Showing papers on "Photosynthesis published in 1990"

The use of chlorophyll fluorescence nomenclature in plant stress physiology.

Abstract: During recent years there has been remarkable progress in the understanding and practical use of chlorophyll fluorescence in plant science. This 'renaissance' of chlorophyll fluorescence was induced by the urgent need of applied research (like plant stress physiology, ecophysiology, phytopathology etc.) for quantitative, non-invasive, rapid methods to assess photosynthesis in intact leaves. Recent developments of suitable instrumentation and methodology have substantially increased these possibilities. Actually, a vast amount of knowledge on chlorophyll fluorescence had already accumulated over more than 50 years, since the discovery of the Kautsky effect in 1931 (Kautsky and Hirsch 1931) (for reviews, see e.g., Lavorel and Etienne 1977, Briantais et al. 1986, Renger and Schreiber 1986). On the one hand this knowledge was mechanistic, resulting from biophysically oriented basic research. On the other hand it was phenomenological, originating from applied plant physiological research. Until recently the phenomenology of whole leaf chlorophyll fluorescence appeared far too complex to find serious attention of biophysicists. Thus, for a long time, there was a gap between applied and basic research in chlorophyll fluorescence. Developments in instrumentation (Ogren and Baker 1985, Schreiber 1986, Schreiber et al. 1986) and methodology (Bradbury and Baker 1981, Krause et al. 1982, Quick and Horton 1984, Dietz et al. 1985, Demmig et al. 1987, Weis and Berry 1987, Bilger et al. 1989, Genty et al. 1989) has succeeded in closing this gap and bringing these two disciplines into sufficiently close contact and in mutually stimulating interaction. Consequently the present "renaissance" of chlorophyll fluorescence may be the product of a fruitful dynamic interaction between three different research disciplines, i.e., basic and applied research linked to new developments in instrumentation and methodology (see scheme in Fig. 1). As a result, measuring chlorophyll fluorescence has become a very attractive means of obtaining rapid, semiquantitative information on photosynthesis, used by an increasing number of researchers not only in the laboratory but also in the field. The wide range of possible applications is reflected by the broad spectrum of contributions to this issue of Photosynthesis Research. The progress made in chlorophyll fluorescence instrumentation and methodology has also induced new developments in the adjacent fields of absorbance spectroscopy (e.g., Klughammer et al. or Harbinson et al. in this issue), photoacoustic spectroscopy (e.g., Canaani, Dau and Hansen, Kolbowski et al. or Snel et al. in this issue) and chlorophyll luminescence (delayed fluorescence) (Bilger and Schreiber in this issue). These new developments are expected to play a role in

Salinity Tolerance of Eukaryotic Marine Algae

Abstract: INTRODUCTION 21 General Scope 21 Definition of Turgor Pressure Regulation and Osmotic Adaptation 23 GENERAL RESPONSES OF THE ALGAE AND RANGE OF TOLERANCE _ 23 Growth Rates 23 Photosynthesis and Respiration........ 25 Effect on the Fine Structure 26 PROCESSES OF OSMOTIC ACCLIMATION 27 Sensing of Turgor Pressure: The Detector Mechanisms 28 Changing the Cellular Concentrations of Osmolytes: The EIfector Processes and Their Regulation 31 MARINE ALGAE FROM AN UNUSUAL HABITAT: SEA ICE ALGAE 42

Carbon and nitrogen economy of 24 wild species differing in relative growth rate.

Abstract: The relation between interspecific variation in relative growth rate and carbon and nitrogen economy was investigated. Twentyfour wild species were grown in a growth chamber with a nonlimiting nutrient supply and growth, whole plant photosynthesis, shoot respiration, and root respiration were determined. No correlation was found between the relative growth rate of these species and their rate of photosynthesis expressed on a leaf area basis. There was a positive correlation, however, with the rate of photosynthesis expressed per unit leaf dry weight. Also the rates of shoot and root respiration per unit dry weight correlated positively with relative growth rate. Due to a higher ratio between leaf area and plant weight (leaf area ratio) fast growing species were able to fix relatively more carbon per unit plant weight and used proportionally less of the total amount of assimilates in respiration. Fast growing species had a higher total organic nitrogen concentration per unit plant weight, allocated more nitrogen to the leaves and had a higher photosynthetic nitrogen-use efficiency, i.e. a higher rate of photosynthesis per unit organic nitrogen in the leaves. Consequently, their nitrogen productivity, the growth rate per unit organic nitrogen in the plant and per day, was higher compared with that of slow growing species.

Expression of a yeast-derived invertase in the cell wall of tobacco and Arabidopsis plants leads to accumulation of carbohydrate and inhibition of photosynthesis and strongly influences growth and phenotype of transgenic tobacco plants.

