Seeds are elliptical or egg shaped, plano convex processing longitudinal furrow on the plane surface. Seed coat is represented by the ‘silver skin’ which is also made up of scleroides. The size, thickness and number of pits in the wall of scleroides are considered as important taxonomic characters in evaluating differences between species. Bulk of the seeds is formed by endosperm which is hard, consisting of polyhedral (many sided) cells. Seeds do not exhibit any dormancy. Viability is also short in coffee. Germination takes place in about 45 days.
Pea berry’ formation in a normal coffee plant is due to abortion of one of the two ovules in a flower at the time of fertilization. The fertilized ovule in the other locule grows into a round seed called ‘pea berry’. At times, three locules having single ovule in each locule are formed in the ovary which gives rise to triangular seeds. Endosperm sometimes shows partial development with or without any embryo. These occur commonly in floats (jollu). Formation of more than one ovule per locule is seen occasionally in Arabica, but quite frequently in S.288 and S.795. Seeds resulting from these ovules are called ‘elephant beans’. There are two types of elephant beans:
- Beans inside the parchment cover inter locked(hollow and bit)
- Seeds occurring side by side (bits). When one of the ovule gets aborted, the other functional ovule usually assumes irregular shapes (defective).
Black or spotted bean is a physiological abnormality where the endosperm is completely or partially blackened.
Arabica (coffea Arabica L)
Popularly known as ‘Arabica coffee’, it is a tetraploid species i.e., 4n=44 chromosomes. Under natural conditions Arabica grows like a small tree, but under regulated conditions looks like a bush. The plants produce profuse branches and the matured leaves are dark green in colour while the young leafs (leaf tips) are either green or bronze. The flower buds are produced in clusters in the axils of leaves at each node. Under South Indian conditions, initiation and subsequent growth of flower buds takes place from September to March. Water is essential for flowering and so the blossoms occur 8 to 10 days after showers. Arabica is self-fertile. The fertilized ovary grows into a fruit (drupe) in about 8 to 9 months and finally ripens into dark a red berry which ranges in
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
Coffee leaves grow in opposite decussate manner on the main stem and orthotropic shoots (growing along the vertical axis). But, in plagiotropic branches (the longer axis inclined away from the vertical axis), the leaf orientation is opposite due to torsion. The leaves are shiny, wavy and dark green in colour with conspicuous veins. The shape of the leaf is usually elliptical. The leaves of Arabica are slender and more delicate than those of Robusta or Liberica. The leaf tip varies from pointed (acuminate) to obtuse. As mentioned earlier, the young leaves of Arabica are either light green or bronze while in Robusta and Liberica, the young leaf colour is generally dark bronze. A silent feature of coffee leaves, as in the case of some other members of Rubiaceae, is the occurrence of ‘domatia’ which are small opening on the lower surface of leaves in the angles of the veins intersecting the mid-rib. They do not appear to have any specific function. In coffee, stomata (minute opening on the leaf for gas exchange) are present only on the lower surface of the leaves and the number of stomata vary from species to species (10000 to 17500cm2). The stomatal level number is negatively correlated to ploidy level with Arabica showing less number of stomata compared to Robusta.
Coffee is a short day plant i.e., floral initiation takes place during short place during short day conditions of 8 to 11 if day light which is prevalent between September to December in South India. Flower buds are produced at the axis of mature green wood on short stalks which are known as peduncles. The group of flowers, technically called ‘inflorescence’ is a condensed cymose (a type of inflorescence where the terminal bud is formed and further flower formation is through lateral buds, as opposed to racemose inflorescence) subtended by bracts (modified vegetative structure at the base of inflorescence which is protective in function). In Robusta, bracts are leafy and expanded whereas they are small and scaly in Arabica,4 to 5 inflorescence of 1 to 4 flowers each other produced per axil while in Robusta more number of flowers per inflorescence(5 to 6) is commonly produced. In Arabica, the axillary buds are indeterminate i.e., they may produce either vegetative shoots or flower buds depending upon the