Salinization plays a primary role in soil degradation and reduced agricultural productivity. We observed
that salt stress reversed photosynthesis and reactive oxygen scavenging responses in leaves or roots of two
rice cultivars, a salt-tolerant cultivar Pokkali and a salt-sensitive cultivar IR-29. Salt treatment (100 mM
NaCl) on IR-29 decreased the maximum photochemical efficiency (Fv/Fm) and the photochemical
quenching coefficient (qP), thereby inhibiting photosynthetic activity. By contrast, the salt treatment on
Pokkali had the converse effect on Fv/Fm and qP, while increasing the nonphotochemical quenching coefficient
(NPQ), thereby favoring photosynthetic activity. Notably, chloroplast or root cells in Pokkali
maintained their ultrastructures largely intact under the salt stress, but, IR-29 showed severe disintegration
of existing grana stacks, increase of plastoglobuli, and swelling of thylakoidalmembranes in addition to
collapsed vascular region in adventitious roots. Pokkali is known to have higher hydrogen peroxide (H2O2)-
scavenging enzyme activities in non-treated seedlings, including ascorbate peroxidase, catalase, and
peroxidase activities. However, these enzymatic activities were induced to a greater extent in IR-29 by the
salt stress. While the level of endogenous H2O2was lower in Pokkali than in IR-29, itwas reversed upon the
salt treatment. Nevertheless, the decreased amount of H2O2 in IR-29 upon the salt stress didn’t result in a
high scavenging activity of total cell extracts for H2O2, as well as O2
and OH species. The present study
suggests that the tolerance to the moderate salinity in Pokkali derives largely from the constitutively
maintained antioxidant enzymatic activities as well as the induced antioxidant enzyme system.