Abstract: Chimeric genes consisting of the coding sequence of the yeast invertase gene suc 2 and different N-terminal portions of the potato-derived vacuolar protein proteinase inhibitor II fused to the 35S CaMV promoter and the poly-A site of the octopine synthase gene were transferred into tobacco and Arabidopsis thaliana plants using Agrobacterium based systems. Regenerated transgenic plants display a 50- to 500-fold higher invertase activity compared to non-transformed control plants. This invertase is N-glycosylated and efficiently secreted from the plant cell leading to its apoplastic location. Whereas expression of the invertase does not lead to drastic changes in transgenic Arabidopsis thaliana plants, transgenic tobacco plants show dramatic changes with respect to development and phenotype. Expression of the invertase leads to stunted growth due to reduction of internodal distances, to development of bleached and/or necrotic regions in older leaves and to suppressed root formation. In mature leaves, high levels of soluble sugars and starch accumulate. These carbohydrates do not show a diurnal turnover. The accumulation of carbohydrate is accompanied by an inhibition of photosynthesis, and in tobacco, by an increase in the rate of respiration. Measurements in bleached versus green areas of the same leaf show that the bleached section contains high levels of carbohydrates and has lower photosynthesis and higher respiration than green sections. It is concluded that expression of invertase in the cell wall interrupts export and leads to an accumulation of carbohydrates and inhibition of photosynthesis.

Predictions of Mn and Fe use efficiencies of phototrophic growth as a function of light availability for growth and of C assimilation pathway

Abstract: SUMMARY Iron is involved in many photosynthetic, respiratory and nitrogen assimilation reactions of plants as Fe bound tightly to polypeptides catalysing redox reactions. Manganese is involved as tightly bound Mn in photoreaction II of photosynthesis and in certain superoxide dismutases, while loosely bound Mn2+ is the unique activator of some enzymes, and is an alternative to Mg2+ in activating many enzymes. This paper uses data on the quantitative role of Fe and Mn in catalysts to predict the efficiency with which Fe and Mn are used in C assimilation [mol C assimilated (mol catalytic metal in enzyme)−1 s−1] and the metal cost of C assimilation [mol catalytic metal in enzyme (mol C assimilated)−1 s−1] in photolithotrophic growth in relation to genetic and environmental variables. The genetic variables were the relative content of thylakoid proteins in major taxa (cyanobacteria and red algae, chlorophytes and chromophytes) and smaller-scale taxonomic differences (various subtypes of C4 metabolism, and C3 metabolism, in terrestrial vascular plants). The environmental variables were the range of photon flux densities in which photolithotrophic growth of O2 evolvers can occur, and the inorganic C supply conditions controlling the repression/de-repression of the inorganic C concentrating mechanism in cyanobacteria and microalgae. The results of the computations yield the following conclusions. The largest predicted difference in Fe and Mn costs of photolithotrophic growth is related to changes in the photon flux density for growth. The predicted Fe cost increased 50-fold, and the Mn cost increased 80-fold, at the lowest extreme of photon flux density compared to the highest found naturally. The increase is partly countered by the larger ratio of light-harvesting pigments to thylakoid protein complexes assumed for the cells grown at low photon flux densities, although the extent of the increase in photosynthetic unit size is limited by considerations of efficiency of excitation energy transfer. However, the major influences are the higher pigment content in biomass enabling a larger fraction of incident light to the absorbed, and the sub-maximal specific reaction rates of redox catalysts (whose content is constrained via excitation energy transfer considerations) at very low photon flux densities. A smaller difference, four-fold or less, in Fe and Mn costs of photolithotrophic growth, is predicted by comparing major taxa (cyanobacteria plus red algae; chlorophytes plus chromophytes) with contrasting ratios of thylakoid redox catalysts. Differences in Fe and Mn costs of growth of less than two-fold are predicted when the Fe- and Mn-efficient organisms with CO2− concentrating mechanisms (C4 land plants; algae with active inorganic C influx) and high requirements for ATP relative to NADPH, are compared with organisms relying on CO2 diffusion from air or air-equilibrated solutions and C3 biochemistry. These predictions of variations in Fe and Mn costs of photolithotrophy have implications for the ecology of phototrophs.

