This week, our city employees cut a diseased tree on my street. You couldn’t design a better experiment to map the location of phloem in a tree! Just look at where the ants are gathering, and that thin ring of congregation adjacent to the bark is above the plant tissue responsible for the transport of sugars from leaves to roots.
How did we first learn about the composition of sap or of its location in plants?
If someone parks a car (or a bike 🙂 ) under a tree in the middle of summer, it could end up being covered with a gluey residue. This substance known as honeydew is a product of phloem-feeding insects such as aphids, whiteflies and scale insects. Members of most aphid species inject their stylets into the sieve tubes of phloem. The sap moves through the small insect’s digestive system and comes out of its posterior end as that sticky product, which not only frustrates parkers but alerts parasites and predators of aphids.
If you analyze the contents of the stylet, you gain insight into what flows through phloem: 75 to 90% water, with its main solute being sugar along with small amounts of amino acids and alkaloids. If it seems that aphids are wasting sugars by excreting them, it’s because of sap’s low nitrogen content (from amino acids) that a lot of the main solute is excreted. The aphids need to ingest a large amount of sap to get enough nitrogen to make protein, and in the meantime they have no use for all the excess sugar.
In other experiments from the past century, bean leaves were exposed to artificial carbon dioxide containing only the radioactive isotope of carbon, 14C. About half an hour later, sugars carrying carbon-14 were found in the phloem’s sieve elements. This is how we first gained solid evidence that some photosynthetic products from leaves are shared via specialized vessels and sent towards the roots.
Some ants live mutualistically with aphids, and the relationship is remarkably sophisticated. The ants spread the aphids and protect them in exchange for the sugars in their honeydew. Some aphids cannot excrete unless they are stroked by the antennae of their farming ants. In certain cases, ants will strip developing wings from aphids not to lose their herd. The ants can also release compounds that slow down aphids’ walking speed. Even slight differences between honeydews in the quantity of plant secondary compounds and in the composition of sugars affect the behavior of honeydew-feeding ant colonies.
Raven, Evert and Curtis. Biology of Plants. 2nd Edition