Seed coat development of Harpagophytum procumbens (Devil's Claw) and the possible role of the mature seed coat in seed dormancy were
studied by light microscopy (LM), transmission electron microscopy (TEM) and environmental scanning electron microscopy (ESEM). Very
young ovules of H. procumbens have a single thick integument consisting of densely packed thin-walled parenchyma cells that are uniform in
shape and size. During later developmental stages the parenchyma cells differentiate into 4 different zones. Zone 1 is the multi-layered inner
epidermis of the single integument that eventually develops into a tough impenetrable covering that tightly encloses the embryo. The inner
epidermis is delineated on the inside by a few layers of collapsed remnant endosperm cell wall layers and on the outside by remnant cell wall
layers of zone 2, also called the middle layer. Together with the inner epidermis these remnant cell wall layers from collapsed cells may contribute
towards seed coat impermeability. Zone 2 underneath the inner epidermis consists of large thin-walled parenchyma cells. Zone 3 is the subepidermal
layers underneath the outer epidermis referred to as a hypodermis and zone 4 is the single outer seed coat epidermal layer. Both zones 3
and 4 develop unusual secondary wall thickenings. The primary cell walls of the outer epidermis and hypodermis disintegrated during the final
stages of seed maturation, leaving only a scaffold of these secondary cell wall thickenings. In the mature seed coat the outer fibrillar seed coat
consists of the outer epidermis and hypodermis and separates easily to reveal the dense, smooth inner epidermis of the seed coat. Outer epidermal
and hypodermal wall thickenings develop over primary pit fields and arise from the deposition of secondary cell wall material in the form of
alternative electron dense and electron lucent layers. ESEM studies showed that the outer epidermal and hypodermal seed coat layers are
exceptionally hygroscopic. At 100% relative humidity within the ESEM chamber, drops of water readily condense on the seed surface and react in
various ways with the seed coat components, resulting in the swelling and expansion of the wall thickenings. The flexible fibrous outer seed coat
epidermis and hypodermis may enhance soil seed contact and retention of water, while the inner seed coat epidermis maintains structural and
perhaps chemical seed dormancy due to the possible presence of inhibitors.
© 2010 SAAB. Published by Elsevier B.V. All rights reserved