Agriculture Reference
In-Depth Information
If there is an excess of light in the blue
wavelength range the length of the inter-
nodes is reduced. On the contrary, the inter-
nodes elongate in excess of far red. Therefore,
equilibrium is necessary between blue and
far red in the radiation spectrum for the nor-
mal development of some plants.
Other daily cycles in plants are the
absorption of ions by the roots (influenced
by transpiration), cell division, respiration
and gutation or water expelled by glands in
the edges of the leaves (Whatley and
Whatley, 1984).
Other pigments
Not all plant movements are primarily
controlled by phytochrome. In phototro-
pism, the curvature of the stem of a plant
towards the light due to lateral illumina-
tion, b-carotene or flavins are mainly
responsible, although phytochrome also has
some influence (Whatley and Whatley,
1984). The phototropic curvature is induced
by blue and not red light.
The duration of the illumination to
induce a response is low, at around 5 min
(Whatley and Whatley, 1984). The relevance
of the phototropic response is the fact that
developing leaves search for the best illumi-
nated position.
There are other photoreceptors (crypto-
chrome) but their effects are less known
(Mohr, 1984).
6.4.4
Photoperiodism
Photoperiodism is the control mechanism
of plant development in response to a
change in the period of illumination (pho-
toperiod) to which plants are exposed each
day (i.e. the duration of the day and the
night in 24 h cycles). In this way, there are
short-day plants that generally flower when
the duration of the day is shorter than its
critical photoperiod, normally less than
12 h, whereas the long-day plants flower
when the duration of the day is longer than
its critical photoperiod, usually more than
12.5 h (Langhams and Tibbitts, 1997). These
photoperiod thresholds are not exact, being
influenced by other factors such as the age
of the plant or the climate conditions (tem-
perature and radiation intensity). Those
plants whose flowering is not dependent
on the duration of the photoperiod are
called day-neutral plants (Vince-Prue,
1986). The majority of vegetables grown in
greenhouse are day neutral. These 24 h
rhythms are known as circadian rhythms
and have similarities to those existing in
animals, among them man (Vince-Prue,
1986). Many of the daily activities of plants
are controlled by this endogenous circadian
rhythm, with which light interacts in dif-
ferent ways (Hart, 1988).
The existing luminosity (with photomor-
phogenic response) before dawn and after
dusk, plus moments of appearance and disap-
pearance of the Sun in relation to the horizon,
means that the duration of the photoperiodic
day, in our latitudes, corresponds to the
6.4.3
Periodic rhythms in plants
In some plants phytochrome also regulates
movements of the leaves from a horizontal
position, in the morning, to a vertical
('sleep') position at night, following a cer-
tain daily rhythm. This regulation is
achieved through the alteration in turgor
(derived from their water content, influ-
enced by the re-distribution of K
+
ions) of
certain special cells in the petiole of the
leaves (Whatley and Whatley, 1984).
The daily opening of some flowers is
also regulated by light. The opening of the
stomata starts at dawn, as the guard cells
become turgid, due to the absorption of
potassium which induces osmotic absorp-
tion of water (Fig. 6.2).
If the night darkness is interrupted,
with artificial light, the stomata start to open
but close when illumination ceases. The
light, obviously, regulates the photosynthe-
sis phase (during the day) and the corres-
ponding translocation of assimilates from
the leaves to the reserve organs (e.g. fruits
and roots) during the night.
Some photo-nastic movements are ini-
tiated by radiation, such as opening and
closing of flowers, movements of the leaves
and stem turn.
