Monday, December 10, 2012

What do plant roots take from the soil?

This is a pretty complex and wide-reaching question, and I've run across it in many forms. From "Does adding sugar to the soil make tomatoes sweeter?" to "Do antibiotic residues in chicken manure end up in my lettuce?", what plants assimilate from the soil is a significant issue for many.

From basic plant biology, we know that plant roots pull several things from the soil they grow in: water, simple ions like nitrate (the major source of nitrogen), and mineral ions like magnesium and calcium. (Ions are charged molecules, carrying a positive or negative charge like one post on a battery.) These are used as building blocks for growth. They're all small molecules, usually readily dissolved in water, with the exception of a few minerals. Few biology courses go beyond that.

Plants don't just suck up everything randomly, though. Roots are highly evolved organs, serving as the gatekeepers for what gets into the plant's circulation. The anatomy of a root varies depending on its age and the type of plant, but the basic structure of an active root is in rings: an outer fleshy part which is able to absorb whatever's around it, an inner core where the circulatory tissue is, and in between, a ring of cells with an impermeable "mortar" cementing them together. That mortar, called the Casparian strip, is what ensures that anything going into the plant's circulation has to go through one of those guardian cells.

They're picky, too. They only let in a certain amount of water, to keep the interiors of the other cells from getting too diluted. They pay attention to the concentration of ions like iron, magnesium, and potassium, and actively manage their uptake. There are specific gates in the cell membrane for several things, and lots of other molecules get left out.

By the nature of those membranes, some types of molecules can just walk right in, however. Cell membranes are mostly made up of oily compounds called lipids, which glob together to make a fatty bubble, dividing the water inside the cell from the water outside the cell. Molecules which resemble those lipids can sometimes sort of phase right through, like a pedestrian making his way across a line of people. He bumps, and jostles, and comes out the other side. A repeat performance on the other side of the cell, and they're into the part of the root where they can enter circulation.

In some cases, plants will intentionally take up things which they don't actually need, for one reason or another.  Many studies have been made of plants which can take up excess minerals, called hyperaccumulators; one such plant managed to accumulate enough nickel to make up a quarter of its sap.1  And food scientists have known for years that rice is very good at accumulating arsenic, to the point that rice from the southeast United States can have significant amounts of it, when grown in old cotton fields where arsenic-related pesticides were once used.2  The reason rice takes up arsenic, even though it's not needed for growth, is that it closely resembles silicon -- and rice uses a lot of silicon, in its common form of sand, to make sturdy stems and leaves.  The gates can't tell the difference, so they pull both in.

To take it one step further, bioremediation is the process of using plants to decontaminate soil; beyond taking up simple elements as mentioned above, some plants can accumulate, break down, or attract microbes that eat more complex hazardous materials like PCBs.3  Plants that naturally take up compounds like that are rare, and often have to be specifically engineered for that purpose.

So the quick answer to "Does X make it from the soil into my food?" is... it depends on what X is like. If it's a small ion like those in table salt, it'll only get taken up if the plant wants it. If it's a larger, but still highly water-soluble chemical like white sugar, it's even less likely. The compounds which are most likely to cruise through the gatekeepers and end up in the rest of the plant are slightly oily, smallish molecules that aren't too afraid of water.4,5

There's a lot of those, including ones that are used as pesticides, some antibiotics, and others that are considered hazardous soil contaminants.  So it's important to know whether they actually make it into what we're eating.  Unfortunately (you saw this coming) it's not simple to figure that out.

Take an antibiotic used in animal feed.  In order to end up in your green salad, it has to travel a long way: from the animal feed, through the animal, into the manure, through manure storage and handling, onto (or into) the soil, into the plant root, through the plant's circulatory system, and finally coming to rest in some edible part.  There are a lot of ways it can get broken down, diverted, or diluted in all of that.  If you want some truly fascinating bedtime reading about the details of animal manure handling, there's at least one good article which summarizes all but the last three links of that chain.6  In short, though results vary widely by the type of antibiotic, a significant amount of drug can make it into animal manure, but washing it into storage ponds or pens and letting it sit there until it's used seems to degrade a lot of residual antibiotics.  From there, if it's put on top of the soil, sun can do a real number on some antibiotics, breaking them down fast.  Plus, some of them are really water-soluble and get washed out of the root zone pretty quickly.

Some amount often persists.  What that amount is depends hugely on the steps up to that point, and what type of soil is involved.  If they're present, and studies have documented fields where they are7,8, some antibiotics have been shown to make it into plant roots9, or even whole plants10... but only in the lab, and sometimes with setups as artificial as soybean plants with the green part lopped off. If they do make it into circulation, the plant may break them down. Plant enzymes have been found which resemble the same ones we have to degrade toxins and drugs, leading to what's called the "green liver" effect.11 Finally, the plant might store antibiotics in some part that we don't eat; if we eat its seeds, storage might be in the leaves.

So when it comes to whether our food plants take up small oily molecules, many of which are also easily assimilated by animals (our cell membranes aren't that different), the answer is... we don't know.  We know it's possible in theory, but we haven't yet determined whether it happens in real life, or whether it affects us.

Could we do testing of plant-based food to determine antibiotic levels?  Sure.  But without also collecting data on all the factors up to that point -- remember all the steps between the feed bag and your table? -- it would be of limited use.  Was the soil sandy or clay?  Was the manure stored before use, and if so, how long?  What was the starting concentration of antibiotic in the manure?  Until we know all the factors, we don't know enough to figure out what to do to reduce the problem.  If it is one.  Maybe it's only an issue in clay soils above pH 6, where pig manure was injected into the soil less than five weeks before planting.

