Sustained photosynthesis parameters, Ca2+signalling pathways and apoplasmic Na+ sequestration
in the roots are the possible salt adaptive mechanisms in Pongamia
Pongamia exhibited significant tolerance to high salinity (500 mM NaCl
(~3% NaCl) like true halophytes and mangroves by
exhibiting sustained leaf morphology and insignificant alterations in
the photosynthetic machinery (Marriboina et al., 2017). The reduced LRWC
and unchanged RRWC indicate that the roots were able to process salt
solution (Marriboina & Attipalli, 2020a). The results regarding
Na+ and Cl- ions accumulation are
rather interesting. High levels of Na+ and
Cl- ions were detected in roots of treated plants
suggesting that the roots may act as potential sink for excessive
Na+ and Cl- ions deposition,
inhibiting their translocation to shoot and alleviate the negative
effects on actively dividing and photosynthesizing cells (Baetz et al.,
2016; Peng et al., 2016; Rahneshan et al., 2018; Wu et al., 2018).
Intriguingly, Ca2+ levels were also increased with
increasing Na+ levels in both leaves and roots of salt
treated plants, which suggest that might possible trigger of vaculoar
Ca2+ reserves to ameliorate the Na+toxicity (Saleh & Plieth, 2013). Ca2+ was found to be
involved in propagation of long distance signalling (root-to-shoot)
under salt stress conditions (Choi et al., 2014).
Our previous fluorescence studies demonstrated that Pongamia roots act
as an effective sequester of Na+ ions (Marriboina &
Attipalli, 2020a). In the present study, the apoplastic/ cell wall
Na+ specific fluorescence intensity was increased with
treatment time in both 300 and 500 mM NaCl treated plants indicating
that there was an apoplastic Na+ sequestration
(Gonzalez et al., 2012; Anower et al., 2017). The carboxylic residues of
pectin in the cell wall may primarily serve as cation-binding matrix for
Na+ ion, contributing to apoplastic
Na+ sequestration (Gonzalez et al., 2012; Marriboina
et al., 2017). The patterns of high apoplastic and vaculoar
Na+ contents might lead to lower Na+ion content in the cytosol (Marriboina & Attipalli, 2020a), which may
facilitate the protection of cytosolic enzymes from sodium toxicity (Wu,
2018).