4.2.2  Transport in cells and tissues

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Net transport through a membrane is normally a balance between inflow and outflow across that membrane. So, the rate of solute absorption by a cell, tissue or organ can be controlled by changes in either the amount going in (influx), the amount coming out (efflux) or a combination of both. Over long periods we observe the net sum of these fluxes but a closer look at membrane physiology shows us that flux in each direction might be under independent control(s). Some examples include gating factors (Section 4.1.3(a)) and high-energy compounds (e.g. PPi) in ion pumps (Section 4.1.2(d)). In this way, each subcellular compartment has characteristic membrane properties which determine the direction and rate of ion flux through that compartment. Resource use by whole tissues is optimised through this fine control of subcellular compartmentalisation and long-distance transport, both contributing to a plant’s capacity to withstand variations in resource supply. For example, orthophosphate concentration in the cytoplasm is maintained at steady levels by ortho-phosphate transport across the tonoplast, with vacuoles acting as an orthophosphate reservoir during phosphorus deficiency (Lee et al. 1990). At the whole-plant level, adequate phosphorus reaches young, growing tissues by long-distance transport in the phloem.

Single cells do not function as isolated entities therefore solute accumulation by whole tissues cannot be predicted solely from the transport properties of its component cells. Gradients in solute concentration can develop across bulky tissues because ions diffuse only slowly through apoplasmic spaces. The proposal that ion uptake capacity at the root epidermis exceeds that in deeper cell layers is consistent with coordination of influx mechanisms and bulk root structure. In many cases we struggle to provide even a broad outline of what happens. This section aims to synthesise a picture of where transport processes fit into whole-tissue behaviour. Examples of how experimentation has led to an understanding of the regulation of nutrient fluxes are given in the hope that this might stimulate the interest of a reader in joining the search. New reasons for optimism come from two powerful techniques, patch clamp methods and genetic manipulation.