Salt stress is considered as the most critical environmental factor which negatively affects the germination, growth and establishment of trees and shrubs. Marine algae extract represents an important option of biostimulants which can improve the crop tolerance to abiotic stresses, such as salinity. In the present study, we investigated the potential of seaweed and microalga extracts as biostimulants priming agent to attenuate the effect of salt stress on seed germination and growth indices of milkweed (Calotropis procera Ait), which was grown under different salinity concentrations. Milkweed seeds were primed by the seaweed extract (SWE) of Sargassum angustifolium, and microalga extract (MAE) of Spirulina platensis at various concentrations (0, 5, 10 and 15 ml l−1) and then exposed to salt stress at four levels (0, 7.5, 15 and 30 dS m−1 of diluted seawater) in factorial experiment and completely randomized design. The results indicated that seeds primed by biostimulant germinate and also tolerate salinity up to 15 dS m−1. Although SWEs (10 ml l−1) increased germination percentage by 42% in seeds exposed to 7.5 dS m−1, the parameter was significantly enhanced using MAEs (5 ml l−1) by 60, 65 and 60% at 7.5–30 dS m−1, respectively. Also, SWE (< 10 ml l−1) improved seed vigor index by 55% and 86.6% as exposed to 0, 7.5 dS m−1, whereas MAE (5 ml l−1) increased it by 54%, 66% and 9% at 0, 7.5 and 15dS m−1, respectively. Moreover, root and shoot length as well as root–shoot ratio was markedly increased using 10 ml of both algae under salinity lower 15dS m−1 than control. SWEs and MAEs enhanced shoot weight by 30.74% and 41.6% in salinity-free control group, respectively. Both priming (< 10 ml) resulted in the raising of root dry weight in stressed-plants versus the control. The highest salt tolerance index in stressed-milkweed was found 85% at 15 dS m−1 while algae priming (< 10 ml l−1) raised it by 118% at the same salinity than control. Totally, this study demonstrates
