Agriculture Reference
In-Depth Information
Box 3.5
Photoinhibition
The photosynthetic systems in leaves have two basic components, one utilizing
chlorophyll and various accessory pigments to capture solar energy, and the
other a series of biochemical pathways that uses the captured energy to build
carbohydrates with carbon derived from atmospheric carbon dioxide. When a
leaf is constructed, these two systems are created in ways suited to the envi-
ronmental regime in which the leaf will function as a photosynthetic organ.
Photoinhibition arises when transient environmental conditions lead to more
solar energy being captured than can be utilized in the biosynthetic reactions.
For example, this can occur in winter for evergreen shrubs in the forest under-
story when photosynthetic enzymes are inactive consequent to low tempera-
ture, but high light levels occur in the usually shaded forest understory because
of leaffall in a deciduous forest canopy (Miyazawa and Kikuzawa 2004, 2006;
Miyazawa et al. 2007).
Revisiting the Basic Concept of Leaf Longevity
In ending this chapter, we return to the concept of leaf longevity itself, which loses
its close functional connection to photosynthesis when leaves survive periods unfa-
vorable to photosynthetic activity such as winter in high latitudes or periods of severe
drought. Considering seasonal variation in conditions favorable to photosynthesis,
we have proposed a concept of functional leaf longevity (Kikuzawa and Lechowicz
2006). Functional leaf longevity is the number of days when a leaf can actually carry
out photosynthesis over its lifetime. In principle, functional leaf longevity is defined
as leaf longevity minus unfavorable days (winter or dry season) during the leaf life-
time. In leaves of deciduous trees or annuals in temperate regions, functional leaf
longevity is generally the same as leaf longevity. In other instances, a favorable
period within a year can be unambiguously defined and recognized. This is the
case for arctic and alpine species associated with snowbeds; the period when plants
are snow covered is considered to be unfavorable for photosynthesis, although some
light penetrates snow to about 30 cm (Starr and Oberbauer 2003). For example,
Kudo (1992) examined the effect of differences in favorable period created naturally
by the timing of snowmelt on the leaf longevity of dwarf evergreen and summer-
green plants on Mt. Daisetsu, central Hokkaido. The snow-free period varied two-
fold, from 60 to 120 days year
−1
, depending on topographically induced variation in
snow depth. In the case of other evergreen species, often it is not as easy to evaluate
functional leaf longevity because some evergreen leaves do photosynthesize during
winter. For example, understory evergreen plants in winter may suffer photoinhibi-
tion (Miyazawa et al. 2007) but still are photosynthetically active in what might at
first be considered an unfavorable season.
Camellia japonica
, an understory ever-
green tree in the deciduous forests of central Japan, actually has higher daily photo-
synthesis in winter than summer when the deciduous canopy is leafless (Miyazawa
