Abstract:

In order to explore the optimal melatonin（MT）concentration for promoting watermelon seed germination under saline-alkali stress，the watermelon cultivar‘Zaochun Hongyu’was used as experimental material to study the effects of complexed saline-alkali（molar ratio of NaCl，Na₂SO₄，NaHCO₃ and Na₂CO₃ was 1︰9︰9︰1）stress on watermelon seed germination and the mitigation effects of pre-soaking seeds with different concentrations of exogenous melatonin（MT）on saline-alkali stress. Results showed that：The relative germination energy（RGE），relative germination rate（RGR），relative germination index（RGI），relative vigor index（RVI），relative root length（RRL），relative shoot length（RSL），relative root fresh weight（RRFW），relative shoot fresh weight（RSFW），relative root dry weight（RRDW），relative total root length（RTRL），relative root projection area（RRPA），relative root surface area（RRSA）and relative root volume（RRV）declined generally with further increase of saline-alkali concentration，but the relative shoot dry weight（RSDW）did not change. Based on the semi-lethal concentration，the mean value of the semi-lethal saline-alkali concentration of all single indicators was obtained by establishing regression equations，which was 66.2 mmol · L^-1 as the suitable concentration for saline-alkali stress test of watermelon. Subsequently，the effects of different concentrations of MT on seed germination and seedling growth under saline-alkali（66.2 mmol · L^-1）stress were investigated. The results showed that exogenous MT soaking increased the RGE，RGI，RVI，RRL，RSL，RTRL，RRPA，RRSA and RRV of watermelon seeds under saline-alkali stress. The 50 µmol · L^-1 MT treatment mainly promoted root morphogenesis，whereas the treatment with 200 µmol · L^-1 MT mainly promoted germination vigor and the growth of radicle and germ. Then all indicators were used to comprehensive evaluate and analyze the effect of exogenous MT by taking the method of fuzzy membership function，and the comprehensive score was ranked as 50 µmol · L^-1 MT > 200 µmol · L^-1 MT > 100 µmol · L^-1 MT > 400 µmol · L^-1 MT > 25 µmol · L^-1 MT > saline-alkali stress. Correlation analysis of all indicators and comprehensive evaluation values of watermelon seeds under exogenous MT treatment was further conducted，the results suggested that 14 of the 15 evaluation indicators had significant positive correlation with the comprehensive evaluation value，indicating that the comprehensive evaluation result was reliable. Among the MT pretreatment，50 µmol · L^-1 treatment had the best effect on promoting watermelon seed germination under saline-alkali stress，which showed the highest activities of SOD，POD，CAT，α-amylase also the highest plant height，relative chlorophyll content，survival rate of seedlings，fresh and dry weight of shoot and root whereas the lowest relative conductivity and content of H₂O₂ and MDA. In conclusion，50 µmol · L^-1 is the optimal melatonin concentration to promote seed germination and seedling growth of watermelon under saline-alkali stress by accelerating starch decomposition and increasing antioxidant levels.

Key words: watermelon, saline-alkali stress, seed germination, regression analysis, melatonin, comprehensive evaluation