Showing papers in "Australian Journal of Plant Physiology in 1995"
TL;DR: It is concluded that soil salinity has not yet become a sufficient agricultural problem, other than on a local scale, to make salt resistance a high priority objective for plant breeders, and that enhancing the salt resistance of at least some crops will be necessary.
Abstract: Soil salinity is widely reported to be a major agricultural problem, particularly in irrigated agriculture, and research on salinity in plants has produced a vast literature. However, there are only a handful of instances where cultivars have been developed which are resistant to saline soils. Reasons for the lack of success in developing salt-resistant genotypes, and for the low impact that plant physiological research has made, are explored. We conclude that soil salinity has not yet become a sufficient agricultural problem, other than on a local scale, to make salt resistance a high priority objective for plant breeders. The limited success of simple selection, where this has been practised in breeding programs, can be accounted for by the fact that research has consistently shown salt resistance is a complex character controlled by a number of genes or groups of genes and involves a number of component traits which are likely to be quantitative in nature. We also conclude that the results of physiological research have been poorly marketed by physiologists and, understandably, have failed to impress plant breeders. We anticipate that the importance of salinity as a breeding objective will increase in the future. Our assessment of reports of the degradation of irrigation systems, together with projections of the future demands of irrigated agriculture, is that enhancing the salt resistance of at least some crops will be necessary. Salinity resistance will both help provide stability of yield in subsistence agriculture and, through moderating inputs, help limit salinisation in irrigation systems with inadequate drainage. It is emphasised that plant improvement and drainage engineering should be seen as partners and not alternatives. We conclude with a personal view of one way forward for developing salt-resistant genotypes, through the pyramiding of physiological characters.
1,019 citations
Journal Article•
570 citations
TL;DR: Despite considerable taxonomic variability within a given plant community, the averaged δ13C signature for that community gives a strong indication of moisture availability.
Abstract: Carbon isotope natural abundance (δ13C) has been previously used as a powerful tool in the study of water-using processes at the leaf, individual and within-community levels. We analysed 348 species from 12 plant communities along a 900 km-long rainfall gradient in southern Queensland. Although the range of δ13C values found in a given community was large, variability in the δ13C signature of plants within a community was relatively small given the large numbers of species sampled (mean n per site of 29) and the taxonomic diversity in each. The community-averaged δ13C signature ranged from -25.60 in a brigalow (Acacia harpophylla) dominated community in western Queensland to - 31.20 in subtropical rainforest in eastern Queensland. A strong relationship was found between the δ13C value averaged for each site and rainfall within the range 350-1700 mm per annum. Foliar δ13C was also related to the number of rain days per annum and moisture balance (rainfall - evaporation). The strength of these relationships varied only slightly according to the rainfall parameter used, with values for r2 of 0.78, 0.70, 0.70 and 0.74 for the relationship between δ13C and long-term rainfall average, 5-year rainfall average, number of rain days and moisture balance, respectively. Despite considerable taxonomic variability within a given plant community, the averaged δ13C signature for that community gives a strong indication of moisture availability.
438 citations
TL;DR: The hypothesis that the growth response to salinity has two phases is supported, and most changes in metabolism or gene expression leading to growth reductions during the first phase relate to the osmotic effect of salinity, not to any salt-specific effect.
Abstract: Wheat and barley genotypes that differed in salt tolerance were used to test a hypothesis that the growth response to salinity has two phases. In the first phase there would be a large decrease in growth rate caused by the salt outside the roots, i.e. an 'osmotic' response. In the second phase there would be an additional decline in growth caused by salt having built up to toxic levels within plants, i.e. a 'salt-specific' response. If this two-phase model is correct, genotypes that differ in their ability to exclude salt or tolerate high internal salt concentrations would not differ in growth rate for some period of time in saline conditions. This hypothesis was tested using many genotypes that had previously been found to differ greatly in salt tolerance, as defined by differences in percent reduction in yield or biomass after prolonged exposure to NaCl. Leaf extension of 15 wheat and barley genotypes was measured daily while the NaCl in the nutrient solution was increased over 10 days to 250 mM. All 15 genotypes showed a similar percentage reduction in leaf extension rate. Dry matter production of four selected wheat genotypes that again differed greatly in salt tolerance was measured for up to 6 weeks in 150 mM NaCl. All genotypes showed the same growth reduction for 4 weeks. After this initial period the more salt- sensitive genotype showed a greater decline in growth. This occurred after 60% of the leaves were dead. These data strongly support the hypothesis that the growth response to salinity has two phases, and indicate that most changes in metabolism or gene expression leading to growth reductions during the first phase relate to the osmotic effect of salinity, not to any salt-specific effect. They also indicate that the salt within the plant reduces growth by causing premature senescence of old leaves and hence a reduced supply of assimilates to the growing regions.
435 citations
TL;DR: The xanthophyll cycle, and the photoprotective energy dissipation process associated with it, would appear to provide plants the flexibility required to deal with the excessive levels of light absorbed by chlorophyll under a wide range of climatic conditions, and can quite possibly account for the 'photoinhibition' observed during winter stress.
