Hardy Pfanz

Non-foliar photosynthesis – a strategy of additional carbon acquisition

Summary In addition to the green leaves, commonly considered as the primary sources of photosynthate production, higher plants can potentially use almost all vegetative and reproductive structures to perform photosynthetic CO 2 assimilation. Green leaves, stems and green sterile flower organs, optimized for light harvesting and photosynthetic performance, are characterized by net photosynthetic assimilation utilizing mainly the atmospheric carbon dioxide. In contrast, chlorophyll-containing bark and wood tissue, most fruit, root and fertile flower organs are principally sub-ordinated to non-photosynthetic functions, but typically perform an effective internal CO 2 recycling using the respiratory released CO 2 . Non-foliar photosynthesis, either manifested as net photosynthesis or internal CO 2 refixation is regarded as an important strategy of additional carbon-acquisition. While chlorophyllous stems or aerial roots even can serve as primary photosynthetic organs, reproductive structures could derive up to 60%; of their total carbon requirement from own CO 2 fixation. In the review, the main strategies of additional carbon acquisition by non-foliar photosynthetic organs are illustrated, presenting an extensive compilation of published data completed with relevant own studies.

Ecology and ecophysiology of tree stems: corticular and wood photosynthesis

Abstract. Below the outer peridermal or rhytidomal layers, most stems of woody plants possess greenish tissues. These chlorophyll-containing tissues (the chlorenchymes) within the stems are able to use the stem internal CO2 and the light penetrating the rhytidome to photoassimilate and produce sugars and starch. Although net photosynthetic uptake of CO2 is rarely found, stem internal re-fixation of CO2 in young twigs and branches may compensate for 60–90% of the potential respiratory carbon loss. Isolated chlorenchymal tissues reveal rather high rates of net photosynthesis (being up to 75% of the respective rates for leaf photosynthesis). Corticular photosynthesis is thus thought to be an effective mechanism for recapturing respiratory carbon dioxide before it diffuses out of the stem. Furthermore, chloroplasts of the proper wood or pith fraction also take part in stem internal photosynthesis. Although there has been no strong experimental evidence until now, we suggest that the oxygen evolved during wood or pith photosynthesis may play a decisive role in avoiding/reducing stem internal anaerobiosis.

Leaf and twig photosynthesis of young beech (Fagus sylvatica) and aspen (Populus tremula) trees grown under different light regime

Summary Sunlight adapted pioneer trees (trembling aspen) and shade-tolerant beeches were exposed to different light conditions (20% and 100% sunlight) throughout an annual cycle. Anatomical and morphological changes of leaves and stem segments were followed besides physiological parameters (photosynthesis, respiration, light transmittance) of these different photosynthesising organs. Buds and leaves of both species responded in differentiation and growth even within the first year of the treatment. While area, stomatal density, and mesophyll thickness clearly responded in leaves, the corresponding parameters in twigs varied only slightly. In the shaded trees plant increment and stem diameter were dramatically reduced. In shade-treated aspen, stem chlorophyll increased by ca. 40%, while only minor changes were recorded in beech. Independent of light conditions during growth, positive net photosynthesis was rarely to be seen in intact twigs and branches. Nevertheless, apparent twig respiration (measured as CO 2 release from the twig) was clearly reduced in the light because of the light-driven carbon re-fixation within the chlorenchymal tissues of twigs and stems. Calculations of net photosynthesis in illuminated current-year and one-year-old twigs revealed stem-internal CO 2 re-fixation to transiently exceed 90%. At least in young twigs and branches, and thus in the outer parts of tree crowns, the respiratory CO 2 losses may efficiently be reduced. Although surely different in young and mature trees, re-fixation of carbon dioxide may be of great importance for carbon budgets in the environmentally controlled leafless states of deciduous trees. An Sonnenlicht angepaste Pionierbaume (Espen) und schatten-tolerante Rotbuchen wurden wahrend eines Jahres unter verschiedenen Lichtklimaten (20% und 100% Sonnenlicht) kultiviert. Sowohl anatomisch-morphologische als auch physiologische Parameter der Blatter und der dazugehorigen Zweigabschnitte wurden untersucht. Die Knospen und Blatter beider Arten zeigten schon im ersten Jahr Anpassungen an das vorherrschende Lichtklima. Wahrend die Blattflache, die Zahl der Stomata und die Mesophylldicke der Blatter deutlich reagierten, waren Veranderungen im Zweigbereich kaum zu erkennen. Bei den beschatteten Baumen waren Zuwachs und Durchmesser des Hauptstammes deutlich geringer. Im Gegensatz zu Rotbuche wiesen beschattete Espen einen um etwa 40% erhohten Chlorophyllgehalt der Rindenchlorenchyme auf. Die Raten der Rindenphotosynthese zeigten sich unabhangig vom Lichtklima wahrend der Anzucht; bei Messungen an intaktem Zweigmaterial wurden selten positive Photosyntheseraten gemessen. Es wurde jedoch eine erhebliche Reduzierung der apparenten Zweigatmung im Lichte festgestellt (als CO 2 -Freisetzung aus dem Zweig), die aus einer Licht-getriebenen Zweig-internen Refixierung des Atmungs-CO 2 im Rindenchlorenchym resultiert. Berechnungen der Netto-Photosyntheseraten von Zweigsegmenten ergaben, das teilweise mehr als 90% des veratmeten CO 2 refixiert werden und damit der Atmungsverlust der Zweige deutlich reduziert werden kann. Zumindest in jungen Zweigen und damit im auseren Bereich einer Baumkrone kann daher der unausweichliche Atmungsverlust verringert und teilweise sogar kompensiert werden. Naturlich werden die effektiven Refixierungsraten der Kronenbereiche zwischen jungen, noch wachsenden und reifen Baumen sehr unterschiedlich sein; trotzdem scheint die Zweigphotosynthese wahrend der blattlosen Zeit der Baume einen nicht unerheblichen Einflus auf den Kohlenstoffhaushalt zu haben.

