Seed germination of hemp (Cannabis sativa L.) cultivars responds differently to the stress of salt type and concentration

Industrial hemp is worldwide, but the effects of salt stress on hemp growth remain unknown, especially the tolerance of different varietal to salt stress during germination. Two important hemp cultivars in China, Yunma 5 (YM5, fiber hemp) and Bamahuoma (BM, seed hemp), were germinated under the conditions with four kinds of salts (NaCl, Na2SO4, Na2CO3, NaHCO3) at 0–300 mM for seven days to test the seed germination responses to salt stress. The germination indexes decreased with the increase of salt concentration for each salt, especially in 300 mM Na2CO3, showing the most toxic effect on germination. However, a low concentration (50 mM) of neutral salt stimulated seed germination, showing a higher germination rate and longer radicles and hypocotyls than the control. The growth of radical and hypocotyl was more salt-sensitive than seed germination. Upon transfer to distilled water, the non-germinated seeds restored germination ability and the recovery percentage increased with the increase of salt concentration. YM5 showed stronger salt tolerance than BM, with higher threshold salinity of 150 mM NaCl and Na2SO4 (100 mM NaCl and 50 mM Na2SO4 for BM), and higher critical salinity of 150 mM Na2CO3 and 250 mM NaHCO3 (100 mM Na2CO3 and 200 mM NaHCO3 for BM) for germination rate. In conclusion, the inhibition of salt stress to hemp seed germination varied with the salt type, salt concentration, and tested cultivars, and both hemp cultivars were more sensitive to alkaline salt stress.