Abstract: Sustained decreases in intrinsic photosystem II efficiency (i.e. Fv/Fm) in response to high light and chilling temperatures were examined in eight species, and were found to be accompanied by the retention of zeaxanthin (Z) and antheraxanthin (A) overnight. The quantitative relationship between changes in Fv/Fm and the A + Z level during these sustained changes on cold days was similar to that obtained for rapidly reversible changes on warm days. Furthermore, upon removal of leaves from the field, recovery from 'photoinhibition' (the reversal of the depression of Fv/Fm) matched the timecourse of the epoxidation of Z and A to violaxanthin (V). These findings suggest that the 'photoinhibition' occuring in these species might be due to the sustained engagement of these de-epoxidised components of the xanthophyll cycle in photoprotective energy dissipation. When examined over the course of several days during the winter, the predawn conversion state of the xanthophyll cycle responded to the daily changes in minimum air (and leaf) temperature, such that the xanthophyll cycle was largely de-epoxidised prior to sunrise on cold nights and was present predominantly as V after nights when the nocturnal temperatures were above freezing. In addition, in some of the species examined, there was a large acclimation of the xanthophyll cycle pool size to the level of excessive light, with a much larger pool present in the leaves examined during the winter and that pool being de-epoxidised to Z and A to a much greater degree at midday than from similar leaves examined during the summer. The xanthophyll cycle, and the photoprotective energy dissipation process associated with it, would thus appear to provide plants the flexibility required to deal with the excessive levels of light absorbed by chlorophyll under a wide range of climatic conditions, and can quite possibly account for the 'photoinhibition' observed during winter stress.
326 citations
TL;DR: The general principles involved in chlorophyll fluorescence quenching analysis by the saturation pulse method are presented, outlining the rationale for using the empirical fluorescence parameters Fm and Fm' as indices for the photosystem II (PSII) photochemical quantum yield, ΦII, in the dark-adapted or illuminated states, respectively.
Abstract: The general principles involved in chlorophyll fluorescence quenching analysis by the saturation pulse method are presented, outlining the rationale for using the empirical fluorescence parameters Fv/Fm and Fv/Fm' as indices for the photosystem II (PSII) photochemical quantum yield, ΦII, in the dark-adapted or illuminated states, respectively. The relationship between ΦII and the quantum yield of photosynthetic electron transport is linear over a wide range of quantum flux densities. However, there is a fraction of PSII contributing approximately 30% to maximal quantum yield, which is closed at rather low quantum flux densities, while at the same time there is only a small drop in ΔF/Fm'. The details of Fm and Fm' determination by application of saturating light are critically examined, with emphasis on the situation in algae where the fluorescence rise to the peak leLel is followed by a rapid decline. For this purpose, the rapid induction kinetics upon onset of strong continuous illumination are investigated. Dark-adapted samples show two distinct intermediate fluorescence levels, I1 and I2, in the polyphasic rise from the O to the P level. The I1 level separates a biphasic 'photochemical' rise, which also can be induced by a saturating single turnover flash, from several 'thermal' phases, induction of which requires multiple turnovers at PSII. Arguments are put forward favouring the I2 level for assessment of Fm or Fm', on which calculation of Fv/Fm or ΔF/Fm' is based. It is shown that although an assessment based on the I1 level, as practised by the so-called pump- and-probe method, does lead to a consistent underestimation of ΔF/Fm, in many cases similar information as with I2 determination is obtained.
308 citations
TL;DR: In this paper, the maximum change in the quantum yield (ΔOmax) of photosystem II (PSII), as well as the effective absorption cross-section of PSII (σPSII) was measured in the open ocean.
Abstract: The ocean is optically thin and lends itself to large-scale measurements of in vivo chlorophyll fluorescence. In the open ocean, however, phytoplankton chlorophyll concentrations average only 0.2 μg L-1, and hence high sensitivity is required for precise measurements of the fluorescence yields. Over the past decade, we have developed two approaches to achieve the required sensitivity; these are the pump- and probe-technique and a fast repetition rate (FRR) method. Both methods have been adapted for in situ studies and are used to rapidly measure the maximum change in the quantum yield (ΔOmax) of photosystem II (PSII), as well as the effective absorption cross-section of PSII (σPSII). Sections of variable fluorescence across the Pacific and Atlantic Oceans reveal the influence of geophysical processes in controlling the quantum yields of phytoplankton photosynthesis. Areas of upwelling, such as off the coast of north-westem Africa, have Fv/Fm values of 0.65, which are close to the maximum achievable values in nutrient-replete cultures. Throughout most of the nutrient-deficient central ocean basins, this quantum efficiency is reduced by more than 50%. In high-nutrient, low- chlorophyll regions of the eastern Equatorial Pacific, the deliberate, large-scale addition of nanomolar iron directly to the ocean leads to a rapid increase in quantum efficiency of the natural phytoplankton community, thereby revealing that in these regions phytoplankton photosynthetic energy conversion efficiency is iron limited. Diel patterns of variation in the upper ocean display midday, intensity- dependent reductions in both upsII and AOmax. We interpret the former as indicative of non- photochemical quenching in the antenna, while the latter is a consequence of both rapidly reversible and slowly reversible damage to reaction centres. From knowledge of the incident spectral irradiance, ΔOmax, σPSII, and photochemical quenching, the absolute photosynthetic electron transport rate can be derived in real-time. Using unattended, moored continuous measurements of in vivo fluorescence parameters, the derived in situ electron transport rates can be related to satellite observations of the global ocean with basin-scale, seasonal estimates of phytoplankton carbon fixation. Thus, unlike any other photosynthetic parameter, chlorophyll fluorescence can be used to bridge the scales of biophysical responses to ecosystem dynamics.
