Abstract
This study examined i) the imbibition behaviour of a wide range of genotypes with
different seed coat characteristics; ii) the use of a polymer to regulate the rate of
water uptake and iii) the mechanism of regulation of water uptake by the seed coat in
soybeans (Glycine max L. Merril.). Seed coat structure was studied by using light,
fluorescence and scanning electron microscopy. The effects of different methanol
and chloroform pre-treatments on seed coat permeability to water were also assessed.
In this study, imbibition damage due to rapid water uptake was well documented in a
wide range of soybean genotypes. In addition a line (VLS-1) was identified that
possessed a delayed-permeability seed coat characteristic that offered protection
against imbibition damage. This characteristic was likely to be due to a lack of pits in
the abaxial region of the seed. In contrast, genotypes with a high proportion of deep
and wide open pits in the abaxial region of the seed offered minimal protection
against imbibition damage.
Coating seeds (24 mg per seed) with a polymer containing vinyl acetate, vinyl
chloride, ethylene and acrylate regulated the rate of water uptake, and offered
protection against imbibition damage. Seedling emergence from polymer coated
seeds was also improved.
Deposits and pits occurred in the surface of the seed coat in most genotypes. Deposits
were shown to be composed of hydrophilic polysaccharide material, since staining
with calcofluor was observed. Water permeability mapping indicated that pits were
the sites o f the initial water penetration. However, in hard seeds, pits appeared to
function in a different way to soft seeds and this is fully discussed within the thesis.
Prolonged methanol pre-treatments were highly effective in increasing the water
uptake when seeds were imbibed immediately after the pre-treatments. However,
drying of seeds after the organic solvent pre-treatments restored permeability to
water to untreated control levels. Results from the absorption spectrum of the
methanol and chloroform supernatants, indicated that the effect of the pre-treatments
were not due to the extraction of UV-absorbed material from the seed coat. In hard
seeds, the location of the water impermeability barrier was near the outermost part of
the palisade cell layer. The nature of the barrier was not identified by comparative
anatomical and histochemical studies between hard and soft seeds.
A mechanism for the water uptake regulation by the soybean seed coat was proposed.
The proposed mechanism involved: i) a diminished role of the cuticle and its
components (epicuticular and intracuticular waxes), ii) a key role for pits as initial
sites of water penetration, and iii) swelling or collapse of the cellulosic and/or pectic
material in the subcuticular and palisade cell layer that could regulate water
penetration through the seed coat. The common behaviour of a wide range of
genotypes tested indicated that the above mechanism of regulation of water uptake
by the seed coat is likely to be universal in soybeans.
