Anatomy and physiology of Apple Blossom.

Although apple is usually self infertile, each flower or blossom contains both male and female parts. The female part, the gynoecium, has five carpels, each comprising a stigma, a style and an ovary. The ovary usually contains two ovules, occasionally 4 or 6. The male part the androecium, consists of a series of filaments each ending in a pollen-producing anther. To fertilise an ovule, an intermediary is needed as the apple tree is not self fertile - the bee. The bee transfers pollen from one flower to another. To entice the bee to do that, at the base of the flower, is the nectary which contains nectar, which is what the bee is looking for. Nectar is based on the sap, and is a sweet liquid consisting of 80% water and the rest complex sugars and amino acids. The latter increase the attractiveness of the nectar to the bee. Some yeasts and bacteria also live in the nectar again altering its scent profile. A complex multi-organism symbiosis. Some yeasts are only found in nectar.

Once the pollen grains have been brushed off onto the stigma, they adhere to its sticky secretion on the top, which then hydrates the pollen grain, enabling it to germinate. A pollen tube starts growing downwards through the style, eventually reaching the ovaries after two or three days.

During this period, in many apple varieties, the style tissue performs a filtering “self-incompatibility” function by producing toxic proteins that can stop the growth of the pollen tubes; if the pollen tubes are genetically identical to the style. Only when the pollen tubes are genetically different can they deactivate the style’s toxic proteins and continue their growth.

Once the pollen tube has reached the ovary, the ovule is fertilised. This is a dual process: the first fertilisation is between the male gamete and the egg creating an embryo, the part of the seed that will generate the new plant; the second fertilisation creates a triploid endosperm one male gamete fused with 2 maternal ones, which provides the initial food supply for the developing embryo, and represents the bulk of the white tissue inside an apple seed. The flower’s ovary then begins its development into a fruit, and the ovules become the apple seeds.

Shown in this diagram is one of the 5 stigma. However all this has to occur ideally for all five stigma simultaneously, in order to fertilise all ten ovules needed to form a perfect apple. This does not always happen, and apples with fewer than its complete set of 10 seeds are generally smaller and perhaps misshapen. Often they fall off the tree early in the fruit’s development., often called the “June” drop in the northern hemisphere.

The nectary, positioned around the developing fruit, now changes it role from attracting bees to help supply the growing apple with nutrients.