bluuebunny/crossref_metadata_2025 huggingface dataset:
Internal Gradients of Chlorophyll and Carotenoid Pigments in Relation to Photoprotection in Thick Leaves of Plants With Crassulacean Acid Metabolism
Authors: Robinson SA , Osmond CB
Published: January 2006

Studies of two species of Crassulacean acid metabolism (CAM) succulent, Cotyledon orbiculata and Cotyledon paniculata reveal external and internal mechanisms of protection against photoinhibitory damage. Both species have thick leaves (up to 8 mm) in which chlorophyll extends throughout the leaf, being somewhat more concentrated towards the outer layers. Epidermal wax on C. orbiculata, which is produced in response to bright light during growth, reflects up to 60% of the incident light and creates a shaded environment within the leaf. These leaves have a low carotenoid/chlorophyll ratio and relatively low levels of the xanthophyll cycle pigments, two of which, antheraxanthin and zeaxanthin, are thought to be involved in controlled dissipation of excess energy and therefore in photoprotection. Under growth irradiance (1300 μmol m-2 s-1) little antheraxanthin or zeaxanthin is found in the epidermal layers and there is none in the interior. However, when the wax is removed by brushing, antheraxanthin and zeaxanthin are found throughout the leaf. In contrast C. paniculata has low reflectivity and is a typical sun plant with a much higher carotenoid/chlorophyll ratio concentrated towards the upper epidermis. In both species the xanthophyll cycle pigments are concentrated towards the exposed surfaces of the leaf, whereas, lutein, β-carotene and neoxanthin are more evenly distributed. Changes in the relative concentrations of the xanthophyll cycle pigments throughout the leaf, measured at various light intensities, show that antheraxanthin and zeaxanthin are always greater at the exposed surfaces and decline towards the centre of the tissue. These results show that variations in the capacity for photoprotection correspond closely with gradients in the light environment within the leaf.
Transpiration, Intracellular Carbon Dioxide Concentration and Carbon-Isotope Discrimination of 24 Wild Species Differing in Relative Growth Rate
Authors: Poorter H , Farquhar GD
Published: January 2006

Various aspects of the water economy were investigated for a range of herbaceous species varying in relative growth rate. Plants were grown in a growth chamber with a non-limiting supply of water and nutrients, and the rate of transpiration, short-term intercellular CO2 concentration and long-term carbon isotope discrimination (Δ) in the leaves were determined. No correlation was found between the relative growth rate of these species, and the transpiration rate per unit leaf area, the intercellular CO2 concentration and the 13C-discrimination. There was a positive correlation, however, with the rate of water uptake per unit root weight. From these observations we infer that the previously observed differences in photosynthetic nitrogen-use efficiency, the rate of photosynthesis per unit leaf nitrogen, can not be explained by variation in intercellular CO2 concentration.
These data were also used to analyse correlations between Δ and both growth parameters and chemical composition. Apart from parameters related to the water economy, Δ only correlated (positively) with the fractional biomass allocation to the roots (root weight ratio) and the specific root length (root length divided by root weight), and not with any other investigated growth parameters.
The Field Pea Crop in South Western Australia - Patterns of Water Use and Root Growth in Genotypes of Contrasting Morphology and Growth Habit
Authors: Armstrong EL , Pate JS , Tennant D
Published: January 2006

