| Rapeseed(Brassica napus)is the most important oil crop in China.Due to the uneven regional precipitation and frequent seasonal drought,the production of rapeseed is seriously restricted.Potassium is involved in a lot of processes of plant growth and development,and plays an important role in crop stress resistance.However,the degree and scope of potassium deficiency in China's arable land were increasing,and the application of potassium fertilizer was seriously insufficient,which led to drought stress added to potassium deficiency in rapeseed production.In this study,soil pot culture experiment was conducted to identify drought resistance based on fifteen rapeseed cultivars.Through PEG6000 simulated drought experiment and soil culture full growth period experiment,effects of potassium fertilizer on drought resistance of rapeseed were studied on water use characteristics,agronomic characteristics,physiology and biochemistry,ionome,metabolism,soil agrochemistry and microorganism.The physiological and biochemical mechanism of potassium improving drought resistance of rapeseed was presented.The main results were as follows:1.Drought stress hindered the growth of rapeseed seedling seriously,resulted in a significant reduction in biomass and water use efficiency,and the effect of drought on growth and water use efficiency could not be eliminated after rewatering.Potassium could promote rapeseed growth and increase of water use efficiency under drought stress.Water use efficiency was positively correlated with transpiration efficiency and negatively correlated with the ratio of evaporation to transpiration.Transpiration efficiency of rapeseed increased significantly under drought stress.However,drought stress led to a significant sharply reduction in transpiration,and the proportion of evaporation in total water consumption increased.Most of the water loss through evaporation rather than direct use of rapeseed,resulted in decrease in water use efficiency.On the one hand,the application of potassium increased the transpiration efficiency of rapeseed,and reduced the ratio of evaporation to transpiration,so as to improve the water use efficiency of rapeseed.Youyan 10,Youyan 57,Youyan817 and Qingyou 1 were drought tolerant cultivars,which showed higher biomass,water use efficiency and absorption and utilization capacity of potassium fertilizer under drought stress.The growth of Chuanyou 36 and Dexinyou 49 was severely inhibited under drought stress,and their water use efficiency and potassium absorption and utilization capacity were low,being sensitive to drought stress.2.The enzyme activity of superoxide dismutase(SOD),peroxidase(POD)and catalase(CAT)were enhanced and malondialdehyde(MDA)content reduced with increased potassium level under PEG6000 simulated drought stress,indicating that potassium could promote antioxidant capacity under drought stress.The content of stress hormone ABA increased significantly under drought stress,and the lack of potassium aggravated the accumulation of ABA,while the growth promoting hormone indole acetic acid(IAA),cytokinin(CTK)and brassinolide(BR)decreased,and the increase of potassium level could promote the increase of IAA,CTK and BR.Compared with Chuanyou 36,Youyan 57 maintained higher POD and CAT activities,higher IAA,CTK and BR contents,showing higher antioxidant capacity and hormone regulatory capacity.3.Photosynthetic rate and stomatal conductance of rapeseed decreased significantly under PEG6000 simulated drought stress,and enhanced photosynthetic rate and stomatal conductance were observed with increase application of potassium.Compared with higher potassium level condition,the activity of Ru BP carboxylase enzyme and PEPC enzyme decreased significantly under potassium deficiency,which could induce the decrease of photosynthetic rate.Drought led to leaf chlorosis and the degradation of chlorophyll reduced with high the level of potassium.Drought stress affected the chlorophyll fluorescence significantly,and significant difference was observed in different potassium levels.Drought stress led to a significant decrease in the maximum photochemical quantum yield(Fv/FM)and electron transfer rate of PSII [ETR(II)],and potassium deficiency aggravated the degree of photo inhibition.Compared with potassium depletion,the regulatory heat dissipation increased at higher potassium level which could consume excess excitation energy so as to reduce the damage to the leaves under drought stress.Pm and Pm' were significantly reduced under drought stress,and increasing the level of potassium fertilizer could promote the increase of Pm and Pm'.Over reduction in receptor side of PSI mainly induced the photo-inhibition of PSI under potassium deficiency.The drought resistant cultivar Youyan 57 showed stronger green holding capacity under drought stress,and Fv/FM,Pm,ETR(II)and ETR(I)were significantly higher than Chuanyou 36.4.The absorption of potassium by rapeseed reduced under drought stress,and potassium content in tissue decreased largely under potassium deficiency which led to disturbance of ionic homeostasis.By contrast,content of Na,Mg and Ca increased significantly,partially compensating for K depletion which could be used as cellular osmolytes.Compared with the normal potassium level treatment,the contents of neutral amino acids and amines,including serine,glycine,ornithine,proline,valine,oxyproline,threonine,alanine,isoleucine,asparagine and putrescine increased significantly,while the contents of aspartic acid and L-glutamic acid which were acid amino acids decreased.The content of organic acids in leaves increased,including ?-ketoglutarate,succinic acid,L-malic acid,oxalic acid,etc.,which participated in the cycle of tricarboxylic acid and involved in the formation of amino acids.The biosynthesis of amino acids and amines could participate in osmotic adjustment and ionization balance which could make up for the depletion of potassium to some extent.By contrast,the content of carbohydrate and carbohydrate conjugates decreased significantly.We deduced that rapeseed responded to potassium deficiency under drought stress by accumulation of amino acids and amines for osmotic adjustment and balance of electric charge,while large biosynthesis of amino acids consumed a lot of carbon skeletons and energy which affected rapeseed vegetative growth.Youyan 57 maintained higher tissue potassium concentration than Chuanyou 36 at potassium deficiency under drought,and was less disturbed in metabolism,resulted in higher biomass.5.The growth and yield of rapeseed decreased significantly under drought stress,and the yield and harvest index increased significantly with the application of potassium fertilizer.However,no significant inrease was observed in rapeseed biomass and yield with continued increase of K application.Drought stress hindered rapeseed root growth significantly,and K application could promoted growth of roots.Root biomass,number of lateral roots,surface area and total root length increased with the increase of K applied under drought stress.The root biomass,total root length and surface area of Youyan 57 were significantly higher than those of Chuanyou 36,and kept higher seed yield at potassium deficiency conditon under drought stress.6.The content of oleic acid and erucic acid in rapeseeds decreased significantly under drought stress,and the increase of potassium fertilizer can significantly promote the increase of oleic acid content and the decrease of erucic acid content,which can effectively improve the quality of rape seeds under drought stress.The oleic acid content of Youyan 57 was significantly higher than that of Chuanyou 36,and the content of glucosinolates and erucic acid was significantly lower than that of Chuanyou 36,indicating that the quality of Youyan 57 was better than that of Chuanyou 36.7.The K uptake and utilization efficiency of rapeseed decreased significantly under stress.The K accumulation increased and the K utilization efficiency of rapeseed decreased significantly with increased K application.Potassium in early senesence leaves caused by drought stress accounting for21% of total potassium accumulation,being not taken part in development of lateral growth,induced the decrerase in potassium use efficiency.Highest potassium content was kept in cuticle,and potassium was distributed to the cuticle as much as possible so as to maintain yield under potassium deficiency,which was an important mechanism for rapeseed to deal with potassium deficiency.8.The content of available K and slowly available K in rhizosphere soil increased with the increase of K application level.The bsorption of potassium by rapeseed mostly sourced from the soil with no application of potassium fertilizer,resulting in a significant decrease in the content of soil available and slow available potassium.However,the microbial metabolic activity,Shannon index,Simpson index and richness index of rhizosphere soil increased significantly under the condition of potassium deficiency,which may promote the transformation of mineral potassium and slow-release potassium into soluble potassium for plant utilization. |
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