Light interception vs. total net photosynthesis
hinckley at u.washington.edu
Thu Oct 12 14:54:18 EST 1995
There are clear advantages of using the APAR approach;
however, from a physiological perspective, it is merely correlation and
not mechanism. The inclusion of the transpiration analog was merely to
demonstrate the weakness of the APAR or IPAR approach. The fact that
each site, each genotype may have its own unique line (APAR vs.
productivity) and that the line may fall apart several years after canopy
closure all suggest weaknesses. A strong correlation does not result in
causation although there is a potential powerful relationship between
absorbing light and the production of sugars. Where the sugars go
becomes more and more important as the organism/stand develops. We see
this with the nice association between 1 and 2 year productivities and
sylleptic branches. We know alot about sylleptic branches and some of
the phenological and translocational advantages they confer; however, by
age 3 or 4, they are probably no longer important and now proleptic
branches become increasing important and then short shoots vs. long
shoots and preformed leaves vs. neoformed leaves. Each of these
developmental changes means a new slope to the APAR vs. productivity
On Wed, 4 Oct 1995, 'Toby' H D Bradshaw wrote:
> It seems to me that advantage of assuming the APAR (absorbed PAR?)
> vs. growth correlation is that absorption can be modeled and used to
> predict performance, and this prediction can be tested experimentally.
> Is the same true of (for example) transpiration?
> If we did construct an ideotype for maximum transpiration, what parameters
> would be needed to make a model? For the light interception model it
> seems we need to know things like leaf area, orientation, branch length,
> internode length, latitudes, etc. Would a transpiration model be simpler?
> The light interception model also makes no attempt to maximize
> photosynthesis (in its simplest incarnation, anyhow); would a "maximum
> transpiration" model try to account for differences in water use
> efficiency among clones, or would the major effects be things like
> stomatal response, ambient humidity, temperature, and omega?
> -Toby Bradshaw
> toby at u.washington.edu
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