Floral Splendour and Shortage of Nutrients - Soil: Reflections on the Basis of our Existence - page 42

Role of Phosphorus Deficiency and Mycorrhizal Fungi
in Floral Blooms
The rich flora of the Öland alvar and Sarek's magnesite quarry epitomises the
resourcefulness of plants in adapting to phosphorus scarcity. Some species secrete
organic acids from their roots that enable them to absorb phosphate ions. Many
enter a symbiosis with mycorrhizal fungi to augment their phosphorus intake in
return for providing access to the carbohydrates the plants produce during photo-
synthesis. Some plants even specialise in parasitising others' adaptations in order
to gain access to phosphorus. Inadequate supplies of phosphorus limit the growth
of all plants but also create scope for more life forms than a nutrient-rich environ-
ment where a handful of fast-growing species can outcompete the others.
Most Scandinavian plants are colonists from calcium-rich areas in central
Europe, from where they spread slowly while gradually evolving into different
species. Acidic soils are less common in central Europe than in more northerly
latitudes, which is why fewer acidic-soil species have evolved in Scandinavia.
The reverse is true in Australia, where calcareous soil is uncommon and there-
fore plays host to fewer species than low-calcium soil. Even the most attractive
soils from a flora perspective have a lack of phosphorus owing to high concen-
trations of aluminium and iron that fix phosphate ions. Many plants here grow
hairy roots that secrete citric acid, which enhances phosphorus availability by
dissolving aluminium and iron phosphate. The strategy has not developed to
an equivalent degree in Europe's calcareous soils because the change in pH is
deleterious to mycorrhizal fungi, which are a more important source of plant
phosphorus.
The symbiosis between mycorrhizal fungi and plants was discovered in the
1930s in Florida during successful attempts to control a parasitic fungus that was
attacking the roots of orange and lemon trees. Scientists found that the saplings
would not grow, even when the fungus was removed, unless large quantities of
phosphorus were added. On closer inspection, they discovered another type of
fungal structures on the wild saplings that were absent in their cultivated conge-
ners. The scientists suspected this fungus was helping the wild saplings to absorb
phosphorus—a hypothesis they proved by adding soil from wild orange and lemon
trees, which allowed the fungus to recolonise and the cultivated saplings to grow
normally again. Since then, Florida's orange and lemon growers always ensure
their trees' roots are colonised by mycorrhizal fungi.
Scientists in the Netherlands have investigated the importance of the role
played by mycorrhizal fungi for plant structure in different places and envi-
ronments. They have been able to demonstrate that if you sterilise earth from
a flower meadow, put it in pots and plant it with meadow flower seeds, it is not
possible to replicate the meadow biotope. All that will grow are a variety of
grass species; the flowers remain conspicuous by their absence. Things change
radically, however, if you add spores of mycorrhizal fungi. All of a sudden, the