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Table 1 Common mechanisms related to symptom relief of different types of abiotic stress (salinity, drought and heavy metal toxicity) by humic substances applied directly to plants or soils

From: Plant chemical priming by humic acids

Salinity
Osmoprotection/osmotic adjustmentAntioxidant activityIonic balanceWater efficiency useReferences
Proline; sugars, phenols[15]
Proline, sugars,CAT, SOD; ascorbic acids[16]
Soluble proteinCAT, POX[17]
Soluble sugars, prolineCAT, SOD, GPOX; ascorbic acid, glut, TOC [18]
Soluble sugars [19]
PigmentsAscorbic acid, phenols, anthocyanins✔ RWC[20]
ProlineSOD, CAT, POX[21]
 [22]
Pigments [23]
MDA, proline, H2O2SOD, POD, CAT[24]
 [25]
MDA. H2O2SOD, APX, GST  [26]
Proline, free sugars [27]
Heavy metal stress
Stress markerAntioxidant activityUptakeToxicityReferences
ProlineCAT, SOD, POXFe[28]
CAT, SOD, POXPb[29]
SOD, CAT+Cd[30]
SOD, POXCd[31]
Protective DNA activity  Mn[32]
 + or − according HA concentrationCd[33]
Decrease on H2O2 production and lipid peroxidation Pb[34]
Proline. MADSOD, CAT, GPOX, GST Cu, Cd[35]
 Pb, Cd[36]
Sugars, soluble proteins RWC Cd[37]
 DecreaseCd[38]
Drought
Osmoprotection/osmotic adjustmentAntioxidant activityWater efficiency useReferences
Improve net photosynthesis[39]
Changes in metabolic profileCAT, POX and SODRWC[40]
Proline, vitamin CCAT, POXRWC and (WEU)[41]
Decrease of MDA; prolineEnhancement of CAT, POX, APX [42]
MDAEnzymatic: POX; decrease of H2O2 contentEnhance of aquaporins gene expression (TIPS)[43]
Proline,Enzymatic: POX; decrease of H2O2 content [44]
  1. CAT: catalase activity; SOD: superoxide dismutase activity; POX: peroxidases; APX: H2O2: hydrogen peroxide concentration; RWC: relative water content; WUE: water use efficiency; GPOX: GST: MAD: malondialdehyde heavy metals: Cu (copper) Cd (cadmium): Fe (iron): Pb: Mn