282 citations
TL;DR: The hypothesis that the Al-stimulated efflux of malate from root apices is involved in a general mechanism for Al tolerance in wheat is supported.
Abstract: Aluminium (Al) can stimulate the efflux of malate and other organic acids from root apices of wheat (Triticum aestivum L.) seedlings. This response has been implicated in a mechanism of Al tolerance since the amount of malate released from an Al-tolerant genotype was 5-10-fold greater than the amount released from a near-isogenic, but Al sensitive, genotype. In the present study, 36 wheat cultivars were screened for Al tolerance and for the amount of malate released from their root apices with a standard A1 treatment. Excised root apices (3.0 mm) were used to measure malate efflux, and the relative tolerance to Al was determined from root growth measurements in 3 and 10μM AlCl3 with 200 μM CaCl2, pH 4.3. There was a significant correlation between relative tolerance of the genotypes to Al and the amount of malate released from their root apices. Growth measurements were also used to investigate the amelioration of Al toxicity by exogenous malate. In the presence of 3 μM Al alone, relative root growth of an Al-sensitive genotype was reduced to 13% of the control. Addition of 10 μM malate to the solution increased relative root growth to 50%, and 20 µM malate completely alleviated the Al-induced inhibition of root growth. The results support the hypothesis that the Al-stimulated efflux of malate from root apices is involved in a general mechanism for Al tolerance in wheat.
278 citations
TL;DR: It is concluded that an increased capacity for xanthophyll cycle-dependent energy dissipation is a key component of the acclimation of Leaves to a variety of different forms of light stress, and that the response of leaves to excess light experienced in the growth environment is thus likely to be qualitatively different from that to sudden experimental exposures to PFDs exceeding the growth PFD.
Abstract: The effect of an acclimation to light stress during the growth of leaves on their response to high photon flux densities (PFDs) was characterised by quantifying changes in photosystem II (PSII) characteristics and carotenoid composition. During brief experimental exposures to high PFDs sun leaves exhibited: (a) much higher levels of antheraxanthin + zeaxanthin than shade leaves, (b) a greater extent of energy dissipation in the light-harvesting antennae, and (c) a greater decrease of intrinsic PSII efficiency that was rapidly reversible. During longer experimental exposures to high PFD, deep-shade leaves but not the sun leaves showed slowly developing secondary decreases in intrinsic PSII efficiency. Recovery of these secondary responses was also slow and inhibited by lincomycin, an inhibitor of chloroplast-encoded protein synthesis. In contrast, under field conditions all changes in intrinsic PSII efficiency in open sun-exposed habitats as well as understory sites with intense sunflecks appeared to be caused by xanthophyll cycle-dependent energy dissipation. Furthermore, comparison of leaves with different maximal rates of electron transport revealed that all leaves compensated fully for these differences by dissipating very different amounts of absorbed light via xanthophyll cycle-dependent energy dissipation, thereby all maintaining a similarly low PSII reduction state. It is our conclusion that an increased capacity for xanthophyll cycle-dependent energy dissipation is a key component of the acclimation of leaves to a variety of different forms of light stress, and that the response of leaves to excess light experienced in the growth environment is thus likely to be qualitatively different from that to sudden experimental exposures to PFDs exceeding the growth PFD.
256 citations
TL;DR: Results of recent research indicate the need for a new sucrose accumulation model based on a dynamic model of rapid cycling and turnover of sucrose between the vacuole and metabolic and apoplastic compartments.
Abstract: Sucrose accumulation has been studied extensively in sugarcane-an example of a highly productive crop plant with the capacity for storing large quantities of sugar. Initial recognition and characterisation of the enzymes involved in sucrose synthesis and cleavage led to widely accepted models of how sucrose transport and accumulation occur. Studies on cells in culture and on isolated cellular fragments initially supported and strengthened these models but more recently have revealed weaknesses in them. Biophysical measurements and anatomical, histochemical, and tracer dye studies further eroded the older models. Molecular studies are beginning to reveal details at the gene and transcriptional levels of the enzymes involved in sucrose transport and metabolism. Collectively, results of recent research indicate the need for a new sucrose accumulation model. A dynamic model of rapid cycling and turnover of sucrose between the vacuole and metabolic and apoplastic compartments explains much of the data, but details of how the cycling is regulated remains to be discovered.
224 citations
TL;DR: Those cultivars most tolerant of high temperature during kernel filling (least reduction of kernel dry weight at maturity) were those where the rate of kernel filling was most enhanced by high temperature, i.e. the increased rate compensated for the reduced duration ofkernel filling.
