I have absolutely no good answer for you, but here's a thought:

Some of the antioxidants and other "detox" helpers we get come from these very plants, right? (Vitamin C, selenium, etc.) We use them as cofactors for enzymes that help us neutralize toxins, drugs, hormones, etc., and we generally get rid of them via excretion in the urine or feces (and maybe even our sweat - not sure about that, though).

So it's possible that plants have similar enzymes and detox systems that help them get rid of wacky stuff they come in contact with, like pesticides and fungicides. My question would be, where do these substances go after the plants neutralize and get rid of them? Back into the soil? Evaporated into the air?

That being said, even if plants do have detox systems, I would imagine the amount of poisons they're exposed to and the frequency would overwhelm these mechanisms.

I dunno...we need a botany expert here. I firmly believe that plants are way more complex than we ever realize. Just because they don't "talk" doesn't mean they don't communicate with each other -- send each other chemical signals that we as humans can't detect. (At least not without specialized equipment. I don't have the details handy, but I saw a documentary about that once -- something about the inner life of plants. Hehheh...makes me wonder what the vegans would be left eating if they understood just how sentient plants really are.)

Scroll down to mechanisms of metal tolerance in plants

http://lqma.ifas.ufl.edu/Publication/Cai-02.pdf

Apparently Plants evolve tolerance and resistance mechanisms. Plants also use chelation mechanisms to deactivate some compounds.

Hope you find your answer, or part of it in this article

I found a potential answer to your question on the wikipedia (yes, I know wikipededia is supposedly a crappy source for a lot of things) article on phytoremidiation in the section of phytotransformation:

"In the case of organic pollutants, such as pesticides, explosives, solvents, industrial chemicals, and other xenobiotic substances, certain plants, such as Cannas, render these substances non-toxic by their metabolism. In other cases, microorganisms living in association with plant roots may metabolize these substances in soil or water. These complex and recalcitrant compounds cannot be broken down to basic molecules (water, carbon-dioxide, etc.) by plant molecules, and, hence, the term phytotransformation represents a change in chemical structure without complete breakdown of the compound. The term "Green Liver Model" is used to describe phytotransformation, as plants behave analogously to the human liver when dealing with these xenobiotic compounds (foreign compound/pollutant).[7] After uptake of the xenobiotics, plant enzymes increase the polarity of the xenobiotics by adding functional groups such as hydroxyl groups (-OH)...similar to the way that the human liver increases the polarity of drugs and foreign compounds (Drug Metabolism). Whereas in the human liver enzymes such as Cytochrome P450s are responsible for the initial reactions, in plants enzymes such as nitroreductases carry out the same role".

This is phase I. Phase II is described next:

"In the second stage of phytotransformation, known as Phase II metabolism, plant biomolecules such as glucose and amino acids are added to the polarized xenobiotic to further increase the polarity (known as conjugation). This is again similar to the processes occurring in the human liver where glucuronidation (addition of glucose molecules by the UGT (e.g. UGT1A1) class of enzymes) and glutathione addition reactions occur on reactive centres of the xenobiotic".

Finally, phase III is the last phase:

"In the final stage of phytotransformation (Phase III metabolism), a sequestration of the xenobiotic occurs within the plant. The xenobiotics polymerize in a lignin-like manner and develop a complex structure that is sequestered in the plant. This ensures that the xenobiotic is safely stored, and does not affect the functioning of the plant".

It appears this process is not the only method of plants detoxifying themselves (it depends on the plant and the toxin), but it's likely a very important one. This paper looks like a good overview of the subject, though it's a bit over my head at times.

Brilliant question - you've set me off on a morning of database and google searching! I don't pretend to fully understand all of the below, but I hope it helps to answer your question!

Were you thinking about plants breaking down herbicides (xenobiotics) and other toxins into harmless substances or excreting toxins and byproducts from the organism itself? I've treated these as two separate questions and tried to do some brief research in them accordingly.

I found the following article which contains the following pertaining to herbicide detoxification in plants:

Research during the last decades revealed that plants contain a multitude of enzymes that metabolize herbicides and other xenobiotics to nonphytotoxic products. Herbicide metabolism often includes a primary step, most commonly an oxidation or hydrolysis, that serves to provide a functional group that is suitable for subsequent conjugation to an endogenous moiety such as GSH (y-glutamylcysteinylglycine), Glc, or an amino acid. The resulting conjugates are (a) generally inactive toward the initial target site; (b) more hydrophilic and less mobile in the plant than the parent herbicide; and (c) susceptible to further processing, which may include secondary conjugation, degradation, and compartmentation.

