Thursday, 19 March 2009

Sock it to 'em oca!

Those of you who glaze over at the sight of charts, figures and arcane jargon may prefer to look away now. Please don't - I won't detain you long.

Ben Gabel of Real Seeds mentioned to me that the crops planted in a bed which had contained oca the previous year seemed to grow less well than expected. My curiosity piqued, I decided to boldly head off at warp factor 10 to the planet Allelopathy.

Allelopathy is one of those concepts in plant science and horticulture that seems to engender a lot of heat, but surprisingly little light. It is, as interesting ideas often are, contentious. Reputations have risen and fallen; promising careers have ended in derision and ignominy. Hans Molisch, who published the first definition of allelopathy in his book Der Einfluss einer Pflanze auf die Anderd-Allelopathie (The Influence of one Plant on Another-Allelopathy) in 1937 - died the same year - suspicious or what?

So what is allelopathy and what's it got to do with oca? Allelopathy is the inhibition of one plant by the chemicals released by another. It's chemical warfare. That's one definition, or maybe two and they'll do for the purposes of this discussion.

Well, knowing a little bit about oca's biochemistry, I wondered whether some of the weird and wonderful compounds it contains might be responsible for these effects. Ones like ocatin, harmine, harmaline and those good old standbys - fluorescent β-carbolines.

So I did a quick bioassay, the so-called "Sandwich Method" using dried oca leaves, some agar and lettuce seeds. The dried leaf material was embedded in the agar and the lettuce seeds sown on the agar surface. Then the whole lot, including a control (no leaf material) were incubated for a few days at 20 C.

Here are the results, recorded as percentage elongation of lettuce roots (radicles) compared to the control:The results seem to indicate that oca leaf material exerts a powerfully inhbitory effect on lettuce root elongation. I'll say one thing though. I'd rather spend my time measuring lettuce radicles than trying to import Excel charts into this blogument. They say practice makes perfect, but strangely, not in my case..........

So perhaps what Ben noticed could be the result of allelochemicals leaching from the leaves or being exuded from the roots. Maybe these chemicals have an inhibitory effect on competitors or following crops. Maybe this is allelopathy. I don't know, but it is interesting and is worth serious investigation. I am available and my rates are very reasonable.

6 comments:

Anonymous said...

This could make sense, I never noticed a reduced fertility the next year, but it surprises me every year again how few weeds the oca bed has. I suspect there could be some elements (ocatin or others) in the ocas (leaves or roots) that reduce the germination rate of weeds (and other seeds). It doesn't seem to be as unfriendly as walnut trees though...
Frank

Ottawa Gardener said...

That is fascinating. And I love the fact that you have agar to play with. Good to know if I ever get a chance to grow some.

W said...

I suppose I just won't pay as much attention to crop rotation with oca, then.

IAP said...

This interests me, as I am experimenting with bi-cropping other crops with Oca. So far I have not noticed any reduction in growth of cordon tomatoes grown closely with Oca, so probably there is no root exudate. Similarly, no noticable effect on peas, and sweetcorn.
My Oca blog is here
http://oca-testbed.blogspot.com/

IAP said...

Further reading here: http://lamar.colostate.edu/~jvivanco/papers/Phytochem/2002Harsh.pdf
documents UV light-activated root exudates (those you listed in the post) located in the rhizosphere of Oca. However the suggestion is that there is little or none in the actual plant tissue (leaves, stems or roots). It is also stated that Harmine is phototoxic to various insects (particularly the Andean Tuber Worm, and the moth Trichoplusia ni [Cabbage Looper]) and who knows what others.
So... oca may protect itself from competing weeds (good!), may suppress bicropped or succeeding veg crops (bad!), and may kill or repel nasty instects (good!).
So when this is all balanced out, does it work well as a bicrop partner or not? Generations of Andean Indians settled on mixing with other root crops, or planting amongst corn, so the exudates cannot be too harmful to (at least these) other crops.

Rhizowen said...

Hi Ian

Good luck with your research and do keep me posted. Like you I believe oca has a bright future. I'm intrigued by the "thinning" idea. Intercropping seems like an excellent idea. Allelopathic effects, if any, are likely to be highly site dependent and influenced by soil nutrient status, length of adaptation of microorganisms/ competitor plants to the putative allelochemicals, water and nutrient stress etc etc. My guess is that leaves are likely to be the most potent source of allelochemicals. As oca becomes more popular, I can see a whole load of anecdotal evidence building up which will support or refute this hypothesis - should be interesting.

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