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1.June

1. January

Time after germination (weeks)

12 -i Arctium tomentosum

Time after germination (weeks)

Fig. 2.4.27. Changes with time in A dry mass and B amount of N in different organs of Arctium tomentosum during the 2 years of growth from germination until death after seed production. Development of the storage roots (R) is particularly noteworthy for it supports early leaf emergence from the rosette in the second year. The large biomass production of Arctium in its second year is mainly due to the "exploitation" of the habitat due to its rosette leaves (/.,) in its first year and the basal rosette in the second (/., and L2), which makes possible the harvest of nitrogen from an area of about 1 m2. This nitrogen gain permits growth of large flowering shoots (S) and of the leaves on the shoot (F) which assimilate C02 after L, and L2 die. All resources are finally transferred to the fruits (V). During flowering, when the rosette and the stem leaves are already dying, the plant receives some 15% of its total N from the root. Exploitation by the basal rosette of the area in which the plant grows is so complete and long-lasting that the seeds of this species can establish themselves in the area without significant competition from other competing species. Thus the biannual Arctium occupies an area for many years (Heilmeier et al. 1986). C Arctium lappa in a ruderal situation in competition with couch grass and thistles. Campus of the University of Bayreuth. (Photo E.-D. Schulze)

m Wmm

• Behaviour analogous to annuals in the first year, but the formation of a storage organ (often the hypocotyl) occurs very early and is genetically fixed. This storage organ is formed during growth (formation of reserve) or is filled with resources only after the termination of growth (accumulation);

• Storage of amino acids predominantly, sometimes lipids;

• Early emergence in the following year because of the stored N products at a time when the root is not yet active;

• Formation of the basal rosette which reduces competition with other species dependent on the size of leaves, and which also enables exploitation of a larger area and also volume of soil, increasing availability of resources, in particular N and water;

• Formation of a flowering shoot in the second year, supplied with nitrogen by the dying leaves of the basal rosette;

• Relocation of all resources to the seed during a period of 1-2 weeks with an harvest index corresponding to that of annuals.

Biennials are, because of the formation of the basal rosette, often very successful in out-competing other species even in the long term. After flowering many seeds drop at the site where the basal rosette of the mother plants died. Seeds germinate and, because of the enormous interspecies competition, only a few are left at the end of the new growing season and then flower again. Thus large-leaved biennials are able to occupy habitats for decades.

Biennial species occur in a very wide range of sites. At nutrient-poor sites these are often biennial legumes able to fix N2 (Melilothus officinalis), at nutrient-rich sites it is usually species with large leaves. Species from nutrient-poor sites often have many small seeds, those from nutrient-rich sites few large seeds.

Biennials are a suitable object to consider strategies and transitions between storage and accumulation. In Arctium, formation of the storage organ and filling occur after the development of the leaf so that the store accumulates excess products. In contrast, for Dipsacus sylves-tris or Daucus carota, the development of the store occurs at the same time as the development of the leaf, so that storage competes with growth (Steinlein et al. 1993).

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