Salicylic Acid-Mediated Abiotic Stress Tolerance - Salicylic Acid: Plant Growth and Development - page 204

treatment with SA also attenuated the UV-B radiation stress in soybean seedlings
(Zhang and Li 2012 ). The concentration of UV-absorbing substances was

increased by increasing levels of SA in maize (Gunes et al. 2007 ).
Like ozone, UV radiation was shown to induce the accumulation of SA, while
also stimulating PR-protein synthesis and inducing virus resistance in tobacco
plants (Yalpani et al. 1994 ). Increase in SA level was accompanied by the accu-
mulation of SA conjugate and an increase in the activity of BA 2-hydroxylase,
which catalyses SA biosynthesis.

2.6.3 Various Light Conditions

Light is one of the most strongly fluctuating environmental parameters. Tests were
made to discover how the varying light conditions found in shade affected the
endogenous SA content, and the possible role of SA in shade avoidance by sun-
flower (Helianthus annuus L.) hypocotyls was examined. A logarithmic increase in
PAR (photosynthetically active radiation) levels caused a roughly 10-fold increase
in endogenous SA levels. Of the far-red (FR), red (R) and blue wavelengths that
make up PAR, only FR light had a significant and positive effect on endogenous
SA levels. Furthermore, a low R/FR ratio significantly increased the endogenous
SA content in hypocotyls compared with normal and high R/FR ratios. Uncoupling
the effect of the R/FR ratio and PAR irradiance on the endogenous SA content
demonstrated that PAR irradiance is a much stronger signal than FR lightenrich-
ment. Thus, while a low R/FR ratio increases the SA content in sunflower hypo-
cotyls, low PAR, the other component of canopy shade, decreases the SA content
much more effectively than a low R/FR ratio increases it. Therefore, it appears that
SA probably has no direct role in shade avoidance effects (Kurepin et al. 2010 ). It
was reported that exogenous SA could protect the photosynthetic apparatus against
strong light-induced photo-damage in the leaves of Satsuma mandarin tree (Citrus
unshiu Marc.) (Qiu et al. 2011 ), and that SA is able to overcome low irradiance
stress in rice (Maibangsa et al. 2000 ). Furthermore, in Arabidopsis the dwarf
phenotype displayed by mutants with high SA content (cpr1-1, cpr5-1, cpr6-1 and
dnd1-1) was less pronounced when these plants were grown in strong light, sug-
gesting that the inhibitory effect of SA on growth was partly overcome at higher
light intensities. On the other hand, mutants with low foliar SA content were less
able to become acclimated to transient exposure to strong light and thus predis-
posed to oxidative stress.These observations implied an essential role of SA in
light acclimation processes (Mateo et al. 2006 ).

2.6.4 Hypoxia

Higher plants are aerobic organisms requiring oxygen for growth and metabolism.
However, low oxygen concentration may occur in the root zone in regions with
higher rainfall due to soil compaction and soil water-logging. It was concluded that