An anatomical study of ovary-to-cuke development in consistently low-producing trees of the ?Fuerte? avocado (Persea americana Mill.) with special reference to seed abortion

Abstract

Cukes develop from female-sterile, cryptically male flowers on consistently low-producing ‘Fuerte’ trees. A hypostase that has, as yet, not been reported for the avocado, is present in the chalazal tissue of the mature ovule and aborting seed. This layer seems to play a role in the degeneration of the peripheral nucellar tissue and the non-development of the intercalary meristem of the pachychalaza. The ultimate cause of cuke formation, however, seemingly lies in the disturbance of the polarity of the primordial nucellar tissue. Additional megagametophytes and non-functional megaspores that develop in the nucellus effect the collapse of the chalazal region of the embryo sac. Degeneration of these gametophytes and megaspores causes the formation of nucellar cavities that isolate the embryo sac from the nutritive tissues and chalazal flow of nutrients. The micropylar region of the embryo sac contains a well-developed egg cell, synergids and central cell nucleus. An embryo and a limited amount of endosperm tissue are formed. Because the endosperm is starved of nutrients, the formation of this tissue is curtailed at an early stage, and embryo development ceases. A meristematic zone that initiates from the inner layers of the outer integument, directly opposite the place where the vascular supply to the chalaza terminates, causes abnormal growth in the outer integument. It is suggested that, due to the absence of meristematic activity in the chalazal region of the embryo sac and the non-developing pachychalaza, resources are redistributed towards the stronger sink, i.e. the outer integument. Consequently, this part of the seed coat proliferates, while the embryo sac and pachychalaza degenerate. In spite of the abortion of the seed, the pericarp of the cuke continues to develop, possibly because the pericarp of the avocado contains phytohormones.