THE TRANSPORT AND FUNCTION OF SILICON IN PLANTS

A number of lines of evidence (M_r, number of -OH groups, measured fluxes at inner mitochondrial membranes) suggest the intrinsic P_Si(OH)₄ of about 10^-10 m s^-1 in the plant cell plasmalemma. While relatively low, such a P_Si(OH)₄ could maintain the intracellular concentration of Si(OH)₄ equal to that in the medium for a phytoplankton cell of 5 μm radius growing with a generation time of 24 h. Such passive entry could not account for SiO, precipitation such as is required for scale (Chrysophyceae) or wall (Bacillariophyceae) production in terms of either the generation of a super-saturated solution or the quantity of SiO₂ required; active transport occurs at the plasmalemma (and possibly at an internal membrane) of such cells. The energy required for silicification, even in a diatom with an Si/C ratio of 0.25, is only some 2% of the total energy (as NADPH and ATP) needed for growth; the energy cost of leakage of Si(OH)₄ due to the intrinsic permeability of lipid bilayers to Si(OH)₄ is never more than 10% of the cost of silicification. In vascular land plants the entry of Si from the soil into the xylem can involve a flux ratio (mol Si/m³ water) that is less than (e.g. Leguminoseae) equal to (e.g. many Gramineae) or greater than (e.g. Oryza, Equisetum) the concentration (mol m^-3) in the bathing solution. Even the low influx of the Leguminoseae cannot be accounted for by the ‘lipid solution’ value of PS(OH)₄, but requires entry coupled (phenomenologically) to water influx with a reflexion coefficient of about 0.9. The situation in most Gramineae is described by such a coupling with a reflexion coefficient near O, while the accumulation of Si (relative to water) in Oryza and Equisetum involves an apparent reflexion coefficient which is negative, i.e. an active transport system stoichiometrically related to water flux. Even in Leguminoseae with a transpiration-stream concentration of Si(OH)₄ of only 20 mmol m^-3 (cf. the soil solution at 200 mmol m^-3), the fact that only I % of the water in the xylem is retained in the plant means that Si(OH)₄ at transpirational termini approaches saturation; super-saturation, and precipitation of SiO, occurs in Gramineae and Equisetum. SiO₂ precipitation occurs mainly near transpirational termini but can also occur in the xylem vessels and endodermis of roots, for example. Si(OH)₂ mobility in the phloem seems to be very restricted. The energy costs of SiO₂ relative to organic compounds as structural and defensive materials are in the ratio of 1:10-1:20 (on the basis of weight of material). The relative rarity of SiO₂ as a structural material is discussed in the context of the evolution of Si(OH)₄-transport mechanisms.