228 – II.3. BRASSICA CROPS (BRASSICA SPP.)
B. napus – Sinapis alba
S. alba is commercially grown as a condiment crop but weedy forms occur in the
Mediterranean region and in some countries where S. alba is used as a green manure
crop. The cross B. napus × S. alba is difficult to make even with hand pollination, usually
requiring embryo or ovule culture (Ripley and Arnison, 1990; Mathias, 1991; Bijral,
Sharma and Kanwal, 1993; Lelivelt et al., 1993; Chèvre et al., 1994; Brown et al., 1997;
Sridevi and Saria, 1996). No field crosses have been reported (Daniels et al., 2005) and
the possibility of such an occurrence is very low.B. napus – Other weedy species
Hand crosses have been made in enclosed environments between B. napus and a
number of weedy species within the tribe Brassiceae (e.g. B. fruticulosa, B. tournefortii,
B. maurorum, Diplotaxis muralis, D. tenuifolia, Rapistrum rugosum, Eruca sativa) while
protoplast fusion and embryo or ovule rescue have produced F 1 plants in B. napus crosses
with B. oxyrrhina, B. barrelieri, B. elongata, B. gravinae, B. souliei and Diplotaxis
tenuisiliqua. No field interspecific or intergeneric hybrids have been reported between
B. napus and the above species (Salisbury, 2002).Ecology
Interactions in natural and agricultural ecosystemsGlucosinolates and their ecological interaction
Virtually all plants of the Brassicaceae produce sulphur compounds called
glucosinolates (Kjaer, 1960). Although there are some 250 of these allelochemicals that
occur in 16 botanical families of the order Brassicales (Verkerk et al., 2009), only
about 20 are commonly found in Brassica species (Sarwar and Kirkegaard, 1998).
A single species will usually contain significant amounts of 4 different glucosinolates but
a single plant may contain as many as 15 different glucosinolates. They are present in
varying amounts in all tissues of the plant and directly or indirectly impact their
biological environment (Brown and Morra, 1997). They are the source of the flavour and
odour of the Brassica vegetables and the hot component in mustards. The kind and
quantity of glucosinolate varies within and among species and even between stages of
plant development as well as between plant parts e.g. cotyledon, leaf, root, flower buds
and seed. The highest concentration of glucosinolates is normally found in flower buds
and seeds.
All glucosinolates have the same basic structure consisting of a β–D-thioglucose
group, a sulphonated oxime group and a side chain “R”, derived from one of the amino
acids, methionine, phenylalamine, tryptophane or a branched-chain amino acid
(Figure 3.40). Glucosinolates accumulate in plant cell vacuoles. They can be broken
down (hydrolyzed) by the enzyme myrosinase which is located separately in the idoblast
cells. When plant cells are crushed or broken, and moisture is present, the glucosinolates
and myrosinase are released and the enzyme catalyses the hydrolysis of the glucosinolates
into glucose, sulphate and thiocyanates, isothiocynates and nitriles plus sulphur
(Figure 3.40). The intact glucosinolates have little biological activity but their thiocyanate
and isothiocynate breakdown products have broad biocidal activity (Brown and Morra,
1997).