Nutrient requirements

A major consequence of the effective mechanism for translocation of nutrients from old to new cells is that the cycling of nutrients is highly efficient in bryophytes. Considerable growth can even occur, temporarily, without further input of inorganic nutrient (Wells & Brown 1996, Bates 1997). Therefore, although bryophytes have, qualitatively, the same nutrient requirements as vascular plants (Bates 2000), they have, quantitatively, extremely low requirements. Their response curve typically exhibits a flat bell-shape, whose optimum corresponds to a solution that is five times less concentrated than the optimum solution used for crop culture (Fig. 8.14). In a series of phosphorus enrichment experiments, Kooijman and Bakker (1993), Steinman (1994) and Martinez-Abaigar et al. (2002) failed to find a clear deficiency zone in the physiological response of different species. Although tissue concentrations increased with rising phosphorus supply, no corresponding increase in production occurred (Steinman 1994). Similarly, Wells and Brown (1996) found no evidence of nutrient (potassium, magnesium and calcium) limitation in the moss Rhytidiadelphus squarrosus.

At the other extreme of the response curve to nutrients (Fig. 8.14), the negative relationship between fresh weight and nutrient concentration indicates that, beyond a certain concentration, several elements, although required in small concentrations, are toxic when they are present in excess.

Fig. 8.14. Average fresh weight (g) of clones of the liverwort Marchantia polymorpha exposed to increasing nutrient concentrations over 27 days. The x-axis represents the factor of dilution or concentration of the optimal nutrient concentration (0.012mollp1 K+, 0.007molT1 Ca2+, 0.0014molr1 Mg2+, 0.0034mollp NOp 0.004mollp PO|P and 0.0008mollp SOp) (plain arrow). By comparison, the optimal nutrient concentration used in crop culture is indicated by a dashed arrow (data from Voth 1943).

Fig. 8.14. Average fresh weight (g) of clones of the liverwort Marchantia polymorpha exposed to increasing nutrient concentrations over 27 days. The x-axis represents the factor of dilution or concentration of the optimal nutrient concentration (0.012mollp1 K+, 0.007molT1 Ca2+, 0.0014molr1 Mg2+, 0.0034mollp NOp 0.004mollp PO|P and 0.0008mollp SOp) (plain arrow). By comparison, the optimal nutrient concentration used in crop culture is indicated by a dashed arrow (data from Voth 1943).

Mineral elements indeed accumulate within cells and on cell walls. Cell walls possess a net negative charge and therefore bind cations, which can be displaced by other cations from the external medium, particularly if the latter are present at higher concentrations or have higher valences. This ability is called the cation exchange capacity (CEC) and has a major effect on mineral retention.

The CEC regulates the occurrence of species depending on soil conditions, which can be one of the most important factors for bryophyte distributions at a regional scale (Bates 1995). In particular, the failure of calcifuge plants to establish on calcareous soils is usually attributed to the fact that calcium, in excess, causes mineral nutrient deficiencies with regard to other elements such as magnesium. The CEC is three to four times greater in calcicolous bryophytes than calcifuges (Fig. 8.15) (Bates 1982). One interpretation of the absence of calcifuges on calcareous soils is that calcifuges do not possess enough binding sites to address the excess of Ca2+ in calcareous environments. However, Bates and Farmer (1990) observed a lack of effect of calcium applications on the growth of Pleurozium schreberi, a species that

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Fig. 8.15. Cation exchange capacity in a range of epilithic calcicole and calcifuge mosses (drawn from Bates 2000).

behaves as a strict calcifuge. Bates and Farmer (1990) therefore suggested that fully developed calcifuge plants may already possess an intracellular capital of essential nutrients and are hence insensitive to changes in external ion supply. Calcium is thus toxic only at the colonizing stage, when the propagules are in intimate contact with the soil and must accumulate a nutrient capital to develop.

A high CEC, which may therefore appear as an advantage for calcicolous plants in calcareous habitats, appears as a limiting factor in acidic environments because of the tendency of calcicoles to concentrate protons and toxic ions (Buscher et al. 1990). This is especially true for aluminium, which is present in the form of highly toxic and mobile Al3+ ions in acidic environments. Calcifuges are, by contrast, thought to possess a physiological system, possibly involving specific membrane transfer proteins, which regulates the difference in proton concentration within and outside the cell. Such a mechanism would explain the ability of species, such as the liverwort Jungermannia vulcanicola, to grow at a pH as low as 1.9 (Satake et al. 1989). In Sphagnum-dominated mires, such a mechanism may be responsible for the adsorption of incoming cations and the release of protons, which are added to those already present in the mire water, increasing the acidity of the latter (see Section 2.2).

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