Root development and seasonal water use of six field pea (Pisum sativum L.) genotypes were studied in a water-limited field environment (Wongan Hills) and in freely watered glasshouse-cultured plants (Perth, WA). From 80-97% of root biomass of the genotypes at peak vegetative growth in the field was located within 20 cm of the soil surface. Roots of one genotype (Wirrega) extended deeper and extracted soil moisture reserves to 2 m, i.e. some 40 cm below that of the other genotypes. Peak evapotranspiration rates (2.7-3.0 mm d-1) were attained in the field at or just beyond flowering after which water consumption decreased sharply parallel with increasing moisture stress and declining green area. By contrast, glasshouse-grown plants increased steadily in cumulative transpiration well into fruiting. Judging from cumulative evaporation and dry matter at peak biomass, field crops of the fully leaved genotypes (Dundale, Wirrega and Progreta) showed significantly better water-use efficiency than the three semi-leafless genotypes (Dinkum, L82 and L80). Effectiveness of water usage was also assessed from regressions of dry matter production against cumulative evapotranspiration (field material), gravimetric measures of transpiration loss versus dry matter gain (glasshouse plants) and 13C isotopic discrimination of shoots of glasshouse-grown plants. All three comparisons showed the tall conventional types (Dundale and Wirrega) to be superior to the four semi-dwarfs. Data are discussed in relation to previous studies of the water-use economy of field pea and other grain crops.
Some Aspects of Photoperiodism in Wheat and Its Wild Relatives
Authors: Evans LT , Blundell C
Published: January 2006

Experiments with plants grown in controlled environment conditions examine three aspects of photoperiodism in wheat. A survey of the flowering responses of 20 genotypes of diploid, tetraploid and hexaploid wheat cultivars and wild relatives to growth under three daylengths (8, 12, 16 h) after three durations of vernalisation (0, 4, 10 weeks at 20�C) showed that all were long-day plants and many responded to vernalisation. The requirement for long days was most stringent among the diploid progenitors and most relaxed among the hexaploid cultivars. However, not even the earliest flowering spring wheat cultivars (among eight) were entirely daylength-neutral. Minimum times to inflorescence initiation appeared to be determined independently of the responses to daylength. Whereas leaf initiation and appearance rates were hardly influenced by daylength, the rate of spikelet initiation responded to it from the beginning of floral induction, well before the appearance of double ridges. Comparison of the times to double ridges among near-isogenic lines of four spring wheat cultivars (Ciano 67, Yaqui 50, Rescue and April Bearded) showed that each of the three dwarfing genes Rht1, Rht2 and Rht3 advanced inflorescence initiation but without changing the response to daylength.
Salt Tolerance Is Not Associated With the Sodium Accumulation of Two Maize Hybrids
Authors: Cramer GR , Alberico GJ , Schmidt C
Published: January 2006

In this report, we test the hypothesis that Na+ accumulation in the shoot in maize is negatively correlated with salt tolerance. Salt tolerance is defined as a percentage of the control on a dry weight basis. Two hybrids (Pioneer hybrid 3578 and Pioneer hybrid 3772) differing widely in Na+ accumulation were compared. Plants were treated with two types of salinity for 15 days (80 mol m-3 NaCl or 80 mol m-3 NaCl plus 8.75 mol m-3 CaCl2). Ion concentrations (Na+, K+, Ca2+ and Cl-) were measured in the roots, stalks, sheaths and leaves of plants harvested every third day. Ion concentrations were significantly affected by the treatments. Na+ and Cl- concentrations increased with salinity treatments; K+ and Ca2+ concentrations decreased. Supplemental Ca2+ increased Ca2+ and decreased Na+ concentrations. Hybrid 3772 maintained very low Na+ concentrations in the shoots, whereas 3578 did not. The largest distinction between the hybrids was in the ability to transport Na+ to the shoot; hybrid 3578 transported Na+ at twice the rate of hybrid 3772. In general, ion transport to the shoot appeared to be a function of root ion concentration. This model could account for the effects of NaCl salinity and supplemental Ca2+ on ion transport, although Na+ transport was complicated by an apparent reabsorption mechanism in the root and mesocotyl. The lack of correlation of Na+ accumulation in the shoot and other ion parameters with growth indicated that the mineral nutrition of the plants was not correlated with salt tolerance. It was concluded that the growth response of maize to salinity was primarily affected by osmotic factors.
Physiological and Morphological Traits Associated With Spring Wheat Yield Under Hot, Irrigated Conditions
Authors: Reynolds MP , Balota M , Delgado MIB , Amani I , Fischer RA
Published: January 2006

