Agriculture Reference
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all the time (Kikuzawa et al. 2004). Mean labor time provides a complement to the
use of
A
max
as a cardinal trait characterizing variation in leaf function. It is essentially
a single, summary variable that subsumes all the environmental and ontogenetic fac-
tors that can reduce photosynthesis below its maximum value over the lifetime of a
leaf. Mean labor time (
m
) expressed as an average per day is defined by
m GG
=
24
/
(2.1)
a
h
where
G
h
is a hypothetical lifetime photosynthetic rate of a leaf, assuming that the
leaf works 24 h at
A
max
throughout its lifetime;
G
a
is the actual photosynthetic rate
of the leaf throughout its lifetime. This definitive equation can be decomposed into
terms representing the various factors that lead to photosynthetic performance
below full capacity:
24
G GG
G
G
pclear
p
pL
m
=
24
a
=
a
(2.2)
G GG GG
h
h
pclear
p
pL
where
G
pclear
is the lifetime carbon gain of a single leaf, supposing that every day
through its life is a clear day. Even if a day is cloudless, the solar angle changes
with time of day, hence the leaf still cannot attain maximum photosynthetic rate
throughout the day; this ratio of
G
pclear
and
G
h
is designated the diel effect. The
term
G
p
represents the lifetime carbon gain under actual weather conditions.
There are cloudy days and rainy days over the lifetime of a leaf when insolation
is reduced compared to a clear sky condition and the photosynthetic rate is
depressed; this ratio of
G
p
and
G
pclear
is designated the overcast effect. The term
G
pL
represents the carbon gain by a leaf under realized insolation over its life-
time, including the effects of shading by surrounding plants and self-shading of
leaves within the plant canopy; this ratio of
G
pL
and
G
p
is designated the shading
effect. The final term is the ratio of actual photosynthesis of a leaf over its life-
time and the potential photosynthetic rate under its realized insolation regime.
The ratio of
G
a
and
G
pL
represents the influence of environmental factors other
than insolation that suppress, such as the midday depression resulting from
water balance limitations or the effects of suboptimal temperatures for maxi-
mum photosynthetic gains. This ratio of
G
a
and
G
pL
is designated the depression
effect. The mean labor time of leaves of
Alnus sieboldiana
was calculated to be
around only 5 h per day on average over their lifetime (Kikuzawa et al. 2004).
Estimates for herbaceous and woody species derived by various methods are
similarly low: for a
Cecropia
species, only 1.0 h day
−1
;
Cleyela
, 1.1 h day
−1
;
Castilla
, 1.5 h day
−1
;
Annona
, 1.9 h day
−1
;
Urera
, 2.5 h day
−1
;
Helocarpus
,
2.6 h day
−1
;
Polygtonatum
, 2.7 h day
−1
;
Fagus
, 2.8 h day
−1
;
Polygonum
, 3.3 h day
−1
;
Antirrhoea
, 3.5 h day
−1
;
Anacardium
, 4.5 h day
−1
; and
Luehea
, 6.1 h day
−1
(calcu-
lated from Kikuzawa et al. 2009; Kitajima et al. 1997, 2002; Ackerly and Bazzaz
1995; Kikuzawa, unpublished data). The average of all these values is 2.9 h day
−1
,
which raises some questions about the use of
A
max
alone as a cardinal value for
characterizing foliar function.
