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).