Salicylic Acid: An Update on Biosynthesis and Action in Plant Response to Water Deficit and Performance Under Drought - Salicylic Acid: Plant Growth and Development - page 5

have increased or decreased levels of SA, to a particular stress factors. Yang et al.
( 2004 ) revealed that SA-deficient transgenic rice exhibited increased susceptibility
to oxidative stress and suggested that SA may play an important role in plant
resistance to oxidative stress caused by biotic and abiotic stress agents. In the
available literature there are no data with regard to transgenic plants having
modified capacity for SA synthesis in terms of the role of this regulator in resis-
tance to water deficit. However, we have shown that SA may play a role in the
mechanism of cross-resistance in plants subjected to the combined action of UV-B
and water deficit. UV-B applied before water deficit induced an increase of
endogenous SA level in the leaves of barley seedlings and alleviated the damaging
effect of water deficit on cell membranes and leaf hydration. This was evidenced
by the lack of membrane damage as well as the lack of a decrease in leaf water
content of plants pre-treated with UV-B before water deficit imposition, in spite of
occurrence of such damage after the sole action of water deficit (Bandurska and
Cie ´ lak 2012 ).
In the studies related to the involvement of SA in plant responses to water
deficit most of the investigations are concentrated on the impact of exogenous
application of SA on plant resistance to stress. However, some research also
focuses on the interplay between changes in the level of SA in water deficit treated
plant and its resistance to water deficit.
3.1 Effect of Water Deficit on SA Level in Plant Tissues
and Stress Resistance
The endogenous SA content increased in the leaves of drought-stressed Mediter-
ranean plant, Phillyrea angustifolia. However, it was revealed that SA level
showed a strong negative correlation with the level of leaf hydration (relative
water content, RWC) and a positive correlation with a-tocopherol level during
drought, which may indicate the possible involvement of SA in the regulation of
water balance as well as activation of antioxidative mechanisms (Munné-Bosch
and Peˇuelas 2003 ). Water deficit induces leaf senescence, which is sometimes
regarded as a negative consequence of stress. However, senescence of mature
leaves permit nutrient remobilization to the youngest leaves, allowing plant sur-
vival during prolonged periods of drought (Munné-Bosch and Alegre 2004 ). It has
been reported that drought stress induced SA accumulation in leaves of field-
grown common sage plant (Salvia officinalis) that may regulate leaf senescence
and plant survival under water deficit conditions (Abreu and Munné-Bosch 2008 ).
Our results provide the first confirmation that water deficit effects a significant
increase of SA content in barley seedlings. We have shown that polyethylene
glycol-induced (PEG 6000) water deficit causes a significant increase of SA
content in barley roots after 6 and 24 h of stress, where as in leaves SA level did