Agriculture Reference
In-Depth Information
Photosynthetic Functionality in Mature Leaves
Once a leaf has attained full photosynthetic function, various factors constrain its
performing to full capacity at all times. The overall situation is illustrated by a
diurnal and seasonal record of photosynthesis in a Mediterranean shrub,
Phlomis
fruticosa
(Fig.
2.8
). The light reactions of photosynthesis obviously are precluded
at night, and the diurnal trace of photosynthesis generally is in proportion to insola-
tion from dawn to dusk if other conditions are favorable. In this evergreen
Mediterranean shrub, photosynthesis is low during the summer dry season and rela-
tively high in winter and spring when water is more available. Leaves function at
their maximum photosynthetic capacity (
A
max
) only near midday in early June, falling
well below their photosynthetic potential throughout midsummer and early fall. In
the transition from late spring to early summer as soil water supplies diminish and
atmospheric vapor pressure deficits increase, first midday and then late afternoon
photosynthesis is depressed despite high levels of insolation (Kyparissis et al.
1997). Such midday depression of photosynthesis in response to limited water
supplies is well known in species from temperate (Ishida and Tani 2003), tropical
(Zots and Winter 1994, 1996; Zots et al. 1995; Ishida et al. 1999), and even arctic
(Gebauer et al. 1998) climates. These and innumerable other examples document
the fact that over their lifetime leaves do not work to their full instantaneous
photosynthetic capacity,
A
max
.
Acknowledging this reality, Kikuzawa introduced the concept of the mean labor
time of a leaf, the cumulative amount of photosynthesis achieved by a leaf over its
lifetime compared to the potential value if a leaf were able to work to its full capacity
25
20
15
10
5
20
0
16
12
8
Fig. 2.8
Diurnal and seasonal record of photosynthesis for mature leaves of an evergreen
Mediterranean shrub,
Phlomis fruticosa
, growing at low elevation. The leaves were produced in
April-May 1992 and measured from June 1992 to April 1993, just before this leaf cohort began
to senesce and abscise. (From Kyparissis et al. 1997)
