FORTY YEARS OF GENECOLOGY
235
extracted from the chambers as required for the measurement of photosynthesis and respiration.
From the many data presented by Mooney and Billings, a selection
may be considered to illustrate some of their principal conclusions.
(a) Plants of northern populations have a higher photosynthetic rate
at lower temperatures and attain a maximum rate at lower temperatures
than do plants of the southern alpine populations, and,
( b ) plants of northern populations have higher respiration rates a t all
temperatures than do plants of the southern alpine populations.
The evidence is contained in Fig. 5, showing pooled results from three
southern alpine and two arctic population samples. The alpine group
shows a temperature optimum for photosynthesis at the light intensity
used of 30-40" C, and the arctic group, an optimum around 15-19" C, the
maximum rates achieved being much the same. The respiration rate of
the northern group is not only generally higher but shows a steeper rise
with temperature-than that for the southern group, indicating a lower
temperature compensating point for the arctic populations.
(c) High-elevation, low-latitude plants attain photosynthetic light
saturation a t a higher light intensity than do low-elevation, highlatitude plants. This is evident from Pig. 6, in which rate intensity
curves for a southern alpine and a northern population are compared at
20" C. It is notable that the southern, high-altitude population did not
achieve complete light saturation even at 5 200 f.c, whilst the arctic
population is saturated at 2 000 f.c.
(d) There is a clinal increase in the photoperiodic requirement for
flowering from southern to northern populations. Data illustrating this
for four populations from different latitudes are summarized in Table
.
I
I
I
t
1
-
-
Y
2
I
I
I
I
I
I
I
500 1000 1500 2000 3000 4000 5000
Foot candles
FIG. 6. Photosynthetic light curves at 20" C for a southern alpine population of Ozyn'a
d i g y w [latitude 39" 40' N] and a northern population [61° 23' N]. From Mooney and
Billings, 1961.
southern alpine population; - - - - - - northern population.
235
extracted from the chambers as required for the measurement of photosynthesis and respiration.
From the many data presented by Mooney and Billings, a selection
may be considered to illustrate some of their principal conclusions.
(a) Plants of northern populations have a higher photosynthetic rate
at lower temperatures and attain a maximum rate at lower temperatures
than do plants of the southern alpine populations, and,
( b ) plants of northern populations have higher respiration rates a t all
temperatures than do plants of the southern alpine populations.
The evidence is contained in Fig. 5, showing pooled results from three
southern alpine and two arctic population samples. The alpine group
shows a temperature optimum for photosynthesis at the light intensity
used of 30-40" C, and the arctic group, an optimum around 15-19" C, the
maximum rates achieved being much the same. The respiration rate of
the northern group is not only generally higher but shows a steeper rise
with temperature-than that for the southern group, indicating a lower
temperature compensating point for the arctic populations.
(c) High-elevation, low-latitude plants attain photosynthetic light
saturation a t a higher light intensity than do low-elevation, highlatitude plants. This is evident from Pig. 6, in which rate intensity
curves for a southern alpine and a northern population are compared at
20" C. It is notable that the southern, high-altitude population did not
achieve complete light saturation even at 5 200 f.c, whilst the arctic
population is saturated at 2 000 f.c.
(d) There is a clinal increase in the photoperiodic requirement for
flowering from southern to northern populations. Data illustrating this
for four populations from different latitudes are summarized in Table
.
I
I
I
t
1
-
-
Y
2
I
I
I
I
I
I
I
500 1000 1500 2000 3000 4000 5000
Foot candles
FIG. 6. Photosynthetic light curves at 20" C for a southern alpine population of Ozyn'a
d i g y w [latitude 39" 40' N] and a northern population [61° 23' N]. From Mooney and
Billings, 1961.
southern alpine population; - - - - - - northern population.