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2006) or m ore f it t han their pa rents ( Klinger & E llstrand 1994; A rnold & H odges 1995;
Linder & Schmitt 1995). Thus, the correlation in plant fitness between interspecific hybrids
and their parents should be studied on a case-by-case basis (Arnold & Hodges 1995).
Influences of seed size on plant fitness
The flowering time and number of flowers of wild mustard was affected by seed category in
the monoculture experiment of B. juncea, but the final seed set was not influenced b y seed
size. It is expected that small-seeded plants have lower plant fitness than large-seeded plants
(Ahmed & Zuberi 197 3; W estoby et a l. 2002; M oles & W estoby 20 04), be cause i t i s
generally accepted that the most obvious influence of seed mass is on the seedling emergence
and development; smaller seeds contain less energy stores for germination and early seedling
growth (Major 1977; Shanmuganathan and Benjamin LR, 1992; Aparico et al. 2002; Wei &
Darmency 2008). As the strong relationship between seed size and morphological traits of the
seed and the embryo considered (Aparico et al. 2002; Diggle et al., 2010), seed size obviously
affects the initial size of the seedling and the provisions available during early seedling life.
For i nstance, i n R. r aphanistrum, l arger s eeds generally p roduce l arger p lants w ith m ore
flowers t han do s maller s eeds (Stanton 1985, C hoe et a l. 1988). H owever, a fter t he e arly
seedling stage, plant growth and development rates are similar regardless of seed size, and the
principal e nergy s ources become phot osynthates f rom e merged l eaves ( Choe et a l. 1988;
Peterson et al. 1989; Aparico et al. 2002). These studies have suggested that the disadvantage
of s mall s eeds va ry w ith ge netic ba ckground and e nvironmental s tresses s uch a s l imited
resources, high plant density, or herbivory.
Influences of seed size on plant fitness affected by plant density
The i nfluence of s eed s ize on pl ant f itness i s a ffected b y popul ation de nsity a nd t he
competition of neighboring plants for limited resources. Wei and Darmency (2008) found that
seed size effect was observed in a direct sowing experiment but not in a transplant experiment.
They suggested t hat t he hom ogeneous e nvironment i n greenhouse, pr evious t o t he
transplantation in the field, had complemented the disadvantage of small seed at early growth
stage. In th e mo noculture tr ial, s mall-seeded pl ants of m ustard pr oduced l ess f lowers a nd
more small seeds than l arge-seeded plants at low density, but that was not the case at high
density. In t he m ulti-culture ex periment, s mall-seeded nt rF1, m ustard, a nd G T pl ants ha d
delayed flowering co mpared w ith l arge-seeded plants a t hi gh d ensity, but t his e ffect w as
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