Abstract: Wheat (Triticum aestivum L.) plants were grown to anthesis at 18/13oC day/night and either retained at 18/13oC or transferred to a higher temperature (24/19 or 30/25oC) for the grain-filling period. It was confirmed that high temperature resulted in a considerable drop in kernel dry weight at maturity and there was significant cultivar variation in the degree of the response. ranging from a 30 to 60% decrease in kernel dry weight at maturity for a rise in temperature from 18/13 to 30/25oC. An analysis of the rate and duration of kernel filling of seven cultivars showed that those cultivars most tolerant of high temperature during kernel filling (least reduction of kernel dry weight at maturity) were those where the rate of kernel filling was most enhanced by high temperature, i.e. the increased rate compensated for the reduced duration of kernel filling. The importance of the rate of kernel filling in determining varietal responses to high temperature illustrates the need to isolate the effect of temperature on processes in the kernel during the linear phase of growth.
TL;DR: SDS-PAGE and monoclonal antibody cross-reactivity experiments suggest that the 100 and 105 kDa polypeptides are absent from starch granules from all other species examined, including other cereals, suggesting that all the major granule proteins are involved in starch biosynthesis.
Abstract: Wheat starch contains two classes of associated proteins: proteins which are embedded within the granule and loosely associated surface proteins. The characterisation of the major proteins that are embedded in the granule are described. Gel electrophoresis on the basis of size resolved these proteins into five bands of molecular weights 60, 75, 85, 100 and 105 kDa. These polypeptides were demonstrated to be within the granule by their resistance to proteinase K digestion when granules were ungelatinised. The N-terminal sequences of these polypeptides are reported. The most prominent polypeptide is the 60 kDa granule-bound starch synthase. The N-terminal sequence obtained from the 75 kDa polypeptide shows homology to rice soluble starch synthase. The 85 kDa band was resolved into at least two types of polypeptides, one of which reacted with polyclonal antiserum to the maize branching enzyme IIb. The 100 and 105 kDa polypeptides were located only in the granule and are related, on the basis of N-terminal sequence similarity and cross-reactivity to monoclonal antibodies. SDS-PAGE and monoclonal antibody cross-reactivity experiments suggest that the 100 and 105 kDa polypeptides are absent from starch granules from all other species examined, including other cereals. It is speculated that all the major granule proteins are involved in starch biosynthesis.
TL;DR: The reason for the inherent vulnerability of PSII to photoinduced damage is discussed in terms of the special nature of P68O and the implications of the role of cytochrome b559 as a versatile protectant against donor and acceptor side photoinactivation is considered.
Abstract: Using isolated reaction centres and cores of photosystem I1 (PSII) it has been possible to elucidate the details of two separate pathways which lead to photoinhibition. The acceptor side pathway involves charge recombination resulting in the formation of the triplet state of the primary electron donor, P680. This triplet state is harmless in the absence of oxygen but in its presence gives rise to highly reactive singlet oxygen. We have shown that this singlet oxygen specifically attacks the chlorophyll of P680 itself. This process, plus other possibilities, gives rise to degradation of Dl protein involving a primary cleavage in the stromal loop joining putative transmembrane regions four and five, to yield 23 kDa N-terminal and 10 kDa C-terminal fragments. In contrast a donor side pathway is oxygen independent and is due to detrimental secondary oxidations brought about by P680+. Oxidation of accessory chlorophyll (C670) and β-carotene are observed and D1 protein is degraded by a primary cleavage in the lumenal loop between the putative transmembrane segments one and two to yield 24 kDa C-terminal and 9 kDa N-terminal fragments. In vivo studies indicate that the acceptor pathway is more common. The reason for the inherent vulnerability of PSII to photoinduced damage is discussed in terms of the special nature of P68O and the implications of the role of cytochrome b559 as a versatile protectant against donor and acceptor side photoinactivation is also considered. The likely dimeric organisation of PSII in vivo adds an additional factor to the general discussion of the molecular processes which underlie the vulnerability of PSII to photoinduced damage.
TL;DR: Evidence points to the importance of the light-harvesting chlorophyll proteins as the site of dissipation of energy, and suggests that the structure and function of these proteins are regulated by protonation and the ratio of zeaxanthin to violaxanthIn.
Abstract: Non-photochemical quenching of chlorophyll fluorescence indicates the de-excitation of light-generated excited states in the chlorophyll associated with photosystem II (PSII). The principle process contributing to this quenching is dependent on the formation of the thylakoid proton gradient and is an important mechanism for protecting PSII from photodamage. Evidence points to the importance of the light-harvesting chlorophyll proteins as the site of dissipation of energy, and suggests that the structure and function of these proteins are regulated by protonation and the ratio of zeaxanthin to violaxanthin. The minor light-harvesting proteins may have a particularly important role as the primary sites of proton binding and because of their enrichment in xanthophyll cycle carotenoids. The dynamic nature of the light-harvesting system is an important part of the process by which plants are able to adapt to different light environments.
TL;DR: This paper reviews recent advances on the regulation of gene expression and activation by protein modification and organic acids, and possible roles of the alternative oxidase are discussed.
Abstract: The alternative oxidase of the inner mitochondrial membrane catalyses cyanide-insensitive respiration in plants and fungi. The molecular biology and regulation of this oxidase have been intensively studied over the past 10 years. Genes have been isolated, expression has been investigated and novel mechanisms for the regulation of activity have been discovered. This paper reviews these recent advances, focusing on the regulation of gene expression and activation by protein modification and organic acids, and possible roles of the alternative oxidase are discussed.
