Guido Aschan

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.

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.

Plants and Geothermal CO2 Exhalations — Survival in and Adaptation to a High CO2 Environment

Modern plants live in a rather “low CO2”- world when compared to CO2 concentrations in the atmosphere during prehistoric evolution (Petit et al. 1999). Similar to Mars and Venus, CO2 on planet Earth might have been as high as 90–98% during the early days of photosynthetic evolution (Emiliani 1992; Raven 1995; Grace and van Gardingen 1997). But concentrations decreased gradually during epochs to reach only a few hundred ppm, although [CO2]1 is again steadily increasing since the last 200 years (Bowes 1993). Values nowadays are dramatically lower by a factor of 3,000 than in those ancient times. Nevertheless, a further increase from presently 360 ppm (0.036% w/v) to ca. 700 ppm (0.07% w/v) is thought to take place within the current century (WMO 1990; Bowes 1993; IPCC 1996). Predictions for the second half of the present century range from 415 to 575 ppm depending on a CO2 emission rate of ±2% (Houghton et al. 1990; Cook et al. 1997). During evolution, plants had to cope with and adapt to a slowly but permanently changing CO2 environment including periods with increasing and others with decreasing CO2. Although ambient CO2 does nowadays not saturate C3 photosynthesis, plants have evolved mechanisms to rather effectively capture and photo-reduce the oxidised carbon to the level of carbohydrates.

Chemical Fractionation and Soil-to-Plant Transfer Characteristics of Heavy Metals in a Sludge Deposit Field of the River Ruhr, Germany

The presented study assessed the heavy metal contamination risk in a former sludge deposit field of the River Ruhr in Essen, Germany. Therefore, the temporal and spatial distribution in soils and plants, chemical fractionation, mobilization potential, and transfer characteristics have been investigated. Soil samples, roots and shoots of rushes (Juncus sp.), and stem wood disks of willows (Salix sp.) were analyzed for Zn, Cu, Pb, Ni, Cr, and Cd. Plant available and mobile heavy metal portions have been determined using a sequential extraction procedure. The results show that the soils and the rushes are highly contaminated, although there is a considerable decrease compared to initial concentrations some 20 years ago. The willows show only small heavy metal enrichment. pH induced mobilization potential in soil is high for Cd, Zn and Ni. Additionally, these elements contain high portions of plant-available fractions. High transfer rates from soil to roots and very high rates from roots to shoots of rushes have been determined for Cd and Zn, indicating an accumulation of these elements in shoots of rushes. The rushes reflect the temporal and spatial heavy metal distribution in soil and might thus be used as a bioindicator or for phytoremediation.