Agriculture Reference
In-Depth Information
6.4
Photomorphogenesis
may adopt different forms depending on the
type of radiation received.
Under the influence of red light (650 nm),
phytochrome (P) adopts the form (P
FR
),
whereas if it is illuminated with far red light
(725 nm), it adopts the form (P
R
) (Whatley
and Whatley, 1984) (Fig. 6.8).
The quantity of phytochrome present
in the plant in the form of P
FR
is expressed
by the relation P
FR
/P
TOTAL
. To produce a cer-
tain morphogenic or biochemical effect a
certain value of P
FR
/P
TOTAL
must be achieved.
The P
FR
coefficient with respect to the total
amount of phytochrome (P
TOTAL
) ranges
between 0.1 when far red (FR) radiation pre-
vails and 0.75-0.89 when red (R) radiation
prevails (Langhams and Tibbitts, 1997).
The ratio of red/far red radiation (R/FR)
is altered as the light is filtered by the leaves
in the upper levels of a canopy. Therefore,
different levels of the canopy receive light
with different values of the R/FR ratio, alter-
ing its phytochrome. This will result in a
different
6.4.1
Introduction
Plants use solar radiation as a supply of
energy and as a source of information (Hart,
1988). Photomorphogenesis is the effect of
radiation on plant development. The mere
presence of light, above a certain minimum,
generates several responses in flowering,
germination or phototropism.
The majority of photomorphogenic
reactions are induced by wavelengths
within the blue region (400-500 nm) or in
the red or far red region (600-700 nm and
700-800 nm, respectively) and controlled
by the pigment 'phytochrome' (Challa
et al.
,
1995). The most relevant wavelengths are
around 660 nm (in the red region) and
725 nm (in the far red).
There are three main pigment groups
associated with the relevant photo-responses
of plants (Whatley and Whatley, 1984):
(i) chlorophylls, involved in photosynthe-
sis; (ii) phytochrome, involved in some
morphogenic changes, in the perception of
light duration and in the daily rhythms
which affect some movements of the plants;
and (iii) b-carotene or flavins, related to
phototropism (Plate 11).
biochemical
or
morphogenic
reaction.
The quality of the light (distribution of
its spectrum) is, therefore, relevant in its
action on phytochrome. Normally low light
intensity is sufficient to obtain a response
induced by phytochrome.
The phytochrome system may detect
the duration of daily illumination (an envi-
ronmental parameter that is constant for
each location) which is relevant in such
latitudes where there are large variations in
daily illumination throughout the year.
The radiation intensity required for
some photomorphogenic responses is only
at the full moon level (0.01 mmol m
−2
s
−1
),
but the majority of the responses are con-
trolled by higher levels: 0.1-1.5 mmol m
−2
s
−1
(Langhams and Tibbitts, 1997).
Phytochrome
Phytochrome is located in the non-green
(and etiolated) parts of the plants.
Phytochrome seems to be involved with
many different types of responses of plants.
It is a very big and complex molecule that
Dark reversion
Destruction
Red light
650-660 nm
Far red light
725-730 nm
P
P
R
P
FR
Biological
responses
Fig. 6.8.
Mode of action of phytochrome (according to Whatley and Whatley, 1984).
