Phloem systemicity

Two complex tissues, the xylem and phloem, provide the conducting network or "circulatory system" of plants. In the xylem or woody tissue, most of the cells are dead and the thick-walled tubes (tracheids) serve to transport water and dissolved minerals from the roots to the stems and leaves. The phloem cells provide the principal means of downward conduction of foods from the leaves. Phloem cells are joined end to end by sieve plates, so-called because they are perforated by numerous minute pores through which cytoplasm of adjoining sieve cells appears to be connected by strands 5-9 pm in diameter.154 Mature sieve cells have no nuclei, but each sieve cell is paired with a nucleated "companion" cell. 

Plants possess a kind of circulatory system by which fluids are transported from the roots upward in the xylem and downward from the leaves through the phloem. Many compounds are carried between cells in this manner, while others are transported across cell membranes and against concentration gradients by active transport. A number of compounds that move between cells in either of these two manners have been classified as hormones.366-369 The major plant... 

These examples clearly show that it is not always desirable to develop fully systemic plant protection compounds, i.e. substances which migrate equally well both in the xylem and in the phloem. Transport in the xylem is often sufficient to effectively protect the plant. Due to the physiology of the plant, the storage organs which... 

It has been nearly a century and a half since Boussingault (1868) presented the hypothesis that the accumulation of assimilates in an illuminated leaf may be responsible for a reduction in the net photosynthetic rate of that leaf. According to the Munch hypothesis for phloem transport, the greater the sink strength, the greater the depression in solute concentration in the phloem at the sink. This increases the concentration differential between the source and sink, creating the hydrostatic pressure head that drives the system. 

An, H., Roussot, C., Suarez-Lopez, P, Corbesier, L., Vincent, C., Pineiro, M., Hepworth, S., Mouradov, A., Justin, S., Turnbull, C., and Coupland, G., CONSTANS acts in the phloem to regulate a systemic signal that induces photoperiodic flowering of Arabidopsis, Development, 131, 3615-3626, 2004. 

Selective systemic herbicide, absorbed by roots and leaves with rapid translocation in xylem and phloem to the meristematic tissues... 

Systemic herbicide absorbed by roots and foliage with translocation in xylem and phloem and accumulation in meristetic regions... 

Although elicitors from insect oral secretions have received special attention, the quality and quantity of HIPVs may also be affected by insect feeding behavior such as continuous or interrupted leaf chewing, phloem sucking, and even egg deposition.193,194 Recently, it was shown that continuous mechanical wounding was sufficient to induce local as well as systemic emission of volatiles that are emitted as HIPVs.195 Thus, the role of insect elicitors in HIPV production still remains largely an open question. 

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