Atmospheric deposition, CO2, and change in the land carbon sink

Concentrations of atmospheric carbon dioxide (CO₂) have continued to increase whereas atmospheric deposition of sulphur and nitrogen has declined in Europe and the USA during recent decades. Using time series of flux observations from 23 forests distributed throughout Europe and the USA, and generalised mixed models, we found that forest-level net ecosystem production and gross primary production have increased by 1% annually from 1995 to 2011. Statistical models indicated that increasing atmospheric CO₂ was the most important factor driving the increasing strength of carbon sinks in these forests. We also found that the reduction of sulphur deposition in Europe and the USA lead to higher recovery in ecosystem respiration than in gross primary production, thus limiting the increase of carbon sequestration. By contrast, trends in climate and nitrogen deposition did not significantly contribute to changing carbon fluxes during the studied period. Our findings support the hypothesis of a general CO₂-fertilization effect on vegetation growth and suggest that, so far unknown, sulphur deposition plays a significant role in the carbon balance of forests in industrialized regions. Our results show the need to include the effects of changing atmospheric composition, beyond CO₂, to assess future dynamics of carbon-climate feedbacks not currently considered in earth system/climate modelling.

Performance of induction module cavity ring-down spectroscopy (IM-CRDS) for measuring δ18 O and δ2 H values of soil, stem, and leaf waters

RATIONALE

Induction module cavity ring-down spectroscopy (IM-CRDS) has been proposed as a rapid and cost-effective alternative to cryogenic vacuum distillation (CVD) and isotope ratio mass spectrometry (IRMS) for the measurement of δ¹⁸ O and δ² H values in matrix-bound waters. In the current study, we characterized the performance of IM-CRDS relative to CVD and IRMS and investigated the mechanisms responsible for differences between the methods.

METHODS

We collected a set of 75 soil, stem, and leaf water samples, and measured the δ¹⁸ O and δ² H values of each sample with four techniques: CVD and IRMS, CVD and CRDS, CVD and IM-CRDS, and IM-CRDS alone. We then calculated the isotopic errors for each of the three CRDS methods relative to CVD and IRMS, and analyzed the relationships among these errors and suites of diagnostic spectral parameters that are indicative of organic contamination.

RESULTS

The IM-CRDS technique accurately assessed the δ¹⁸ O and δ² H values of pure waters, but exhibited progressively increasing errors for soil waters, stem waters, and leaf waters. For soils, the errors were attributable to subsampling of isotopically heterogeneous source material, whereas for stems and leaves, they were attributable to spectral interference. Unexpectedly, the magnitude of spectral interference was higher for the solid samples analyzed directly via IM-CRDS than for those originally extracted via CVD and then analyzed by IM-CRDS.

CONCLUSIONS

There are many types of matrix-bound water samples for which IM-CRDS measurements include significant errors from spectral interference. As a result, spectral analysis and validation should be incorporated into IM-CRDS post-processing procedures. In the future, IM-CRDS performance could be improved through: (i) identification of the compounds that cause spectral interference, and either (ii) modification of the combustion step to completely oxidize these compounds to CO₂ , and/or (iii) incorporation of corrections for these compounds into the spectral fitting models used by the CRDS analyzers. Copyright © 2016 John Wiley & Sons, Ltd.

Mercury distribution across 14 U.S. Forests. Part I: spatial patterns of concentrations in biomass, litter, and soils

Results from a systematic investigation of mercury (Hg) concentrations across 14 forest sites in the United States show highest concentrations in litter layers, strongly enriched in Hg compared to aboveground tissues and indicative of substantial postdepositional sorption of Hg. Soil Hg concentrations were lower than in litter, with highest concentrations in surface soils. Aboveground tissues showed no detectable spatial patterns, likely due to 17 different tree species present across sites. Litter and soil Hg concentrations positively correlated with carbon (C), latitude, precipitation, and clay (in soil), which together explained up to 94% of concentration variability. We observed strong latitudinal increases in Hg in soils and litter, in contrast to inverse latitudinal gradients of atmospheric deposition measures. Soil and litter Hg concentrations were closely linked to C contents, consistent with well-known associations between organic matter and Hg, and we propose that C also shapes distribution of Hg in forests at continental scales. The consistent link between C and Hg distribution may reflect a long-term legacy whereby old, C-rich soil and litter layers sequester atmospheric Hg depositions over long time periods. Based on a multiregression model, we present a distribution map of Hg concentrations in surface soils of the United States.