Effects of supplementary ultraviolet-B radiation on photosynthesis

Abstract: L., cv. Greenfeast) were exposed to supplementary UV-B light (up to 8 days) starting on the 17th day after sowing. The effects of this exposure on photosynthesis and the content and activities of some chloroplast components of the mature leaves of these #ants were studied. (i) The total chorophyil content of pea leaves was approximately 4~o of that in the control leaves on the 8th day of UV-B exposure. Chlorophyll a levels decreased to a greater extent than the content of chlorophyll b. The decrease in carotenoids paralleled the decrease in chlorophyll b. (ii) On a chlorophyll basis, the contents of Photosystem I and cytochrome / were stable, whereas Photosystem II, ATP hydrolysis by the ATP synthase and the maximum ribulose-l,5-bisphosphate carboxylase (Rubisco) activity decreased by 55, 47 and 80%, respectively, when compared with the controls at the end of the 8-day illumination period. (iii) On a leaf-area basis, Photosystem I and cytochrome f content decreased by 58%, Photosystem II by 80%, ATP hydrolysis by 80%, and Rubisco activity by 90%, when compared with the controls. The in vivo activation of Rubisco was markedly increased in UV-B-treated pea leaves. The underlying mechanisms for these results are discussed. Introduction Solar radiation is essential for all plant fife. Light is not only the driving force of photosynthesis but it also triggers and regulates many morphogenic responses. Excessive photon flux density, however, is potentially harmful, if not lethal, for plants. Deleterious effects to photosynthesis may be caused by excess visible light (photoinhibition; Ref 1) and ultraviolet (UV) radiation [2,3]. Global ultraviolet irradiance on the earth's surface Abbreviations: ATPase, ATP hydrolytic activity of the ATP syn- thase/hydrolase; Bicine, N,N-bis(2-hydroxyethyl)glycine; Car, caro- tenoids; CF1, the catalytic component of the chloroplast ATP syn- thase/hydrolase; Chl, chlorophyll; C a, plants fixing carbon by the tricarboxylic acid pathway; D1, the herbicide-binding polypeptide of Photosystem II; F m, maximum chlorophyll fluorescence; Fo, initial chlorophyll fluorescence when Photosystem II reaction centres are open; F v, variable chlorophyll fluorescence (Fm-Fo); LHCII, the light-harvesting components associated with Photosystem II; PAR, photosynthetically active radiation; PS I, Photosystem I; PS II, Photo- system II; P700, the reaction centre chlorophyll a of Photosystem I; Rubisco, ribulose-l,5-bisphosphate carboxylase; RuP2, ribulose 1,5- bisphosphate; UV-B, ultraviolet-B radiation (280-320 nm); UV-C, ultaviolet-C radiation ( < 280 nm). Correspondence: W.S. Chow, CSIRO, Division of Plant Industry, GPO Box 1600, Canberra, ACT 2601, Australia. fluctuates with changes in solar altitude, cloud cover, atmospheric turbidity and stratospheric ozone con- centration. Little UV radiation below 290 nm is de- tected at the earth's surface due to the absorption of solar radiation in the 200-290 nm region by strato- spheric ozone. Whereas UV-C radiation (< 280 nm; e.g., the mercury lamp with mostly 254 nm emission) causes damage to photosynthesis, primarily to the water-splitting apparatus of Photosystem II (PS II), and by the destruction of plastoquinone, it is not relevant in the natural environment [4]. However, UV-B radiation (280-320 nm) is readily absorbed by nucleic acids, proteins, pigments and lipids to a lesser extent [2]. UV-B primarily inhibits photosynthesis, as well as damaging many other plant processes [2,3]. The effects of UV-B radiation have been studied on plant communities, individual plants, leaves, isolated chloroplasts or thylakoid membranes using a plethora of UV-B radiation procedures ranging from UV-B radi- ation only, to various light treatments supplemented with UV-B [2,3,5]. The present study was initiated to evaluate the effects of moderate levels of supplementary UV-B radiation on photosynthesis. The composition and function of chloroplasts from fully expanded ma- ture leaves of peas were compared following exposure of one-half of the pea plants to additional UV-B radia- tion from the 17th day after planting. Peas were shown 0005-2728/90/$03.50 © 1990 Elsevier Science Publishers B.V. (Biomedical Division)

A gas-exchange study of photosynthesis and isoprene emission in quercus rubra l.

Abstract: We have investigated the signals which affect the rate of isoprene emission from photosynthesizing leaves of red oak (Quercus rubra L.) using analytical gas-exchange techniques, chlorophyll-fluorescence measurements, and inhibitor feeding. Isoprene emission increased with increasing photon flux density at low CO2 but much less so at high CO2 partial pressure. Photosynthetic CO2 assimilation exhibited the opposite behavior. In CO2-free air, isoprene emission was reduced; above 500 μbar CO2 partial pressure, isoprene emission was also reduced. The high-CO2 effect appeared to be related to low ATP levels which can occur during feedback-limited photosynthesis. At high temperature, which can prevent feedback limitations, isoprene emission remained high as CO2 partial pressure was increased. After exposing the leaves to darkness, isoprene emission declined over 15 min, while photosynthesis stopped within 2 min. Adding far-red light to stimulate cyclic photo-phosphorylation during the post-illumination period stimulated isoprene emission. These analyses lead us to propose that the rate of isoprene emission is regulated by ATP. Analysis of transients indicated that isoprene emission is also related to photosynthetic carbon metabolism. Inhibitor feeding indicated that 3-phosphoglyceric acid and 1,3-bisphosphoglyceric acid are possible candidates for the link between photosynthetic carbon metabolism and the regulation of isoprene emission. Given the ATP dependence, we suggest that the concentration of 1,3-bisphosphoglyceric acid may exert control over the rate of isoprene emission from oak leaves.