Stomatal patchiness in Mediterranean evergreen sclerophylls : Phenomenology and consequences for the interpretation of the midday depression in photosynthesis and transpiration.

Midday depression of net photosynthesis and transpiration in the Mediterranean sclerophylls Arbutus unedo L. and Quercus suber L. occurs with a depression of mesophyll photosynthetic activity as indicated by calculated carboxylation efficiency (CE) and constant diurnal calculated leaf intercellular partial pressure of CO2 (Ci). This work examines the hypothesis that this midday depression can be explained by the distribution of patches of either wide-open or closed stomata on the leaf surface, independent of a coupling mechanism between stomata and mesophyll that results in a midday depression of photosynthetic activity of the mesophyll. Pressure infiltration of four liquids differing in their surface tension was used as a method to show the occurrence of stomatal patchiness and to determine the status of stomatal aperture within the patches. Liquids were selected such that the threshold leaf conductance necessary for infiltration through the stomatal pores covered the expected diurnal range of calculated leaf conductance (g) for these species. Infiltration experiments were carried out with leaves of potted plants under simulated Mediterranean summer conditions in a growth chamber. For all four liquids, leaves of both species were found to be fully infiltratable in the morning and in the late afternoon while during the periods leading up to and away from midday the leaves showed a pronounced patchy distribution of infiltratable and non-infiltratable areas. Similar linear relationships between the amount of liquid infiltrated and g (measured by porometry prior to detachment and infiltration) for all liquids clearly revealed the existence of pneumatically isolated patches containing only wide-open or closed stomata. The good correspondence between the midday depression of CE, calculated under the assumption of no stomatal patchiness, and the diurnal changes in non-infiltratable leaf area strongly indicates that the apparent reduction in mesophyll activity results from assuming no stomatal patchiness. It is suggested that simultaneous responses of stomata and mesophyll activity reported for other species may also be attributed to the occurrence of stomatal patchiness. In Quercus coccifera L., where the lack of constant diurnal calculated Ci and major depression of measured CE at noontime indicates different stomatal behavior, non-linear and dissimilar relationships between g and the infiltratable quantities of the four liquids were found. This indicates a wide distribution of stomatal aperture on the leaf surface rather than only wide-open or closed stomata.

Evidence for the functioning of photosynthetic CO2-concentrating mechanisms in lichens containing green algal and cyanobacterial photobionts

