Long‐term functioning of enucleate sieve elements: possible mechanisms of damage avoidance and damage repair

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Abstract
The sieve‐tube elements of long‐lived arborescent monocotyledons which lack secondary thickening remain functional for many decades despite lacking a nucleus. A minimal requirement for transport by mass flow powered by the Munch mechanism is maintenance of semi‐permeability of the plasmalemma of the sieve tube elements; loading and re‐loading could be deputed to nucleate cells symplastically linked to the sieve elements. An additional requirement in the long‐term relates to replacement of components damaged by mechanical, chemical or radiation intrusions. Minimizing the damage from radiation and chemical agents can be related to a number of commonly observed features of sieve tubes. Damage from O2 and radicals derived there from is minimized in these essentially aerobic cells by (1) the absence of intercellular gas spaces in the phloem combined with the lower O2 solubility and diffusivity in concentrated disaccharide (or sugar alcohol) solutions, (2) the absence of photosynthetic machinery which could generate singlet oxygen, and (3) the presence of at least some components of scavenging mechanisms (glutathione, peroxidase, abscorbate). Non‐enzymic glycosylation (exacerbated by O2) of proteins is minimized by the low concentrations of reducing sugars in sieve‐tubes. Ultraviolet damage is minimized by UV‐absorbing materials between the plant surface and the sieve tubes, including the selerenchymatous cap of fibres on the vascular bundles. The extent to which repair involves symplastic import of polypeptides from nucleate companion cells, thus breaching the −800‐Da limit on symplastic transport, is unclear, but it could occur in fully differentiated companion cell‐sieve element associations without necessarily upsetting development.