A high affinity pyruvate decarboxylase is present in cottonwood leaf veins and petioles: a second source of leaf acetaldehyde emission?

Considerable evidence indicates that acetaldehyde is released from the leaves of a variety of plants. The conventional explanation for this is that ethanol formed in the roots is transported to the leaves where it is converted to acetaldehyde by the alcohol dehydrogenase (ADH) found in the leaves. It is possible that acetaldehyde could also be formed in leaves by action of pyruvate decarboxylase (PDC), an enzyme with an uncertain metabolic role, which has been detected, but not characterized, in cottonwood leaves. We have found that leaf PDC is present in leaf veins and petioles, as well as in non-vein tissues. Veins and petioles contained measurable pyruvate concentrations in the range of 2mM. The leaf vein form of the enzyme was purified approximately 143-fold, and, at the optimum pH of 5.6, the K(m) value for pyruvate was 42 microM. This K(m) is lower than the typical millimolar range seen for PDCs from other sources. The purified leaf PDC also decarboxylates 2-ketobutyric acid (K(m)=2.2mM). We conclude that there are several possible sources of acetaldehyde production in cottonwood leaves: the well-characterized root-derived ethanol oxidation by ADH in leaves, and the decarboxylation of pyruvate by PDC in leaf veins, petioles, and other leaf tissues. Significantly, the leaf vein form of PDC with its high affinity for pyruvate, could function to shunt pyruvate carbon to the pyruvate dehydrogenase by-pass and thus protect the metabolically active vascular bundle cells from the effects of oxygen deprivation.

Induction of poplar leaf nitrate reductase: a test of extrachloroplastic control of isoprene emission rate

Several recent studies have suggested that control of isoprene emission rate is in part exerted by supply of extrachloroplastic phosphoenolpyruvate to the chloroplast. To test this hypothesis, we altered PEP supply by differential induction of cytosolic nitrate reductase (NR) and PEP carboxylase (PEPC) in plants of Populus deltoides grown with NO3- or NH4+ as the sole nitrogen source. Growth with 8 mM NH4+ produced a high leaf nitrogen concentration, compared with 8 mM NO3-, as well as slightly elevated rates of photosynthesis and significantly enhanced rates of isoprene emission and content of dimethylallyl diphosphate (DMAPP, a precursor to isoprene biosynthesis), chlorophyll (a+b) and carotenoids. Growth with 8 mM NO3- resulted in parallel reductions in both leaf isoprene emission rate and DMAPP. The differential effects of growth with NH4+ or NO3- were not observed when plants were grown with 4 mM nitrogen. The effects of reduced DMAPP availability were specific to isoprene emission and were not propagated to higher isoprenoids, as the correlations between nitrogen content and either leaf chlorophyll (a+b) or total carotenoids were unaffected by nitrogen source. Biochemical analysis revealed significantly higher levels of NR and PEPC activity in leaves of 8 mM NO3- -grown plants, consistent with their fundamental roles in nitrate assimilation. Taken together, these results support the hypothesis that foliar assimilation of NO3- reduces isoprene emission rate by competing for carbon skeletons (mediated by PEPC) within the cytosol and possibly reductant within the chloroplast. Cytosolic competition for PEP is a major regulator of chloroplast DMAPP supply, and we offer a new "safety valve" hypothesis to explain why plants emit isoprene.

Temperature as a control over ecosystem CO2 fluxes in a high-elevation, subalpine forest

