| Foxtail millet known as the first of five cereals grain has high nutritional value.It is second only to wheat and maize in dry farming of north China and characterized by small genome,short growth cycle and tolerance to drought stress.Foxtail millet has become a model plant for resistance mechanism research of gramineous crops,but drought is still one of the important factors which restrict the development of millet.Based on the natural conditions of less rain and more mountains and deep trenches in Shanxi Province,the present study took millet and humic acid as research material and object,respectively.Studied on the physiological mechanism of humic acid on millet from seed germination,photosynthetic mechanism,antioxidant system,osmotic regulation,and meanwhile quality and yield components of millet under dry farming conditions.The research purpose is to explore the optimal concentrations of humic acid applied to foxtail millet and to analyze the regulation pathway and physiological mechanism of foxtail millet under drought stress.It can not only provide theoretical basis for elucidating the mechanism of humic acid on the growth regulation of millet,but also play an important role in reducing fertilizer and alleviating drought stress.The main research results are as following:（1）In order to research the effects of humic acid on seed germination and seeding growth of millet under drought stress,an experiment was conducted to soak seed using foxtail millet cultivar ‘Jingu 21’ and ‘Zhangza 10’ by different concentrations of humic acid(50、100、200、300、400 mg·L-1)under 18% PEG-6000 of drought stress.The results showed that drought stress significantly inhibited seed germination and seedling growth of millet.The germination potential,germination rate,germination index and activity index of millet under drought stress were significantly lower than that of the CK,and meanwhile the shoot length,root length,fresh weight and dry weight of the seedlings were significantly decreased.Under drought stress,HA of 50-300 mg·L-1 could significantly improve the germination potential,activity index,shoot length and root length of millet of the two varieties,and significantly promoted the drought resistance index of ‘Zhangza 10’.The comprehensive evaluation of multiple indexes by using the membership function value method showed that the optimal concentration was 100 mg·L-1 of humic acid to promote grain germination.The results showed that the suitable concentration of humic acid could significantly promote the germination and seedling growth of millet under drought stress,effectively alleviate the damage of drought stress and enhance the drought resistance of millet.（2）In order to investigate effects of humic acid on chlorophyll content（SPAD）,gas exchange parameters,chlorophyll fluorescence parameters（PSII）and P700 parameters（PSI）of millet under drought stress.An experiment was conducted to soak seed using ‘Jingu 21’ and ‘Zhangza 10’ by different concentrations of humic acid,and measured index under normal water supply（0d）,early drought（5d）and late drought（10d）respectively when millet growed 3-5 leaf stage.When compared with the CK,the SPAD of ‘Jingu 21’and ‘Zhangza 10’ were significantly increased by 100 mg·L-1 HA.A 100 mg·L-1 HA significantly increased the Pn（net photosynthetic rate）and Gs（stomatal conductance）of ‘Jingu 21’ and ‘Zhangza 10’,and reduced the Ci（intercellular CO2 concentration）of ‘Jingu 21’ under drought stress.‘Jingu 21’ and ‘Zhangza 10’ treated by 100 mg·L-1 HA exhibited significantly increased Y（II）（effective quantum yield of photosystem II）,ETR（II）（relative photosynthetic electron transfer rate of photosystem II）,and q P（photochemical quenching）,and decreased NPQ（non-photochemical quenching）under drought stress.Meanwhile,200 mg·L-1 HA significantly increased the Pm（maximum P700）,Y（I）,and ETR（I）in ‘Zhangza 10’,and decreased the Y（NA）in ‘Jingu 21’ and ‘Zhangza 10’ under drought stress.In a word,100-200 mg·L-1 HA significantly improves the photosynthetic characteristics of millet seedlings under drought stress.Principal component analysis was performed on 16 photosynthetic indexes,and 2 principal components were extracted.Fv/Fo,Fv/Fm,NPQ,ETR（I）,q P,Y（II）and ETR（II）were used as the first principal components,and SPAD,Ci as the second principal components.The cumulative contribution rate reached 88.1571%.（3）Under drought stress,100 mg·L-1 HA significantly increased the stem diameter,leaf area,fresh weight,dry weight and chlorophyll content of millet seedlings,increased the relative water content and water potential of the leaves,and improved the water situation of millet.100-200 mg·L-1 HA significantly increased soluble protein,proline content,antioxidant enzymes activity,antioxidants content of millet under drought stress,but O2?? production rate,H2O2 content,MDA content,cell electrolyte permeability were significantly decreased,effectively relieved the accumulation of reactive oxygen species.When concentration of HA was beyond 400 mg·L-1,no mitigation effect was observed on the drought stress of millet.The results showed that appropriate concentrations of humic acid(100 mg·L-1)could effectively improve osmotic regulation,antioxidation and alleviate the damage of drought stress so as to promote the growth of millet seedlings.（4）In 2017 and 2018,200 mg·L-1 HA had the most significant effect on the stem diameter of ‘Zhangza 10’,100-200 mg·L-1 HA had an obvious promoting effect on the underground dry weight and root-shoot ratio of millet,and significantly increased the yield of millet.The average yield of ‘Jingu 21’ and ‘Zhangza 10’ increased by 16.96% and 14.48% respectively.100-200 mg·L-1 HA ignificantly increased the content of protein,Cu,Zn and Mg in millet.200-400 mg·L-1 HA had a significant effect on Fe content.When humic acid was 400 mg·L-1,the content of Fe was the highest.The order of the effect of humic acid on the micromineral concentrations of millet was different because of cultivals.‘Jingu 21’order was Fe > Mg > Cu > Zn,‘ Zhangza 10’order was Fe > Cu > Zn > Mg.To sum up,in Shanxi province,the recommended dosage of humic acid is 200 mg·L-1 in cultivation of millet.Field trials results showed that humic acid could promote the quality and yield of millet. |
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