Geoscience Reference
In-Depth Information
overlap in time such that the short-term processes usually
constitute the initial responses to a stress while the long-term
processes are usually detected later in the acclimation process.
Long-term response to changes in light, water availability and
temperature that are part of the acclimation process results in
phenotypic alteration.
photoacclimation
The process whereby adjustments are made to the structure and
function of the photosynthetic apparatus in response to changes
in growth irradiance is called photoacclimation. One conse-
quence of photoacclimation is a change in pigment composition
which results in an altered visible phenotype. It is important to
know that photoacclimation requires growth and development.
For example, photoautotroph grown under high light typically
exhibit a decrease in total chlorophyll per leaf area compared
with the same plants grown at low irradiance. Thus, the leaves
of high-light plants are usually a pale green or yellow green
compared with a dark green phenotype of the same species
grown at low light. Functionally, high-light plants exhibits a
photosynthetic light response curve for CO
2
assimilation that
is distinct from plants grown under low light, when measured
as net photosynthesis. Typically, plants grown under high light
have a higher photosynthetic capacity, that is, a higher light-sat-
urated rate of photosynthesis than low-light plants. In contrast,
high-light plants may have a lower initial slope, compared with
the same plants grown at low light.
Gene
expression is
regulated by
light
Many green algae may exhibit an even more dramatic change in
phenotype in response to growth at either high or low light than
terrestrial plants. Experiments utilising single-cell green algal
species such as
Dunaliella tertiolecta
and
Chlorella vulgaris
indicate that the light-dependent change in the content of LHCII
(light-harvesting complex II) is modulated in response to the
redox state of the plastoquinone pool. Photosynthetic electron
transport can be inhibited specifically at the
Cyt b
6
lf
complex
with a compound called DBMIB (2,5-dibromo-6-isopropyl-3-
methyl-1,4-benzyquinone). In the presence of this compound,
there is a net accumulation of PQH
2
because, although PSII
is able to convert PQ to PQH
2
, PSI cannot oxidise this pool
because of the chemical block at the condition, the PQ pool
remains largely reduced and transcription of the nuclear
Lbcb
genes coding for the major LHCII polypeptides is repressed.
This results in an inhibition of the biosynthesis of LHCII poly-
peptides, which decreases the LHCII polypeptide content.
As a consequence, this results in yellow phenotype typical of