We evaluated the hypothesis that CO(2) uptake by a subalpine, coniferous forest is limited by cool temperature during the growing season. Using the eddy covariance approach we conducted observations of net ecosystem CO(2) exchange (NEE) across two growing seasons. When pooled for the entire growing season during both years, light-saturated net ecosystem CO(2) exchange (NEE(sat)) exhibited a temperature optimum within the range 7-12 degrees C. Ecosystem respiration rate ( R(e)), calculated as the y-intercept of the NEE versus photosynthetic photon flux density (PPFD) relationship, increased with increasing temperature, causing a 15% reduction in net CO(2) uptake capacity for this ecosystem as temperatures increased from typical early season temperatures of 7 degrees C to typical mid-season temperatures of 18 degrees C. The ecosystem quantum yield and the ecosystem PPFD compensation point, which are measures of light-utilization efficiency, were highest during the cool temperatures of the early season, and decreased later in the season at higher temperatures. Branch-level measurements revealed that net photosynthesis in all three of the dominant conifer tree species exhibited a temperature optimum near 10 degrees C early in the season and 15 degrees C later in the season. Using path analysis, we statistically isolated temperature as a seasonal variable, and identified the dynamic role that temperature exhibits in controlling ecosystem fluxes early and late in the season. During the spring, an increase in temperature has a positive effect on NEE, because daytime temperatures progress from near freezing to near the photosynthetic temperature optimum, and R(e )values remain low. During the middle of the summer an increase in temperature has a negative effect on NEE, because inhibition of net photosynthesis and increases in R(e). When taken together, the results demonstrate that in this high-elevation forest ecosystem CO(2) uptake is not limited by cool-temperature constraints on photosynthetic processes during the growing-season, as suggested by some previous ecophysiological studies at the branch and needle levels. Rather, it is warm temperatures in the mid-summer, and their effect on ecosystem respiration, that cause the greatest reduction in the potential for forest carbon sequestration.

Biochemistry and physiology of foliar isoprene production

Leaves of some plants emit isoprene, a volatile hydrocarbon. This is formed by a novel chloroplastic isoprenoid biosynthetic pathway, the 1-deoxy-D-xylulose-5-phosphate pathway. The thermoprotection hypothesis suggests that isoprene protects thylakoids from damage at high temperatures. In this article, we discuss the most recent discoveries about the metabolic pathway underlying isoprene biosynthesis, explore the experimental evidence surrounding thermoprotection and advance some alternative hypotheses about the adaptive role that isoprene biosynthesis might play.

Thermotolerance of leaf discs from four isoprene-emitting species is not enhanced by exposure to exogenous isoprene

The effects of exogenously supplied isoprene on chlorophyll fluorescence characteristics were examined in leaf discs of four isoprene-emitting plant species, kudzu (Pueraria lobata [Willd.] Ohwi.), velvet bean (Mucuna sp.), quaking aspen (Populus tremuloides Michx.), and pussy willow (Salix discolor Muhl). Isoprene, supplied to the leaves at either 18 &mgr;L L-1 in compressed air or 21 &mgr;L L-1 in N2, had no effect on the temperature at which minimal fluorescence exhibited an upward inflection during controlled increases in leaf-disc temperature. During exposure to 1008 &mgr;mol photons m-2 s-1 in an N2 atmosphere, 21 &mgr;L L-1 isoprene had no effect on the thermally induced inflection of steady-state fluorescence. The maximum quantum efficiency of photosystem II photochemistry decreased sharply as leaf-disc temperature was increased; however, this decrease was unaffected by exposure of leaf discs to 21 &mgr;L L-1 isoprene. Therefore, there were no discernible effects of isoprene on the occurrence of symptoms of high-temperature damage to thylakoid membranes. Our data do not support the hypothesis that isoprene enhances leaf thermotolerance.

Isoprene emission, photosynthesis, and growth in sweetgum (Liquidambar styraciflua) seedlings exposed to short- and long-term drying cycles

Isoprene emissions were studied in one-year old sweetgum (Liquidambar styraciflua L.) seedlings during nine drying-rewatering cycles extending over five months. Each drying cycle lasted to the point of leaf wilting. Growth was essentially stopped in response to the first drying cycle, though seedling survival and capacity to recover turgor on rewatering remained high throughout the entire nine cycles. Photosynthetic rates of leaves were inhibited by the drying treatments. Under severe drought, isoprene emission rates of leaves were also inhibited, though isoprene emission was generally less sensitive to drought than photosynthesis. The lower drought sensitivity of isoprene emission compared with photosynthesis resulted in a higher percentage of fixed carbon lost as isoprene as seedlings became more stressed. During the recovery phase of the drying-rewatering cycles, isoprene emission rates in several seedlings were higher than in well-watered control seedlings. Following the ninth drying-rewatering cycle, sustained daily watering resulted in recovery of isoprene emission rates to control values within four days. Photosynthetic rates only recovered to 50% of control values after seven days. We conclude that the mechanisms regulating photosynthetic rate and isoprene emission rate are differentially influenced by limited water supplies. The results are consistent with past studies that predict a protective role for isoprene emission during stress, particularly protection from excessive leaf temperatures during drought.