Sixteen spring wheat genotypes were grown under hot, irrigated, low latitude conditions, during the 1990-1991 and 1991-1992 winter cropping cycles in Mexico, Egypt, India and the Sudan, and in the 1990 and 1991 winter cycles in Brazil. The genotypes were chosen to represent a range of genetlc diversity within modem wheat varieties. In addition to grain yield, in Mexico a number of morphological and physiological traits were measured throughout the growing season on two sowing dates (December and February), to evaluate their correlation with yield in the other countries. The morphological traits - above-ground biomass at maturity, grains m-2, days to anthesis and days to maturity - measured on both sowing dates in Mexico showed generally significant correlations with yields measured at the other sites. Moreover, ground-cover estimates early in establishment, and at anthesis, also showed some association with performance at these sites, but only when measured on the hotter, February sowing date in Mexico. Membrane thermostability (MT), measured on field-acclimated flag leaves showed generally significant correlations with performance at all sites, and with MT measurements made on heat- acclimated seedlings of the same genotypes grown in growth chambers. Flag-leaf photosynthesis measured at booting, anthesis and during grain filling on both sowing dates, was generally significantly correlated with grain yield at all sites, as was rate of loss of leaf chlorophyll content during grain filling. Stomatal conductance was also significantly correlated with performance at all three stages. Canopy temperature depression (CTD) measured with an infrared thermometer was significantly positively correlated with performance at the international sites when measured between 1200 and 1600 hours, after full canopy establishment. The correlation of CTD with yield was not affected by the irrigation status of the crop under well-watered conditions. The possible use of these traits in selection for yield under hot conditions is discussed.
The Effect of High Temperature on Kernel Development in Wheat: Variability Related to Pre-Heading and Post-Anthesis Conditions
Authors: Wardlaw IF
Published: January 2006

In wheat, mean temperatures greater than 15-18�C following anthesis can result in a decrease in kernel weight at maturity, and breeding for high temperature tolerance during kernel filling could provide a significant increase in yield in large parts of the Australian wheat belt. The response of kernel filling to high temperature, however, varies from planting to planting and this variation has been shown to be related to both pre-heading and post-anthesis conditions. Thus high temperature (27/22�C), or low light (50% shade) during ear development can reduce the response of the developing grain to high temperature (30/25�C) following anthesis. In contrast, low light during kernel filling enhances the response to high temperature, resulting in a relatively greater reduction in kernel size. The latter response suggests that the slightly greater sensitivity to high temperature of grains from plants allowed to tiller freely in comparison with the responses observed using single culms, may be related to differences in light penetration of the canopy. This variation in response to high temperature, although not appearing to change the order of tolerance across cultivars, can create difficulties in selecting for high temperature tolerance over a number of generations, and can account for the apparent low heritability (h2 = 0.2) of high temperature tolerance determined here from a cross between the cultivars Kalyansona and Pinnacle.
Heat-Induced Alterations in Thylakoid Membrane Protein Composition in Barley
Authors: Takeuchi TS , Thornber JP
Published: January 2006

Biochemical and spectroscopic studies on the effects of high temperatures (45-47� C) over a 1 h period on the protein composition, fluorescence and photochemical activities of the barley thylakoid membrane were made. Photosystem II (PS II) activity decreased as expected, and photosystem I (PS I) activity also unexpectedly decreased. Our data support previous conclusions that the decrease in PS I activity is largely due to inactivation (or loss) of a component between the two photosystems. A two-dimensional electrophoretic system permitted first the separation of the thylakoid pigment-protein complexes of unstressed and stressed plants, followed by a determination of their subunit composition. The changes in the protein composition of each pigment-protein complex in response to elevated temperatures were monitored. Heat changed the quaternary structure of PS II and resulted in removal of the oxygen-evolving enhancer proteins from the thylakoid, but did essentially no damage to the PS I complex. The PS II core complex dissociated from a dimeric form to a monomeric one, and the major LHC II component (LHC IIb) changed from a trimeric to a monomeric form. The pigments that are lost from thylakoids during heat stress are mainly removed from the PS II pigment-proteins.
Starch Synthesis in the Kernel of Wheat Under High Temperature Conditions
Authors: Jenner CF
Published: January 2006

