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

This function is asymptotic to zero respiration
at both low and high temperatures an d
exhibits an exponential increase in respiratio n
followed by a rapid decline at high
temperatures .
The completed functional expression fo r
Psn(L,T) is :
Psn(L,T) = -Boexp[B 1 (T-B2 )-exp(B 1 (T-B2 ))]
+ [ B 2 + B3 ( T -B4 ) 2 ] [1 - exp(B s L ) ]
(4)
The intent of this model is to predict ne t
photosynthesis in light and CO 2 losses in
darkness. The model is not useful for predicting
gross photosynthesis without including
some function for respiration in the light .
Until the experimental techniques for obtaining
such measurements are developed, th e
light respiration term must remain implicit i n
the gross photosynthesis function . This limit s
the use of the model although most C 3 plant s
will respond similarly . A different model may
be required for C 4 plants such as grasses that
require a much higher light level to saturat e
photosynthesis .
Methods and Materials
Values for the six parameters in the model
were determined with a least squares fit of th e
data using an algorithm developed by Marquardt
(1963) . Net photosynthetic data were
taken with a controlled environment syste m
developed by Webb (1971) . The test organis m
for the model was red alder (Alnus rubra
Bong.) .
Forty red alder seedlings, 1- to 2-years-old ,
were removed from the field and propagated
in a nutrient culture before transferring them
to the gas-tight controlled environment chamber.
The root system of the seedlings was en -
closed in a nutrient flow system with temperature
controlled at 11°C ± 1°C . The plants
were maintained in this system for 1 month at
conditions of light and temperature consisten t
with those in the greenhouse .
Carbon dioxide absorption rates were measured
by monitoring the CO 2 depletion in the
gas-tight environment chamber with a Beckman
IR gas analyzer. Atmospheric CO 2 levels
in the chamber were maintained between 32 0
and 335 p .p.m. Light energy was varied from
0.06 to 0 .68 ly/min (total shortwave radiation)
at air temperatures from 5 to 30°C . For
each of five light levels, CO 2 uptake was
measured while temperature was changed a t
the rate of 1° per 5 minutes beginning at
15°C and proceeding upscale to approximately
30°C . Measurements were then made
while decreasing temperature at the same rat e
until near 5°C at which time temperature wa s
increased again up to 16°C . All CO 2 uptak e
measurements were made during 3 successiv e
days. Relative humidity varied between 6 5
percent at low temperatures to 75 percent at
high temperatures .
Results
A portion of the data is plotted in figures 3
and 4 . The photosynthetic response to radiation
in figure 3 is characterized by a linea r
response at low light followed by the usual
light-saturated response at high radiation . Thi s
is consistent with the findings of many other
investigators . The saturation value for ne t
photosynthesis increases with temperature ,
but the increase is not linear .
Figure 4 shows the net photosynthetic
response to temperature for constant radiation
of 0 .19 ly/min . A quadratic function wa s
fit to the data and the parameters with their
respective standard errors are listed in figure
4 . At 0.19 ly/min, net photosynthesis has a
maximum of 20 .5°C. Although these data
represent the temperature range between 5 .5 °
and 27°, the data of Pisek et al . (1969) indicate
a nearly symmetrical response betwee n
5° and 40°C. Extrapolation of the quadratic
beyond this temperature range should b e
done cautiously although Phillips an d
McWilliams (1971) have measured zero CO 2
fluxes at high temperatures . It may be that
high temperatures increase respiration until it
exceeds the photosynthetic capacity .
Figure 5 illustrates the response surface
generated by expression (4) which is the net
239