Pollination
Pollination takes place within 6h of the flower opening, under bright light and warm windy conditions. Rain, during morning hours, before or after flower opening, affects pollination and thereby lowers fruit set. Wind, gravity and bees are the agents of pollination.
Arabica coffee is autogamous (self-fertilization) with different degrees of natural cross-pollination in contrast to Robusta coffee which is strictly allogamous (cross fertilization). Self-fertilization in Robusta coffee is avoided by gemetophytic self-incompatibility which is genetically controlled. Robusta has the adaptive advantage in having longer styles compared to Arabica. This may also facilitate cross pollination. Sometimes in Arabica, self-pollination occurs before the opening of the flower bud itself. But in interspecific hybrids between Robusta and Arabica, the tendency for cross pollination is high.
Fertilization, fruit and seed formation
Fertilization takes place when a pollen grain germinates to produce a pollen tube on the stigamatic surface. The pollen tube reaches the embryosac by growing through micropyle and subsequently bursts open realeasing the two male nuclei, of which one units with the egg to form zygote and the other fuses with the secondary nucleus to form the primary endosperm nucleus (double fertilization).
The process of fertilization is completed within 24 to 48h after pollination. The zygote and endosperm nucleus formed as a result of fertilization, undergo a resting period of nearly 45 days in Arabica and 60 days in Robusta. Meanwhile, the integument (protective coat) of the ovule begins to increase in size to perform nutritive function for zygote. The endocarp or the parchment cover is laid down after blossom. This determines the size of the integument (perisperm) grows to its maximum size in 100 to 120 days after blossom. This determines the size of the future bean. Finally, the endosperm grows into space that has been previously occupied by the integument and restricts it to a thin layer surrounding the endosperm. This forms the ‘silver skin’ or the seed coat of bean. Commensurating with the growth of endosperm, the zygote grows into an embryo with a hypocotyl and two (sometimes 3 to 4) cotyledons. Embryo is situated at one end of the bean on its convex surface. The normal duration of a flower to fruit development is about 8 to 9 months in Arabica and 10 to 11 months in Robusta. Ripe fruits have a thick fleshy pericarp with a mucilageneous layer surrounding the parchment which is made up of stone cells or scleroides.