Isoprene Emission from Velvet Bean Leaves (Interactions among Nitrogen Availability, Growth Photon Flux Density, and Leaf Development)

Although isoprene synthesis is closely coupled to photosynthesis, both via ATP requirements and carbon substrate availability, control of isoprene emission is not always closely linked to photosynthetic processes. In this study we grew velvet bean (Mucuna sp.) under different levels of photon flux density (PFD) and nitrogen availability in an effort to understand better the degree to which these two processes are linked. As has been observed in past studies, we found that during early leaf ontogeny the onset of positive rates of net photosynthesis precedes that of isoprene emission by 3 to 4 d. Other studies have shown that this lag is correlated with the induction of isoprene synthase activity, indicating that overall control of the process is under control of that enzyme. During leaf senescence, photosynthesis rate and isoprene emission rate declined in parallel, suggesting similar controls over the two processes. This coordinated decline was accelerated when plants were grown with high PFD and high nitrogen availability. The latter effect included declines in the photon yield of photosynthesis, suggesting that an unexplained stress arose during growth under these conditions, triggering a premature decline in photosynthesis and isoprene emission rate. In mature leaves, growth PFD and nitrogen nutrition affected photosynthesis and isoprene emission in qualitatively similar, but quantitatively different, ways. This resulted in a significant shift in the percentage of fixed carbon that was re-emitted as isoprene. In the case of increasing growth PFD, isoprene emission rate was more strongly affected than photosynthesis rate, and more carbon was lost as isoprene. In the case of increasing nitrogen, photosynthesis rate increased more than isoprene emission rate, and leaves containing high amounts of nitrogen lost a lower percentage of their assimilated carbon as isoprene. Taken together, our results demonstrate that, although the general correlation between isoprene emission rate and photosynthesis rate is consistently expressed, there is evidence that both processes are capable of independent responses to plant growth environment.

Isoprene emission rate and intercellular isoprene concentration as influenced by stomatal distribution and conductance

Isoprene emission in relation to stomatal distribution and conductance was determined for the hypostomatous species, aspen and white oak, and the amphistomatous species, cottonwood. For aspen and oak, isoprene emission from the adaxial (nonstomatal) surface was <2% of that from the abaxial (stomatal) surface, even when stomata were closed by addition of abscisic acid (ABA). When treated with ABA, the total flux rate of isoprene emission from leaves of these two hypostomatous species was unchanged, despite decreases in stomatal conductance of over 90%. The lack of control over isoprene emission rate by stomatal conductance, despite the apparent movement of isoprene through the stomatal pores, was due to increases in the intercellular isoprene concentration that compensated for the decreased stomatal conductance and restored the equilibrium between the isoprene synthesis rate and emission rate. This relationship was demonstrated by (a) an experiment in which the decrease in the internal isoprene pool following the imposition of darkness took longer in the presence of ABA than in its absence, and (b) direct measurements of the internal isoprene concentration through vacuum extraction, which revealed substantially higher values in the presence of ABA than in its absence. In the amphistomatous species, cottonwood, isoprene was emitted from both surfaces and addition of ABA caused an increase in isoprene emission from one surface coupled with a decrease from the other surface. The specific surface exhibiting an increase varied among leaves, with some leaves exhibiting an increase from the adaxial surface and other leaves from the abaxial surface. We interpret this as indicating nonuniform stomatal closure with concomitant emission of isoprene at the greatest rate from the surface with the highest stomatal conductance. We also observed an increase in the total isoprene emission rate from cottonwood leaves following treatment with ABA. We interpret this as indicating a stimulation of isoprene synthesis in response to ABA or stomatal closure, with unknown cause.