TL;DR: The high salt tolerance of Rangpur lime appears to be associated with low rates of net uptake and transport of Cl, feedback control of net root to shoot transport by Cl levels in the shoot, a low interaction between water flow and Cl and Na fluxes, and a reduction in the leafiroot ratio in response to salinity.
Abstract: This paper describes the ion relations of two citrus genotypes, Etrog citron (Citrus medica L.), a salt-sensitive genotype, and Rangpur lime (Citrus reticulata var. austera hybrid?), a salt-tolerant genotype, under conditions of high NaCl concentrations. Root morphology was modified by establishing seedlings for 6 months in solution or sand culture. When established in sand culture, both genotypes displayed a high efficiency, relative to plants established in solution culture, to exclude Na from leaves, but differed in their capacities to exclude Cl; this capacity was much lower in Etrog citron than Rangpur lime. The net Cl root to shoot transport rates over 7 weeks for Etrog citron and Rangpur lime, treated with 50 mol m-3 NaCl, were 0.25 and 0.07 μmol g-1 root FW h-1, respectively. The net transport rates of Cl in Etrog citron were 10 times those in Rangpur lime between weeks 4 and 7. In Rangpur lime Cl reached pseudo steady-state levels in root and leaf tissue by week 4, thereafter, the net Cl root to shoot transport rate of Rangpur lime decreased while the rates in Etrog citron increased. The accumulation of Na and Cl in leaves of Rangpur lime, in contrast to Etrog citron, was not affected by transpiration. The high salt tolerance of Rangpur lime appears to be associated with low rates of net uptake and transport of Cl, feedback control of net root to shoot transport by Cl levels in the shoot, a low interaction between water flow and Cl and Na fluxes, and a reduction in the leafiroot ratio in response to salinity. When plants were established in solution culture the pioneer roots became elongated and the normal development of the laterals into a fibrous root system was suppressed. Plants of both genotypes, previously established in solution culture and then treated with 50 mol m-3 NaCl, accumulated levels of Na and Cl in leaves up to 200-400 mol m-3. Under these conditions the rates of net Cl root to shoot transport over the 7 weeks were 0.53 and 0.56 μmol g-1 root FW h-1 for Etrog citron and Rangpur lime, respectively. These high rates of Cl uptake and transport were attributed to changes in root permeability and increases in passive ion fluxes. Keywords: citrus, root morphology, root medium, transpiration, Cl, Na, salt accumulation, salinity.
TL;DR: Legumes are a particularly rich source of flavonoid compounds, so the diversity of these agriculturally important molecules in agronomic lines and cultivars could be exploited for increased food production.
Abstract: Flavonoids affect how plants interact with (Brady)rhizobium and vesicular-arbuscular mycorrhizae (VAM) microsymbionts, microbial pathogens, insect predators, insect pollinators and herbivores. Legumes are a particularly rich source of flavonoid compounds, so the diversity of these agriculturally important molecules in agronomic lines and cultivars could be exploited for increased food production. Also, the widespread distribution of flavonoids in plants, especially in unselected native flora of developing countries, could be explored for their increased use in medicine and disease control.
TL;DR: In this article, the photochemical yield of photosystem II (PSII) was estimated by using pixel-by-pixel analysis of leaf chlorophyll fluorescence data during steady-state photosynthesis and during a transitory saturation of photochemistry.
Abstract: A method has been developed for routine, non-invasive monitoring of the topography of leaf photochemistry. The method uses video images of leaf chlorophyll fluorescence, taken during steady-state photosynthesis and during a transitory saturation of photochemistry, to construct, pixel by pixel, an image of the photochemical yield of photosystem II (PSII). The photochemical yield of PSII was estimated according to Genty et al. (1989) (Biochimica et Biophysica Acta 990, 87-92). The effectiveness of the method was shown by mapping the heterogeneous distribution of photosynthetic activity after treatment with either a herbicide (DCMU), abscisic acid, or during the course of the induction of photosynthesis. Leaf CO2 assimilation was simultaneously monitored under non- photorespiratory conditions to estimate the average quantum yield of linear electron transport. A unique proportional relationship was found between the mean photochemical yield of PSII calculated from images of the photochemical yield of PSII, and the average quantum yield of linear electron transport in three plant species exposed to a wide range of treatments or conditions. This new ability to quantitatively visualise leaf photochemistry provides a powerful tool to probe the spatial distribution of leaf photosynthesis. Possible errors in estimating the photochemical yield of PSII from mean fluorescence measurements are discussed.
TL;DR: Fluorescence techniques provide a powerful means of linking photosynthesis with higher levels of plant functioning and have great potential for research in forest ecology.
