Agriculture Reference
In-Depth Information
). They then decline, much more steeply than is the case for other plant
parts, to less than
ng g
−
fresh wt s
−
by late September. This is followed by
the climacteric rise at maturity discussed in a later chapter. Bepete and Lakso
(
ng g
−
fresh
) found some apples to have
R
d
values of more than
weight s
−
when only about
mm diameter, falling to less than
when over
mm diameter. Indeed, maximum respiration per fruit occurred at only
mm diameter. Fruit respiration showed an exponential temperature
response under controlled conditions, the equation for the response curve
being
Y
-
.
X
) where
Y
is net carbon dioxide evolution and
X
is
the temperature of the fruit. Despite this, exposed fruits, with temperatures as
much as
=
.
(
◦
C above shaded fruits, have much lower 'respiration' as measured
by CO
output. In the morning they showed no output, the CO
presumably
being re-fixed by fruit photosynthesis, and even later in the day gave out less
than a quarter of the CO
given out by shaded fruits.
Net CO
exchange by canopies
Net carbon dioxide exchange (NCE) of canopies, measured by enclosing whole
trees in plastic chambers (cuvettes) and recording the changes in CO
between
the air pumped in and that coming out, integrates the effects of photosynthesis
and respiration of leaves, stems, buds, flowers and fruits including the ways in
which organs affect the CO
exchanges of other organs.
Canopy light response curves
Canopy net photosynthesis light response curves show lower maxima at satu-
rating light intensity than do individual well-exposed leaves or shoots (Proctor
et al.
,
). This is because whole trees or branches have more respiratory
losses from non-photosynthetic tissues and include shade-adapted leaves with
low photosynthetic potential as well as having low light intensities in parts of
the canopy.
Daily and seasonal patterns
The whole canopy carbon exchange follows the diurnal pattern of incoming
radiation,withsomeevidenceofhigherratesofnetcarbonassimilationperunit
of light in the morning than in the afternoon (Figure
). It also follows a pat-
tern that reflects changes in incident radiation and leaf area through the season
(Figure
.
).Netphotosynthesisandcarbonassimilationperunitleafareapeak
very early in the season with maximum rates of
.
µ
mol CO
m
−
s
−
.
and
.
in April at Bonn in northern Europe (Wibbe
et al.
,
), but net photosynthe-
mol CO
m
−
s
−
),
and dark respiration, although also high in mid-summer, peaked in October
gCO
tree
−
h
−
(
sis per tree peaked in August at
.
.
µ
