The correlation analysis between metabolites provides new insights for salinity studies in Pongmaia
In leaves, a strong correlation was observed among carbohydrates, fatty acids and polyols in treated plants suggesting that these metabolites may serve as first line ‘osmolyte defense’ against Na+ion osmotic imbalance, protecting the structural and functional integrity of the cell and cell membrane from osmotic imbalance (Gao et al., 2013; Conde et al., 2015) (Figure 7). Further, the positive interaction between carbohydrates, intermediates of TCA cycle and polyols could serve as second line ‘ion homeostasis defense’ to maintain the intracellular pH and ion balance under salt stress (Guo et al., 2015) (Figure 7). The positive correlation, observed between carbohydrates, polyols, amino acids and organic acids might serve as third line ‘quick energy defense’ suggesting an anaplerotic role for the TCA cycle and provide immediate carbon energy source under salt stress (Zhang et al., 2017). The association between carbohydrates and amino acids showed maintain C:N balance, which may ameliorate salt-induced stress in Pongamia (Nasir et al., 2010;Naliwajski & Skłodowska, 2018).
In roots, the positive correlation between organic acids, carbohydrates and fatty acids may serve as line of ‘ion homeostasis and osmolyte defense’, regulating cellular pH, osmotic potential and membrane fluidity (Sakamoto & Murata, 2002; Guo et al., 2015). The enhanced correlation of secondary metabolites with these metabolites could serves as second line of ‘antioxidant and cell wall barrier defense’, where it enhances the tolerance to salinity by regulating pathways such as ROS detoxification and cell wall barrier synthesis (lignin biosynthesis) (Liu et al., 2018; Yang et al., 2018) (Figure 7). The positive association of polyols may provide a continuous osmoticum at cellular level to mitigate the osmotic imbalance raised by the increase uptake of Na+ ions into the vacuole (Slama et al., 2015).