ECOLOGY OF PLANTS 247
associated with dormancy. He found that it or abscisic acid
increased hardiness when applied to branches after a harden-
ing period of 3 weeks at 5C. There are several cellular chemi-
cal changes associated with cold hardening (Klages, 1947);
(1) increase in bound water, (2) decrease in total water,
(3) increase in osmotic pressure, (4) change of starch to
sugars, (5) increase in pentosans, and (6) change of certain
proteins to amino acids. Nothing new has been added to this
list since Klages’ publication in 1947.
Temperature obviously affects rates of all organic pro-
cesses in plants, but a comprehensive discussion of such
effects is outside the scope of this discussion. The relationship
of temperature conditions at the geographic point of origin of
plants to the optimum temperatures for photosynthesis and
respiration in those plants is very important in explaining
their distribution. Mooney and Billings (1961) studied rela-
tionships of temperature to photosynthesis and respiration
in Oxyria digyna (alpine sorrel), a species with a circumbo-
real distribution of wide latiudinal range. The sources of the
populations studied varied from an elevation of 12,300 feet
at 39 40 N latitude to a zero elevation at 68 56 N. They
found that plants of northern populations had higher respi-
ration rates at all temperatures than did plants of southern
alpine populations. Moreover, they found that plants of north-
ern populations had higher photosynthetic rates at lower tem-
peratures and attained maximum rates at lower temperatures
than did plants of the southern alpine populations. Obviously,
there are definite temperature ecotypes in this species.
Mooney, Wright, and Strain (1964) investigated the gas
exchange capacity of plants in relation to elevational zona-
tion in the White Mts. of California. Field measurements
over an elevational span of 4000 to 14,000 feet indicated
that the ratio of photosynthesis to dark respiration decreased
with altitude primarily due to increased respiration rates with
increased elevation. When similar measurements were made
on plants grown in uniform greenhouse conditions, from
seeds collected at different elevations, no differences in res-
piration rates correlated with elevation were obtained indi-
cating considerable environmentally conditioned plasticity
in gas exchange capacity. However, the high elevation plants
generally displayed peak photosynthetic activity at temper-
atures lower than the low elevation plants even though all
were grown in the same conditions.
Mooney and West (1964) studied the photosynthetic and
respiratory acclimation of five species in the White Mts. of
California (Table 1). Seeds were collected at the indicated ele-
vations, planted in pots in the greenhouse, and after establish-
ment of the seedlings, they were taken to stations at various
elevations in the mountains for acclimation (Table 2).
They were kept in the same pots and soil and subirrigated
at each place. Water was thus not limiting and the soil type
was constant. Gas exchange measurements were made after a
minimum of 3 weeks acclimation. The dark respiration rates
of plants of all species from all stations were similar. Plants
of all species acclimated to the colder subalpine environment
were more efficient in photosynthesis at colder temperatures
than were plants grown at the warmer environment of the
lowest elevation. Conversely, plants grown in the desert weremore efficient in photosynthesis at high temperatures than
were plants grown in the subalpine region. Plants of the two
species with the widest natural distributional range in eleva-
tion in the White Mts. showed the greatest plasticity in their
photosynthetic response as indicated by shifts in the optimum
photosynthetic temperatures dependent on treatment.
Miller (1960) compared the effects of temperature on
the photosynthetic rate in Agrostis palustris (creeping bent)
which is adapted to cooler regions of the US, and Cynodon
dactylon (bermuda grass) which is adapted to the warmer
regions. The relative rate of apparent photosynthesis in
creeping bent increased from 64.6% at 15C to a maximum
of 100% at 25C, then decreased to 62.2% at 40C. On the
other hand, the relative rate of apparent photosynthesis in
bermuda grass increased form 54.9% at 15C to a maximum
of 100% at 35C, and only dropped to 97.7% at 40C.
In summary, it is evident that plants vary genetically in
their photosynthetic and respiratory responses to temperature
changes, and they vary also in such responses depending on
the temperature conditions during acclimation.EFFECTS OF LIGHTAccording to Johnson (1954) solar radiation at the outer
surface of the earth’s atmosphere has a virtually constant
intensity of 1400 watts m^2 or 2cal cm^2 min^1. About 98%
of the energy is in the wavelength interval of 0.2–4.5 ,
including about 40–45% in the visible range of 0.4–0.7 .
About one-third of the energy is reflected back to space by
the earth’s atmosphere and smaller amounts are absorbedTABLE 1
Natural distribution of experimental plantsSpeciesElev. range
(m)Elev. origin of
seeds used (m)Artemisia tridentata 1829–3200 3078
Chamaebatiaria millefolium 1980–3200 3200
Haplopappus apargioides 3048–3962 3658
Artemisia arbuscula 3200–3719 3475
Encelia virginensis ssp. actoni 1372–1829 1402TABLE 2
Acclimation stationsMean air temp. Ca
Natural vegetation Elev. (m) Max. Min.Desert scrub 1402 38.9 11.7
Pinyon woodland 2408 33.4 10.3
Subalpine forest 3094 19.5 –2.4
a Based on weekly maximum and minimum temperatures during period of
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