tissue the lack of aerenchyma and therefore low rates of internal oxygen diffusion restricts root growth in anaerobic media and presumably contributes to the sensitivity of Brassica napus to waterlogging.
Abstract: The morphology and physiology of the response of two cultivars of Brassica napus to an anaerobic root medium was investigated. The cultivars Chikuzen and Topas showed a large reduction in growth rate when their roots were exposed to a de-oxygenated stagnant nutrient solution containing 0.1% w/v agar. Older seedlings (11 d old) were more sensitive to stagnant agar, expressed as biomass accumulation, than younger ones (5 d old). Brassica napus was characterized by a constitutively low root porosity (3–5%), typical for plant species with a low tolerance to waterlogging. A hypoxia pre- treatment (16 h; 2.25% O2) before exposure to de-oxygenated stagnant agar had no effect on the final number or length of lateral roots and adventitious roots. Brassica napus cv. Chikuzen is characterized by radial oxygen loss being most at the basal portion of the root, when a strong oxygen sink surrounds the root. Oxygen profiles through laterals of Brassica napus cv. Chikuzen show a typical pattern with low oxygen concentrations in the stele and somewhat higher levels in the cortex. Despite the continuum of intercellular air spaces in the root cortical tissue the lack of aerenchyma and therefore low rates of internal oxygen diffusion restricts root growth in anaerobic media and presumably contributes to the sensitivity of Brassica napus to waterlogging.