Relationships among Isoprene Emission Rate, Photosynthesis, and Isoprene Synthase Activity as Influenced by Temperature

Isoprene emissions from the leaves of velvet bean (Mucuna pruriens L. var utilis) plants exhibited temperature response patterns that were dependent on the plant's growth temperature. Plants grown in a warm regimen (34/28 degrees C, day/night) exhibited a temperature optimum for emissions of 45 degrees C, whereas those grown in a cooler regimen (26/20 degrees C, day/night) exhibited an optimum of 40 degrees C. Several previous studies have provided evidence of a linkage between isoprene emissions and photosynthesis, and more recent studies have demonstrated that isoprene emissions are linked to the activity of isoprene synthase in plant leaves. To further explore this linkage within the context of the temperature dependence of isoprene emissions, we determined the relative temperature dependencies of photosynthetic electron transport, CO(2) assimilation, and isoprene synthase activity. When measured over a broad range of temperatures, the temperature dependence of isoprene emission rate was not closely correlated with either the electron transport rate or the CO(2) assimilation rate. The temperature optima for electron transport rate and CO(2) assimilation rate were 5 to 10 degrees C lower than that for the isoprene emission rate. The dependence of isoprene emissions on photon flux density was also affected by measurement temperature in a pattern independent of those exhibited for electron transport rate and CO(2) assimilation rate. Thus, despite no change in the electron transport rate or CO(2) assimilation rate at 26 and 34 degrees C, the isoprene emission rate changed markedly. The quantum yield of isoprene emissions was stimulated by a temperature increase from 26 to 34 degrees C, whereas the quantum yield for CO(2) assimilation was inhibited. In greenhouse-grown aspen leaves (Populus tremuloides Michaux.), the high temperature threshold for inhibition of isoprene emissions was closely correlated with the high temperature-induced decrease in the in vitro activity of isoprene synthase. When taken together, the results indicate that although there may be a linkage between isoprene emission rate and photosynthesis, the temperature dependence of isoprene emission is not determined solely by the rates of CO(2) assimilation or electron transport. Rather, we propose that regulation is accomplished primarily through the enzyme isoprene synthase.

Isoprene emission from aspen leaves : influence of environment and relation to photosynthesis and photorespiration

Isoprene emission rates from quaking aspen (Populus tremuloides Michx.) leaves were measured simultaneously with photosynthesis rate, stomatal conductance, and intercellular CO(2) partial pressure. Isoprene emission required the presence of CO(2) or O(2), but not both. The light response of isoprene emission rate paralleled that of photosynthesis. Isoprene emission was inhibited by decreasing ambient O(2) from 21% to 2%, only when there was oxygen insensitive photosynthesis. Mannose (10 millimolar) fed through cut stems resulted in strong inhibition of isoprene emission rate and is interpreted as evidence that isoprene biosynthesis requires either the export of triose phosphates from the chloroplast, or the continued synthesis of ATP. Light response experiments suggest that photosynthetically generated reductant or ATP is required for isoprene biosynthesis. Isoprene biosynthesis and emission are not directly linked to glycolate production through photorespiration, contrary to previous reports. Isoprene emission rate was inhibited by above-ambient CO(2) partial pressures (640 microbar outside and 425 microbar inside the leaf). The inhibition was not due to stomatal closure. This was established by varying ambient humidity at normal and elevated CO(2) partial pressures to measure isoprene emission rates over a range of stomatal conductances. Isoprene emission rates were inhibited at elevated CO(2) despite no change in stomatal conductance. Addition of abscisic acid to the transpiration stream dramatically inhibited stomatal conductance and photosynthesis rate, with a slight increase in isoprene emission rate. Thus, isoprene emission is independent of stomatal conductance, and may occur through the cuticle. Temperature had an influence on isoprene emission rate, with the Q(10) being 1.8 to 2.4 between 35 and 45 degrees C. At these high temperatures the amount of carbon lost through isoprene emission was between 2.5 and 8% of that assimilated through photosynthesis. This represents a significant carbon cost that should be taken into account in determining midsummer carbon budgets for plants that are isoprene emitters.