As temperature rises above 18-22�C, the observed decrease in the duration of deposition of dry matter in the kernel is not accompanied by a compensating increase in the rate of grain filling with the result that grain weight (and yield) is diminished at high temperature. Reduced starch content accounts for most of the reduction in grain dry matter at high temperature. Responses to temperature in the low temperature range, 20-30�C (the LTR), could possibly be ascribed to the temperature response characteristics of the reaction catalysed by soluble starch synthase (SSS), the enzyme synthesising starch. However, the rate of cell enlargement and the rate of accumulation of nitrogen in the grain also do not increase much as temperature rises, so other explanations are conceivable for the temperature responses in the LTR. Variation amongst cultivars of wheat in tolerance of high temperature is evident in the LTR. At temperatures above 30�C (in the high temperature range (HTR) between 30 and 40�C), even for short periods, the rate of starch deposition is slower than that observed at lower temperatures, an effect which is carried over after transfer from high to lower temperatures. This response is attributable to a reduction in the activity, possibly due to thermal denaturation, of SSS. Several forms of SSS are found in cereal endosperm, and some forms may be more tolerant of high temperature than others. Loss of enzyme activity at high temperature is swift, but is partly restored some time after transfer from hot to cool conditions. There appear to be two distinct mechanisms of response to elevated temperature, both resulting in a reduced grain weight through reduced starch deposition, but one of them is important only in the range of temperature above 30�C.
Temperature Variation During Grain Filling and Changes in Wheat-Grain Quality
Authors: Wrigley CW , Blumenthal C , Gras PW , Barlow EWR
Published: January 2006

There have been a few notable occasions when the Australian wheat segregation system (mainly based on specification of variety and protein content) has failed to produce grain which gives dough properties expected for the wheat grade. The reasons for this are likely to relate to growing and storage conditions; of these, variations in temperature during grain filling appear to be a major factor. Observations of crop statistics, field and glasshouse experiments indicate that as growth temperatures increase up to 30°C, there is a general increase in dough strength (as indicated by Extensograph maximum resistance, Rmax, and as Farinograph development time and stability). However, a decline in dough strength is observed following periods of heat stress (e.g. a few days with maxima of over 35°C). Increasing temperatures during grain filling have also been observed to produce grain with a higher protein content, but this observation is not as consistent nor as marked as the effects on dough strength.
We have sought to identify genotypes that do not follow this general trend in response to heat stress, and thus could be used as parents to breed for heat tolerance and greater stability of dough quality. A glasshouse experiment involving 45 genotypes has indicated that there is some variation in the response to heat stress, with a few genotypes being promising sources of tolerance. A second important approach to minimising the effects of heat stress is to develop a model to predict grain-quality changes, thus enabling a marketing authority to be forewarned of significant variation from the quality attributes normally expected for a wheat grade, and assisting breeders to better interpret the results of quality testing of lines grown at various sites.
Plant Flavonoids: Biological Molecules for Useful Exploitation
Authors: Dakora FD
Published: January 2006

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.
Salt Tolerance, Ion Relations and the Effect of Root Medium on the Response of Citrus to Salinity
Authors: Storey R
Published: January 2006

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.
Fluorescence Induction From Photosystem II: Analytical Equations for the Yields of Photochemistry and Fluorescence Derived From Analysis of a Model Including Exciton-Radical Pair Equilibrium and Restricted Energy Transfer Between Photosynthetic Units
Authors: Trissl HW , Lavergne J
Published: January 2006

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.
The Photosystem II Heterogeneity of Chlorophyll b-Deficient Mutants of Rice: a Fluorescence Induction Study
Authors: Hsu Bandar , Lee Yaulee
Published: January 2006

