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

The important interaction between light
and temperature can best be shown by numerical
example . Table 1 shows the predicted
temperature of maximum net photosynthesis ,
or optimum temperature, and the peaks of
net photosynthesis for each of five levels of
light. Both the optimum temperature and th e
peaks of net photosynthesis increase with
light. The latter is exemplary of photosynthetic
light saturation .
Table 1. Predicted effect of light on
maximum net photosynthesis
Light Relative Temperature of
ly/min maximum Psn maximum Psn
Figure 5 . Response surface of net CO 2 exchange generated
by Psn(L,T) = -513.3 exp[ .088(T-47.7) -
exp .088(T-47.71] + [187 .3 - .105(T-41 .41 2 1
[1 - exp(-9.59L)] .
photosynthetic response to both light an d
temperature . The functional form of the surface
is included in figure 5 along with th e
values of the 6 parameters obtained from a
least-squares fit of the CO 2 exchange data.
The average deviation of the function fro m
the data was ±9 .1-pct . and the r 2 was 0 .97 .
Note that the light saturation phenomeno n
of photosynthesis is conserved in this model
as well as the symmetrical response to temperature.
Although there are no data below 5 °
and above 30°, the model seems well-behave d
in the regions outside the data. As light decreases
the gross photosynthesis functio n
tends toward 0, and the dark respiration function
begins to dominate . Below the light
compensation point CO 2 fluxes become negative.
At zero light only respiration occurs, an d
because the function goes asymptotically t o
zero at both low and high temperatures, the
model never predicts an uptake of CO 2 in
darkness .
0 .07 27 .8 1 6
.15 65 .5 2 0
.19 76 .2 2 1
.25 90 .8 2 2
.68 100 .0 23
The increased temperature of maximu m
net photosynthesis associated with additiona l
light can be explained as an interaction between
the gross photosynthesis function an d
dark respiration . At low light, photorespiration
is minimal and photosynthesis is largely
independent of the temperature increases that
step-up dark respiration . Therefore, dark
respiratory CO 2 losses from cellular maintenance
are not compensated by photosynthesis
as temperatures rise and maximum net photo -
synthesis (photosnythesis minus respiration )
occurs at a relatively low temperature . As
light increases, photosynthesis and its depend -
ence on temperature increases . This allows
photosynthesis to keep pace with temperature-controlled
dark respiration and maxi -
mum net photosynthesis then shifts to a
higher temperature .
Although the model is empirical, it has
several noteworthy features . The model is
continuous over a wide range of independent
variables and is therefore more mathemati-
24 1