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

The phloem-loading-related effects of temperature on leaf ultrastructu re were studied in seven species having numerous plasmodesmatal connec tions between the mesophyll and phloem (symplasmic minor-vein configur ation). The response to temperature (between 5 and 30 degrees C) was c haracterized 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 d egrees C, the ER system in the intermediary cells reached its maximal volume, while the vacuole in bundle-sheath cells was positioned centri petally (proximal to the intermediary cell). With decreasing temperatu re, the ER labyrinth in intermediary cells gradually contracted till t he ER was fully collapsed at 10 degrees C and the vacuole in bundle-sh eath cells moved to a more centrifugal position. The apparent eliminat ion of photosynthate transport via the ER and plasmodesmata at tempera tures lower than 10 degrees C led to starch accumulation in the chloro plasts of bundle-sheath cells and mesophyll cells. All of these change s were fully temperature-reversible and probably reflect changes in th e balance between photosynthate transport and storage. The ultrastruct ural 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/plasm odesmata 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.