It has been shown that the fluorescence induction curve of DCMU-poisoned spinach thylakoids can be resolved into three kinetically different phases, a rapid sigmoidal phase (�) followed by two slower exponential phases (β and γ), by using a mathematical analysis method previously described (Hsu, B. D., Lee, Y. S. and Jang, Y. R. (1989). Biochimica et Biophysica Acta 975, 44-49). There is evidence suggesting that the a-phase originates from the major 'normal' photosystem II (PSII) centres, while the β and γphases arise from the two minor groups of 'abnormal' PSII centres with low quantum efficiencies due to their slow electron donation systems (Hsu, B. D. and Lee, J. Y. (1991). Biochimica et Biophysica Acta 1056, 285-292). The same kinetic analysis was made on several chlorophyll �-deficient mutants of rice. PSIIα was identified by its responses to the variation in excitation light intensity, the addition of ferricyanide and the depletion of Mg2+, whereas the slower PSIIβ and PSIIγ were identified by their reaction to the addition of electron donors like hydroxylamine. It was found that the three types of PSII were present in all the mutants studied, irrespective of the amounts of chlorophyll b and the light-harvesting complexes associated with PSII (LHCII). The results suggest that the PSII heterogeneity cannot be attributed to a difference in the antenna size. The variable content of LHCII in mutants mainly affects a single type of PSII, PSIIα. They also suggest that the presence of sigmoidicity in the a-phase (i.e. cooperation between PSIIα units) does not require LHCII, but the effects brought about by Mg2+-depletion (e.g. lowering of fluorescence yield and slowing down of fluorescence rise) are LHCII-dependent.
Molecular Basis of the Vulnerability of Photosystem II to Damage by Light
Authors: Barber J
Published: January 2006

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.
Assessment of Photosystem II Photochemical Quantum Yield by Chlorophyll Fluorescence Quenching Analysis
Authors: Schreiber U , Hormann H , Neubauer C , Klughammer C
Published: January 2006

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.
Regulation of Non-Photochemical Quenching of Chlorophyll Fluorescence in Plants
Authors: Ruban AV , Horton P
Published: January 2006

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.
Mechanism of Non-Photochemical Chlorophyll Fluorescence Quenching. II. Resolution of Rapidly Reversible Absorbance Changes at 530 Nm and Fluorescence Quenching by the Effects of Antimycin, Dibucaine and Cation Exchanger, A23187
Authors: Mohanty N , Gilmore AM , Yamamoto HY
Published: January 2006

The putative relationship between the light-induced absorbance increase at 530 nm (ΔA530), the so-called light-scattering change, and non-photochemical chlorophyll fluorescence quenching (NPQ) was examined by the effect of inhibitors. Antimycin at a low concentration (350 nM) completely inhibited fluorescence quenching while only partially inhibiting A530. This effect was independent of the mode of thylakoid energisation and preinduction of violaxanthin de-epoxidation. Dibucaine at 20 FM abolished NPQ but had little effect on ΔA530. Moreover, the light-induced ΔA530 signal was present even in the absence of de-epoxidised xanthophylls. The cation exchanger A23187 blocked the development of NPQ as well as relaxed fluorescence quenching at steady state without involving a major portion of ΔA530. Thus, the relationship between energy-dependent A530 changes and fluorescence quenching was non-linear under all conditions tested. The light-induced absorbance increase at 530 nm, therefore, is insufficient for NPQ. The differential effects of inhibitors are explained schematically, depicting three phases for NPQ: (a) formation of zeaxanthin and antheraxanthin by the xanthophyll cycle; (b) formation of a state reflected by A530 that is induced by the transthylakoid ApH, possibly involving aggregation of LHCII; and (c) fluorescence quenching by the combined effect of both steps and by the H+-cation exchange properties of thylakoid membranes.
Xanthophyll Cycle-Dependent Energy Dissipation and Flexible Photosystem II Efficiency in Plants Acclimated to Light Stress
Authors: Demmig-Adams B , Adams WW Iii , Logan BA , Verhoeven AS
Published: January 2006

