Biology Reference
In-Depth Information
not in roots from seedlings grown in continuous white light. In contrast, PAL1
and PAL2 were upregulated in the hypocotyl and root of seedlings exposed to
white light for 5 h (
Molas et al., 2006
).
Changes in light conditions have been shown to be associated with an
increase in lignin content in a number of different developmental processes.
For example, 2 days of continuous light induced a reduction in the length of
mungbean (Phaseolus radiatus) hypocotyls. This reduction was correlated to
an increase in lignin content and linked to significant increases in peroxidase
and laccase activities (
Chen et al., 2002
). Callus cultures of P. radiata grown
with a 16 h photoperiod showed an enhanced rate of tracheary element
differentiation as well as increased PAL and CAD enzyme activities and
lignin content in comparison with those of dark-grown conditions (
M
ยจ
ller
et al., 2006
).
The observed alteration in leaf morphology in response to changing light
conditions/gradient could be the consequence of modifications to the structural
components of the leaf. The lignin content in leaves has been positively linked
to irradiation (
Niinemets, 1999
)andthesunleavesofcoffeeplantshavebeen
shown to exhibit an increase of bulk modulus elasticity that could be explained
by the higher content of lignin when compared with shade leaves. The decrease
in tissue elasticity could also affect water uptake (
Cavatte et al., 2011
). The
comparison of two Mediterranean Quercus species showed that leaves of Q. ilex
had a lower specific leaf area and higher lignin content per leaf area than
Q. suber leaves (
Vaz et al., 2011
). Such structural differences might contribute
to reducing leaf transpiration under drought conditions.
Light intensity has also been shown to exert an effect on the activity of
enzymes involved in shikimate, phenylpropanoid and lignin pathways. Ana-
lyses of orchid (Phalaenopsis) plants exposed to different light intensities (60,
160 and 300
mol m
2
s
1
) after transfer from in vitro to soil plantation
revealed that SKDH, PAL and CAD enzyme activities were induced in leaves
(
Ali et al., 2006
). It is important that precursors of the phenylpropanoid
pathway are available in sufficient (non-limiting) quantity to supply this
pathway whatever the light intensity. Moreover, a positive correlation
between PAL, CAD activity and lignin concentration was observed. The
authors conclude that lignin synthesis probably induces defence against
radiation stress. In certain situations, the amount of light energy exceeds
the photosynthetic capacity of the plant and induces an oxidative stress. The
induction of the phenylpropanoid pathway, as well as activation of different
antioxidant systems can help to limit cell damage. In one study, the transfer of
10-day-old Arabidopsis seedlings developed under low light conditions to high
light conditions for 3 h was associated with the induction of PAL, 4CL,
CCoAOMT, CCR and CAD gene expression. Stimulation of phenylpropanoid
m
