Light interception vs. total net photosynthesis

[T. Hinckley]

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 
line. 

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