The control of germination in melia volkensil seeds

Abstract

Seeds of Melia volkensii Gurke. do not germinate when placed under favorable conditions of moisture, air and warm temperature. To study factors which cause and maintain seed dormancy, and conditions that lead to its release, three studies were carried out. These were: (1) to describe flower and fruit external morphology and structure of the ovule~ (2) to classify the dormancy type and prescribe seed germination treatments that might break it, and~ (3) to propose dormancy mechanisms by which integuments, perisperm and endosperm operate. Seeds of M.volkensii mature In 11 to 13 months, but phases of fruit development lack a seasonal pattern. Change in fruit colour from green to yellowish-green appeared to be the most practical indicator of seed maturity. Lignification and cutinization of integuments, and their growth to form a caruncle starts early In ontogeny, but major structural changes like deposition of nutrient reserves in the embryo and endosperm, persistence and crushing of the nucellus in mature seed, the thickening of endosperm cell-walls that characterize mature seeds do not occur until late in ontogeny. The embryos and endosperm main constituents are crude fat (54% and 49%, respectively) and crude proteins (35% and 26%, respectively). Integuments are mainly composed ot cutin and lignin (60%). These changes appear to playa leading role in establishing and maintaining the state of dormancy. Seeds with their integuments, perisperm and endosperm removed reached 81% germination, compared to only 9% in intact seeds. The few intact seeds that could germinate probably did so because they had small cracks in the seed coats which were not detected on X-rays. These cracks may have developed during seed extraction because the seed coat is quite brittle. Germination started after seeds with damaged integuments absorbed over 60% water (dry weight basis). Intact seeds can absorb 48% water but most of this is held in the seed coats. Only 15% of this water is taken up by the embryo tissues themselves. This shows that integuments, peri sperm and endosperm are not completely impermeable to water, but that they limit rate and amount of water absorbed by the embryo. The endosperm, and/or peri sperm seem to be the tissues involved in limiting the amount of water absorbed by the embryo. Consequently, one way in which integuments, perisperm and endosperm inhibit germination IS by not allowing the embryo to absorb sufficient water for the start of cell division. Seeds with damaged integuments could germinate In elevated CO2 (0.65%) and depressed O2 (15%), but seeds in 50% O2 germinated only to 50% while controls (21% O2) ceached 82%. Therefore, gaseous inhibitior. of germination seems insufficient to account for the failure of intact seeds to germinate. Normal germination was induced when integuments, perisperm and endosperm were cut longitudinally at the micropylar end, but when cut horizontally in the centre, or longitudinally at the chalazal end, they germinated abnormally. Radicles tended to get trapped in the seed coats. This suggests that th~ permeability of integuments, perisperm and endosperm to water or gases alone may not account adequately for dormancy in the species. Therefore, in addition to limiting the amount of water absorbed by the embryo, they also restrict radicle protrusion mechanically. To account for the germination behaviour of seeds subjected to various integuments, perisperm and endosperm treatments and experimental conditions, three mechanisms are proposed: (1) that the endosperm, and/or perisperm do not allow the embryo to absorb adequate water for cell division to start; (2) that the embryo does not acquire sufficient imbibition pressure to break mechanical restraining action of the integuments and/or; (3) that in nature, a mechanical weakening of the integuments is needed in addition to the removal of the perisperm effects, in order that embryos be able to absorb enough water to start cell division.