This is a classic case of the frustration built into science.  It feels like a basic question, but the answer is incomplete and unsatisfying... perhaps now you can see that it's just because the question is so big.  To really answer it, we'd have to go through and test each animal antibiotic under all the common situations, for a large sample of the plants we eat.  It's equivalent to taking students from a dozen elementary schools all around the country, and forty years later, trying to find out whether they play golf.  You could make guesses, based on generalities; those from whiter, wealthier neighborhoods are more likely to play golf.  You could ask a bunch of random people, and use those results to estimate what percentage of those people would play golf.  But until you track each one down and ask them, you don't know for sure.  Repeat that process for each crop plant, with each antibiotic, and it becomes a huge task.

If there's anything to take away from this, it's that some of the research on antibiotics and other contaminants is being done.  Enough people feel that it's an issue worth exploring that they have done studies, and those studies have been wrapped into review articles, and those reviews have prompted people to ask more questions.  Hopefully you now have a better idea of what questions are worth asking, and what might make a good answer.

Meanwhile, you can put away the sugar container when planting your tomatoes. :)

Wednesday, December 5, 2012

What kinds of pollen cause hay fever?

Hay fever is a malady we generally don't associate with winter.  Even in Northern California, the rainy season damps it down for a while (just in time to catch respiratory viruses instead).  Still, depending on what you're allergic to, hay fever can strike almost year-round in warmer climes.  If the humidity is down and the temperature is mild, get ready to buy allergy medication.

Assorted pollen grains (public domain, from
Dartmouth Electron Microscope Facility)

Allergic responses in the airways are caused by all sorts of microscopic particles: dust, mold spores, animal dander, and pollen are major players.  When it comes to pollen, you often hear people blaming various plants -- goldenrod, daisies, sunflowers, wisteria, and even roses have been named by sufferers.  They don't usually include the most likely culprits, such as oaks, evergreens, and grasses.

The major deciding factor in whether a plant triggers hay fever is how it's pollinated.  Plants tailor their pollen, like their flowers, to their pollinator of choice.  For a large class of plants, the main pollinator is simply the wind; they're called anemophiles (anemo means "wind", phile means "loving").  The rest rely on animals of some kind, self-pollinate, or clone themselves some other way.  Insect-pollinated plants are referred to as entomophiles (you might recognize entomo from "entomology", the study of insects).

In order for pollen to cause respiratory allergies, it has to be airborne.  For anemophiles, that's the whole idea: make light, powdery, often small pollen that is picked up and carried easily by the wind.  Some conifers go so far as to put "wings" on their pollen grains.  Flowers are small and quite simple, as their task is largely to get as much exposure to air currents as possible.  Male flowers throw lots of pollen out in the hope that a grain or two will fall on a receptive female flower, somewhere.  (Fun fact: airborne pollen is quite effective at encouraging cloud formation.)1

Entomophiles, on the other hand, have insects to handle distribution for them.  They make less pollen, and it's more nutritious (to encourage foragers); that added protein and fat content makes the grains heavier.  They're covered with a sticky substance called pollenkitt2, which is barely present on pollen made to be windborne.3 The size of the grains doesn't seem to have much to do with the type of pollinator4 (in fact the smallest pollen commonly listed is from forget-me-not, an insect-pollinated plant)5, but the heavy, sticky nature of insect-borne pollen makes it much less likely to be wafted up into the air.  These plants also make showier flowers, with bigger structures to serve as billboards for their pollinators.  Those structures are what we typically think of as flowers, with color, petals in various arrangements, and sometimes insect-luring scents and nectar.  Sweet-smelling flowers are insect- or bird-pollinated, so unless you stick your nose deep in and take a big whiff, the pollen is unlikely to bother you.

That isn't to say animal-pollinated flowers can't cause hay fever at all... when there's enough of a particular entomophile around, airborne pollen can build up to the point that it can sensitize some people.6 But it's unusual.

So what should you be leveling your glare at between tissues?  Look for the plants which don't sport showy flowers.  Conifers, like pine, juniper, and spruce.  Olives, oaks, sycamores, mulberries, ginkgoes, all those trees where you never see a "real" flower on them.  Grasses, with their spiky panicles that rise way above all the leaf blades to get a good shot at the wind (that includes food plants like wheat, rice, and corn).  The rattier weeds, like pigweed, ragweed, Russian thistle, and many other roadside nuisances.  Some culprits are widely planted landscape plants, such as xylosma or London plane tree.  Take care with the plants you choose for your own landscape, and try to lean toward those with conspicuous flowers, or strictly female plants.

As a side note, many people believe that buying local honey can help with seasonal allergies -- but if bees don't visit the plants that put out the worst pollen, how could it?  Not all the pollen that bees collect is from the plants they visit; some windblown pollen settles on them and their preferred plants, and gets picked up by the static charge on their body hairs.7 So honey that is largely from clover or orange blossoms will still have small amounts of pine, ragweed, or other anemophilous pollen.  As to whether it helps, there's only a little research out there on honey and hay fever, using rather small patient groups, and the results seem to be mixed.8,9The bottom line: if allergies bother you, and you don't have a problem with honey, it's worth a try.  Worst case, you get a sweet little silver lining to your seasonal sniffles.