Symplasmic transport and phloem loading in gymnosperm leaves

Despite more than 130 years of research, phloem loading is far from being understood in gymnosperms. In part this is due to the special architecture of their leaves. They differ from angiosperm leaves among others by having a transfusion tissue between bundle sheath and the axial vascular elements. This article reviews the somewhat inaccessible and/or neglected literature and identifies the key points for pre-phloem transport and loading of photoassimilates. The pre-phloem pathway of assimilates is structurally characterized by a high number of plasmodesmata between all cell types starting in the mesophyll and continuing via bundle sheath, transfusion parenchyma, Strasburger cells up to the sieve elements. Occurrence of median cavities and branching indicates that primary plasmodesmata get secondarily modified and multiplied during expansion growth. Only functional tests can elucidate whether this symplasmic pathway is indeed continuous for assimilates, and if phloem loading in gymnosperms is comparable with the symplasmic loading mode in many angiosperm trees. In contrast to angiosperms, the bundle sheath has properties of an endodermis and is equipped with Casparian strips or other wall modifications that form a domain border for any apoplasmic transport. It constitutes a key point of control for nutrient transport, where the opposing flow of mineral nutrients and photoassimilates has to be accommodated in each single cell, bringing to mind the principle of a revolving door. The review lists a number of experiments needed to elucidate the mode of phloem loading in gymnosperms

Assimilation von Kohlenstoff und der Assimilattransport in Coniferen unter dem Einfluss von Luftschadstoffen. Photosyntheseleistung und Assimilatverteilung in jungen Fichten (Picea abies [L.] Karst.) unter kontrollierter Ozon-Exposition Abschlussbericht

In this project, effects of long-term, low-level ozone exposure on CO_2fixation and assimilate partitioning in 4-years spruces old were studied. 1. In October, ozone exposure resulted in a 40% decrease of photosynthesis. Chlorophyll content was not altered. Depression of net photosynthesis by ozone exposure fluctuates. At least partly, this depression of net photosynthesis might reflect an increase of respiration. After 6 weeks of ozone exposure, both stomatal conductivity and transpiration seemed to be lowered. However, internal [CO_2] and water use efficiency were not significantly altered. 2. Compared with the control, needles of ozone-exposed plants exhibited a shift of the "1"4C-ratio of #beta#-carotene/xanthophylls which effect faded away with the season (October). 3. In August and October, translocation of "1"4C-assimilates from the "1"4C-fed source leaves seems to be impeded by ozone exposure. 4. No O_3-related differences were found for the lipophilic as well as for the fractions of organic or amino acids. For the sink tissues (twig, stem, root), less label was found in non-starch residue in August (main growth season), in O_3-exposed plants. 5. Starch and sugar content of cortex, cambium, xylem, and of the root follow annual cycles. Under ozon exposure, levels of "1"4C-starch and "1"4C-sucrose are lowered whereas in the needles, build-up of "1"4C-starch was detected. 6. Obviously, long-term low-level ozone affects allocation of assimilates. Secondary effects on chloroplasts by the treatment cannot be excluded. (orig./UWA)Unter Labor- bzw. Gewaechshausbedingungen wurde der Einfluss einer kontrollierten Ozonexposition auf die Photosyntheseleistung und die Assimilatverteilung in 4-jaehrigen Fichten untersucht. 1. Unter Ozon-Exposition wird die Netto-Photosyntheserate in den jungen Nadeln zum Herbst hin reduziert, waehrend der Chlorophyllgehalt unbeeinflusst bleibt. Die Photosyntheserate der jungen Nadeln sinkt nach O_3-Einwirkung offenbar in Schueben. Teilweise scheint die verminderte Netto-Photosynthese Folge einer erhoehten Atmung zu sein. Nach 6-woechiger Ozonbegasung erschienen die stomataere Leitfaehigkeit und die Transpiration leicht abgesenkt, die mesophyllinterne CO_2-Konzentration und die 'Water-Use-Efficiency' aber kaum veraendert. 2.) Unter Ozon-Exposition wird zum Herbst hin die prozentuale Verschiebung in der "1"4C-Markierung #beta#-Carotin zur Xanthophyll-Fraktion schwaecher. 3.) Der unter Ozon erwartete 'Assimilatstau' in den Nadeln ist im August und im Oktober gut erkennbar, wobei im Herbst ein erhoehter Verlust and "1"4C gemessen wird. 4.) In den Fraktionen der lipophilen Substanzen, der organischen Saeuren und Aminosaeuren lassen sich keine ozonbedingten Unterschiede erkennen. Die Markierung der sogenannten Rueckstands-Fraktion in den 'Sink'-Geweben und Organen Rinde, Holz und Wurzel erscheint in ozonexponierten Pflanzen dagegen deutlich reduziert. 5.) Staerke-und Zuckergehalt unterliegen in Rinde, 'Kambial'-Bereich, Holz und Wurzel jahreszeitlichen Aenderungen. Unter Ozonwirkung bleiben in Rinde, 'Kambial'-Zone und Wurzel die Gehalte deutlich geringer. Dagegen kommt es in den Nadeln zu einem Anstieg an ("1"4C-)Staerke. 6.) Die Ozon-Exposition wirkt sich offensichtlich auf die Allokation von Assimilaten aus. Die Moeglichkeit einer Sekundaerschaedigung der Chloroplasten ist nicht auszuschliessen. (orig./UWA)Available from TIB Hannover: RO 7299(32)+a / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEMinisterium fuer Umwelt, Raumordnung und Landwirtschaft des Landes Nordrhein-Westfalen, Duesseldorf (Germany)DEGerman