Favouring NO over H2O2 production will increase Pb tolerance in Prosopis farcta via altered primary metabolism

- Accumulation of free phenolic compounds in Prosopis was related to its growth inhibition.
- Exogenous lead and ascorbic acid reduced phenolic acids and improved growth.
- Accumulation of excess total amino acid is an important adaptive response in plants to stresses.
- Different amounts of formation of NO and H2O2 can be channeled into specific pathways: survival and growth in conflicting demands.

Reactive oxygen species (ROS) and nitric oxide (NO) are known in triggering defense functions to detoxify heavy metal stresses. To investigate the relevance of ROS production, Pb treatment (400 µM) alone and in combination with 400 µM sodium ascorbate (Asc: as H2O2 scavenger) were given to hydroponically grown Prosopis farcta seedlings over a time course of 72 h. Data presented here indicate that, the low extent of H2O2 due to scavenging by ascorbate, together with high level of NO improved Pb+Asc- treated Prosopis growth. Following the evoked potential of both the signals, significant increases in phenolic acids; caffeic, ferulic and salicylic acid were observed with Pb treatment; which are consistent with observed increase in lignin content and consequently with growth inhibition. In contrast, Pb+Asc treatment induced more flavonoids (quercetin, kaempferol, luteolin), diminished phenolic acids contents and also lignin. Elicited expression rate of phenylalanine ammonia-lyase gene (PAL) and also its enzymatic activity verified the induced phenylpropanoid metabolism by Pb and Pb+Asc treatments. In comparison with Pb stress, Asc+Pb application induced the high expression of arginine decarboxylase gene (ADC), in polyamines biosynthesis pathway, and conducted the N flow towards polyamines and γ-amino butyric acid (GABA). Examining the impact on enzyme activities, catalase, and guaiacol peroxidase; Pb+Asc reduced activity but this increased ascorbate peroxidase, and aconitase activity. Our observations are consistent with conditions favouring NO production and reduced H2O2 can improve Pb tolerance via wide-ranging effects on a primary metabolic network.