Welcome & Rhododendrons
Firstly, welcome. Since my work over the summer’s largely going to be computer related, I thought I’d try my hand at science blogging, to ensure that I keep reading about science. I’m interested in plants, particularly the ecological side, so that’s what I’ll try to focus on. It won’t necessarily be what’s in the news, just what fascinates me. I’m hoping that whatever I write should make sense to the intelligent general reader—targeting only plant scientists might limit my readership a bit—so please let me know (using the comments) if I’m not being clear enough.
So, without further ado, on to the first piece of research:
For us, recycling is largely a selfless matter, saving the planet for little personal benefit. But rhododendrons recycle in a much more selfish way, according to Nina Wurzburger and Ronald Hendrick in the USA. Specifically, they make sure they can get back nitrogen that they’d otherwise lose.
Nitrogen is one of the key elements for making living things. It’s used in DNA, and in proteins, which are the main ‘machinery’ both in animals and plant cells. If you’ve used NPK fertiliser, the N is Nitrogen (P and K are Phosphorus and Potassium). Without fertiliser, many plants are short of nitrogen, and looking for ways to get as much as possible from the soil. That’s made worse by dropping leaves—although plants can pull much of the nitrogen out before shedding a leaf, there’s always some left, and their roots then have to compete with all the other plants around to take it up again as the leaf decomposes.
So, what do rhododendrons do about this? The key is in some fungi that plants work with. The smallest roots that plants can make are still relatively thick, but fungi are very good at making microscopically thin tubes, which allow them to explore more soil with the same amount of energy. So the two team up: the plant makes carbohydrates using sunlight (photosynthesis) and the fungus grows through the soil to find nutrients. They can then trade nutrients for carbohydrates. This is a pretty popular move: something like 90% of plant species today team up with fungi like this (although there are some exceptions, like cabbages), and fossils show the partnership 400 million years ago, fairly soon after plants moved out onto land.
We call these fungi mycorrhizae, and there are a few different groups. Rhododendrons are in the heather family (Ericaceae), which uses a relatively specific group known as ‘ericoid mycorrhizae’. This probably helps them survive in relatively poor soils.
The new research found that the nitrogen left in rhododendron litter was partially protected against decomposition, in proteins coupled with tough tannins. But, crucially, the rhododendron’s ericoid mycorrhizae could break it down and absorb the nitrogen better than the two common types of mycorrhizae connected to competing plants. So, although it has to lose some nitrogen in shedding its leaves, the rhododendron ensures that it can get more of that back than its competitors will take.
How did they work this out? The key was to trace where nitrogen was going, and the way to do that was with nitrogen-15. This is a ‘stable isotope’ of nitrogen: it’s almost exactly the same as normal nitrogen (nitrogen-14), but a little bit heavier, which lets us distinguish it in a lab. They chose four sites with rhododendrons, and four without, and took samples of the leaf litter at each. Then they made proteins using nitrogen-15 instead of nitrogen-14, and mixed them with tannins that they extracted from each type of leaf litter. They added the two mixtures (tannins from rhododendron sites vs. tannins from non-rhododendron sites) to the soil at the sites, and left it there.
Three months later, and again after a year, they came back and took samples to see where the nitrogen was. The rhododendron tannins protected the proteins more effectively—more nitrogen was left in the soil than with the other tannins. At the rhododendron sites, on the other hand, more of the nitrogen was taken back into roots than at the other sites, suggesting that the rhododendron’s fungal partners could get at it better. Finally, comparing the different types of roots (apparently it’s possible to tell them apart), they could show that more of the rhododendron roots’ nitrogen came from the mixtures they’d added.
Reference: Wurzburger, N., & Hendrick, R. (2009). Plant litter chemistry and mycorrhizal roots promote a nitrogen feedback in a temperate forest Journal of Ecology, 97 (3), 528-536 DOI: 10.1111/j.1365-2745.2009.01487.x