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.

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