EFFECTS OF TEMPERATURE ON THE CONFORMATION OF THE ENDOPLASMIC-RETICULUM AND ON STARCH ACCUMULATION IN LEAVES WITH THE SYMPLASMIC MINOR-VEIN CONFIGURATION

The phloem-loading-related effects of temperature on leaf ultrastructure were studied in seven species having numerous plasmodesmatal connections between the mesophyll and phloem (symplasmic minor-vein configuration). The response to temperature (between 5 and 30 degrees C) was characterized by drastic changes in the endoplasmic-reticulum labyrinth (ER labyrinth) of intermediary cells, in the position of the vacuole in bundle-sheath cells, and in the starch content in the chloroplasts of bundle-sheath cells and mesophyll cells. At temperatures above 20 degrees C, the ER system in the intermediary cells reached its maximal volume, while the vacuole in bundle-sheath cells was positioned centripetally (proximal to the intermediary cell). With decreasing temperature, the ER labyrinth in intermediary cells gradually contracted till the ER was fully collapsed at 10 degrees C and the vacuole in bundle-sheath cells moved to a more centrifugal position. The apparent elimination of photosynthate transport via the ER and plasmodesmata at temperatures lower than 10 degrees C led to starch accumulation in the chloroplasts of bundle-sheath cells and mesophyll cells. All of these changes were fully temperature-reversible and probably reflect changes in the balance between photosynthate transport and storage. The ultrastructural shifts appear to be correlated with the passage of photosynthate through the intermediary cells and, as a consequence, with the rate of phloem loading at various temperatures. A contraction of the ER/plasmodesmata system imposed by cytoskeletal reorganisation is discussed as the reason for the blockage of phloem loading at low temperatures in association with the general chilling sensitivity of these species.