The cacao tree enigma

Date:
April 6, 2022
Source:
University of Wu"rzburg
Summary:
The tropical tree, from whose seeds chocolate and other sweets
are produced, has its secrets. Unravelling them is not so easy.



FULL STORY ========================================================================== Cacao has long been a sought-after raw material for the worlds food
industry.

At first glance, it therefore seems surprising that biology knows little
about the pollination of the cacao tree -- although it is precisely this process that is the basis for fruit set and ultimately for the yield.


==========================================================================
At second glance, however, one quickly understands why the pollination
of this tropical crop holds so many secrets: cacao flowers are very
small and are usually found by the thousands on a tree. The insects
that gather at the flowers are also tiny and very diverse in terms of
species. All these factors make systematic observations difficult.

Study in the north and south of Peru A new study now brings more
clarity. It was conducted in Peru by an international research
team at the Chair of Animal Ecology and Tropical Biology at Julius-Maximilians-Universita"t (JMU) Wu"rzburg, Germany. The leading
project organization was Bioversity International, the Deutsche
Gesellschaft fu"r internationale Zusammenarbeit (GIZ) funded the project.

South America is the native region of the cacao tree, which occurs there
in the undergrowth of tropical rainforests. In agriculture, too, the cacao
tree is planted in the shade of larger trees, in so-called agroforestry systems. The researchers applied glue to cacao flowers in 20 such systems
in northern and southern Peru to investigate which animals visit the
flowers. They also analysed the influence of the degree of shading and
the distance to the nearest forest on visitor activity at the flowers.

The results have been published in the journal Ecological Solutions
and Evidence. The first author is biologist Justine Vansynghel, who has
been a doctoral student at JMU under Professor Ingolf Steffan-Dewenter
since 2018.



==========================================================================
A wide variety of insects cavort on the blossoms In the cacao agroforests
in the dry north of Peru, aphids (38 %), ants (13 %) and thrips (10
%), were the most frequent flower visitors. In the more humid south,
on the other hand, thrips (65%), midges (14%) and parasitic wasps (10%)
were the most common.

In the north, the researchers counted more insects on the cacao flowers
the more shaded the plantations were. In the south, on the other hand,
the insects preferred to stay in less shaded plantations, at least
during the rainy season in which the study took place. The distance to
the nearest forest did not play a role in the extent of flower visitation
in either region.

Pollen transfer and fruit set remain poor Justine Vansynghel's team
also observed that only two percent of the pollinated cacao flowers set
fruit. Transferring pollen by hand tripled fruit set to seven percent,
which is still very poor.



==========================================================================
The doctoral researcher can only speculate about the factors that
limit fruit set. One reason could be that there are simply no efficient pollinators for cacao in Peru. This is suggested by the fact that only
very few pollen grains were counted on most of the cacao flowers, an
average of 30. Four times that amount would be needed for successful pollination, according to the literature.

Another reason for low fruiting in cacao could be that the individual
cacao plants are genetically incompatible with each other.

Many questions remain to be clarified So there remain major gaps in our knowledge of the biology of the cacao tree.

"Among other things, it would be important to identify the main
pollinators," says Justine Vansynghel. Then it would also be possible
to develop higher- yielding agroforestry systems and improved management strategies in the Peruvian native regions of the cacao tree.

Why are cacao yields in Peru so much worse than in Africa or Asia? "In Indonesia, you can achieve a fruit set of around 50 percent with hand pollination. This is probably because the plantations there do not
use the native South American cacao clones, but higher-yielding ones,"
explains the JMU researcher.

On the other hand, cacao plantations in Africa and Asia are threatened
by so many diseases and pests that large monocultures can be eliminated
at once.

Another problem with the high-yielding, non-native clones is that
they only produce good harvests for five to ten years. After that,
old plantations are abandoned, and existing forest is cut back for
new plantations.


========================================================================== Story Source: Materials provided by University_of_Wu"rzburg. Original
written by Robert Emmerich. Note: Content may be edited for style
and length.


========================================================================== Journal Reference:
1. Justine Vansynghel, Carolina Ocampo‐Ariza, Bea Maas, Emily A.

Martin, Evert Thomas, Tara Hanf‐Dressler, Nils‐Christian
Schumacher, Carlos Ulloque‐Samatelo, Teja Tscharntke,
Ingolf Steffan‐Dewenter. Cacao flower visitation: Low pollen
deposition, low fruit set and dominance of herbivores. Ecological
Solutions and Evidence, 2022; 3 (2) DOI: 10.1002/2688-8319.12140 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/04/220406101643.htm

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