Effect of cadmium and nickel on photosynthesis and the enzymes of the photosynthetic carbon reduction cycle in pigeonpea (Cajanus cajan L.).

Abstract: The effect of Cd2+ and Ni2+ on the rate of photosynthesis and activities of key enzymes of the photosynthetic carbon reduction cycle was examined in leaves from pigeonpea (Cajanus cajan L., cv. UPAS-120) grown in nitrogen free sand culture. Two different concentrations of Cd2+ and Ni2+ were applied through the rooting medium at two growth stages. The application of Cd2+ and Ni2+ (0.5 and 1.0 mM) at an early vegetative stage (30 days after sowing) resulted in about 50% and 32% reduction in net photosynthesis, respectively. However, enzyme activities were decreased to different levels (2–61%) depending upon the enzyme and the concentration of the metal ion.

The mechanisms contributing to photosynthetic control of electron transport by carbon assimilation in leaves

Abstract: ‘Photosynthetic control’ describes the processes that serve to modify chloroplast membrane reactions in order to co-ordinate the synthesis of ATP and NADPH with the rate at which these metabolites can be used in carbon metabolism. At low irradiance, optimisation of the use of excitation energy is required, while at high irradiance photosynthetic control serves to dissipate excess excitation energy when the potential rate of ATP and NADPH synthesis exceed demand. The balance between ΔpH, ATP synthesis and redox state adjusts supply to demand such that the [ATP]/[ADP] and [NADPH]/[NADP+] ratios are remarkably constant in steady-state conditions and modulation of electron transport occurs without extreme fluctuations in these pools.

Elevated atmospheric partial pressure of CO2 and plant growth. II. Non-structural carbohydrate content in cotton plants and its effect on growth parameters.

Abstract: Cotton plants were grown in late spring under full sunlight in glasshouses containing normal ambient partial pressure of CO2 (32±2Pa) and enriched partial pressure of CO2 (64±1.5Pa) and at four levels of nitrogen nutrition. Thirty-five days after planting, the total dry weights of high CO2-grown plants were 2- to 3.5-fold greater than plants grown in normal ambient CO2 partial pressure. Depending on nitrogen nutrition level, non-structural carbohydrate content (mainly starch) in the leaves of plants grown in normal CO2 was between 4 and 37% of the total leaf dry weight compared to 39 to 52% in the leaves of high CO2-grown plants. Specific leaf weight calculated using total dry weight was 1.6- to 2-fold greater than that based on structural dry weight. In high CO2-grown plants the amount of non-structural carbohydrate translocated from the leaves at night was between 10 and 20% of the level at the end of the photoperiod. This suggests that the plant was unable to utilize all the carbohydrate it assimilated in elevated CO2 atmosphere. While there was a 1.5-fold enhancement in the rate of CO2 assimilation in plants grown in 64 Pa CO2, there was, however, some evidence to suggest that the activities of other metabolic pathways in the plants were not stimulated to the same extent by the enriched CO2 atmosphere. This resulted in massive accumulation of non-structural carbohydrate, particularly at low level of nitrogen nutrition.

Chlorophyll regulates accumulation of the plastid-encoded chlorophyll apoproteins CP43 and D1 by increasing apoprotein stability.

Abstract: Chlorophyll apoprotein accumulation in higher plant chloroplasts is controlled by light-dependent chlorophyll formation. Dark-grown plants lack chlorophyll and chlorophyll apoproteins. However, the plastid genes encoding the chlorophyll apoproteins are transcribed; chlorophyll apoprotein mRNA accumulates and associates with polysomes in plastids of dark-grown plants. Pulse-labeling assays revealed a population of short-lived proteins in plastids of dark-grown plants. One of these transiently labeled proteins was CP43, a chlorophyll apoprotein associated with photosystem II. Pulse-chase assays showed that newly synthesized CP43 was rapidly degraded in plastids of dark-grown plants, which lack chlorophyll. In contrast, CP43 synthesized in plastids from illuminated plants was stable. The synthesis of D1, a chlorophyll apoprotein of the photosystem II reaction center, was also analyzed in plastids of dark-grown and illuminated plants. Radiolabel accumulation into full-length D1 was only detected in plastids of illuminated plants. However, D1 translation intermediates of 15-25 kDa were detected in both plastid populations. Pulse-chase assays showed that the 15- to 25-kDa D1 translation products were precursors of mature D1 in plastids of illuminated plants. In contrast, in plastids of dark-grown plants, the 15- to 25-kDa translation intermediates were converted into a 23-kDa polypeptide previously suggested to be a proteolytic product of D1. These results indicate that chlorophyll produced in illuminated plants stabilizes D1 nascent polypeptides, which allows accumulation of mature D1.