Abstract: Functioning of photosystem II (PSII) is the most sensitive indicator of environmental stress in plants. Changes in PSII activity can be assayed rapidly and non-destructively by measurement of chlorophyll fluorescence. While there have been many laboratory studies of chlorophyll fluorescence, fluorescence techniques have seldom been applied to questions in forest ecology. Most studies have emphasised the fluorescence parameter, Fv/Fm, which is well correlated with the quantum efficiency of photosynthetic carbon dioxide assimilation or oxygen evolution. This parameter reveals information which can be related to diurnal and seasonal variation in photosynthesis, plant growth and community dynamics. Thus, fluorescence techniques provide a powerful means of linking photosynthesis with higher levels of plant functioning and have great potential for research in forest ecology.
TL;DR: The accuracy of the approximation to the basic integral of leaf photosynthesis over the canopy and over time is illustrated by applying the algorithm to compute the seasonal variation of daily canopy photosynthesis and comparing these data with corresponding values obtained by numerical integration.
Abstract: This paper presents a simple algorithm for calculating daily canopy photosynthesis given parameters of the single-leaf light response, the canopy extinction coefficient, canopy leaf area index, daylength, daily solar irradiance and daily maximum and minimum temperatures. Analytical expressions are derived for total daily production by a canopy of leaves whose light response is either a rectangular hyperbola or a Blackman response. An expression which gives an excellent approximation to canopy photosynthesis for an arbitrary hyperbolic light response is then derived. These expressions assume photosynthetically active radiation (PAR) within the canopy follows Beer's law, light-saturated photosynthetic rate at any point in the canopy is proportional to the ratio of local PAR to full-sun PAR, diurnal variation of PAR is sinusoidal, and parameters of the single-leaf photosynthetic light response do not vary diurnally. It is shown how these expressions can be used to accommodate diurnal temperature variation of photosynthesis in a simple manner. The accuracy of the approximation to the basic integral of leaf photosynthesis over the canopy and over time is illustrated by applying the algorithm to compute the seasonal variation of daily canopy photosynthesis and comparing these data with corresponding values obtained by numerical integration.
TL;DR: Developing seeds of cereals and grain legumes have proven to be useful experimental models to examine post-sieve element assimilate transport in sink tissues and the role of phytohormones as modulators of sucrose transport is uncertain.
Abstract: Developing seeds of cereals and grain legumes have proven to be useful experimental models to examine post-sieve element assimilate transport in sink tissues. Morphologically, these seeds offer well-defined sinks in which the processes of sucrose import plus efflux and influx plus metabolism may be examined independently. In all cases, sucrose is delivered through the phloem to the maternal seed tissues. Unloading from the sieve element-companion cell complexes is symplastic. Subsequently, sucrose moves through a symplastic route to cells responsible for sucrose efflux to the seed apoplast. The efflux cells are located at, or near, the maternal/filial interface. Sucrose is retrieved from the seed apoplast by the outermost cell layers of the filial tissues. Subsequent transfer of sucrose to the sites of storage in the filial tissues is confined principally to a symplastic route. Sucrose efflux from the maternal tissues appears to be passive in cereals and energy dependent in grain legumes, possibly through a sucrose/proton antiport system. Sucrose influx across the plasma membranes of the filial cells is energy dependent and, for grain legumes, is energy coupled through a sucrose/proton symporter. Studies on the control of post-sieve element transport of sucrose have focused largely on the membrane transport steps. The role of phytohormones as modulators of sucrose transport is uncertain in grain legumes, efflux from the maternal cells could be regulated by rates of sucrose utilisation in the filial tissues through a turgor homeostat mechanism located in the efflux cells.
TL;DR: The calculus of variations is used to formally prove Field's assertion that total canopy photosynthesis will be a maximum for a fixed total canopy leaf nitrogen provided the derivative δA/δN, where A is photosynthetic rate and N is leaf nitrogen concentration, has the same value throughout the canopy.
Abstract: On the basis of detailed numerical simulations, Field (1983. Oecologia 56, 341-347) stated that total canopy photosynthesis will be a maximum for a fixed total canopy leaf nitrogen provided the derivative δA/δN, where A is photosynthetic rate and N is leaf nitrogen concentration, has the same value throughout the canopy. This paper uses the calculus of variations to formally prove Field's assertion. It shows that if the single-leaf light response is a first-degree homogeneous function of both light-saturated photosynthetic rate Amax and intensity I of photosynthetically active radiation and if Amax is linearly related to N, then the optimal distribution of leaf nitrogen is linearly related to the decline in I with canopy depth, and Amax is proportional to this decline. The nature of photosynthetic gains due to optimisation of canopy nitrogen distribution is illustrated numerically for a simple model canopy. It is found that, for canopies with fixed mean leaf nitrogen, canopy photosynthesis is approximately proportional to canopy leaf area index (LAI), and the gain due to canopy optimisation compared with a uniform canopy is small for shallow canopies but pronounced for deep canopies. It is also found that, for canopies with fixed total leaf nitrogen, there is a canopy LAI which maximises canopy photosynthesis, and that this LAI and the corresponding canopy photosynthesis are approximately proportional to total canopy nitrogen.
TL;DR: Experimental data are more consistent with a model of 'connected units' for PSIIα than with the pure 'lake' model, and the sigmoidicity of the induction kinetics is shown to depend on both the connectivity of the photosynthetic units and on reaction centre parameters.