The photosynthetic properties of a range of lichens containing both green algal (11 species) and cyanobacterial (6 species) photobionts were examined with the aim of determining if there was clear evidence for the operation of a CO2-concentrating mechanism (CCM) within the photobionts. Using a CO2-gas-exchange system, which allowed resolution of fast transients, evidence was obtained for the existence of an inorganic carbon pool which accumulated in the light and was released in the dark. The pool was large (500–1000 nmol · mg Chl) in cyanobacterial lichens and about tenfold smaller in green algal lichens. In Hypogymnia physodes (L.) Nyl., which contains the green alga Trebouxia jamesii, a small inorganic carbon pool was rapidly formed in the light. Carbon dioxide was released from this pool into the gas phase upon darkening within about 20 s when photosynthesis was inhibited by the carbon-reduction-cycle inhibitor glycolaldehyde. In the absence of this inhibitor, release appeared to be obscured by carboxylation of ribulose bisphosphate. The kinetics of CO2 uptake and release were monophasic. The operation of an active CCM could be distinguished from passive accumulation and release accompanying the reversible light-dependent alkalization of the stroma by the presence of saturation characteristics with respect to external CO2. In Peltigera canina (L.) Willd., which contains the cyanobacterium Nostoc sp., a larger CO2 pool was taken up over a longer period in the light and the release of this pool in the dark was slow, lasting 3–5 min. This pool also accumulated in the presence of glycolaldehyde, and under these conditions the CO2 release was biphasic. In both species, photosynthesis at low CO2 was inhibited by the carbonic-anhydrase inhibitor ethoxyzolamide (EZ). Inhibition could be reversed fully or to a considerable extent by high CO2. In Peltigera, EZ decreased both the accumulation of the CO2 pool by the CCM and the rate of photosynthesis. Free-living cultures of Nostoc sp. showed a similar effect of EZ on photosynthesis, although it was more dramatic than that seen with the lichen thalli. In contrast, in Hypogymnia, EZ actually increased the size of the CO2 pool, although it inhibited photosynthesis. This effect was also seen when glycolaldehyde was present together with EZ. Surprisingly, EZ did not alter the kinetics of either CO2 uptake or release. Taken together, the evidence indicates the operation in cyanobacterial lichens of a CCM which is capable of considerable elevation of internal CO2 and is similar to that reported for free-living cyanobacteria. The CCM of green algal lichens accumulates much less CO2 and is probably less effective than that which operates in cyanobacterial lichens.

A FLUORESCENCE METHOD FOR THE DETERMINATION OF THE APOPLASTIC PROTON CONCENTRATION IN INTACT LEAF TISSUES

Summary A non-destructive fluorescence method is described for determining the extracellular proton concentration in plant tissues. After infiltrating leaves and tissues with unbuffered solutions of coumarins, fluorescence emission upon excitation by ultraviolet light is a function of the apoplastic proton concentration. For calibration, coumarin solutions were strongly buffered to obtain fluorescence emission intensities for known pH values. Experiments were performed with leaves from various plant species. Apoplastic pH values ranged between 5.3 in Taxus baccata (young needles) and 6.4 in young leaves of Spinacia oleracea .

Photosynthetic performance of vegetative and reproductive structures of green hellebore (Helleborus viridis L. agg.)

Photosynthetic irradiance response of vegetative and reproductive structures of the green-flowered deciduous perennial green hellebore was studied by the comparative use of chlorophyll (Chl) fluorescence techniques and gas exchange measurements. All the Chl-containing organs (leaves, sepals, stalks, and fruits) examined were photosynthetically active showing high intrinsic efficiencies of photosystem 2 (Fv/Fm: 0.75–0.79) after dark adaptation. Even in the smaller fertile and sterile parts of the flower (nectaries and anthers) a remarkable photosynthetic competence was detected. With increasing photon flux densities (PFD) electron transport rates, actual quantum yields, and photochemical quenching coefficients of the main photosynthetic organs decreased in the order: leaf>sepal>fruit>stalk. At moderate to high PFDs the sepals achieved maximum electron transport rates corresponding to about 80 % of concomitant mature leaves. In contrast, maximum net photosynthetic rate of the sepals [2.3 μmol(CO2) m−2 s−1] were less than one fourth of the leaves [10.6 μmol(CO2) m−2 s−1]. This difference is explained by a 70–80 % lower stomatal density of sepals in comparison to leaves. As the basal leaves emerge late during fruit development, the photosynthetically active sepals are a major source of assimilates, contributing more than 60 % of whole-plant CO2 gain in early spring. The ripening dehiscent fruits are characterized by an effective internal re-fixation of the respirational carbon loss and thus additionally improve the overall carbon budget.

Air Pollution and Lichen Physiology. Physiological Responses of Different Lichens in a Transplant Experiment Following an SO2-Gradient