Carbon-isotope discrimination by leaves of Flaveria species exhibiting different amounts of C3-and C 4-cycle co-function

Carbon-isotope ratios were examined as δ(13)C values in several C3, C4, and C3-C4 Flaveria species, and compared to predicted δ(13)C, values generated from theoretical models. The measured δ(13)C values were within 4‰ of those predicted from the models. The models were used to identify factors that contribute to C3-like δ(13)C values in C3-C4 species that exhibit considerable C4-cycle activity. Two of the factors contributing to C3-like δ(13)C values are high CO2 leakiness from the C4 pathway and pi/pa values that were higher than C4 congeners. A marked break occurred in the relationship between the percentage of atmospheric CO2 assimilated through the C4 cycle and the δ(13)C value. Below 50% C4-cycle assimialtion there was no significant relationship between the variables, but above 50% the δ(13)C values became less negative. These results demonstrate that the level of C4-cycle expression can increase from, 0 to 50% with little integration of carbon transfer from the C4 to the C3 cycle. As expression increaces above 50%, however, increased integration of C3- and C4-cycle co-function occurs.

Photosynthesis in Flaveria brownii A.M. Powell : A C(4)-Like C(3)-C(4) Intermediate

Leaves of Flaveria brownii exhibited slightly higher amounts of oxygen inhibition of photosynthesis than the C(4) species, Flaveria trinervia, but considerably less than the C(3) species, Flaveria cronquistii. The photosynthetic responses to intercellular CO(2), light and leaf temperature were much more C(4)-like than C(3)-like, although 21% oxygen inhibited the photosynthetic rate, depending on conditions, up to 17% of the photosynthesis rate observed in 2% O(2). The quantum yield for CO(2) uptake in F. brownii was slightly higher than that for the C(4) species F. trinervia in 2% O(2), but not significantly different in 21% O(2). The quantum yield was inhibited 10% in the presence of 21% O(2) in F. brownii, yet no significant inhibition was observed in F. trinervia. An inhibition of 27% was observed for the quantum yield of F. cronquistii in the presence of 21% O(2). The photosynthetic response to very low intercellular CO(2) partial pressures exhibited a unique pattern in F. brownii, with a break in the linear slope observed at intercellular CO(2) partial pressure values between 15 and 20 mubar when analyzed in 21% O(2). No significant break was observed when analyzed in 2% O(2). When taken collectively, the gas-exchange results reported here are consistent with previous biochemical studies that report incomplete intercellular compartmentation of the C(3) and C(4) enzymes in this species, and suggest that F. brownii is an advanced, C(4)-like C(3)-C(4) intermediate.

Co-function of C3-and C 4-photosynthetic pathways in C3, C 4 and C 3-C 4 intermediate Flaveria species

The potential for C4 photosynthesis was investigated in five C3-C4 intermediate species, one C3 species, and one C4 species in the genus Flaveria, using (14)CO2 pulse-(12)CO2 chase techniques and quantum-yield measurements. All five intermediate species were capable of incorporating (14)CO2 into the C4 acids malate and aspartate, following an 8-s pulse. The proportion of (14)C label in these C4 products ranged from 50-55% to 20-26% in the C3-C4 intermediates F. floridana Johnston and F. linearis Lag. respectively. All of the intermediate species incorporated as much, or more, (14)CO2 into aspartate as into malate. Generally, about 5-15% of the initial label in these species appeared as other organic acids. There was variation in the capacity for C4 photosynthesis among the intermediate species based on the apparent rate of conversion of (14)C label from the C4 cycle to the C3 cycle. In intermediate species such as F. pubescens Rydb., F. ramosissima Klatt., and F. floridana we observed a substantial decrease in label of C4-cycle products and an increase in percentage label in C3-cycle products during chase periods with (12)CO2, although the rate of change was slower than in the C4 species, F. palmeri. In these C3-C4 intermediates both sucrose and fumarate were predominant products after a 20-min chase period. In the C3-C4 intermediates, F. anomala Robinson and f. linearis we observed no significant decrease in the label of C4-cycle products during a 3-min chase period and a slow turnover during a 20-min chase, indicating a lower level of functional integration between the C4 and C3 cycles in these species, relative to the other intermediates. Although F. cronquistii Powell was previously identified as a C3 species, 7-18% of the initial label was in malate+aspartate. However, only 40-50% of this label was in the C-4 position, indicating C4-acid formation as secondary products of photosynthesis in F. cronquistii. In 21% O2, the absorbed quantum yields for CO2 uptake (in mol CO2·[mol quanta](-1)) averaged 0.053 in F. cronquistii (C3), 0.051 in F. trinervia (Spreng.) Mohr (C4), 0.052 in F. ramosissima (C3-C4), 0.051 in F. anomala (C3-C4), 0.050 in F. linearis (C3-C4), 0.046 in F. floridana (C3-C4), and 0.044 in F. pubescens (C3-C4). In 2% O2 an enhancement of the quantum yield was observed in all of the C3-C4 intermediate species, ranging from 21% in F. ramosissima to 43% in F. pubescens. In all intermediates the quantum yields in 2% O2 were intermediate in value to the C3 and C4 species, indicating a co-function of the C3 and C4 cycles in CO2 assimilation. The low quantum-yield values for F. pubescens and F. floridana in 21% O2 presumably reflect an ineffcient transfer of carbon from the C4 to the C3 cycle. The response of the quantum yield to four increasing O2 concentrations (2-35%) showed lower levels of O2 inhibition in the C3-C4 intermediate F. ramosissima, relative to the C3 species. This indicates that the co-function of the C3 and C4 cycles in this intermediate species leads to an increased CO2 concentration at the site of ribulose-1,5-bisphosphate carboxylase/oxygenase and a concomitant decrease in the competitive inhibition by O2.

