Posted on

banana with seeds

The seed has two chambers: the larger one containing the endosperm and the embryo, and the smaller one the chalazal mass. Apart from a small discontinuity of parenchymatic tissue, the chambers are separated by the inner integument and cuticle. At the micropylar end, the inner integument projects into the micropylar canal [2] , forming a cap to the larger chamber. In Musa acuminata, the larger chamber occupies just over 50% of seed volume, while the smaller chamber, the micropylar plug and other outer integument material account for the remaining volume [3] [4] .

In dry mature seeds, the hard, leaky outer layer of the coat (the outer integument) acts like a cage, while the thin inner layer of the coat (the inner integument), which is lined with a cuticle, is impermeable to water and provides an almost complete envelope surrounding the embryo and endosperm. Only wild species of bananas — and under certain circumstances cultivars that have residual fertility — produce seeds.
While size, colour and shape varied, a study of 20 species of wild and edible bananas found this basic seed structure and anatomy in all of them [5] .

Opinions on dormancy in banana seeds differ. The embryo of Musa balbisiana, for one, has been said not to have dormancy [11] because it is easy to culture embryos excised from fresh seeds [12] . Usually, more than 90% of these excised embryos of Musa balbisiana grow into plantlets [13] . On the other hand, only 68 to 75% of embryos excised from fresh seeds of Musa acuminata developed into plantlets [13] ; whereas in studies using seeds from the products of open-pollinated wild bananas and interspecific crosses [13] , dried seeds of Musa acuminata [14] , and crosses between East African highland banana cultivars and Calcutta 4 [15] , the success rate of embryo culture was well below 50%. These observations suggest that, in edible bananas and some wild banana species, dormancy may have a role in preventing embryo growth into plantlets.
Water must enter the chamber containing the embryo and endosperm before germination can occur. Experiments have shown that the seed’s outer integument is leaky since rupturing the outer integument of Musa acuminata seeds by sandpapering did not increase the absorption of water. During imbibition, the weight of the sandpapered seeds increased by the same proportion (33%) as the one of intact seeds (see graph) [8] .
On the other hand, piercing with a needle the outer and inner integument (with its cuticle) led to a more rapid absorption of water and an additional 7% increase in weight [8] , consistent with the inner integument being a barrier to water movement into the chamber containing the embryo and endosperm.
It is not known, however, how seeds sense temperatures that lead to germination. Work on Arabidopsis thaliana showed that increasing gibberellin (GA) content and decreasing abscisic acid (ABA) content in the seed promoted germination, and vice versa, and that interactions between ABA and GA played key roles in regulating germination at different temperatures [19] . Incubating intact fresh hybrid AA diploid seeds (Pisang Jajee x Musa acuminata ssp. burmannicoides) in GA for several days before embryo culture resulted in an increase in embryo germination rate in vitro [20] , although the mechanism is unclear.
In laboratory-based studies, the exudation of a drop of brownish fluid from the micropyle has been observed to precede the expulsion of the micropylar plug [6] [8] . This drop may be the consequence of a build-up of hydrostatic pressure in the chamber containing the embryo and the endosperm as the embryo begins to expand, forcing fluid in the seed cavity through breakage points around the micropylar plug. This is loosely analogous to guttation in non-transpiring leaves in that hydrostatic pressure is the driving force for the exudation.

While seeds will not germinate without imbibing water, imbibition does not necessarily lead to germination. Non-viable seeds, for one, will not germinate. Seed viability in a narrow sense refers to viability of the embryo, but it can be extended to include absence of the embryo or the endosperm, or absence of a functional connection between the embryo and endosperm.

ProMusa is a platform for sharing news and knowledge on bananas.