Through out the developed world the avocado is regarded as a luxury item. As one of very few fruits that are high in fat, being up to 30 percent oil, it is often considered an extravagance. However, in much of central and southern America the avocado has been a staple of the rural poor for thousands of years. Its everyday usage has resulted in it being termed ‘the butter of the poor’, vegetable butter and midshipman’s butter.
The avocado is a tropical or subtropical tree which grows to about 10 metres tall. It exists as three distinct types, sometimes considered as sub-species. These are: the Mexican, the Guatemalan and the West Indian (which is actually originally from Colombia in northern South America). These different types may well be the descendants of independent cultivation events from the wild. Archaeological records indicate that humans have been consuming avocados for as long as 9,000 years and have been cultivating them for as much as 7,000. These three types are adapted to the different climatic zones of their origins, with the Mexican being the most cold tolerant and the West Indian being truly tropical. The majority of avocados finding their way to the tables of Europe and north America are of the Mexican type which are smaller and richer in fat than the tropical form. Over time, however, hybridisation between these three traditional types is resulting in a blurring of these distinctions.
In addition to being divided into geographic races avocado trees are split into two types with respect to their time of flowering. The species exhibits a unique flowering behaviour, which is technically referred to ‘protogynous, diurnally synchronous dichogamy’. So what does this botanical jargon mean? All trees have flowers, which look more or less identical, with both female and male parts within them. Nothing unusual here. The odd thing about the avocado is that when a tree flowers, all of its flowers do so at the same time and later that day they all close again. However, in about half the trees, those of the A group, the flowers open first in the morning and only their female parts are mature. Later that afternoon these flowers, which may by then have been pollinated by visiting bees, close for the night. The next morning the same flowers reopen. This time their female parts have past their prime and no longer function, but the male anthers are now mature and shedding pollen. A second groups of trees, the B group, not surprisingly do the reverse. They open their flowers first in the afternoon. At this first opening these flowers are again functionally female. As night approaches, they too close on mass until the next morning. When the B flowers reopen they have also miraculously changed from being female to become male. The whole elaborate exercise seems to have evolved to increase the chances of cross-pollination, while limiting the amount of self -pollination. Group A flowers are female in the morning and open to pollination from group B which are in their male phase, and by afternoon the situation has reversed. Wonderful, but does it work? Well sort of, certainly isolated avocado trees do not set a lot of fruit. The little there is appears to result from some environmentally induced variation with a few flowers opening out of synchrony with the rest. Even without this cheating the system does not seem very efficient. When groups of A and B trees are grown together less than 0.1 percent of all flowers actually result in fruit. However, this ratio of flowers to fruit produced is not unusually poor for a tree crop, and presumably the trees are usually successful in avoiding the ‘sin’ of inbreeding.