The influence of inorganic phosphate on photosynthesis in intact chloroplasts from Mesembryanthemum crystallinum L. plants exhibiting C3 photosynthesis or crassulacean acid metabolism

Intact chloroplasts were obtained from mesophyll protoplasts isolated from Mesembryanthemum crystallinum in the C3 or Crassulacean acid metabolism (CAM) photosynthetic mode, and examined for the influence of inorganic phosphate (Pi) on aspects of bicarbonate-dependent O2 evolution and CO2 fixation. While the chloroplasts from both modes responded similarly to varying Pi, some features appear typical of chloroplasts from species capable of CAM, including a relatively high capacity for photosynthesis in the absence of Pi, a short induction period, and resistance to inhibition of photosynthesis by high levels of Pi. In the absence of Pi the chloroplasts retained 75-85% of the (14)CO2 fixed and the total export of dihydroxyacetone phosphate was low compared with the rate of photosynthesis. In CAM plants the ability to conduct photosynthesis and retain most of the fixed carbon in the chloroplasts at low external Pi concentrations may enable storage of carbohydrates which are essential for providing a carbon source for the nocturnal synthesis of malic acid. At high external Pi concentrations (e.g. 10 25 mM), the amount of total dihydroxyacetone phosphate exported to the assay medium relative to the rate of photosynthesis was high while the products of (14)CO2 fixation were largely retained in the chloroplasts which indicates starch degradation is occurring at high Pi levels. Starch degradation normally occurs in CAM plants in the dark; high levels of Pi may induce starch degradation in the light which has the effect of limiting export of the immediate products of photosynthesis and thus the degree of Pi inhibition of photosynthesis with the isolated chloroplast.

Photosynthetic Characteristics of C(3)-C(4) Intermediate Flaveria Species : I. Leaf Anatomy, Photosynthetic Responses to O(2) and CO(2), and Activities of Key Enzymes in the C(3) and C(4) Pathways

Four species of the genus Flaveria, namely F. anomala, F. linearis, F. pubescens, and F. ramosissima, were identified as intermediate C(3)-C(4) plants based on leaf anatomy, photosynthetic CO(2) compensation point, O(2) inhibition of photosynthesis, and activities of C(4) enzymes. F. anomala and F. ramosissima exhibit a distinct Kranz-like leaf anatomy, similar to that of the C(4) species F. trinervia, while the other C(3)-C(4) intermediate Flaveria species possess a less differentiated Kranz-like leaf anatomy. Photosynthetic CO(2) compensation points of these intermediates at 30 degrees C were very low relative to those of C(3) plants, ranging from 7 to 14 microliters per liter. In contrast to C(3) plants, net photosynthesis by the intermediates was not sensitive to O(2) concentrations below 5% and decreased relatively slowly with increasing O(2) concentration. Under similar conditions, the percentage inhibition of photosynthesis by 21% O(2) varied from 20% to 25% in the intermediates compared with 28% in Lycopersicon esculentum, a typical C(3) species. The inhibition of carboxylation efficiency by 21% O(2) varied from 17% for F. ramosissima to 46% for F. anomala and were intermediate between the C(4) (2% for F. trinervia) and C(3) (53% for L. esculentum) values. The intermediate Flaveria species, especially F. ramosissima, have substantial activities of the C(4) enzymes, phosphoenolpyruvate carboxylase, pyruvate, orthophosphate dikinase, NADP-malic enzyme, and NADP-malate dehydrogenase, indicating potential for C(4) photosynthesis. It appears that these Flaveria species may be true biochemical C(3)-C(4) intermediates.

