Agriculture Reference
In-Depth Information
30
20
10
0
−
5
500
1000
PPFD (
µ
mol m
−
2
s
−
1
)
1500
2000
Figure 8.2
Light response curves of mid-shoot leaves from young
'Fuji' apple trees grown under full daylight (
—
),52.9% daylight
(
—
),34.2% daylight (
—
),15.8% daylight (
—
) and 5.0%
daylight (
•
•
). From Asada and Ogasawara (1998). Reproduced
with permission. Under natural light conditions 2000
µ
mol m
−
2
s
−
1
PAR corresponds to
c.
1000 W m
−
2
of total radiation measured by
radiometer or 100 Klux measured with a light meter.
—
complex. 'Sun' leaves in exposed (exterior) canopy positions have higher net
photosynthesis per unit light and higher stomatal conductance (Campbell
et al.
,
), but also have numerous other adaptations leading to higher photosyn-
thetic potential. Humidity may control stomatal conductance directly (see
Chapter
),hencephotosynthesis,andinothercasesphotosynthesis
appears to control stomatal behaviour rather than vice versa (Lakso,
,pp.
-
).
Light response curves
Apple leaf photosynthesis is of the C
type with a hyperbolic light response that
typically saturates at
mol quanta m
−
−
s
(Figure
). The light
compensation point, i.e. the light level below which net CO
exchange is nega-
tive,withrespirationexceedingphotosynthesis,is
-
µ
.
molquantam
−
s
−
.
Good rates of photosynthesis per unit leaf area for healthy exposed leaves are
around
-
µ
µ
mol CO
m
−
s
−
(Lakso,
) although much higher rates
mg CO
dm
−
h
−
by Avery (
have been recorded, e.g. over
), about
mg CO
dm
−
h
−
by Bravdo (
mg CO
dm
−
h
−
by
) and
-
Looney (
).