Growth and Photosynthetic Responses of Spinach to Salinity: Implications of K+ Nutrition for Salt Tolerance

Abstract: To compare the effects of K+ under high and low salinity, spinach plants (Spinacia oleracea) were grown in nutrient solutions containing either 50 mM NaCl (low salinity) or 250 mM NaCl (high salinity), with a diurnal regime of 10 h light (~300 μmol photons m-2 s-1, 23°C) and 14 h dark (15°C). At each level of salinity, the nutrient KCl concentration was 0.01, 0.1, 1 or 10 mM. The plant and shoot biomass was greater at low salinity than high salinity and increased with the logarithmic increase in nominal K+ concentrations supplied to the roots. Plant and shoot growth were related to the K+ uptake into the leaves, with leaves having a higher K+ content under low salinity than high salinity. Variation of the K+ content in the leaves, induced by the combinations of nutrient KCl concentrations with high or low salinity, were accompanied by changes in the photosynthetic capacity at light- and CO2-saturation per unit leaf area; there was a greater decrease in photosynthetic capacity with decreasing K+ supply to the roots under high salinity than under low salinity. The photosynthetic capacity was in turn highly correlated with the contents of cytochrome f and ATP synthase per unit leaf area. Under conditions of high salinity and low K+ supply, a reduction in the quantum yield of oxygen evolution also occurred, due to malfunction of photosystem II and, apparently, an increased proportion of light absorbed by non-photosynthetic tissue. The decreases in photosynthetic capacity and quantum yield partly account for the lower plant and shoot biomass at high salinity and low nutrient KCl concentrations. Our results suggest strongly that there are higher K+ requirements for shoot growth under high than low salinity conditions, and that high concentrations of Na+ in the leaves may help to maintain turgor, but cannot substitute for adequate K+ levels in the leaves, presumably because K+ is specifically required for protein synthesis. Increasing the K+ supply at the roots can ameliorate reductions in plant and shoot biomass imposed by an increase in salinity.

Carbonic Anhydrase Activity in Leaves and Its Role in the First Step of C4 Photosynthesis

Abstract: In C4 plants carbonic anhydrase catalyzes the critical first step of C4 photosynthesis, the hydration of CO2 to bicarbonate. The maximum activity of this enzyme in C4 leaf extracts, measured by H+ production with saturating CO2 and extrapolated to 25°C, was found to be 3,000 to 10,000 times the maximum photosynthesis rate for these leaves. Similar activities were found in C3 leaf extracts. However, the calculated effective activity of this enzyme at in vivo CO2 concentrations was apparently just sufficient to prevent the rate of conversion of CO2 to HCO3− from limiting C4 photosynthesis. This conclusion was supported by the mass spectrometric determination of leaf carbonic anhydrase activities.

Chlorophyll Fluorescence as a Measure of Photosynthetic Carbon Assimilation

Abstract: It has long been supposed that there must be an inverse relation between chlorophyll fluorescence and photosynthetic carbon assimilation Excitation of chlorophyll, by light, initiates the electron transport that drives photosynthesis Excess excitation energy is dissipated as heat and as light (fluorescence) but, at least in regard to carbon assimilation, the actual fluorescence signal was once regarded as too complex to permit interpretation More recently, the concept of two major fluorescence-quenching mechanisms and the development of techniques and devices that allow these to be resolved, has advanced understanding Concepts and procedures based on linear relations between fluorescence parameters and carbon assimilation have allowed good predictions of photosynthetic rates Here we show that a single curvilinear relation can be derived for several disparate plant species and suggest that this could allow photosynthesis and the effect of light on `dark respiration9 to be measured by non-intrusive fluorescence techniques without recourse to analysis of gaseous exchange between leaf and atmosphere

The Effect of External Nutrient Resources on the Optical Properties and Photosynthetic Efficiency of Stylophora pistillata

Abstract: Enrichment of the hermatypic Red Sea coral Stylophora pistillata with dissolved inorganic nitrogen, inorganic nitrogen+phosphorus, and feeding on Artemia, all led to increases in areal pigmentation in comparison with control colonies. These increases, unlike photoadaptive ones, resulted from growth in cell numbers ranging from $\times $ 2.75 in the Artemia-fed to $\times $ 4.85 in the N+P-enriched corals. The treated corals absorbed 51-85% of incident light, whereas the controls absorbed only 33%. Areal photosynthesis increased with treatment, although to a lesser degree than absorptivity. This difference resulted in reduced photosynthetic efficiencies in the treated colonies. Photosynthetic rates, calculated on a percell basis, were inversely correlated with algal densities, indicating possible competition among the algae for CO$_{2}$.

Photosynthesis and Productivity during High‐Temperature Stress of Wheat Genotypes from Major World Regions

Abstract: (...) objectives were to ascertain relationships and genotypic variability among single-leaf photosynthesis, thylakoid activity, and productivity during high-temperature stress at vegetative and reproductive stages. Ten genotypes from major world wheat-producing regions were grown under moderate (22/17 °C day/night) and high (32/27 °C day/night) temperatures for 2 wk as seedlings or from anthesis to maturity. Net photosynthesis, chlorophyll variable fluorescence (Fv), and productivity were measured at both stages (....)