The quantum yield for CO2 uptake in C 3 and C 4 grasses

The quantum yield for CO2 uptake was measured in C3 and C4 monocot species from several different grassland habitats. When the quantum yield was measured in the presence of 21% O2 and 340 cm(3) m(-3) CO2, values were very similar in C3 monocots, C3 dicots, and C4 monocots (0.045-0.056 mole CO2 · mole(-1) quanta absorbed). In the presence of 2% O2 and 800 cm(3) m(-3) CO2, enhancements of the quantum yield values occurred for the C3 plants (both monocots and dicots), but not for C4 monocots. A dependence of the quantum yield on leaf temperature was observed in the C3 grass, Agropyron smithii, but not in the C4 grass, Bouteloua gracilis, in 21% O2 and 340 cm(3) m(-3) CO2. At leaf temperatures between 22-25°C the quantum yield values were approximately equal in the two species.

Temperature Dependence of Photosynthesis in Agropyron smithii Rydb. : I. FACTORS AFFECTING NET CO(2) UPTAKE IN INTACT LEAVES AND CONTRIBUTION FROM RIBULOSE-1,5-BISPHOSPHATE CARBOXYLASE MEASURED IN VIVO AND IN VITRO

As part of an extensive analysis of the factors regulating photosynthesis in Agropyron smithii Rydb., a C(3) grass, we have examined the response of leaf gas exchange and ribulose-1,5-bisphosphate (RuBP) carboxylase activity to temperature. Emphasis was placed on elucidating the specific processes which regulate the temperature response pattern. The inhibitory effects of above-optimal temperatures on net CO(2) uptake were fully reversible up to 40 degrees C. Below 40 degrees C, temperature inhibition was primarily due to O(2) inhibition of photosynthesis, which reached a maximum of 65% at 45 degrees C. The response of stomatal conductance to temperature did not appear to have a significant role in determining the overall temperature response of photosynthesis. The intracellular conductance to CO(2) increased over the entire experimental temperature range, having a Q(10) of 1.2 to 1.4. Increases in the apparent Michaelis constant (K(c)) for RuBP carboxylase were observed in both in vitro and in vivo assays. The Q(10) values for the maximum velocity (V(max)) of CO(2) fixation by RuBP carboxylase in vivo was lower (1.3-1.6) than those calculated from in vitro assays (1.8-2.2). The results suggest that temperature-dependent changes in enzyme capacity may have a role in above-optimum temperature limitations below 40 degrees C. At leaf temperatures above 40 degrees C, decreases in photosynthetic capacity were partially dependent on temperature-induced irreversible reductions in the quantum yield for CO(2) uptake.

Ecophysiological studies of sonoran Desert plants : V. Photosynthetic adaptations of Machaeranthera gracilis, a Winter annual

Photosynthetic responses to light and temperature are compared for two genetically related chromosomal races of Machaeranthera gracilis. The ancestral foothills race occurs in cooler, more mesic environments, while the derived desert race occurs in more arid environments. The desert race exhibited greater rates of net photosynthesis at all levels of irradiance than did plants of the foothills race. This enhancement is due primarily to a greater quantum yield and leaf density. High light pretreatment significantly increased the photosynthetic capacity of the desert race, with little or no effect on the foothills race. Furthermore, the leaf density of the desert race was affected proportionately more than the foothills race by light pretreatment. The desert race also possessed higher stomatal and mesophyll conductances to CO₂. Both races exhibited enhanced photosynthetic capacities when grown in a warm thermoperiod (35/25 C), relative to a cool thermoperiod (25/15 C), concomitant with slight increases in leaf density. There was a lack of thermal acclimation for both photosynthesis and respiration of the two races. The greater photosynthetic capactity of the desert race is suggested as an evolutionary modification which could account for invasion into and survival in the sonoran Desert.