Agriculture Reference
In-Depth Information
mol CO
m
−
s
−
). The high level of respiration
late in the season was attributed to the energy needs of translocation of car-
bohydrates from the leaves to the woody parts of the trees. Dark respiration
levels are very low in the winter months, following leaf shed.
gCO
tree
−
h
−
(
at
.
.
µ
Measured effects of light and temperature
Over any short period it is expected that high light intensities will have a
positive effect on net carbon exchange, through effects on photosynthesis, and
high temperatures a negative effect, through effects on respiration.
Francesconi
et al.
(
%offullsunreducedthenet
carbon exchange rate (NCER) of potted trees of 'Gala'/'M.
)foundthatshadingto
' by about
%.
◦
C had approximately
◦
C
Trees at
.
% lower NCER than those at
.
◦
C about
and trees at
% higher NCER than the latter at around noon.
The differences in temperature had substantial effects on vapour pressure
deficit, which may have been involved in the effects on NCER. They were not
accompanied by differences in irradiation such as would frequently, though
not invariably, occur in nature in the daytime but may explain the negative
relationship between night temperature and NCE found by Wunsche and
Palmer (
.
b). They were obtained over single days.
Over longer periods of time the effects of higher temperatures may be less
negative. In a number of crops it has been found useful to divide total respi-
ration,
R
t
, into two components, maintenance (
R
m
) and synthetic respiration
(
R
s
).
R
m
is associated with the maintenance of established physiologically ac-
tive tissue and increases in proportion to the amount of tissue to be maintained.
As the metabolic rate of cells increases rapidly with temperature,
R
m
is strongly
dependent on temperature.
R
s
is associated with the synthesis of new cellu-
lar material and on a daily basis is governed by two main factors, the level
of respiratory substrate which is normally determined by the photosynthetic
rate, and the temperature, which governs the maximum rate of synthesis that
can occur. When photosynthesis is limited, e.g. by dull weather or drought,
R
s
is governed mainly by the rate of assimilate supply. Dewar
et al.
(
)have
concluded that in general over the long term, the respiration to photosynthesis
ratio is relatively insensitive to temperature, but more information is needed
for apples and pears. This is important in relation to understanding effects of
latitude, and climate in general, on orchard productivity and also the use of
shading and orchard misting treatments in areas of high insolation.
Higher temperatures in spring and autumn increase leaf area duration, with
shootsbecomingnetexportersofphotosynthatesearlierintheseason( Johnson
and Lakso,
).
Both of these effects can be expected to have positive impacts on net carbon
exchange.
) and leaf senescence being delayed (Lakso and Lenz,