Abstract: The theoretical relationships between the fluorescence and photochemical yields of photosystem II (PSII) and the fraction of open reaction centres are examined in a model based on the following assumptions: (a) a homogeneous, infinite PSII domain; (b) exciton-radical pair equilibrium; and (c) different rates of exciton transfer between 'core' and 'peripheral' antenna beds. Simple analytical relations are examined for the yields and their time-courses in induction experiments. Variation of the inter-unit transfer rate allows continuous transition from the case of 'separated units' to the pure 'lake' model. Widely used relations for estimating the fraction of closed reaction centres from the complementary area of the fluorescence, or the photochemical yield from fluorescence levels are derived. An experimental induction curve is analysed, considering its composition of 'α' and 'β' centres. The sigmoidicity of the induction kinetics is characterised by a single parameter J (corresponding to Joliots' 'p'), that is shown to depend on both the connectivity of the photosynthetic units and on reaction centre parameters. On the other hand, the relation between J and the extreme fluorescence levels (or the deviation from the linear Stern-Volmer dependence of 1 /Φf, on the fraction of open traps) is only controlled by antenna connectivity. Experimental data are more consistent with a model of 'connected units' for PSIIα than with the pure 'lake' model.
TL;DR: Measurements of time-resolved fluorescence decay, laser-flash-induced absorption changes in the UV and at 820 nm and of the relative fluorescence quantum yield in different preparations from spinach led to a number of conclusions.
Abstract: Measurements of time-resolved fluorescence decay, laser-flash-induced absorption changes in the UV and at 820 nm and of the relative fluorescence quantum yield in different preparations (thylakoids, photosystem II (PSII) membrane fragments and PSII core complexes) from spinach led to a number of conclusions. (1) Light is transformed into Gibbs energy with trapping times of 250 ps and 130 ps in open reaction centres of PSII membrane fragments and PSII core complexes, respectively. Assuming rapid Boltzmann distribution of excitation energy and taking into account the antenna properties (size and spectral distribution), the molecular rate constant of primary charge separation is estimated to be about (3 ps)-1. (2) The electron transfer from Pheo- to QA is characterised by a rate constant of (300 ps)-1. (3) The QA- reoxidation kinetics are significantly retarded in D2O suspensions. These H/D isotope effects are interpreted as to reflect hydrogen-bond dependent changes in the protein dynamics that are relevant to electron transfer. (4) In PSII reaction centres closed for photochemical trapping the yield of a primary radical pair with lifetimes exceeding 1 ns is comparatively small (c 30%) at room temperature. Short illumination in the presence of Na2S2O4 changes the radical pair dynamics. (5) Photoinhibition under aerobic conditions impairs the primary charge separation and leads to formation of quencher(s) of excitation energy.
Journal Article•
TL;DR: In this paper, the structure and the catalytic cycle of oxygen reduction of the protonmotive haem-copper terminal oxidases are reviewed and a new working model of the essential elements of the chemiosmotic mechanism of coupling of the oxygen reduction chemistry to vectorial proton translocation is presented.
Abstract: Advances in understanding of the structure and the catalytic cycle of oxygen reduction of the protonmotive haem-copper terminal oxidases are reviewed This information has been combined with our recent recognition of the need for electroneutrality of stable catalytic intermediates to produce a new working model of the essential elements of the chemiosmotic mechanism of coupling of oxygen reduction chemistry to vectorial proton translocation
TL;DR: Photoprotective energy dissipation activity was examined in spinach leaves grown outside during the winter versus leaves that had developed at moderate temperatures in a glasshouse, and characteristics suggest that the leaves exposed to high light on colder days during theWinter exhibited sustained energy Dissipation activity that remained engaged throughout the night.
Abstract: Photoprotective energy dissipation activity, that was largely associated with the de-epoxidation of the xanthophyll cycle, was examined in spinach leaves grown outside during the winter versus leaves that had developed at moderate temperatures in a glasshouse. On a leaf area basis the rates of photosynthesis were higher in leaves from the field at all temperatures examined, but were similar in both sets of leaves on a chlorophyll basis. The rate at which energy dissipation activity increased upon sudden exposure to high light was similar for the warm-grown leaves and those growing outside. This rate was futhermore similar to that of the rate of antheraxanthin and zeaxanthin formation, and was similar throughout the winter as long as the pre-dawn level of photosystem II (PSII) efficiency was at a normal high level. Whereas energy dissipation activity developed more rapidly at higher temperatures, the final extent of energy dissipation activity was greater at lower temperatures, where the rate of energy utilisation through photosynthetic electron transport was much lower. On colder days leaves collected pre-dawn from plants growing outside exhibited sustained decreases in PSII efficiency, which were associated with sustained decreases in both maximal and minimal levels of fluorescence. Such characteristics suggest that the leaves exposed to high light on colder days during the winter exhibited sustained energy dissipation activity that remained engaged throughout the night. It is likely that the xanthophyll cycle was involved in this response, as the sustained high levels of energy dissipation activity were found to be associated with sustained high levels, and thus the retention of, zeaxanthin and antheraxanthin overnight.
TL;DR: Improvements to the technique have produced a gas-phase system which allows measurements of alternative pathway flux in intact tissues in less than an hour, and the development and application of these techniques and the potential for future experiments are discussed.
