Impact of Salicylic Acid on the Transport and Distribution of Sugars in Plants - Salicylic Acid: Plant Growth and Development - page 85

Two main processes primarily affect sugar accumulation: (1) the rate at which
they are formed in the process of photosynthesis and (2) shift in their consumption
at the site of their metabolism. The data on SA influence on these processes are
very scarce. There are some reports about SA-induced activation of photosynthesis
(Khan et al. 2003 ; Aldesuquy et al. 2012 ), chlorophyll and carotenoid synthesis in
soybean (Zhao et al. 1995 ), maize (Khodary 2004 ), and wheat (Singh and Usha
2003 ; Arfan et al. 2007 ). One of the reasons for photosynthesis activation may be
decrease in the stomatal resistance which facilitated CO 2 transport to the meso-
phyll cells. The effect of SA on the stomatal apparatus is well known (Rai et al.
1986 ; Khodary 2004 ). Several researchers reported about the increase in the
content of chlorophylls in common bean and wheat leaves (Zhao et al. 1995 ; Sinha
et al. 1993 ; Khodary 2004 ; Farouk and Osman 2011 ; Amin et al. 2008 ; Uzunova
and Popova 2000 ). El Tayeb and Ahmed ( 2010 ) presented the results of Hamade
and Al-Hakimi experiments where seed treatment with 100 lM SA enhanced
photosynthesis in the plants and suppressed dark respiration under drought con-
ditions. Some authors observed SA-induced changes in leaf anatomy: an increase
in the thickness of the leaf blade, the thickness of palisade and spongy paren-
chyma, changes in the vascular bundle density, the diameter of the midrib (Fa-
roukand and Osman 2011 ; Maslenkova et al. 2009 ). However, Najafian et al.
( 2009 ) reported that spraying of thyme (Thymus vulgaris) leaves with SA at the
concentrations of 100, 300, and 400 ppm resulted a decline in the transpiration rate
(i.e. increase in stomatal resistance).
Under stress conditions, the direction of SA action got a shift. For example,
Najafian et al. ( 2009 ) reported that SA suppressed photosynthesis in thyme plants,
not subjected to stress; however, under salinity SA increased the rate of photo-
synthesis. Salinity reduced the content of pigments in wheat leaves, but short-term
seed soaking treatment with SA solution prevented this reduction (Aldesuquy et al.
2012 ). SA affected photosynthetic activity: Rubisco activity, Hill reaction, and
quantum yield of photosynthesis. In addition, changes are also observed in the
functioning of the stomatal apparatus, the content of chlorophylls, and gas
exchange during photosynthesis. The tolerance of photosynthetic apparatus to
salinity increased (Khodary 2004 ). According to Maslenkova et al. ( 2009 ), the SA
influence on photosynthesis occurs on the membrane level. Since SA is well
soluble in lipids, its action may be unspecific, via change in the lipid—protein
interactions in membranes. In fact, Uzunova and Popova ( 2000 ) observed SA-
induced changes in both the composition of the chloroplast envelope and the
granum membrane structure. The authors assume that change in the photosystem II
functioning, they observed, may be explained just by SA impact on chloroplast
membranes.
Photosynthesis activation may be one of the reasons for increased sugar pro-
duction in the presence of SA (Barakat 2011 ). Another reason may be SA-induced
activation of enzymes synthesizing sucrose, e.g., sucrose phosphate synthase and
sucrose synthase (Dong et al. 2011 ; Gadi and Laxmi 2012 ). The additional factor
affecting the contents of soluble sugars in plants is their metabolization. This
aspect of SA action has not been studied in depth. It is observed that SA improved