210 – II.3. BRASSICA CROPS (BRASSICA SPP.)
1997; Gruber, Pekrun and Claupein, 2004) and China (Momoh et al., 2002) suggested
that genotypes differ in their predisposition to undergo secondary dormancy. Indeed, it
has been clearly shown that genotype is the principal factor controlling its potential in
B. napus (Gulden, Thomas and Shirtliffe, 2004; Pekrun et al., 1997; Gruber, Emrich and
Claupein, 2009; Gruber, Pekrun and Claupein 2004). Gulden, Thomas and Shirtliffe
(2004) found seed size was of secondary importance, with large seed more likely to
undergo secondary dormancy, while maturity and pre- and post-harvest environment had
little influence. The occurrence of secondary dormancy is reduced by alternating
temperatures (Pekrun, Potter and Lutman, 1997; Momoh et al., 2002), while cold
stratification readily releases secondary dormancy as does exposure to continuous light
(Schlink, 1995). Exogenous applications of gibberellic acid (0.2 mg 1-1) will also reverse
secondary dormancy (Pekrun, Lutman and Baeumer, 1998).
In Germany, Gruber, Pekrun and Claupein (2004) evaluated the persistence and
secondary dormancy in the seed of four winter oilseed rape varieties. They found that of
the 3 000-3 500 seeds/m^2 lost during harvest, 60-75% of that seed either died or was
scavenged within a few months. Similar levels of seed disappearance were observed by
Gruber, Pekrun and Claupein (2003) when investigating the effect of different tillage
treatments on seed persistence. Six months after harvest, no seed of the variety Artus
could be detected in the soil seed bank while the other three varieties – Bristol, Liberator
and Capital – respectively contributed 4.3%, 9.3% and 11% of their lost seed to the seed
bank. Laboratory tests for the presence of secondary dormancy closely corresponded to
that observed in the field. Gruber, Emrich and Claupein (2009) also laboratory tested seed
from over 40 varieties for their tendency to undergo secondary dormancy. The seed was
harvested from one site for three years and a second site for two years. They found that,
over several years, varieties consistently ranked high, medium or low in percentage of
seed exhibiting secondary dormancy. However, the rate of secondary dormancy varied
significantly with harvest years, dry years having the lowest incidence. They concluded
variety rank, rather than the actual percentage of secondary dormancy, should be used to
characterise a variety. Thus, selection for varieties without secondary dormancy could be
easily achieved and would greatly reduce the incidence of B. napus volunteers in
subsequent crops. It should probably be made mandatory for all new B. napus varieties to
be free of the secondary dormancy trait.
At shallow burial depths, B. napus and closely related species exhibit low seed bank
persistence (Schlink, 1995; Pekrun and Lutman, 1998; Sparrow, Knight and Conn, 1990;
Gulden, Shirtliffe and Thomas, 2003a). At 10 cm depth Gulden, Thomas and Shirtliffe
(2004) found seed-bank populations shifted from a germinable to an ungerminable state
and no seedling recruitment was observed. Masden (1962) reported that 1% of buried
B. napus seed germinated after five years, and that trace amounts of B. rapa seed
emerged after ten years. Schlink (1998) and Lutman, Freeman and Pekrun (2003) found
that approximately 1% of B. napus seed in undisturbed soil could survive for ten years.
Jørgensen, Pavlo Hauser and Bagger Jørgensen (2007), sampling a deep soil layer,
identified viable seeds of a variety sown in the field 17 years earlier. In Canada, Beckie
and Warwick (2010) reported a small population of volunteers resistant to the herbicide
bromoxynil in a field that had not grown oilseed rape since the sowing of a
bromoxynil-resistant variety seven years previously. The volunteers persisted in
low-lying areas of the field which were too wet to plant or spray with herbicides between
2001 and 2007. No volunteers were detected in either 2008 or 2009. There is general
agreement that secondary dormancy will be induced in a significant percentage of deeply
buried B. napus seed.