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.
Chloroplast Cytochrome b6/f and ATP Synthase Complexes in Tobacco: Transformation With Antisense RNA Against Nuclear-Encoded Transcripts for the Rieske FeS and ATPδ Polypeptides
Authors: Price GD , Yu JW , Caemmerer SV , Evans JR , Chow WS , Anderson JM , Hurry V , Badger MR
Published: January 2006

Tobacco plants with reduced amounts and activities of both the chloroplast cytochrome b6/f and ATP synthase complexes have been produced using antisense RNA techniques. Antisense constructs were generated from tobacco cDNA clones coding for the Rieske FeS protein and the δ subunit of the b6/f and ATP synthase complexes respectively. Transformants with altered activities were selected using pulse-modulated fluorescence measurements. The b6/ftransformants showed high levels of steady-state fluorescence and reduced levels of both photochemical and non-photochemical quenching. In striking contrast, the ATP synthase transformants showed low levels of steady-state fluorescence and greatly increased levels of non-photochemical quenching. Transformants with a range of suppression were isolated for both constructs, in some cases with photosynthesis reduced to less than 10% of wild-type values. The most severely affected transformants showed extremely slow growth and in some cases they were unable to grow and produce seed. Progeny from the R1 seed from several cytochrome b6/f transformants have been analysed and show segregation of phenotypes ranging from intermediate to severe in repression. Intermediate and severe phenotype plants showed a reduction in Rieske FeS mRNA of more than 90% while FeS polypeptide was reduced to 60 and 86% of wild type. There was a strong correlation between photosynthesis at air and Rieske FeS polypeptide content in the antisense plants suggesting that the cytochrome b6/f complex was a major determinant of photosynthetic rate under these conditions. Photoinhibition studies of FeS antisense plants showed that there was a reduced activity of the xanthophyll cycle in the most severe plants, consistent with a reduction in the transthylakoid pH gradient and a lowered non-photochemical quenching. Preliminary studies of the ATPδ antisense plants showed that they also had reduced levels of mRNA and ATPδ polypeptide.
Effect of Canopy Position on the Susceptibility of Kiwifruit (Actinidia deliciosa) Leaves on Vines in an Orchard Environment to Photoinhibition Throughout the Growing Season
Authors: Greer DH
Published: January 2006

Kiwifruit (Actinidia deliciosa (A. Chev) C.F. Liang & A.R. Ferguson) plants grown in an orchard were studied over several seasons to assess the impact of photoinhibition on the leaves using chlorophyll fluorescence and photosynthetic measurements. Leaves above were compared with those below the vine canopy. In addition, temperature and photon flux densities above and below the canopy were monitored. A gradient of sun to shade photosynthetic characteristics developed in leaves from above to below the canopy. There was a 10% higher Fv/Fm ratio in the shade leaves (0.810) but a 30% lower photosynthetic capacity and a 30% higher photon yield than in sun leaves. In addition, Fo and Fm were both higher (20-60%) in shade leaves. Little variation in Fv/Fm occurred throughout the growing season, except during spring, when Fv/Fm was about 0.4-0.5, especially in small, rapidly expanding leaves. Changes in Fv/Fm during spring were correlated with leaf diameter, indicating development of photosynthetic competence was an important factor in the rise in Fv/Fm. However, increasing night temperatures also correlated with the increase in Fv/Fm during spring.
Light-Induced Enhancement in Proline Levels in Vigna Radiata Exposed to Environmental Stresses.
Authors: Arora S , Saradhi PP
Published: January 2006

