Exogenous 5-aminolevulinic acid promotes plant growth and salinity tolerance of grape rootstocks in coastal areas

Salinity stress is a key factor affecting grape production in coastal areas. Two grape rootstock cultivars, ‘Beta’ (sensitive to salt stress) and ‘3309 C’ (resistant to salt stress), were used to investigate the physiological role of 5-aminolevlinic acid (5-ALA) in salinity tolerance. Functional capacities in the rhizosphere, leaf phenotype, photosynthetic parameters, antioxidant enzymes activities and certain physiological indexes were determined after treatment with seawater or seawater followed by 100 mg/L 5-ALA application. The results showed that the functional capacities of the rhizosphere were distinct between the two management regimes. Microbial biomass and soil enzyme activities were reduced by salinity but promoted by 5-ALA. Salinity also suppressed the overall plant growth, leaf chlorophyll content, photosynthetic rate, maximal photochemical efficiency (Fv/Fm), photochemical quenching (qP), electron transport rate (ETR), root activity, ATP content, ATPase activity, and activities of the antioxidant enzymes ascorbate peroxidase (APX), peroxidase (POD), catalase (CAT), and superoxide dismutase (SOD) in both cultivars. Although plant growth, chlorophyll content, and activities of antioxidant enzymes in Beta were decreased to a greater extent than those in 3309 C under salinity stress, exogenous application of 5-ALA significantly improved plant growth along with enhancement of all the above photosynthetic parameters and activities of the four antioxidant enzymes in both cultivars. Seawater treatment alone significantly increased non-photochemical quenching (NPQ), relative electrical conductivity (REC), and malondialdehyde (MDA) contents in leaves, especially for Beta. However, 5-ALA treatment decreased the levels of NPQ, REC, and MDA, but increased ATP content and ATPase activity in roots and leaves of both cultivars. Thus, application of 5-ALA would be beneficial to improve the salt tolerance of grape rootstocks grown in coastal areas.