Christiane Wittmann

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.

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.

Bark and woody tissue photosynthesis: a means to avoid hypoxia or anoxia in developing stem tissues

In woody plants, oxygen transport and delivery via the xylem sap are well described, but the contribution of bark and woody tissue photosynthesis to oxygen delivery in stems is poorly understood. Here, we combined stem chlorophyll fluorescence measurements with microsensor quantifications of bark O2 levels and oxygen gas exchange measurements of isolated current-year stem tissues of beech (Fagus sylvatica L.) and pedunculate oak (Quercus robur L.) to investigate how bark and woody tissue photosynthesis impairs the oxygen status of stems. Measurements were made before bud break, when the axial path of oxygen supply via the xylem sap is impeded. At that time, bark O2 levels showed O2 concentrations below the atmospheric concentration, indicating hypoxic conditions or O2 deficiency within the inner bark, but the values were always far away from anoxic. Under illumination bark and woody tissue photosynthesis rapidly increased internal oxygen concentrations compared with plants in the dark, and thereby counteracted against localised hypoxia. The highest photosynthetic activity and oxygen release rates were found in the outermost cortex tissues. By contrast, rates of woody tissue photosynthesis were considerably lower, due to the high light attenuation of the bark and cortex tissues, as well as resistances in radial oxygen diffusion. Therefore, our results confirm that bark and woody tissue photosynthesis not only play a role in plant carbon economy, but may also be important for preventing low oxygen-limitations of respiration in these dense and metabolically active tissues.

More than just CO2‐recycling: corticular photosynthesis as a mechanism to reduce the risk of an energy crisis induced by low oxygen

Reassimilation of internal CO2 via corticular photosynthesis (PScort ) has an important effect on the carbon economy of trees. However, little is known about its role as a source of O2 supply to the stem parenchyma and its implications in consumption and movement of O2 within trees. PScort of young Populus nigra (black poplar) trees was investigated by combining optical micro-optode measurements with monitoring of stem chlorophyll fluorescence. During times of zero sap flow in spring, stem oxygen concentrations (cO2 ) exhibited large temporal changes. In the sapwood, over 80% of diurnal changes in cO2 could be explained by respiration rates (Rd(mod) ). In the cortex, photosynthetic oxygen release during the day altered this relationship. With daytime illumination, oxygen levels in the cortex steadily increased from subambient and even exhibited a diel period of superoxia of up to 110% (% air sat.). By contrast, in the sapwood, cO2 never reached ambient levels; the diurnal oxygen deficit was up to 25% of air saturation. Our results confirm that PScort is not only a CO2 -recycling mechanism, it is also a mechanism to actively raise the cortical O2 concentration and counteract temporal/spatial hypoxia inside plant stems.

Respiratory adaptations to a combination of oxygen deprivation and extreme carbon dioxide concentration in nematodes

To examine physiological adaptations to the two combined stressors O2 deprivation and extreme CO2 concentrations, we compared respiratory responses of two nematode species occurring in natural CO2 springs. The minimum O2 concentration allowing maintenance of respiration in both species was 0.0176μmol O2ml-1 (corresponds to 1.4% O2 in air). After exposure to anoxia, individuals resumed respiration immediately when O2 was added, but on a lower level compared to control and without showing a respiratory overshoot. A species-specific response was found in respiration rate during 20% CO2: the more tolerant species maintained respiration rates, whereas the sensitive species showed a decreased respiration rate as low as after anoxia. The results indicate that during 20% CO2 the sensitive species undergo a survival state. We conclude, that the ability to maintain respiration even under low oxygen and high CO2 concentrations may allow the better adapted species to occupy an ecological niche in the field, where others cannot exist.