Posted on

banana seed

A seed has one living component, the embryo. To germinate, not only does the embryo have to come into contact with water, the seed also has to be non-dormant.

The basic external requirements for germination are water, oxygen and an appropriate temperature. The germination of a seed begins with water uptake (imbibition), followed by embryo expansion along its axis through cell elongation, and ends with the breaking-through of the seed coat by the embryo root (radicle), then shoot (plumule). In the banana seed, the endosperm disappears within two weeks and within three weeks, the seedling has a lateral root system, and a prominent shoot [6] , in which the first leaves have elongated beyond the cotyledonary sheath [7] . By the fourth week, the seedling is dependent on the root system for inorganic nutrition [6] .
It is likely that there are different types of dormancy among Musa species, with contributions from the seed coat and the embryo. The necessary experimentation to clearly define the roles of the coat, embryo and other tissues in dormancy have not been done, but that banana seeds can become dormant appears beyond doubt [16] .

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 their natural habitat, humid tropical forests, banana seeds may lie viable for years, often germinating at the same time and in large numbers when soil is disturbed or vegetation removed [17] . The higher temperature of exposed soil in gaps in the forest suggests that temperature is a cue for banana seeds to germinate. Under greenhouse conditions, the germination of Musa balbisiana seeds was greatest when temperature fluctuations were large [18] .
The first visual evidence of germination is the expulsion of the micropylar plug from the micropylar canal. The plug is forced out cleanly along a line of “predetermined breakage” (the abscission layer) [6] [1] by pressure exerted by the axial expansion of the embryo, perhaps assisted, at least in fresh seeds, by pressure resulting from the absorption of water from its environment by the chalazal mass.
Where, and under what circumstances, water penetrates the inner integument is unknown. In many non-banana species it happens via water gaps [9] . In bananas, water gaps might develop around the micropylar plug [10] , or in the area of the chalazal mass, where the inner integument is discontinuous, and parenchymatic tissue separates the outer and inner chambers [2] .
The structure of the banana seed with its leaky outer integument means that the seed imbibes but the embryo and endosperm might not absorb water because of the impermeable inner integument and non functional water gaps.

Studies using ruptured seed coats show that the expanding embryo emerges through the rupture [18] [8] , the path of least resistance, suggesting that some force is needed to dislodge the micropylar plug [16] . In nature, the need to exert a force might select against embryos that would otherwise grow into seedlings that are “less fit” [16] . If this is the case, bypassing this step by excising embryos and culturing them will include any such less fit seedlings [16] .

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