Four-day-old Vigna radiata (L.) Wilczek seedlings were exposed to water stress (5% mannitol), salt stress (200 mM sodium chloride) and heavy metal stress (2.5 mM cadmium nitrate), in continuous light and darkness. The seedlings exposed to any of these stresses accumulated higher levels of proline in their shoots compared with control seedlings. Irrespective of the type of stress, accumulation of proline was higher in light than in the dark, which suggests that the photosynthetic activity of the seedlings might be responsible for light-induced enhancement of proline levels. In a second set of experiments, seeds were germinated under water stress, salt stress and heavy metal stress in continuous light and darkness. The plumule did not emerge from seeds germinated under stress, even after day 7. However, a 1.0-1.5 cm long radicle did emerge in light and in darkness. In spite of the lack of photosynthetic organs, seedlings raised under stress in light accumulated more proline than those raised in continuous darkness. In a third set of experiments, roots excised from 7-day-old light-grown seedlings were exposed to salt stress in light and in continuous darkness. The roots exposed to stress accumulated considerably more proline in light than in darkness. The results obtained in the last two sets of experiments suggest, for the first time, that some factoris) or process(es) other than photosynthesis plays an important role in light-induced enhancement of proline accumulation.
The Response of Wheat to High Temperature Following Anthesis. I. The Rate and Duration of Kernel Filling.
Authors: Wardlaw IF , Moncur L
Published: January 2006

Wheat (Triticum aestivum L.) plants were grown to anthesis at 18/13�C day/night and either retained at 18/13�C or transferred to a higher temperature (24/19 or 30/25�C) 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 30/25�C. 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.
Possibility of Increasing Yield Potential of Rice by Reducing Panicle Height in the Canopy. I. Effects of Panicles on Light Interception and Canopy Photosynthesis.
Authors: Setter TL , Conocono EA , Egdane JA , Kropff MJ
Published: January 2006

A new rice (Oryza sativa L.) plant type is proposed which is aimed at greater light interception by the leaves during grain filling and reduced susceptibility to lodging. This plant type is based on lowering panicle height in the canopy so that leaves are able to intercept more radiation. Panicle height of four cultivars with high yield potential ranged from 80 to 95% of canopy height at 14 days after flowering (DAF). Harvested panicle area index of two of these which were two tropical cultivars amounted to 0.45-0.57 m2 m-2, while the leaves above the panicles intercepted only 4-12% incident radiation at midday in these cultivars at 14 DAF. The potential impact of lowering panicle height in the canopy on canopy photosynthesis was evaluated by panicle removal. When panicles were removed, the irradiance at the bottom of the panicle layer of the canopy increased from 52 to 80%. At 11 DAF, net canopy photosynthesis of two cultivars similarly increased by 42-52% following removal of panicles. Increases in canopy photosynthesis were mainly due to increased light interception by leaves within the canopy since there were little or no changes in canopy dark respiration or in net leaf photosynthesis rates at the same irradiance following panicle removal. Optimum panicle height in the canopy and the effect of different panicle area index is further examined using a physiological model for light competition, INTERCOM. Reduced panicle height in the canopy is discussed in terms of considerations for and impacts of lowering panicle height in the canopy on canopy photosynthesis and yield of rice.
Diurnal Time Course of Leaf Gas Exchange Rates and Related Characters in Drought-Acclimated and Irrigated Trifolium Subterraneum.
Authors: Vadell J , Cabot C , Medrano H
Published: January 2006

The effects of drought acclimation on the diurnal time courses of photosynthesis and related characters were studied in Trifolium subterraneum L. leaves during two consecutive late spring days. Leaf CO2 assimilation rate and transpiration rate followed irradiance variations in irrigated plants. Under drought, a bimodal pattern of leaf CO2 assimilation rate developed although stomatal conductance remained uniform and low. Instantaneous water-use efficiency was much higher in droughted plants during the early morning and late evening, while during the middle of the day it was close to the value of irrigated plants. Net carbon gain in plants under drought reached 40% of the carbon gain in irrigated plants with a significant saving of water (80%). Average data derived from midday values of leaf CO2 assimilation rates and instantaneous water-use efficiency did not provide good estimates of the daily carbon gain and water-use efficiency for droughted leaves. Coupled with the morphological changes as a result of acclimation to progressive drought, modifications of diurnal patterns of leaf gas exchange rates effectively contribute to a sustained carbon gain during drought. These modifications significantly improve water-use efficiency, mainly by enabling the plant to take advantage of morning and evening hours with high air humidity.
Beyond Sham and Cyanide: Opportunities for Studying the Alternative Oxidase in Plant Respiration Using Oxygen Isotope Discrimination.
Authors: Robinson SA , Ribas-Carbo M , Yakir D , Giles L , Reuveni Y , Berry JA
Published: January 2006

