The function of a solute in a cell depends, in part, on the particular solute and, in part, on its location. Clearly it is not possible to ascribe here a function to the myriad of solutes that occur in plant cells.
Vacuoles are the largest compartments in mature plant cells. All cells are derived from meristematic cells and generally undergo a massive expansion of the vacuole during development. Meristematic cells have a volume of about 0.5-8 pL (a picolitre is 10-12 of a litre), and as development proceeds, cell volumes increase and reach values between 50 and 5000 pL. The vacuoles are the sites of storage of most (in terms of quantity - that is volume times concentration) of the solutes present in mature cells.
A particularly important aspect is the storage of energy in the form of a reservoir of protons within the vacuole. These protons can be exchanged in the acquisition of sucrose or other mineral elements.
A large number of plant extracts have been analysed since the 1880s, primarily to obtain estimates of the osmotic pressure within cells: these range from fractions of an MPa to 5 or more MPa (Steiner, 1939; Flowers et al., 1977). The accumulation of solutes in the vacuole is essential for the generation of turgor pressure in cells (see Section 3.2.3), which in turn provides the shape and form of non-woody species and the driving force for the growth in all plants.
Many plant extracts have also been analysed for the nature of the solutes present; the major components of vacuoles are sugars, potassium, calcium, magnesium and nitrate ions. However, since there can be large differences in the composition of sap between plants, depending on the species and the environment in which the plant is growing, the analysis is valuable only in context. For example, in the roots of beet, it is sugars that constitute the major solutes stored for the generation of energy for new growth after winter. Where plants are growing on saline soils, the sap is commonly dominated by sodium and chloride ions whose accumulation adjusts the plant water potential (see Section 3.2.2) to that of the external medium. In plants utilising crassulacean acid metabolism (see Section 13.6.2), malic acid may be present in high concentrations, particularly at the end of the night.
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