Abstract: Discrimination against 18O during dark respiration forms the basis of a new technique for measuring- flux through the alternative pathway during plant respiration. This technique, first reported by Guy and coworkers, is the first to allow measurements of the alternative oxidase in vivo under steady-state conditions. Improvements to the technique have produced a gas-phase system which allows measurements of alternative pathway flux in intact tissues in less than an hour. The development and application of these techniques and the potential for future experiments are discussed in this review.
TL;DR: It is believed that litchi pericarp browning is due to highly localised oxidative activity in the epicarp and upper mesocarp and the relative role of POD activity needs to be re-appraised.
Abstract: Cellular localisation of visual browning and oxidative activity studies were conducted to determine the relative significance of polyphenol oxidase (PPO) and peroxidase (POD) activities during pericarp browning. Pericarp browning was first observed on the protuberance apices and subsequently extended uniformly over the entire pericarp surface. Anatomically, browning was highly localised and restricted to the epicarp and the upper mesocarp. PPO and POD activities were highest in the epicarp, with progressively less activity in both the mesocarp and endocarp. In situ localisation of oxidative activity using tissue blots confirmed high epicarp PPO activity. POD activity, although primarily restricted to mesocarp vascular tissue, was also detected in the epicarp. We believe that litchi pericarp browning is due to highly localised oxidative activity in the epicarp and upper mesocarp. As PPO and POD activities were significantly higher in this tissue and browning was not observed when both enzymes were selectively inhibited, it is postulated that both PPO and POD activities are associated with litchi pericarp browning. The current theory that litchi pericarp browning is only caused by PPO activity needs to be re-appraised to determine the relative role of POD activity.
TL;DR: Overall, the data indicated that the positive response to CO2 in grain yield is likely to increase at approximately 1.8% per 1°C in wheat crops that are not limited by water.
Abstract: Clear, plastic-coated, temperature gradient tunnels (TGTs), 8 × 1.25 × 1-25 m were designed and built to examine how temperature and CO2 affect the yield of wheat in the field. Each of the three modules of each TGT was maintained at a different temperature above the ambient temperature using solar heating during the day and electric heating at night. The maximum day-time increment above ambient for the warmest module was 5oC and full-season averages were close to 2oC. TGTs were paired, with air in one being enriched to 700 μL L-1 CO2, and in the other being maintained at ambient CO2. Crops were planted in the TGTs at two sites in either summer (December) or winter (April and July) and they remained there until maturity. CO2 enrichment increased the yield in summer plantings by up to 36%. In winter plantings, with mean temperatures between sowing and anthesis of around lVC, the responses to CO2 were small averaging only 7% (range 1-12%). Though yield declined with increasing temperature in the TGTs in summer, there was a clear trend for an increasing response to CO2 at these higher temperatures, i.e. yield declined less. In summer, there was no convincing evidence for a different relative response to CO2 in two isolines which differed in maturity date, though the later line yielded more under the highest temperature regime (mean of 22-24oC between sowing and anthesis). In winter there was a strong trend for the isoline requiring less vernalisation to respond more to CO2. It is suggested that early progress towards flowering might predispose wheat to a greater CO2 response. Overall, the data indicated that the positive response to CO2 in grain yield is likely to increase at approximately 1.8% per 1°C in wheat crops that are not limited by water. Extrapolation indicated that the temperature at which there was no response to CO2 was 5oC. All yield responses reflected biomass responses as harvest index was unchanged by CO2
TL;DR: Potted apple trees were subjected to water stress in a greenhouse to determine whether active osmotic adjustment occurred and if water stress affected carbohydrate metabolism, and sorbitol was the major component in all organs.
Abstract: Potted apple (Malus domestica Borkh. cv. Jonathan) trees were subjected to water stress in a greenhouse. Midday leaf water potential (ΨW), osmotic potential (ΨS), soluble carbohydrates, and starch content of expanding and mature leaves, stems, and roots were measured to determine whether active osmotic adjustment occurred and if water stress affected carbohydrate metabolism. Mature leaves had the highest total soluble carbohydrate level (357 mM) and lowest Ψ (-1.85 MPa), followed by young leaves (278 mM, -1.58 MPa), stems (115 mM, -1.02 MPa), and roots (114 mM, -0.87 MPa). Sorbitol was the major component in all organs ranging from 53% of total soluble carbohydrate in young leaves to 73% in mature leaves. When ΨW decreased from -1.0 to -3.2 MPa, active osmotic adjustments of 0.3-0.4 MPa were observed in mature leaves, stems, and roots while a significantly higher adjustment of 1.0 MPa was detected in young leaves 5 days after the initiation of water stress. Sorbitol levels in leaves and stems gradually increased as ΨW decreased from -1.0 to -2.5 MPa, and then remained relatively stable or decreased slightly as ΨW decreased from -2.5 to -3.2 MPa. However, the percentage of soluble carbohydrate as sorbitol in roots decreased in response to water stress. Sucrose concentration decreased in mature leaves and stems, but increased in young leaves and roots as ΨW decreased. Starch concentrations in stems and roots also decreased as water stress developed. The sorbitol to sucrose ratios increased in mature leaves, but decreased in roots in response to water stress.