Ultrastructural features of well-preserved and injured sieve elements: minute clamps keep the phloem transport conduits free for mass flow

After chemical fixation following two different preparation procedures, the ultrastructure of mature sieve elements (SEs) was systematically compared in the transport phloem of Vicia faba leaves and Lycopersicon esculentum in ternodes. The SEs in samples obtained by gentle preparation were well prese rved, while those in conventionally prepared samples were generally injured . (1) In well-preserved SEs, parietal P-proteins were associated with ciste rnae of the SE endoplasmic reticulum (ER). Additionally, the V. faba SEs ha d crystalline P-proteins, and a homogeneous network of filamentous P-protei ns occurred in the lumen of the L. esculentum SEs. In injured SEs, all P-pr oteins were dispersed. (2) In well-preserved SEs, stacked ER cisternae asso ciated with P-proteins lay also on the sieve-plate walls, but passages were kept free in front of the sieve pores. Injured SEs lacked these orderly ar ranged deposits. Instead, irregular filamentous and membranous materials oc cluded the: sieve pores. (3) In well-preserved SEs. the sieve-pore lumen wa s foe of obstructions, apart from small, lateral coatings of P-proteins. Si eve pores in injured SEs were always occluded. (4) The SE organelles and, i n tomato SEs, also the parietal ER located at the longitudinal walls were f irmly attached in the SE periphery and stayed in place after injury. The st able parietal attachment is likely exerted by minute, clamplike structures which link the outer membranes of the SE components with one another or to the SE plasma membrane. Single, straight clamps with a length of about 7 nm anchored the SE components directly to the SE plasma membrane. The connect ions between adjacent SE organelles and/or parietal ER cisternae were mostl y twice as long (about 15 nm) and often were branched. Presumably, the long , branched clamps were constituted by the interaction of opposite short cla mps. The ultrastructural results are discussed with respect to SE functioni ng.