site on C-3 & C-4 respiration.
http://www.ultranet.com/~jkimball/BiologyPages/C/C4plants.html
The basics: onC4 Plants
Several groups of angiosperms have developed adaptations which minimize the losses to photorespiration. They all use a supplementary method of CO2 uptake which forms a 4-carbon molecule instead of the two 3-carbon molecules of the Calvin cycle. Hence these plants are called C4 plants. (Plants that have only the Calvin cycle are thus
C3 plants.)
Many grasses/ C4 plants have structural changes in their leaf anatomy so that
their C4 and C3 pathways are separated in different parts of the leaf with
RUBISCO sequestered where the CO2 level is high; the O2 level low
The details of the C4 cycle
After entering through stomata, CO2 diffuses
into a mesophyll cell.
Being close to the leaf surface, these
cells are exposed to high levels of O2,
but have no RUBISCO so cannot start
photorespiration (nor the dark
reactions of the Calvin cycle).
Instead the CO2 is inserted into a 3-carbon
compound (C3) called
phosphoenolpyruvic acid (PEP) forming
the 4-carbon compound oxaloacetic acid
(C4).
Oxaloacetic acid is converted into malic acid or
aspartic acid (both have 4 carbons), which is
transported (by plasmodesmata) into a bundle
sheath cell. Bundle sheath cells
are deep in the leaf so atmospheric
oxygen cannot diffuse easily to them.
often have thylakoids with reduced
photosystem II complexes (the one
that produces O2).
Both of these features keep oxygen
levels low.
Here the 4-carbon compound is broken down into
carbon dioxide, which enters the Calvin cycle to form sugars and starch.
pyruvic acid (C3) - which is transported back to a mesophyll cell where it is converted back into
PEP.
These C4 plants are well adapted to (and likely to be found in) habitats with
high daytime temperatures
intense sunlight.
Some examples:
crabgrass
corn (maize)
sugarcane
sorghum
--------
so its the daytime nighttime respiration shift that I was considering in the Q's.
Thanks for the A's
In Light & Love
Markess
From: Hugh Lovel <[EMAIL PROTECTED]>
Reply-To: [EMAIL PROTECTED]
Date: Thu, 3 Jan 2002 10:59:01 -0500
To: [EMAIL PROTECTED]
Subject: Re: Plant brix testing
Dear Hugh,
thanks for weighing in on this discussion.
Several Q's.
Does this rhythm, release of dissolve solids (sugars & amino acids) apply to all families of plants? i.e. do broccoli or potatoes, say, shed carbon from their roots?
Dear Mark,
In general I would say yes. Definitely broccoli and potatoes shed carbon. Different plants shed a different mix. Corn, for example, sheds a clear, jelly-like mix rich in polysaccarides, which are polymer (chain) sugars. Lettuce, which can be a vigorous carbon shedder, gives off a milky latex. Buckwheat sheds a mix that really stimulates the microorganisms that make phosphorous available, while hemp gives off a mix that stimulates those microbes that make potash available. Potatoes, need plenty of available potash but don't give off the rights stuff to make it available in a soil that is low in accessible potash. So potatoes give a much better yield following hemp--if we could only grow it in the US! But from this I guess you'll see some of the major reasons for crop rotations, cover cropping and the like.
What are the ramifications in term of C4 synthesis, in other words do C4 plants constantly hold their Brix high under high light & temperatures?
While I've heard of C4 synthesis I'm not clear at the moment on this terminology. Could you bring me up to speed?
What portion of the plant do you sample?
Any portion I choose (generally not the root) depending on what I'm looking for. Usually I would sample where I think the best photosynthesis is occurring or where I might expect sugars to be most concentrated at any particular time of day and/or growth cycle.
Are these principles continuous into flowering and fruits periods?
In general where sugars are being concentrated in fruit the plant is NOT sending these to the roots. That's only at the end of the growth cycle, and by then they aren't needed in the root zone anymore anyway. This is an example of sampling where I'd expect sugars to concentrate.
I have always read that Boron is toxic to many crops but "puts weight into hay"?
Boron tends to put weight in anything from apples and pears to radishes and beets. Without boron calcium (there's the weight) is not fully accessed by the plant. And, yes, rare though it seems to be, boron may be toxic. This is mostly a western problem where rainfall is sparse. Boron is one of the most soluble elements in the soil, and may leach easily if not tied up in living organisms. A good case in point is alfalfa, which is particularly boron sensitive. When deficient its leaves will be deformed, when toxic it's leaves will discolor.
In general one does not want nutrients to be soluble in the soil. That is a terrible misconception promulgated by the fertilizer industry. What you want is insoluble but available nutrients--which generally means keeping the microlife of the soil robust and healthy. I would call this a no-brainer, but most of modern, university level agriculture is looking for nutrient solubility in their soil tests and everywhere else. Tch, tch!
Hugh
Thanks
In Love & Light
Markess
