Uptake Of Inorganic Nutrients

Plants absorb the inorganic nutrients they require from soils almost entirely as ions. An ion is an atom, or a group of atoms, which has gained either a positive charge (a cation) or a negative charge (an anion). Inorganic nutrients are added to plant culture media as salts. In weak aqueous solutions, such as plant media, salts dissociate into cations and anions. Thus calcium, magnesium and potassium are absorbed by plant cells (normally those of the root) as the respective cations Ca2+, Mg2+ and K+; nitrogen is mainly absorbed in the form nitrate (the anion, NO3-) although uptake of ammonium (the cation, NH4+) may also occur, phosphorus as the phosphate ions HPO42- and H2PO4-; and sulphur as the sulphate ion SO42-. In tissue culture, uptake is generally proportional to the medium concentration up to a concentration of twice MS (Williams, 1993). For specific elements this may be different. For example, Leiffert et al, (1995) found only a small increase in Zn uptake with increasing medium concentration indicating that the concentration of Zn in the cultured tissues was adequate, not requiring further uptake. Selective uptake also suggests active uptake.

Table 3.2 Content (mmol/kg) of elements in various agar brands. [agar 1-7: Scholten and Pierik (1998); agar 8 and gelrite: Scherer et a/.(1988)]. Na = not analysed, nd = not detected. It should be noted that some elements present in agar are not present in MS. This is particularly relevant for Ni which is an essential element

Table 3.2 Content (mmol/kg) of elements in various agar brands. [agar 1-7: Scholten and Pierik (1998); agar 8 and gelrite: Scherer et a/.(1988)]. Na = not analysed, nd = not detected. It should be noted that some elements present in agar are not present in MS. This is particularly relevant for Ni which is an essential element

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