Humic acid overcomes salinity barriers and stimulates growth of Urochondra setulosa by altering ion-flux and photochemistry

Humic acid (HA) is a known bio-stimulator, but its role in salinity tolerance is not well reported. This study examined the effect of HA (0, 250 and 500 mg L −1 ) on growth, water relation, ion-flux and chlorophyll fluorescence of Urochondra setulosa treated with 0 (control), 300 (moderate salinity) and 600 (high salinity) mM NaCl. HA application significantly promoted plant growth at all salinities. Shoot growth of HA-treated plants increased up to 58% (FW) and 50% (DW) at 600 mM NaCl as compared to HA untreated non-saline controls. HA application protected U. setulosa leaf from membrane damages by decreasing hydrogen peroxide and malondialdehyde contents, while it increased soluble sugars and proline accumulation. HA application helped U. setulosa to get rid of toxic ions by secreting more Na + , while retaining more K + in the leaf via selective trans-membrane transport. HA application also increased leaf pigments, which helped to alleviate the negative effects of salinity on chlorophyll fluorescence parameters as shown in improvement of maximum quantum yield of photosystem II (YII), photochemical quenching (qP) and electron transport rate (ETR) at high salinity. Leaf OP decreased linearly with increasing salinity, while HA application slowed the degree of OP reduction at higher salinity. HA application improves water/minerals uptake, leaf Na + secretion, selective K + transport and photochemical efficiency, while minimizing leaf damages in U. setulosa grown up to seawater equivalent salinity. Therefore, HA could be used as a natural growth stimulator to enhance salt tolerance of plants and get higher biomass through saline agriculture.