Four lichen species, Parmelia sulcata Taylor, Hypogymnia physodes (L.) Nyl., Cetraria islandica (L.)Ach., and Bryoria fuscescens (Gyelnik) Brodo andHawksworth were exposed during autumn and winter at differentsites of the Erzgebirge (Ore Mountains), the Fichtelgebirge andcontrol sites. All lichens tested became visibly damaged withtime. Thallus bleaching started from the edges and went on tothe centre of the thallus. Sites of facilitated gas exchangelike the soralia of P. sulcata and the pseudocyphelles ofC. islandica became preferentially bleached. The sulfateconcentration increased with exposure time reaching 200% ofunpolluted controls. In contrast to coniferous trees (e.g. Picea abies), further exposure lead to a reduction in the S-concentration in the lichens, as sulfate and otherintracellular metabolites were leached from the thalli due tomembrane damage. The changes in the K-concentration proved tobe an excellent measure for membrane leakiness; it wascorrelated with time of exposure and with SO2 concentrations at the different sites. Photosynthetic capacityand respiration were also strongly affected. Depending on theSO2-dose, the Bryoria species were unable tophotosynthesize as early as 4–8 weeks after exposure, whereasCetraria and Hypogymnia showed clear reduction intheir ability to photoreduce CO2 within 8–10 weeks ofexposure in the field. Parmelia sulcata was found to bethe most tolerant species. After 3–4 months, photosynthesis wasreduced by only 30%. The bioindicative value of theseobservations is discussed.

Leaching and Uptake of Ions Through Above-Ground Norway Spruce Tree Parts

Increasing air pollution in Central Europe during the last century has resulted in a substantial input of inorganic ions into the forest ecosystem. Deposition mainly occurs through snow and rainfall, fog and cloudwater impaction, dry deposition of particles, and upon solution of acidic gases in the water films of wet surfaces, e.g., after dewfall (see Part 1).

Sulfur-dioxide fluxes into different cellular compartments of leaves photosynthesizing in a polluted atmosphere. II: Consequences of SO2 uptake as revealed by computer analysis

A computer model is used to analyze fluxes of SO2 from polluted air into leaves and the intracellular distribution of sulfur species derived from SO2. The analysis considers only effects of acidification and of anion accumulation. (i) The SO2 flux into leaves is practically exclusively controlled by the boundary-layer resistance of leaves to gas diffusion and by stomatal opening. At constant stomatal opening, flux is proportional to the concentration of SO2 in air. (ii) The sink capacity of cellular compartments for SO2 depends on intracellular pH and the intracellular localization of reactions capable of oxidizing or reducing SO2. In the mesophyll of illuminated leaves, the chloroplasts possess the highest trapping potential for SO2. (iii) If intracellular ion transport were insignificant, and if bisulfite and sulfite could not be oxidized or reduced, leaves with opened stomata would rapidly be killed both by the accumulation of sulfites and by acidification of chloroplasts and cytosol even if SO2 levels in air did not exceed concentrations thought to be permissible. Acidification and sulfite accumulation would remain confined largely to the chloroplasts and to the cytosol under these conditions. (iv) Transport of bisulfite and protons produced by hydration of SO2 into the vacuole cannot solve the problem of cytoplasmic accumulation of bisulfite and sulfite and of cytoplasmic acidification, because SO2 generated in the acidic vacuole from the bisulfite anion would diffuse back into the cytoplasm. (v) Oxidation to sulfate which is known to occur mainly in the chloroplasts can solve the problem of cytoplasmic sulfite and bisulfite accumulation, but aggravates the problem of chloroplastic and cytosolic acidification. (vi) A temporary solution to the problem of acidification requires the transfer of H+ and sulfate into the vacuole. This transport needs to be energized. The storage capacity of the vacuole for protons and sulfate defines the extent to which SO2 can be detoxified by oxidation and removal of the resulting protons and sulfate anions from the cytoplasm. Calculations show that even at atmospheric levels of SO2 thought to be tolerable, known vacuolar buffer capacities are insufficient to cope with proton production during oxidation of SO2 to sulfate within a vegetation period. (vii) A permanent solution to the problem of acidification is the removal of protons. Protons are consumed during the reduction of sulfate to sulfide. Proteins and peptides contain sulfur at the level of sulfide. During photosynthesis in the presence of the permissible concentration of 0.05μl·l-1 SO2, sulfur may be deposited in plants at a ratio not far from 1/500 in relation to carbon. The content of reduced sulfur to carbon is similar to that ratio only in fast-growing, protein-rich plants. Such plants may experience little difficulty in detoxifying SO2. In contrast, many trees may contain reduced sulfur at a ratio as low as 1/10 000 in relation to carbon. Excess sulfur deposited in such trees during photosynthesis in polluted air gives rise to sulfate and protons. If detoxification of SO2 by reduction is inadequate, and if the storage capacity of the vacuoles for protons and sulfate is exhausted, damage is unavoidable. Calculations indicate that trees with a low ratio of reduced S to C cannot tolerate long-term exposure to concentrations of SO2 as low as 0.02 or 0.03 μl·l-1 which so far have been considered to be non-toxic to sensitive plant species.