In the whole plant, nutrients are either taken up passively, or through active mechanisms involving the expenditure of energy. Active uptake is generally less dependent on ionic concentration than passive uptake. Both systems are however influenced by the concentration of other elements, pH, temperature, and the biochemical or physiological status of the plant tissues. These factors can in turn be controlled by the solution presented to the roots, or they may dictate the ionic balance of an ideal solution. For example, Mg2+ competes with other cations for uptake. Under conditions of high K+ or Ca2+ concentrations, Mg deficiency can result, and vice versa. Active uptake of phosphate falls off if the pH of the solution should become slightly alkaline when the (H2PO4)- ion becomes changed to (HPO4)2-. There is some evidence that ammonium is utilised more readily than nitrate at low temperatures and that uptake may be enhanced by high carbohydrate levels within plant cells. Calcium is not absorbed efficiently and concentrations within plant tissues tend to be proportional to those in the soil. Plants are comparatively insensitive to sulphate ions, but high concentrations of dissolved phosphate can depress growth, probably through competitively reducing the uptake of the minor elements Zn, Fe and Cu. Although the biochemistry and physiology of nutrient uptake in tissue cultures may be similar, it is unlikely to be identical.
In vivo, plants take up mineral ions with their roots. No studies have been made on how uptake of nutrients occurs in shoot cultures. For IAA, it has been shown that most uptake is via the cut surface and that only a small fraction is taken up via the epidermis (Guan and De Klerk, 2000). The same likely holds for minerals. It should be noted, though, that in tissue culture the stomata are always open in the portion of the explant exposed to the gaseous phase (De Klerk and Wijnhoven, 2005) and the same may apply for tissues that are exposed to semi-solid or liquid medium. Uptake via the stomata is well possible.
Once taken up, transport within the plant occurs in the mass flow via the xylem. In in vivo plants there are two mechanisms for driving the water flow, root pressure and water potential gradient between at one end the soil and at the other end the atmosphere. Under normal conditions, the latter is the far more important, but water flow resulting from root pressure is in itself sufficient for long-distance mineral supply (Tanner and Beevers, 2001). Plants without roots are often cultured in vitro where the atmosphere is very humid, and the flow driven by a difference in water potential consequently reduced. In spite of this, in tissue culture there still seems to be sufficient water flow (Beruto et al., 1999) which may be favoured by the stomata being continuously open (De Klerk and Wijnhoven, 2005). There are no indications that the structure of the xylem is altered in such a way as to reduce transport of ions.
When explants are first placed onto a nutrient medium, there may be an initial leakage of ions from damaged cells, especially metallic cations (Na+, Ca2+, K+, Mg2+) for the first 1-2 days, so that the concentration in the plant tissues actually decreases (Chaillou and Chaussat, 1986). Cells then commence active absorption and the internal concentration slowly rises. Phosphate and nitrogen (particularly ammonium) are absorbed more rapidly than other ions. In liquid medium, almost all phosphorus and ammonium are taken up in the first two weeks of culture (e.g. by 5 microshoots of Dahlia in 50 ml stationary liquid medium; G. de Klerk, unpublished results). After uptake, phophorus is massively redistributed to tissues that are formed after the initial two weeks. Nitrate in a medium very similar to that of Wood and Braun (1961) B medium, was reduced by Catharanthus roseus suspensions from 24 mM to 5 mM in 15 days, while Na+, K+, and SO42-, fell to only just over half the original concentrations in the same time (MacCarthy et al., 1980). Carnation shoot cultures were found to use 31-75% Mg2+, and 2941% Ca2+ in MS medium during a 4 week period (Dencso, 1987).
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