Re: [compost_tea] Exudates (was: Re: Brassicas and vam fungi)

From: <soilfoodweb_at_aol.com>
Date: Mon, 01 Nov 2004 00:34:33 -0500

Ted wrote:
I have found no one who uses any kind of root enhancement materials such as VAMs to enhance root growth and plant health but I will keep asking. 

As far as I know, there has never been any report that inoculation with VAM benefitted brassica that cannot become mycorrrhizal, and there are reports that VAM from other plants harmed some brassica species.  As with most things biological, there are exceptions to this, but I am not the best person to ask about the exceptions, because people like Mike Amaranthus, or Bob Linderman, or Mike Allen, are more up on these interactions than I am. 


Ted wrote: 
But I need something cleared up.  Are we using exudates as a term for any exuded material from the plant? 

That is the technical definition of exudate.

Ted wrote:
For example, there has been a lot of research in regards to sap rising from capillary action in the leaf vs. root pressure.  (I am oversimplifying here for brevity.) Sometimes the root pressure in certain plants becomes too much and the plants exude sap from the bark and leaf surfaces.  I am unsure if this is the same material or process involved with exudates sent out through roots and leaves that communicate with bacteria and fungi for feeding. 

No, this is not the same process.  Sap being expressed from a wound is a function of turgor pressure, I believe.  Exudates are released from roots through a different process, but we are getting out of my area of expertise in being able to tell you what exactly the differences are. 

Ted wrote:
I thought the exudates were selective and not general and were sent out in response to plant needs. 

Through many, many years of selection, plants that release exudates that enahnce the growth of beneficial organisms typically survived better, and were less stressed.  There has therefore been selection for exudate release by plants that enhance beneficials. 

Ted worte: 
Elaine:
Are you stating that the nature of any exudate (exuded material) from the plant is inhibitive because the materials that make up the plant are natural herbicide/fungicies? 

No, I am not saying that.  In the case of brassicas, SOME of them make glycosinolates which when decomposed by certain bacteria, produce compounds that are iso-thiocyanates.  Sudan grass makes a MITC, for example, so it is not just broccoli, or caulifkower, or cole crops that do this.  Again, it is possibly related to actinobacterial growth in the rhizosphere.  So, is it the plant that makes the glycosinolate, or the bacteria?  Or are the bacteria just using the glycosinolate made by the plant, and both the exudate and bacteria have to be present in order for the inh8ibitory compound to be made?  The latter seems to be the case. 

But there are many other compounds that roots release that are not inhibitory in of and by themselves.  But, do those exudates select for bacterial or fungal growth that will inhibit other organisms?  Maybe.  Do all of the exudates made select for beenficial organisms which may inhibit the growth of "bad guys".  Hum......

I feel safest in saying, it could be the case, but it doesn't HAVE TO BE the case.  Some exudates may just be neutral,m and get used by neutral bacteria or fungi.  Or maybe the bacterial or fungal species that are supposed to be present aren't, and then that exudate doesn't get used by the beneficial organism(s), and inhibition doesn't occur. 

Lots yet to know. 

--
Elaine Ingham
President, Soil Foodweb Inc.
SFI Corvallis, OR
SFI Port Jefferson, NY
SFI Lismore, NSW, Australia
SFI Cambridge, New Zealand
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Received on Mon Nov 01 2004 - 01:13:08 EST

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