Salicylic Acid: Physiological Roles in Plants - Salicylic Acid: Plant Growth and Development - page 19

However, contrary to these results, the transpiration rate decreased significantly
in Phaseolus vulgaris and Commelina communis after the foliar application of SA
and this decrease in transpiration rate was attributed to SA induced closure of
stomata (Larque-Saavedra 1978 , 1979 ; Khokon et al. 2010 ). Moreover, SA pre-
treatment alleviated the loss of net photosynthetic rate under heat stress, apparently
in part through maintaining a higher Rubisco activation state and greater PSII
efficiency (Wang et al. 2010 ). SA also accelerated the increase of net photosyn-
thetic rate mainly through the more rapid recovery of PSII function after heat
stress and may be related to higher levels of HSP21 (Wang et al. 2010 ). Other
mechanisms by which SA mediated protection of photosynthetic machinery are
still to be determined.
2.3 Nitrate Metabolism
All the living organisms are basically composed of carbon, hydrogen, oxygen,
nitrogen and minor quantities of other elements. These elements contribute to
finally organize various biomolecules of the cell. Nitrogen is next to carbon in
importance to living organisms. In a living cell, nitrogen is an important constit-
uent of amino acids, proteins, enzymes, vitamins, alkaloids and some growth
hormones. Therefore, study of nitrogen metabolism is absolutely essential because
the entire life process is dependent on these nitrogen-containing molecules. In this
section, we will learn about various effects of SA on nitrogen metabolism
including nitrogen fixation in plants. Nitrogen metabolism is an important aspect
of legume-Rhizobium symbiosis. The exogenous SA affects the activities of the
enzymes of nitrate/nitrogen metabolism as well. The activity of enzyme nitrate
reductase (NR) was enhanced in the leaves of wheat following the exogenous
application of SA. The treatment also protected the enzyme from the action of
proteinases and trypsin (Rane et al. 1995 ). The total protein content was increased
in soybean plants sprayed with SA and this increase might be due to enhanced
activity of NR following the SA treatment (Kumar et al. 1999 ). A significant
increase in the activity of nitrate reductase was observed both in roots and leaves
of the plants raised from the wheat grains soaked in lower concentration (10 -5 M)
of SA (Hayat et al. 2005 ). Such a lower concentration of SA when sprayed to the
foliage of mustard plants enhanced their NR activity (Fariduddin et al. 2003 ).
However, at higher concentrations (10 -3 or 10 -4 M), SA proved to be inhibitory.
Mabood and Smith ( 2007 ) showed that exogenous SA inhibited the growth of
Rhizobia and production of nod factors by them and also delayed the nodule
formation, thereby decreasing the number of nodules per plant. However, SA level
in the roots of Medicago sativa, inoculated with specific strain of Rhizobia, either
decreased or remained close to the basal levels (Martinez-Abarca et al. 1998 ).
Moreover, Medicago sativa plants when inoculated with an incompatible strain of
Rhizobia, resulted in a marked accumulation of SA in the roots of host plant. It was