Seed Coat Structure and Histochemistry of Abelmoschus esculentus. Chalazal Region and Water Entry

The seed coat structure and histochemistry of Abelmoschus esculentus were studied by bright-field, fluorescence and scanning electron microscopy. The seed coat was typical of species of the Malvaceae. The endotesta cells had inner tangential walls which were very thick and autofluorescent. The occurrence of phenolic substances at this level has been related to seed coat imposed dormancy. The palisade cells were composed of three differently shaped parts: an upper ‘prismatic part’, a medium ‘transition part’ and a lower ‘twisted part’. The former was rich in hydrophilic substances, the latter was lignified. The swelling of the ‘prismatic parts’ was related to seed coat cracks. The region controlling onset of water entry was thought to be the chalazal area. Thanks to the presence of a large amount of highly acidic polysaccharide, water was able to penetrate from the permeable maternal tissue, through the chalazal cap and plug as far as the boundary between the palisade and underlying mesophyll. During imbibition of water a kidney-shaped ‘blister’ was seen to rise, formed by separation of the palisade cells from an underlying single layer of subpalisade cells. The palisade layer forming the blister roof showed the same histochemical characteristic of other seed regions. The single layer of the blister floor showed an affinity with Toluidine Blue O and Alcian blue. Both blister roof and floor were strongly autofluorescent.