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SHORT‐ AND LONG‐DISTANCE TRANSPORT OF BORIC ACID IN PLANTS
Author(s) -
RAVEN J. A.
Publication year - 1980
Publication title -
new phytologist
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 3.742
H-Index - 244
eISSN - 1469-8137
pISSN - 0028-646X
DOI - 10.1111/j.1469-8137.1980.tb04424.x
Subject(s) - boric acid , xylem , transpiration , membrane , chemistry , transpiration stream , permeation , biophysics , partition coefficient , phloem , steady state (chemistry) , intracellular , membrane transport , plant cell , biochemistry , botany , chromatography , biology , photosynthesis , organic chemistry , gene
SUMMARY The molecular weight and ether‐water partition coefficient of boric acid are consistent with a P B (OH) 3 in plant cell membranes of at least 10 ‐6 cm s ‐1 . This permeability coefficient is high enough to account for the measured magnitude of boric acid fluxes at many plant cell membranes. The use of active transport of boric acid to maintain B distribution across a membrane away from thermodynamic equilibrium is consequently likely to be energetically expensive. The content of m‐diols (with which boric acid can form complexes) in cell walls and inside the cells varies widely between different plant species without any obvious correlation with either total B content or with the B content at which deficiency (in B‐requirers) or toxicity symptoms are manifested. B distribution at the cell level depends on the relative extents of passive permeation, active transport and cisdiol formation; regulations may be in response to total intracellular B rather than free boric acid. The net uptake of boric acid by intact vascular land plants is influenced by the rate of transpiration; while transport of B within the xylem is probably directly proportional to the rate of transpiration, neither whole plant B uptake nor transfer from root tissue into the xylem exhibit such a simple relationship. Redistribution of B in the phloem from transpirational termini is very limited. This limitation could result from the toxicity of B (which requires a low B concentration in the translocation stream relative to other nutrients and to sink requirements) or to ‘counter‐current distribution’ of B from the phloem to the adjacent xylem stream (low in B) through the B‐permeable sieve‐tube plasmalemma. The transport of B is discussed in relation to the evolution of multi‐cellular algae and of vascular land plants.