Kreuz, K, Tommasini, R, Martinoia, E (1996) Old enzymes for a new job: herbicide detoxification in plants. Plant Physiology 111: 349-353

So herbicides can be metabolised by the plant in processes which alter their chemical structure. The article goes on to describe the biochemical pathways by which this process occurs. If I understand the article correctly, the by-products of these detoxification reactions are then sequestered in the vacuoles inside the plant's cells or in the cell walls and material surrounding the cells (the 'extracellular matrix'):

Herbicide metabolites derived from both Glc and GSH conjugation either can be deposited as so-called "bound residues" in the extracellular matrix or stored as water-soluble metabolites in the large central vacuole (Lamoureux et al., 1991)

ibid.

The article also compares the similarities between plant and animal detoxification:

Striking similarities can be found between plants and animals in their detoxification mechanisms with respect to the involvement of Cyt P450 monooxygenases and conjugation of electrophilic substrates to GSH. Differences are observed for the other major conjugation step in that xenobiotics are often glucuronated in animals but glucosylated in plants.

ibid.

However, this doesn't answer the question of what happens to the waste products once they are transported into the vacuole or extracelluar matrix.

According to this article, it's likely that the waste products of detoxification accumulate within the plant and stay there for long periods of time:

In plants, excretion is replaced by internal compartmentation or apoplastic deposition of metabotites (bound residues). These residues may persist in plant tissues for considerable periods, and may have toxicological implications for consumers of plant tissues.

Coleman, J, et al. (1997) Trends in Plant Science 2(4): 144-151

Pondering it myself, the only major method I can think of for plants to physically excrete these waste substances is through 'abscission' where plants shed old leaves (and flower structures) periodically. The most obvious example is leaf senesence in deciduous trees in autumn but even tropical and non-deciduous plants shed leaves periodically.

Maddeningly, I haven't been able to uncover much about this so far. I found the following in an article about heavy metals and senesence in beech trees:

The annual leaf cycle in deciduous trees is the rational consequence of renewing frail organs of photosynthesis annually instead of making them durable and hard against frost. The same consequence can be ascribed to winter-green trees and shrubs of arid zones, where leaf-shedding reduces water loss. However, leaves also are shed periodically in tropical forests where no frost occurs, and where water supply is not problematic (Dingier 1911; Volkens 1912). Therefore, an additional function could be ascribed to the annual leaf cycle. The observed transport of heavy metals into autumn leaves of the beech suggests an organized action of waste elimination.

Fromm J, Essiamah S, Eschrich W (1987) Displacement of frequently occurring heavy metals in autumn leaves of beech (Fagus sylvatica). Trees 1: 164–171

If plants can eliminate heavy metals via leaf abscission I suppose it's theoretically possible that other toxins could be eliminated this way as well.

This guy appears to be very keen on the idea of plants excreting toxins via leaf abscission (at least he was back in the '80s). However, he hasn't produced much scientific support for this and it looks more like the musings of a 'gentleman scientist'/media presenter than anything else.

Looking at it though, I think this mechanism would be more significant for plants with longer life cycles (e.g. trees) rather than annual plants. The potential to accumulate large quantities of toxins must be greater for plants which live for many years as opposed to annuals with lifecycles of just a few months.

I can't find much research to back up (or refute) any of the theory about leaf abscission, although I find it hard to believe that the evidence isn't out there. Perhaps someone with a more time and a more up-to-date grasp of botany will be able to shed some light on it!

I don't know if plants do it directly themselves or if it is the symbiotic relationship with fungus in the soil like mycorrhizae that might do it for them. I suspect healthy soil with a good fungal population, as much as lack of pesticides render organic vegetables less toxic overall.

Just one example, check out what oyster mushrooms can do converting diesel fuel contaminated soil into a field of edible goodness: http://www.islaearth.org/show.php?_sid=1260345600

This is pretty good too: http://www.ted.com/talks/paul_stamets_on_6_ways_mushrooms_can_save_the_world.html

As far as I know, they don't. They tend to concentrate toxins, because they simply don't have hepatic and renal system. One example that comes to mind though is mangrove trees. They excrete salt from their leaves because they are found in saltwater environments. Other plants, when confronted with simple salt, just die.

Apparently some can make their own antioxidants http://www.purdue.edu/uns/html4ever/2004/040901.Salt.antioxidant.html

"We were able to clearly establish for the first time that plants that create and accumulate high cellular levels of the antioxidant glutathione are much more nickel tolerant," said David Salt, associate professor of plant molecular physiology in Purdue's horticulture department.