3 The sugar beet biology, geographical distribution and gene flow

The genus Beta is classified into four genetically differing sections, namely Beta, Corollinae, Nanae and Procumbentes (Lange et al., 1999; Frese, 1998*). The species B. vulgaris ssp. vulgaris comprises the cultivated forms sugar beet (var. altissima), fodder beet (var. crassa), Swiss chard (var. vulgaris) and red beet (var. conditiva). The sugar beet has cultivar, wild and weed forms. Wild beet species are quite common among the Beta section, comprising the subspecies Vulgaris, Maritima, Adanensis and the closely related species B. patula and B. macrocarpa. They occur along the coasts of northern and western Europe and in the Mediterranean area including north-west Africa and the Canary Islands, the Balkans, the Caucasus and from Asia Minor to Bangladesh (Table 1) (Frese, 1998*).

Sugar beets are most often self-incompatible and usually wind-pollinated, although insect pollination is also possible (Bartsch et al., 1999*). Sugar beet hybridises easily with cultivated and wild forms of Beta vulgaris. It has normally vigorous and fertile progeny without incompatibilities with members within the Beta section (OECD, 2006*). Natural interspecific hybridisation between the species of section Beta is also possible, but is very unlikely with species of the other three Beta sections. If hybridisation between B. vulgaris and other sections occurs either naturally or through artificial techniques, the resulting progeny is not viable and does not reach the generative phase (Van Geyt et al., 1990*). Thus, a gene transfer from cultivated beets to wild beets is likely only for the Beta section.

Figure 2: Intra- and interspecific gene flow of cultivated, weedy, wild sugar beet and related wild species within and between agricultural fields and the environment (Bartsch et al., 2003*, modified).

Sugar beets are biennial plants forming a beet in their first season. If the beets are not harvested, flowering shoots appear in the second year after vernalisation. However, the formation of flowering shoots and completion of the whole life cycle in the first year is possible (bolters) (probability < 0.05%) due to the genetic constitution of the plants and/or certain weather conditions such as drought or frost (Keller et al., 1999*; Geisler, 1980*). In practice, bolters are usually removed or destroyed before flowering. Seeds from bolters that have not been removed before seed maturity fall off and pass into the soil seed bank. From these seeds, weed beets can emerge within and between rows in the crop stand in the following years (May, 2009*). The seeds can have a life span of over 10 years, but are depleted with time or may germinate under favourable conditions. Small beets or beet sections left in the field after harvesting can regenerate (groundkeepers) (Elliott and Weston, 1993*) depending on the part of the plant, size, depth of placement, survival ratio, and management of the following crop (Buddemeyer and Petersen, 2002*). Bolters and volunteers in some years with special conditions can become overwhelming, hindering their management or eradication by farmers. Hence, gene flow via seed, pollen or clonal plant parts within and between agroecosystem and the environment is possible in various ways and directions (Figure 2*).