| Rapeseed（Brassica napus L.）is an important oil cash crop in the world and the third largest source of edible oil in the world after soybean and palm oil.Rapeseed is sensitive to drought and the annual loss of production due to drought is more than 30%.Potassium is the most demanding mineral element in plants.It maintains the osmotic potential of cells and regulates photosynthesis and the transport of assimilates to organs.Application of potassium can effectively improve the tolerance of crops to water stress.Aiming at the current situation of frequent seasonal drought in the Yangtze river basin,this study carried out PEG6000 simulated drought test and potted water control test with two rapeseed varieties with different drought resistance of Chuanyou 36 and Youyan 57 as materials.（1）Under the condition of sand culture,15%PEG6000 was added into Hogland nutrient solution to simulate drought.Five potassium fertilizer gradients of 0,0.1,1.0,10 and 100 m M K2SO4 were set,which were recorded as KI,KII,KIII,KIV and KV respectively.Without PEG6000 and 1m M K2SO4 as control（CK）,the effects of different potassium supply levels on seed germination,root morphogenesis and root exudates under drought conditions were investigated.（2）Under potted soil culture,two water gradients of 40%field water holding capacity（W1）and 75%field water holding capacity（W2）were set,and three potassium fertilizer gradients of 0 mg·kg-1 K2O（K1）,80 mg·kg-1 K2O（K2）and 160 mg·kg-1 K2O（K3）were set.By separating the rhizosphere soil from the non-rhizosphere soil with a root bag in the pot,the effects of water-potassium coupling on the growth,soil physical and chemical properties,soil enzyme activity and microbial metabolism of rapeseed were explored.This paper investigated the relationship between root-soil-microorganism of rapeseed under water-potassium coupling,and revealed the mechanism of action of potassium fertilizer on drought relief.The main research results of this paper are as follows:1.Rapeseed germination and seedling growthDrought stress inhibited seed germination,seedling growth and root morphogenesis of rapeseed.Water deficit restricted the growth of taproot and favored the growth of lateral root of seedling,which indicated that the growth of lateral root increased the water absorption area.Potassium input could alleviate the stress,which had a certain concentration effect.With the increase of potassium supply level,the germination index and RI of KIV level of CY36 reached the maximum level.The variation of YY57 was consistent with that of CY36.The best relieving effect of YY57 on germination index and RI was at KIII level,which proved that YY57 was a potassium efficient utilization variety.The total root length and number of root tips were maximized in KIV,while the average diameter,number of branches and crosses reached the peak level in KIII.The alleviation of drought by potassium did not continue to be strengthened with the potassium fertilizer gradient,and high concentration of potassium（KV）caused significant inhibition on rapeseed,which indicated that the overall potassium showed a rule of"Low concentration promotion&high concentration inhibition".2.Activation of organic acid and rapidly available potassium in root exudatesThe results showed that drought restricted the release of organic acid content and species from rapeseed root exudates,and the addition of potassium promoted the secretion of organic acid.There were 9 organic acids was detected in YY57,such as oxalic acid,lactic acid,citric acid,succinic acid,malonic acid,acetic acid,propionic acid,formic acid and malic acid.Among them,water deficit restricted the secretion of malic acid,potassium deficit inhibited the secretion of formic acid and propionic acid,and the total amount of organic acid showed the rule of CK>KIII>KII>KIV>KI.However,only 8 acids（except malic acid）were detected in CY36,and drought stress inhibited the release of formic acid,while the high potassium supply of KIV caused the root system to secrete more propionic acid,and the total organic acid showed the rule of KIV>CK>KIII>KII>KI.Rapeseed root exudates activated available potassium in soil.The activation increased gradually with the potassium supply level,and the phenomenon was more obvious in CY36.3.Biomass and nutrient distribution strategy at seedling stage of rapeseedCompared with overground part of rapeseed,the undergound part of rapeseed,the biomass accumulation of rapeseed root was not sensitive to the change of external environment.Drought stress significantly inhibited the accumulation of above-ground biomass in rapeseed seedling stage,while the application of potash fertilizer alleviated the trend of stress effect showing K3>K2>K1.The mineral calcium and magnesium in rapeseed are always distributed mainly in the underground part.The distribution strategy of main nutrients was influenced by the difference of water gradient.The distribution of NPK in the underground part is balanced.Under water deficit,rapeseed transported more nutrients to the above-ground part,thus increasing the photosynthesis of the plant.Potassium fertilizer input significantly increased the accumulation of nitrogen and potassium in rapeseed.4.Photosynthetic and chlorophyll fluorescence characteristics of rapeseedWater deficit reduced the net