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.
Regulation of the Alternative Oxidase in Plants and Fungi.
Authors: Day DA , Whelan J , Millar AH , Siedow JN , Wiskich JT
Published: January 2006

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.
Heterotrophic Carbon Gain of the Parasitic Angiosperm Tapinanthus oleifolius
Authors: Richter A , Popp M , Mensen R , Stewart GR , Willert DJV
Published: January 2006

Tapinanthus oleifolius is a xylem-tapping mistletoe, parasitising a wide range of host trees including acacias, which are potentially nitrogen fixing, and Euphorbia virosa, which acquires CO2 mainly by nocturnal fixation via phosphoenolpyruvate carboxylase. Heterotrophic carbon gain by T. oleifolius was estimated by two different approaches. Firstly, δ13C values of leaves were calculated from cI/ca values and compared with measured δ13C ratios of these leaves and corresponding host xylem saps. Secondly, carbon intake from the host was determined by measuring carbon content of the host xylem sap and transpiration of the mistletoe leaves. This taken together with the photosynthetic CO2 net uptake allowed the calculation of a carbon budget and hence the proportion of heterotrophically-derived carbon. There was good agreement between the methods of determining heterotrophic carbon gain. For young leaves of T. oleifolius, heterotrophic carbon gain was around 55% on E. virosa and Acacia nebrownii. In old leaves parasitising E. virosa, heterotrophic carbon accounted for more than 80% of the total carbon. Although nitrogen content of mistletoe and host leaves was highly correlated, we could not confirm earlier findings that the difference in δ13C values of parasite-host associations depends on the nitrogen content of the host. The photosynthetic characteristics (carboxylation efficiency, photosynthetic capacity) of the parasite were only little affected by the host species being parasitised.
Variation in Carbon Isotope Discrimination and Its Relationship to Specific Leaf Area and Ribulose-1,5-Bisphosphate Carboxylase Content in Groundnut Genotypes
Authors: Rao RCN , Udaykumar M , Farquhar GD , Talwar HS , Prasad TG
Published: January 2006

Variation in cahn isotope discrimination (Δ) and ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) content per unit leaf area was examined in leaves from upper and lower positions in the canopy of six groundnut (Arachis hypogaea L.) genotypes, grown under irrigated and mild water-deficit conditions in the field. The leaf mass per unit leaf area (ρL) and soluble proteins in leaves were determined at 80, 96, 111 and 127 days after sowing (DAS), while Δ and Rubisco were determined at 80 DAS only. The mean Δ ranged from 18.2 to 20.20 among genotypes, representing a significant (P < 0.01) variation. Rubisco content per unit leaf area also varied significantly (P < 0.01) with genotype and leaf position. There was a trend to an increase in Rubisco content under water deficit, but the effects were not significant. Leaves at the top of the canopy had a higher Rubisco content and lower Δ, than leaves at the bottom of the canopy. Genotype × leaf position interaction was significant for Δ and Rubisco, indicating the importance of leaf position in selecting for water-use efficiency (W), using leaf traits in groundnut. Rubisco content and Δ were negatively related (r2 = 0.65, P < 0.01). There was a significantly positive correlation between Rubisco content and ρL in the upper leaves (r2 = 0.60, P < 0.01 ), but not in the lower leaves in the canopy. However, the overall relationship between Rubisco and ρL (r2 = 0.40) was not as strong as it was between Rubisco and Δ. The results suggest that, in groundnut, the basis of genotypic variation in was mostly (> 60%) attributable to Rubisco content per unit leaf area. In view of the leaf positional effects on Δ and Rubisco, the upper leaves in the canopy should be used for selecting genotypes for W based on leaf traits like ρL or Δ.