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emerged from any cultivar up to 40 days after oviposition. There were no positive
correlations between thickness, pigmentation or surface texture of cultivars' seed coats
and the larval ability of crossing this tissue. A delay of up to 116 % in the time for the
larvae to cross the seed coats was observed. Some laid eggs showed abnormalities and
others were completely withered. The surviving larvae that crossed the tissue, in the
artificial soybean seed coat-cowpea covered system, reached 34 % of the mass of a
normal larva. The incorporation of seed coat flour into artificial seeds revealed that the
UFV 20 Florestal was the most toxic cultivar (WD
50
[dose that reduced larval weight to
50%] = 1.5%). Lowest levels of toxicity were observed for the UFUS 2005, Conquista,
UFUS 2003 and Elite cultivars (WD
50
varying from 10.5 to 12%). LD
50
(doses that
reduced the surviving larvae number to 50%) were also variable, ranging from 1% to
14% among the cultivars. Despite all variations, soybean seed coats were highly
restrictive to the bruchid suggesting that the tissue plays an important role for
evolutionary discrimination of legumes by this bruchid.
Keywords
: bruchid, cowpea, legume discrimination, seed coat, seed defense, soybean
I
NTRODUCTION
The seed coat consists of several layers of specialized maternal cell types that provide an
important interface between the embryo and the external environment during embryogenesis,
dormancy and germination (Haughn & Chaudhury, 2005). In certain species, seed coat is
sometimes represented by a rudimentary testa, where the first external covering is the
pericarp, derived from the ovary wall. In the Fabaceae, the seed coat originates from the two-
ovule integuments (De Souza & Marcos-Filho, 2001). During seed ontogeny, the outer
integument gives rise to several distinct layers transforming itself into the testa, while in
many species, the inner integument disappears (Esau, 1977; Miller et al. 1999). Especially
among
Glycine
and
Phaseolus
species, the seed coat is a true testa. Special features described
for
Glycine
seed coats comprise: two waxy layers instead a single outermost cuticle layer
(Ragus, 1987); small openings denominated pores, or pits, (or antipits [Ma et al. 2004])
unevenly distributed throughout the testa surface (La Scala Jr. et al. 1999); and the adherence
of the membranous inner endocarp epidermis of the pod wall to the seed coat surface (Gijzen
et al. 1999). The vast majority of paper work published on soybean seed coat relates to water
uptake issues (Calero et al. 1981; Yaklich et al. 1984: Harris, 1987; Koizumi et al. 2008).
Among the major seed coat functions are: preservation of the integrity of seed parts,
regulation of aqueous and gaseous exchanges between the embryo and the external
environment, dispersion process of some seeds and in the protection of the embryo against
mechanical damage and attacks of pests and pathogens (Zeng et al. 2004).
Insect injury to leguminous seeds poses a serious problem for agriculture and food
processing. Despite the abundance of defensive chemicals, such as lectins, proteinase
inhibitors and secondary metabolites (tannins, alkaloids, cyanogenic glucosides) found in
these seeds (Carlini & Grossi-de-Sá, 2002), several members of the seed-eating Bruchidae
family are major pests of cultivated legumes, developing within and consuming seed tissues,
which would be utilized for human consumption (Southgate, 1979), without suffering any
severe damages. Among these insect species,
Callosobruchus maculatus
(F.) is of
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