Even if it didn’t give us chocolate, Theobroma cacao would be a fascinating tree. Viewed from a purely botanical perspective there is much to admire and wonder about, from its minute delicate flowers to its glossy green leaves that move to follow the sun’s path across the sky. In today’s post we look deeper at the pollination habits of this native of the Amazon rain forest.
The cacao tree is a member of the Malvaceae family, a large grouping of plants which include fruit trees such as durian and jackfruit. Other members of the genus Theobroma such as Theobroma cupuacu and Theobroma bicolor can also be made into chocolate-like products, although they are not yet commonplace. Could jackfruit seeds be fermented, dried and roasted into something resembling chocolate? Someone is probably trying that right now.
Cacao and it’s many related trees flower in an unusual way – they burst a spray of very small flowers right out of growth nodes along the trunk and main branches. This feature is called a “cauliflory”. The petals of cacao flowers are typically creamy white with pink edges and hang on a stem similar to a fuscia. They are unscented, and bear five stamens, five petals and a central pistil. When open they look like little starbursts all over the trunk of the tree. When closed they resemble tiny cacao pods! On a mature tree there may be over 5000 flowers at one time.
The primary pollinator of cacao flowers is a tiny flying insect, the forcipomyia midge. They live in the decaying matter at the base of the cacao trees and swarm in huge clouds all around the lower branches. Inadvertently, they contact the pollen bearing stamens of the flowers and then transfer this pollen to the pistil of another flower, perhaps many meters away. Other insects aid in pollination too. Ants, flies and parasitic wasps all play a role although like the midge their efforts are entirely accidental.
Cacao trees developed this rather inefficient method of reproduction because they evolved in a rainforest. They had no need for large showy flowers or potent attractive scents. The sheer numbers of insects present ensured enough flowers were successfully pollinated to allow for future generations. And really, that’s all a plant is trying to do. While this haphazard trait works fine when trees are left to their own devices in the wild, it poses many problems for cacao farmers.
Cacao flowers are resistant to self-pollination just like most of the world’s flowering plants. It is a method of encouraging bio-diversity and promoting stronger trees while eliminating recessive genes that might occur with constant self-pollination. Botanists call this characteristic “self-incompatablilty”. Most of the pollen distributed by midges comes, obviously, from the flowers of the same tree however. Unlike bees or butterflies they don’t tend to roam far. The ovary of each cacao flower is able to recognize its own pollen and the tree will reject that flower, causing it to wither and drop from the tree. Only pollen from a different tree will be allowed to form a cacao pod. This can lead to very low amounts of fruit actually “setting”. Cacao farmers get paid by the kilo for their beans so yield per tree is an important consideration. Growers tend to prize trees which are generous yielders while shy bearing ones are pretty quickly relegated to the firewood pile.
Recent research into flowering and pollination has lead to some very interesting discoveries that will be impacting growers decisions in the coming decades. It turns out that most cacao trees will allow at least a few flowers to self pollinate while some varieties permit as much as 50 percent of them to self-pollinate. Environmental factors and tree stress can change these percentages, too. Obviously a grove of trees which allow a high degree of self-pollination will vastly out produce a similar sized grove where the trees do not, or do so very marginally. As long as propagation of those trees occurs via softwood cuttings and not by seed, genetic flaws will not be allowed to develop. The focus of research efforts now is on identifying varietals which are not only vigorous self-pollinators but produce fine flavour cacao beans. It will come as no surprise that the CCN-51 varietal, a huge yielding workhorse of the cacao industry is highly self-compatable. Sadly, the cacao from this tree is poorly flavoured and better suited to industrial uses. More promising are trees like TSH-1188, a trinitario hybrid and Ocumare-67, a very fine criollo varietal. Both are just as self-compatable as CCN-51 and yield far finer quality cacao.
Most of the scientific focus on cacao breeding over the past twenty years has been on disease resistance, and rightfully so as this is still a major problem. Little, if any, thought was given to the taste of the chocolate made from these new disease resistant trees though. Now we are beginning to take a much deeper look at the correlation between pollination levels, yield and superior varietals of fine flavour cacao trees. Can a disease resistant variety be crossed with a high self-compatable tree to give a hybrid that has the best of both parents? Will that tree produce cacao that craft chocolate makers want to pay a premium for? This will be a major topic of discussion for years to come.
The tiny, beautiful cacao flower and the microscopic forcipomyia midge are almost ready for their close-up.