PE EIE[R-Rg RESEARCH ON - HJ Andrews Experimental Forest

Proceedings-Research on Coniferous Forest Ecosystems-A symposium .
Bellingham, WashingtonMarch 23-24, 197 2
A model of light and temperature
controlled net photosynthetic rates
for terrestrial plants
Warren L . Web b
Forest Research Laboratory
School of Forestr y
Oregon State University
Corvallis, Orego n
Abstract
Steady-state relative net CO2 exchange was modeled in terms of a temperature-dependent respiration function
and light- and temperature-dependent photosynthesis function. The parameters of the model were
evaluated using the laboratory CO 2 exchange data of a group of 40 red alder seedlings (Alnus rubra Bong.). Th e
model is continuous and well-behaved in the temperature region of 0-50°C for light energy between 0 .0 and 1 . 0
ly/min total short-wave radiation.
Introduction
Consumer populations depend upon the
chemical energy that is converted from sola r
energy by plants in the ecosystem . The rate of
conversion, or net photosynthesis, is in tur n
dependent upon the genetic information avail -
able to each plant and its immediate environment.
Many factors influence net photosynthesis
but, as Schulze (1970) found from hi s
work in a beech stand, when water stress is
not appreciable, the radiation and temperature
regimes of the plant largely regulate net
photosynthesis . This paper presents an empirical
model of steady-state net CO 2 exchange i n
terms of a light (L) and temperature (T) con -
trolled gross photosynthesis function (Ps) an d
a temperature-controlled dark respiration (Rs )
function .
Net CO 2 flux entering the leaf, or ne t
photosynthesis (Psn), is conceptualized as th e
difference between carbon fixed in the photo -
synthetic process and that lost during respiration,
Psn = Ps - Rs (Larcher 1969) . The two
terms, net photosynthesis and net CO2 exchange,
are used synonymously in this paper .
Although the former is less general in that it
usually applies only to CO 2 exchanges occurring
in the light, it is more mechanically
viable .
Fluxes of CO 2 are easily measured and
many investigators have reported on the ne t
photosynthetic response of plants to temperature
and light (Heath 1969, Rabinowitc h
1969, Milner and Hiesey 1969) . Figure 1 illustrates
the net photosynthetic response of
small plants or individual leaves to increasin g
temperature or light . The linear portion of the
light curve represents the rate of the photo -
chemical reaction at the chloroplast and is
largely invariant among species (Rabinowitc h
1969) . This linear slope can be extrapolated
through zero CO2 flux, called the light compensation
point, and the negative absciss a
intercept interpreted as dark respiration
(Chartier 1969) . At high light levels, photo -
synthesis becomes saturated and its rate i s
dependent upon factors such as temperatur e
237