Monday, August 29, 2011

Caught in a Web of Suberin

It's melon season.  We're searching through the melon fields to collect the ripe ones on the farm right now.  Testing for ripeness is an art, and it varies for each type of melon.  The cantaloupes are perfectly ripe when the stem easily peels off the top of the fruit.  We farmers refer to this as 'full slip', as in, "That melon's ready to go - it's full slip."  The reason melons from our farm (and, to be fair, other local farms) taste infinitely superior to supermarket melons is that mass-market melons are harvested at half-slip or even earlier, so the fruit flesh doesn't mature into as complicated of a combination of sweet and aromatic notes.  In addition, supermarket melons are treated with a compound to prevent their rinds from molding.  Unfortunately this treatment also prevents the melons from smelling like melons.  Go buy a melon from the farmer's market or your local organic farmer (my favorite is here: Fresh Harvest Cooperative).  Even if you hated canteloupe as a kid, like I did, you will not be able to stop eating a delicious local ripe cantelouple. 

The farm I'm working on grows a variety of melons.  Here is what I harvested one day this week:
Genetic diversity in melons on our farm.
The picture above contains two varieties of watermelons, cantaloupes, an heirloom variety of cantaloupes, and honeydews.  All the types of melons above except the watermelons are types of muskmelons.  Muskmelons are tropical melons originating from Africa or the Near East.  They require a long warm growing season.  They are in the same plant family (Cucurbitaceae) as squash and cucumbers, so they have to deal with squash bugs, too. 

Have you ever noticed the netting on the rind of cantaloupes?  If not, take a look at the cantaloupe below.  If you did notice the netting, did you ever wonder what it was doing there?  Not me, but when I started reading about cantaloupes for this post, I found out the most fascinating thing about that netting.
Netting on cantaloupe rind.
The netting on cantaloupe rind is made of a substance called suberin.  Suberin is a fantastic molecule produced by many plants.  It is waxy and waterproof.  On the cantaloupe rind, anywhere the rind cracks as it grows, the cracks are quickly filled in with suberin.  The suberin helps keep the juiciness in and the moldiness out.  It's basically very much like a scab on our skin, but not gross.  Fortunately for me, but not for the farm owner, suberin fails on occasion and a crack becomes too large to grow a suberin scab.  These cracked melons cannot be sold, but they are fine to eat as long as they don't get moldy and aren't inhabited by an ant colony.   I've been stuffing myself with melon culls for two weeks now.

But back to suberins.  You may already know suberins from their most famous location.  Cork is pretty much solid suberin.  Cork is the almost-outmost layer of bark of the cork oak (Quercus suber).  It provides waterproofing for the cork oak, enabling the cork oak to survive in the very dry areas around the Mediterranean Sea.  It provides the same function when humans peel it from those trees, cut it into cylinders and cram it into wine bottles. 

You will probably be surprised to learn that roots of almost all plants contain microscopic layers of suberin under their surfaces.   This seems ludicrous at first, since roots are supposed to absorb water. You'd think the last thing a plant would want is a layer of waterproofing around their roots.  In roots, suberin helps the plant regulate what can get into the plant.  There are doors and windows in the suberin layer, and those openings let in water plus the minerals the plant needs.  The openings keep out anything the plant doesn't need, such as too much salt, harmful bacteria or lots of other things that could be dissolved in groundwater.  Mangrove roots are highly suberized to keep out the salt of the ocean water in which they grow.

 Suberins are actually a group of complicated molecules.  All suberins have a similar structure, but they can vary slightly in their components.  Suberins are long polymers of smaller molecules, arranged variously but allowing crosslinks.  These molecules have a lot in common with oils, which makes them hydrophobic and water-repellent.  A little-known fact about suberins is that they have their own facebook page

Many varieties of melons don't have suberin netting, including most honeydew melons.  Their rind is smooth and net-free. The lack of suberin is not due to an inability to make suberin or an absence of the gene for suberin.  In melons lacking netting do not express the gene for suberin in their rinds.  The difference between netted and non-netted melons is a genetic difference, but not in what the DNA says.  The difference is in how the DNA is read.  Imagine the DNA for melons as a book.  If you read the pages in a different order, you get a different story.  For honeydews, the suberin chapter is skipped over altogether, except in the root cells.  (Source)  I don't know why honeydew melons don't have cracks on them.  Their rind must be able to expand around the fruit without cracking. 

I'll leave you with this picture of last week's melon harvest. 

Nice melons!  Ready for the market.

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