Field Study of the Interaction between Solar Ultraviolet-B Radiation and Drought on Photosynthesis and Growth in Soybean

Abstract: Soybean, Glycine max (L.) Merr. cv Essex, plants were grown in the field in a 2 × 2 factorial design, under ambient and supplemental levels of ultraviolet-B (UV-B) radiation (supplemental daily dose of 5.1 effective kilojoules per square meter) and were either well-watered or subjected to drought. Soil water potentials were reduced to −2.0 megapascals by the exclusion of natural precipitation in the drought plots and were maintained at approximately −0.5 megapascal by supplemental irrigation in well-watered plots. Plant growth and gas exchange characteristics were affected under both drought and supplemental UV-B radiation. Whole-leaf gas exchange analysis indicated that stomatal limitations on photosynthesis were only significantly affected by the combination of UV-B radiation and drought but substrate (ribulose bisphosphate) regeneration limitations were observed under either stress. The combined effect of both drought and UV-B radiation on photosynthetic gas exchange was a reduction in apparent quantum efficiency and the rapid appearance of biochemical limitations to photosynthesis concomitant with reduced diffusional limitations. However, the combination of stresses did not result in additive effects on total plant growth or seed yield compared to reductions under either stress independently.

A Model Describing the Regulation of Ribulose-1,5-Bisphosphate Carboxylase, Electron Transport, and Triose Phosphate Use in Response to Light Intensity and CO2 in C3 Plants

Abstract: A model of the regulation of the activity of ribulose-1,5-bisphosphate carboxylase, electron transport, and the rate of orthophosphate regeneration by starch and sucrose synthesis in response to changes in light intensity and partial pressures of CO2 and O2 is presented. The key assumption behind the model is that nonlimiting processes of photosynthesis are regulated to balance the capacity of limiting processes. Thus, at CO2 partial pressures below ambient, when a limitation on photosynthesis by the capacity of rubisco is postulated, the activities of electron transport and phosphate regeneration are down-regulated in order that the rate of RuBP regeneration matches the rate of RuBP consumption by rubisco. Similarly, at subsaturating light intensity or elevated CO2, when electron transport or Pi regeneration may limit photosynthesis, the activity of rubisco is downregulated to balance the limitation in the rate of RuBP regeneration. Comparisons with published data demonstrate a general consistency between modelled predictions and measured results.

Adaptation of the Photosynthetic Apparatus in Maize Leaves as a Result of Nitrogen Limitation : Relationships between Electron Transport and Carbon Assimilation

Abstract: In maize (Zea mays L., cv Contessa), nitrogen (NO3−) limitation resulted in a reduction in shoot growth and photosynthetic capacity and in an increase in the leaf zeaxanthin contents. Nitrogen deficiency had only a small effect on the quantum yield of CO2 assimilation but a large effect on the light-saturated rate of photosynthesis. Linear relationships persisted between the quantum yield of CO2 assimilation and that of photosystem II photochemistry in all circumstances. At high irradiances, large differences in photochemical quenching and nonphotochemical quenching of Chl a fluorescence as well as the ratio of variable to maximal fluorescence (Fv/Fm) were apparent between nitrogen-deficient plants and nitrogen-replete controls, whereas at low irradiances these parameters were comparable in all plants. Light intensity-dependent increases in nonphotochemical quenching were greatest in nitrogen-deficient plants as were the decreases in Fv/Fm ratio. In nitrogen-deficient plants, photochemical quenching decreased with increasing irradiance but remained higher than in controls at high irradiances. Thermal dissipative processes were enhanced as a result of nitrogen deficiency (nonphotochemical quenching was elevated and Fv/Fm was lowered) allowing PSII to remain relatively oxidised even when carbon metabolism was limited via nitrogen limitation.

Response of the Photosynthetic Apparatus in Dunaliella salina (Green Algae) to Irradiance Stress.

Abstract: The response of the photosynthetic apparatus in the green alga Dunaliella salina, to irradiance stress was investigated. Cells were grown under physiological conditions at 500 millimoles per square meter per second (control) and under irradiance-stress conditions at 1700 millimoles per square meter per second incident intensity (high light, HL). In control cells, the light-harvesting antenna of photosystem I (PSI) contained 210 chlorophyll a/b molecules. It was reduced to 105 chlorophyll a/b in HL-grown cells. In control cells, the dominant form of photosystem II (PSII) was PSIIα(about 63% of the total PSII) containing >250 chlorophyll a/b molecules. The smaller antenna size PSIIβ centers (about 37% of PSII) contained 135 ± 10 chlorophyll a/b molecules. In sharp contrast, the dominant form of PSII in HL-grown cells accounted for about 95% of all PSII centers and had an antenna size of only about 60 chlorophyll a molecules. This newly identified PSII unit is termed PSIIγ. The HL-grown cells showed a substantially elevated PSII/PSI stoichiometry ratio in their thylakoid membranes (PSII/PSI = 3.0/1.0) compared to that of control cells (PSII/PSI = 1.4/1.0). The steady state irradiance stress created a chronic photoinhibition condition in which D. salina thylakoids accumulate an excess of photochemically inactive PSII units. These PSII units contain both the reaction center proteins and the core chlorophyll-protein antenna complex but cannot perform a photochemical charge separation. The results are discussed in terms of regulatory mechanism(s) in the plant cell whose function is to alleviate the adverse effect of irradiance stress.