photosynthetic rate and inhibited the levels of stomatal conductance and transpiration rate,but had little effect on chlorophyll fluorescence.It indicated the existence of water-potassium coupling.The response of chlorophyll fluorescence parameters to potassium supply level was more sensitive in CY36.Potassium not only promoted the increase of maximum quantum yield and photoprotection ability of PS（II）,but also affected photosynthetic activity to some extent.At the same time,the light damage degree of CY36 was significantly reduced by potassium fertilizer5.Enzyme activity in rhizosphere and non-rhizosphere soilsThe effect of soil water content on soil enzyme activity was higher than that of potassium fertilizer.In non-rhizosphere soils,the increase of soil water promoted the activity of sucrase and inhibited the activity of amylase.Under the rhizosphere,with the increase of water content,catalase activity increased significantly and urease activity decreased significantly,and catalase activity increased significantly,urease activity decreased significantly.Potassium fertilizer only significantly promoted the activity of amylase in the rhizosphere,but did not significantly affect in the non-rhizosphere.However,potassium increased the soil enzyme index and promoted the overall activity of the soil enzyme system.6.Diurnal variation of respiration in rhizosphere and non-rhizosphere soilsPotassium fertilizer had a significant effect on the mean daily respiration rate of rhizosphere and non-rhizosphere.The rhizosphere showed as K2>K3>K1,and the non-rhizosphere showed as K3>K2>K1.There is coupling effect of water and potassium on diurnal variation of soil respiration.Different genotypes of rapeseed rhizosphere soil respiration had different responses to water gradient:under drought stress,the peak of respiration of CY36 at K2（11:00）appeared 3 hours earlier than that of other treatments（14:00）,while the maximum of respiration under normal water was at 8:00.The soil respiration pattern of YY57 was not disturbed by water and potassium fertilizer,and it peaked at 8:00.The peak respiration time of non-rhizosphere soil was generally later,and response of different genotypes to water gradient was the same:K3 peak respiration time（14:00）appeared 3 hours earlier than other treatments under drought stress,while the potassium supply treatment（14:00）appeared 3hours earlier than the potassium deficiency treatment under normal water.7.Physical and chemical properties of rhizosphere and non-rhizosphere soilsSoil p H value is consistent with water gradient,presenting W2>W1,and p H of rhizosphere soil is lower than that of non-rhizosphere soil.The application of potassium inhibited the total nitrogen content of soil in the non-rhizosphere,but promoted the total nitrogen level in the rhizosphere.This indicates that potassium ensures the efficient utilization of nutrients by pushing nitrogen nutrients from non-rhizosphere to rhizosphere.The effect of potassium fertilizer on available nutrients was consistent between rhizosphere soil and non-rhizosphere soil,which indicated that the way of potassium fertilizer’s effect on available nutrients was not by changing their distribution in soil but increasing their decomposition rate and activation degree.The response of soil nutrients to potassium in rapeseed of different genotypes presented CY36>YY57,which indicated that YY57 was not easily disturbed by external factors due to more stable soil system.8.Carbon source utilization capacity of rhizosphere soil microorganismsDrought stress inhibited the diversity index of rhizosphere microbial community.The water gradient significantly affected the utilization capacity of microorganisms to the 6 kinds of carbon sources.The effect of potassium fertilizer on carbon source was amplified by water deficit,which showed that potassium fertilizer only significantly increased the metabolic degree of polymer,amino acid and carboxylic acid under drought stress.The effects of potassium fertilizer on phenolic acid carbon sources were different under different water gradients:with the increase of potassium supply,the utilization of phenolic acid carbon sources continued to decrease under drought stress,while the utilization of phenolic acid carbon sources continued to increase under normal water.The two varieties had different patterns of carbon source utilization,which was further confirmed by principal component analysis.The effect of potassium fertilizer on carbon source utilization of rhizosphere microorganisms under water deficit:CY36>YY57,while the opposite was true under normal moisture:YY57>CY36.Combining the above experimental results,we found that potassium addition was beneficial to seed germination,seedling establishment,root morphogenesis,and release of organic acids from root exudates.Potassium had a concentration effect on drought relief,which showed the phenomenon of"low promoting and high inhibiting".The application of potassium fertilizer could improve the photosynthesis,soil respiration and available nutrient level of rapeseed seedling,which affected nutrient distribution strategy and carbon source utilization capacity of soil rhizosphere microorganisms.Many of these effects were more significant under drought stress. |
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