Question

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How do angiosperms seal off wounded sieve tube elements to prevent bleeding? Describe the short-term and long-term mechanisms?

Answer 1

Callose is deposited into the tangled mass in the sieve pores of damaged sieve tubes, which serves to seal off the damaged sieve elements. Callose proliferates when there is a pressure drop, which helps to seal the sieve pores.

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.