The influence of N metabolism and organic acid synthesis on the natural abundance of isotopes of carbon in plants

Abstract: SUMMARY This paper relates the 13C/12C ratio of C3 plant material relative to that of source CO2 to the N source for growth, the organic N content of the plant, and the extent of organic acid synthesis. The 13C/12C ratio is quantified as Δ, defined as (δ13C substrate –δ13C product)/(1+δ13C product), where δ13C values of substrate or product (i.e. the samples) are defined as [13C/12C]sample]/[(13C/12C)standard]−1. The computation is performed by relating differences in plant composition as a function of N nutrition and acid synthesis to the fraction of plant C which is acquired via Rubisco and via other carboxylases. The fractional contribution of the different carboxylases to C gain is then related, using the known isotopic fractionations exhibited by these carboxylases, in a model to predict the final Δ of the plant (relative to atmospheric CO2). Application of this approach to a ‘typical’ C3 land plant yields predictions of the decrease of Δ relative to a hypothetical case in which all C is fixed via Rubisco. The predicted decreases range from 0–24 %, for NH4+ assimilation (which always occurs in the roots) to 2–80%, for NO3− assimilation in shoots with the organic acid salt which results from acid-base balance, plus any additional organic acid salts plus free acids for a plant with a basal C:N molar ratio in organic material of 15. Intermediate values are predicted for symbiotic growth with N2, or where NO3− assimilation in root or shoot is accompanied by some acid-base regulation via OH- loss to the root medium. Comparison with published data on the difference in Δ of Ricinus communis cultured with NH4+ or NO3− shows that the measured influence of nitrogen source is in the right direction (NO3− grown plants with a smaller Δ, i.e. a larger deviation from the value predicted for the absence of non-Rubisco carboxylations) to be explained by the observed difference in composition and hence fractional C contribution by the various carboxylases. However, the effect of N source on Δ is greater than that predicted by the model, i.e. a 2.1 % decrease as opposed to a 0.10 % decrease. It is likely that the major cause of the difference in δ13C of the plants grown on the two N sources is a change in the ratio of transport and biochemical conductances of leaf photosynthesis. Such a change is quantitatively consistent with the lower water use efficiency of NH4+ -grown plants. The predicted, and observed, changes in Δ as a function of N source are of the same magnitude as those found for C3 terrestrial species grown at different temperatures or photon flux densities, or in environments yielding different water use efficiencies by changing root water supply relative to shoot evaporation potential. Variations in N source should be added to the factors which might alter δ of plants growing in the field.

The physiological significance of photosystem II heterogeneity in chloroplasts.

Abstract: Photosystem II in green plant chloroplasts displays heterogeneity both in the composition of its light-harvesting antenna and in the ability to reduce the plastoquinone pool. These two features are discussed in terms of chloroplast development and in view of a proposed photosystem II repair cycle.

CHLOROPHYLL BINDING PROTEINS WITH ANTENNA FUNCTION IN HIGHER PLANTS and GREEN ALGAE

Abstract: The absorption of light by antenna pigments creates mobile singlet state excitons which may migrate within the complex array of chlorophylls to either Photosystem I (PSI)* or PSII reaction centers. When an exciton reaches the reaction center, its energy is converted into charge separation, the exciton disappears, and a chain of redox reactions is triggered by which radiant energy is transduced. Excitons reaching the reaction centers are said to be trapped if charge separation occurs. Alternatively, they may return to the antenna pigments. During migration of excitons in the antenna matrix, the probability that they may decay as fluorescence quanta or by means of other non-radiative decay processes is finite. Owing to the high efficiency by which light is converted in the photosynthetic apparatus, energy losses are small and very little energy (approx. 2%) is emitted as fluorescence. As the purpose of photosynthesis is to convert excited state energy into chemical free energy with maximum efficiency, it might seem that any light energy emitted as fluorescence from the photosynthetic apparatus is “wasted”. However, at the present state of knowledge it cannot be excluded that fluorescence emission, at least in part, is the result of molecular control of the amount of energy reaching the reaction centers in order to avoid damage due to overexcitation and photoinhibition. In any case, the small fraction of energy emitted as fluorescence is of great importance in the investigation of excitation energy distribution and exciton migration among chlorophyll-protein complexes. The proportion of energy which is not utilized in photosynthesis and is emitted as fluorescence varies, depending mark-

Effects of phosphorus nutrition on photosynthesis in Glycine max (L.) Merr.

