Agriculture Reference
In-Depth Information
20
15
10
5
Drought
Crop load
0
0
100
Leaf conductance (mmol m
−
2
s
−
1
)
200
300
400
500
Figure 8.1
The relationship between photosynthesis and leaf
conductance in separate studies of drought stress and of varying
crop loads with 'Empire'/'M.9' apple trees in the field. From Lakso
(1994). Reproduced with permission.
with the isolation from apple fruits of a sorbitol dehydrogenase that oxidizes
sorbitol to fructose.
Traditionally, leaf photosynthetic rate (
A
) was expressed as the mass of
CO
fixed per unit of leaf area per unit of time (mg CO
dm
−
h
−
). More
recently it has been expressed in molar units (
mol m
−
s
−
) with an approx-
µ
mg CO
dm
−
h
−
= µ
moles m
−
s
−
(Flore
imate conversion of
.
×
and Lakso,
). Photosynthesis responds to light in the visible wavelengths
(
% of total solar radiation, with some
variation depending on cloudiness and solar position relative to the earth.
This is measured in W m
−
or
-
nm), which makes up about
Em
−
s
−
. Commonly photosynthetically
active radiation (PAR) or photosynthetic photon flux(PPF) within or below the
canopy is expressed as a percentage of above-canopy irradiance, but it must
be remembered that the latter varies from site to site and through the season.
µ
Stomatal conductance and photosynthesis
Clearly the rate of entry of CO
into the leaf is potentially a major limit-
ing factor to photosynthesis. Effects of both environmental and within-plant
factors on photosynthesis may therefore be mediated by effects on stomatal
conductance, measured as leaf conductance or canopy conductance. Stomata
and stomatal aperture control are discussed in Chapter
.
There is frequently a close relationship between stomatal conductance and
net photosynthesis of apple leaves (Palmer,
and Chapter
; Lakso,
(cf. Figure
.
);
Giuliani
et al
.,
b). The pattern of causality in this relationship may be