Biology / Science

A plant that doesn’t photosynthesise

Is there such a thing as a plant that doesn’t photosynthesize (without dying)?

Thanks Robert for the question!

The short answer is yes! These plants have evolved a parasitic lifestyle, and have such tiny amounts of chlorophyll that they couldn’t photosynthesise if they wanted to. The only examples I’ve found are all in the genus Cuscuta, or “dodder”, with common names like “devil’s guts” and “devil’s hair”. Sounds pretty appropriate for a parasitic plant.

Once a dodder seedling has germinated, it has to attach itself to a host plant within about a week, or it will just die. It does this by detecting chemical signals in the environment and growing towards them. It can parasitise a huge number of plants, which sounds like a sensible survival mechanism to me — potatoes, alfalfa, clover, flax and ivy are among its victims.

Cuscuta europaea bgiu
Cuscuta europaea parasitising Sambucus ebulus

There are also various different kinds of photosynthesis which allow plants to survive in difficult circumstances, which means they aren’t actively photosynthesising all the time. Traditional photosynthesis is referred to as “C3” photosynthesis, and it’s extremely inefficient, as discussed in this post. However, there’s also “C4” and “CAM” photosynthesis, which improve on the process not by improving Rubisco — a chancy proposition, since it needs to be shaped exactly right to do its job — but by separating the processes out so that Rubisco catalyses the right reaction (remember that Rubisco can either oxygenate or carboxylate — photosynthesis requires it to carboxylate). Restricting photosynthesis like this can improve its efficiency and prevent loss of valuable water in desert environments.

Still, C3, C4 and CAM plants all photosynthesise. Only plants like dodder have (so far) bucked the trend — which is a good thing for us, as we owe our atmospheric oxygen supply to photosynthesising organisms.

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