Morphology of seed testa in wild Cucumis species: potential contribution to physical seed dormancy
Seed dormancy in wild watermelon (Cucumis africanus) and wild cucumber (C. myriocarpus) seeds, within the Cucurbitaceae family was attributed to allelochemicals referred to as cucurbitacins. However, erratic germination in leached seeds of C. africanus, as well as inhibited germination of leached C. myriocarpus seeds could be attributed to the existence of impermeable seed testa layers. The objective of this study was to determine whether the testa in seeds of the two Cucumis species possess structures, that interfere with imbibition and movement of water to the endosperm or watergap structures that amplify imbibition. Seeds were cut into longitudinal and transverse sections using a microtome blade. Sections were mounted on aluminium stubs using sticky tabs, coated with carbon and examined, measured and photographed using a field emission scanning electron microscope. In C. myriocarpus, five distinct layers were observed: epidermis, hypodermis, sclerenchyma, aerenchyma and collenchyma. In contrast, only four layers occurred in C. africanus, as the hypodermis was missing. Most of the layers were heavily lignified and therefore, served as candidates for impermeability to water. In both Cucumis species, canals were observed at the micropyle and chalaza regions. Scarification on both regions unblocked the canals and enhanced germination of the two Cucumis species. In addition to chemical dormancy, seeds of both Cucumis species also have physical dormancy. Therefore, to enhance germination, both types of dormancy must be ameliorated.
Maila, M.Y., Mashela, P.W. and Baker, C. (2018). Morphology of seed testa in wild Cucumis species: potential contribution to physical seed dormancy. Acta Hortic. 1204, 17-24
cucurbitacin, ethnomedicinal plant, microbial degradation, secondary metabolite