Sites of synthesis and transport of photosynthetic products within the leaf cell

Abstract1.1. After illumination of leaves in the presence of 14CO2 for various times and subsequent freeze drying, chloroplasts were isolated using a nonaqueous procedure. The time-course of the distribution of a number of compounds between chloroplasts and the remainder of the cell was calculated from the 14C-incorporation into the fractions obtained.2.2. Labelled ribulose diphosphate, sedoheptulose diphosphate and sedoheptulose monophosphate occurred, at least during the first minutes of photosynthesis, solely in the chloroplasts. At the beginning of photosynthesis phosphoglyceric acid, fructose diphosphate, fructose monophosphate and glucose monophosphate appeared first in the chloroplasts, but were found later also in the non-chloroplastic part of the cell. The major part of glucose diphosphate, uridine diphosphoglucose, sucrose, malic acid and citric acid was always located in the non-chloroplastic part of the cell.3.3. From the results it is concluded that the photosynthetic carbon cycle operates exclusively in the chloroplasts. Sugar phosphates, which are not needed in the cyclic regeneration of the CO2-acceptor, are directly translocated into the cytoplasm. The synthesis of uridine diphosphoglucose takes place mainly in the cytoplasm. Glucose diphosphate and possibly also sucrose seem to be formed in the cytoplasm of the leaf cell

Photosynthesis dependent acidification of perialgal vacuoles in theParamedum bursaria/Chlorella symbiosis. Visualization by monensin

After treatment with the carboxylic ionophore monensin theChlorella containing perialgal vacuoles of the greenParamecium bursaria swell. TheParamecium cells remain motile at this concentration for at least one day. The swelling is only observed in illuminated cells and can be inhibited by DCMU. We assume that during photosynthesis the perialgal vacuoles are acidified and that monensin exchanges H+ ions against monovalent cations (here K+). In consequence the osmotic value of the vacuoles increases. The proton gradient is believed to drive the transport of maltose from the symbiont into the host. Another but light independent effect of the monensin treatment is the swelling of peripheral alveoles of the ciliates, likewise indicating that the alveolar membrane contains an active proton pump

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