Sunday, May 5, 2013

Holy guacamole!

Species name: Persea americana

Common name: avocado

Location: Dominican Republic

The avocado is not only a popular tree crop but also a popular ornamental plant species. It has huge morphological variation in the shape and colour of the fruit (one of the reasons why I didn't clue in right away that this was an avocado tree) based on the variety grown, and can also either be a dwarf tree (like this one) or can grow up to 20 meters tall. A bit awkward to pick the fruit! The avocado is native to Mexico and prior to the domestication process was actually a purple pear-shaped fruit with yellow-brown flesh. Very different from the purple oval-shaped fruit with green flesh that we get now! I'd say that the avocados we buy in the store now are slightly more pleasing to the eye, although I doubt they taste any different.

Considering how common the product is, this tree took me forever to identify! Part of the reason is because of its very unusual leaves, which are in part a product of something called "phenotypic plasticity": a trait can vary along some sort of morphological continuum given different environmental conditions. This is incredibly useful in most cases, like being able to produce more surface hairs on your leaves when experiencing conditions of drought, which actually reduces the amount of water lost from your leaves. Other traits, like leaf shape, I'm not sure how it would confer some sort of benefit to the plant. But obviously it does, or else the plant wouldn't bother! The type of leaf this tree was producing at the tips of the branches are called cuneate leaves, which are very wide at the tips and gradually taper towards the base of the leaf. This is a relatively uncommon leaf shape, but a plant commonly grown in temperate areas that has this shape of leaf is the flowering magnolia tree. Another interesting characteristic about the avocado is how the seeds are dispersed. They aren't. Probably not all that successful for an avocado tree! Where these trees are growing in forests in the tropics and subtropics, animals may investigate the fruit and carry them off to eat them elsewhere, but that's not how the fruit would have originally been dispersed. The thought for how avocados were once dispersed stems from the idea that way back in geological and evolutionary time, many more megafaunal species roamed the earth in all habitats (think of the wooly mammoths that North America used to have). In Central and South America there were enormous mega-sloths that were probably, along with gomphotheres or elephant-like herbivores, the main dispersal organisms of this and many other large-seeded plants. These animals would have been big enough to eat the avocados whole, passing the seeds intact through their digestive systems and excreting them in their poo. Their intestines would have scoured the outer seed coat and kept them nice and moist, and the animal's excrement would have acted like a pile of natural fertilizer to kick-start the growing process. Next time you eat an avocado, think about how big your stomach would have to be to not notice that you're eating avocado seeds. Those would have been enormous animals!

Avocados are a great example of a product that is becoming more expensive to purchase because of our growing practices. Avocados are "made" to be grown in Mexico: that's where they evolved and that's where they're best adapted for growing. Sure, we CAN grow them in the United States, which is regularly done in California and Florida, but why would we bother? The plant requires deep watering in order to bear fruit, and water is a hot commodity in California, and the right growing conditions are rare in Florida. This drives the cost of avocados up, only because we're trying to grow them in suboptimal conditions. So why don't we just import them from Mexico? Well, we do now. But up until recently this was not allowed because it was believed that all Mexican avocados were infested with type of fruit fly that would completely destroy crops in the southern United States. I wonder why the government wouldn't assume the fly would travel across the border on its own since it could fly, but apparently that wasn't ever a possibility. The border was completely closed to Mexican-grown avocados until almost the year 2000, which was when the US set up a foreign fruit inspection station outside American borders. It's amazing what we're prepared to do for some guacamole! Today, the US and Canada both accept avocados grown in Mexico, Peru, Chile, South Africa, and New Zealand.

Despite these being tasty fruits to humans, you should always be very careful when using avocados in your kitchen if you have house pets of any kind. Avocado skins are deathly toxic to cats and dogs (as are all parts of the plant if you choose to grow it as an indoor species, which many people do), and they don't seem to understand to stay away from it. A good rule of thumb, especially if you have a dog that likes to raid your garbage can and you like making guacamole, is to rub lemon juice all over the skin before you throw it out. Dogs and cats HATE lemon juice, so that will usually get them to stay away. If you use a compost pile and like to compost avocado skins you might want to make sure you have some sort of deterrent around the outside of your compost pile; rabbits, squirrels, horses and cows (if you live on a farm), and birds are all also deathly allergic to avocado skins.