Abstract: The effects of phosphorus nutrition on various aspects of photosynthetic metabolism have been examined for soybean plants (Glycine max) grown in growth chambers. Orthophosphate was supplied at two levels in 0.5-strength Hoagland's solution. At the end of the 19-d growth period, plants grown at 10 μM KH2PO4 (low-P plants) had undergone a 40% drop in net CO2 exchange (averaged over a 16-h light period), as compared with control plants grown with 200 μM KH2PO4. Low-P resulted in reductions in the initial activities of five, and in the total activities of seven, Calvin-cycle enzymes. Notable exceptions were the initial and total activities of chloroplastic fructose-1,6-bisphosphatase (EC 3.1.3.11) which were increased by 85 and 53%, respectively, by low-P. Low-P decreased leaf 3-phosphoglycerate (PGA) levels most (by 80%), ribulose-1,5-bis-phosphate (RuBP) less (by 47%) while triose-phosphate (TP) was not significantly changed. The results indicate that photosynthetic CO2-fixation in low-P plants was limited more by RuBP regeneration than by ribulose-1,5-bisphosphate carboxylase/oxygenase (EC 4.1.1.39) activity. Ribulose-1,5-bisphosphate regeneration in low-P plants did not appear to be limited by ATP and-or NADPH supply because ATP/ADP and NADPH/ NADP+ ratios were increased by 60 and 37%, respectively, by low-P, and because TP/PGA ratios were higher in low-P plants. Low-P may diminish RuBP regeneration, and hence photosynthesis, by reducing Calvin-cycle enzyme activity, in particular, the initial activity of ribulose-5-phosphate kinase (EC 2.7.1.19) (44% reduction), and by enhancing the flux of carbon into starch biosynthesis.

Photosynthetic Decline from High Temperature Stress during Maturation of Wheat : I. Interaction with Senescence Processes.

Abstract: Photosynthetic capacity decreases rapidly when temperate species are exposed to heat stress during reproductive development. We investigated whether injury in wheat (Triticum aestivum L.) resulted from general acceleration of senescence processes or specific heat-induced lesions. In situ photosynthetic capacity of leaf discs and thylakoid reactions were measured using flag leaf tissue from two cultivars maintained at 20 and 35°C during maturation. Photosynthetic rates of leaf discs decreased faster at 35 than at 20°C and were more photolabile in cv Len than in cv Waverly at high temperature. Patterns of thylakoid breakdown also differed in the two wheat genotypes at 20°C: intersystem electron transport and photosystem II activity decreased linearly during postanthesis development in Len wheat, whereas coupling of photophosphorylation to electron transport declined late during senescence in Waverly wheat. Heat stress induced early loss of intersystem electron transport followed sequentially by decreased silicomolybdic acid, + 3-(3,4-dichlorophenyl)-1-dimethylurea-mediated photosystem II activity and 2,5-dichloro-p-benzoquinone-mediated photosystem II activity in Len. Stress accelerated the uncoupling process, but loss of intersystem electron transport and photosystem II activities was slower in Waverly than in Len. We conclude that high temperature initially accelerated thylakoid component breakdown, an effect similar to normal senescence patterns. Thylakoid breakdown may induce a destabilizing imbalance between component reaction rates; an imbalance between photosystem II and cytochrome f/b6-mediated activities would be particularly damaging during heat stress.

Effect of nodulation, nitrogen fixation and CO2 enrichment on the physiology, growth and dry mass allocation of seedlings of Alnus rubra bong

Abstract: SUMMARY Inoculated and uninoculated Alnus rubra Bong, seedlings were grown for 47 days in atmospheres containing ambient (350 μ1 CO2 1−1) and elevated (650 μl CO2 1−1) levels of CO2, with and without combined nitrogen (20 mg 1−1) supplied as ammonium nitrate. Five plants from each treatment were harvested 15, 30, and 47 days after exposure to CO2 treatments began. Evidence for the presence of a positive feedback loop between nitrogen fixation and photosynthesis was observed in nodulated plants growing at elevated CO2. These plants had greater whole-plant photosynthesis and nitrogenase activity, leaf area and nitrogen content, as well as nodule and plant dry mass, relative to nodulated plants grown at ambient CO2 and non-nodulated plants grown at both CO2 levels. This feedback may be an important way in which the potential carbon drain of nitrogen fixation on the host plant could be compensated; increased nitrogen availability resulting in stimulated leaf area growth and whole-plant photosynthesis. The relative amount of dry mass allocated below ground decreased for all seedlings over time, and the amount allocated above ground increased. This shift in allocation occurred slowly and at a constant rate in non-nodulated plants and more rapidly and abruptly when plants were nodulated. The proportion of dry mass allocated below ground was consistently greater in non-nodulated plants. Dry mass allocation to the stem was increased when combined nitrogen was applied and was greatest in nodulated plants grown at high CO2. Dry mass partitioning among other organs was not directly affected by nodulation, CO2 enrichment, or other treatment interactions.