Agriculture Reference
In-Depth Information
Although stomata are closed in the dark they open fully at light levels well
below photosynthetic light saturation and the photosynthesis light response
curve does not reflect changes in stomatal conductance (Kriedemann and
Canterford,
).
When solar irradiance and air temperatures are very high, shading may
actually increase photosynthesis. Bravdo (
; Lakso,
), working in Israel, found that
when the maximum radiation flux density at noon was
Wm
−
or
µ
E
m
−
s
−
PAR shading by
◦
C,
reduced stomatal resistance to about a third and increased photosynthesis by
up to around
◦
Cto
% reduced leaf temperatures from
times. This shade effect should be regarded as overcoming
temperature and water stress effects rather than indicating supraoptimal PAR
levels.
The net photosynthesis of
Pyrus communis
cultivars at saturating light levels
is similar to that of apples: Kriedemann and Canterford (
.
) found 'Bartlett'
Wm
−
of PAR
under the conditions of the experiment, at which net photosynthesis of single
leaves was approximately
leaves to be light saturated at a light intensity equivalent to
mg CO
dm
−
h
−
. The maximum rate in the
mg CO
dm
−
h
−
, achieved early in the day with
solar radiation at approximately
orchard was around
-
Wm
−
and PAR about half of this. The
Wm
−
of PAR).
Several studies have shown
P. serotina
to have relatively low maximum photo-
synthetic rates. Honjo
et al.
(
light compensation point was
fc (
) found net photosynthesis to increase slowly
mol m
−
s
−
under conditions where
P. communis
cv. 'Bartlett' reached a near maximum of
mol m
−
s
−
at PPF
to
.
µ
=
µ
µ
mol m
−
s
−
at
.
mol m
−
s
−
. Higgins
et al.
(
PPF
=
µ
) also found the light compen-
sation point of the Asian pear
P. serotina
cv. '
th Century' was, at
µ
mol
quanta m
−
s
−
, about half that of the apple cv. 'Granspur'.
The typical rates for apple and European pear at saturating light intensity
(
A
max
) are comparable to those of
Prunus
species and grapes but higher than
those of citrus (Flore and Lakso,
). Comparisons of individual
published rates can be difficult to interpret because of the effects of a large
number of plant factors, e.g. fruiting and leaf type, on
A
max
and the differing ef-
fects of e.g. temperature on
A
max
and of leaf-to-air vapour pressure differences
on stomatal conductance and net photosynthesis of different species (Higgins
et al.
,
; Lakso,
).
Conversion of light energy to dry matter by apple trees, at
gMJ
−
of
PAR, is slightly higher than that calculated for forest stands (Palmer,
.
).
Effects of temperature
The response curves of apple net photosynthesis to temperature show broad
optima over the range
◦
C (Lakso
et al.
,
◦
C (Watson
et al.
,
-
),
-
