| Leymus chinensis,a dominant species in Songnen grassland,has strong ability to resist grazing as well as tolerate saline-alkali stress.Compensatory growth is a positive response of plants to cope with grazing or clipping.Previous studies have mainly focused on the mechanism of compensatory growth from the aspects of morphological compensation,density compensation and photosynthetic compensation.Although resources(e.g.C and N)allocation between plant organs can strongly influence the extent of compensatory growth,it has rarely been studied.Nitrogen is an important restricted resource in plants,and it plays an important role in plant physiological activities and resistance to stress.However it is a pity that the nitrogen allocation strategy of plants after clipping in saline-alkaline conditions remains largely unknown.Therefore,an experiment with different saline-alkali and clipping intensities was conducted in L.chinensis grassland of Songnen Plain.We labelled nitrogen with 15N-urea to trace nitrogen allocation of L.chinensis between plant organs under different clipping and saline-alkali conditions,further to explore the effects of different nitrogen allocation strategy.Our projected results may add new information for the theory of plant compensation growth,and also can provide scientific basis for rational use and restoration of saline-alkali meadow steppe.The experiment was conducted in a completely randomized block design with four blocks.Each block has two saline-alkali levels and three clipping levels.Saline-alkali treatments include no saline-alkali addition(S0)and saline-alkali addition(S1,mixed saline-alkali addition,NaCl: Na2CO3: NaHCO3=2:1:1);clipping treatments include no clipping(C0),moderate clipping(C1,removal 40% of aboveground height)and severe clipping(C2,removal 80% of aboveground height).Nitrogen labelling was conducted by foliar brushing and underground injecting with(CO15NH2)2 solution(15N enrichment: 5.18 atom%).15 N recovery ratio and allocation ratio of different parts of L.chinensis were determined.The morphological parameters(e.g.biomass,plant height,plant density,and physiological parameters(e.g.photosynthetic rate,proline content,soluble sugar content,Na+,K+)were measured at the same time.The main results are shown as follows:(1)Generally,shoot was the main nitrogen pool of L.chinensis: more than 65.4%(foliar labelling)and 61.5%(underground labelling)of newly absorbed nitrogen were distributed in shoot,irrespective of clipping and saline-alkali treatments,indicating nitrogen allocation strategies in response to stresses are similar under two different labelling methods.(2)Saline-alkali stress inhibited L.chinensis' s nitrogen uptake,total 15 N recovery ratio was decreased by 18.1%(foliar labelling)and 14.8%(underground labelling)after saline-alkali application under no clipping conditions,while the allocation of newly absorbed nitrogen in fine root was significantly increased by 52.6%(foliar labelling)and 59.0%(underground labelling).The excess Na+ was compartmentalized into rhizome to buffer the saline-alkali stress,and L.chinensis regulated fine root osmotic pressure by increasing the content of soluble sugar and proline to ensure underground resources absorption under saline-alkali treatment.The fine root biomass accumulation was significantly increased by 36.8%(foliar labelling)and 66.2%(underground labelling)under saline-alkali treatment,irrespective of clipping levels.(3)Moderate clipping significantly increased total 15 N recovery ratio of L.chinensis by 27.5%(foliar labelling)and 10.0%(underground labelling)under saline-alkali stress,suggesting the effects of saline-alkali and moderate clipping on nitrogen uptake were antagonistic.L.chinensis showed positive regrowth strategy in response to moderate clipping under saline-alkali stress: 82.7%(foliar labelling)and 80.0%(underground labelling)of labelling 15 N were allocated to shoot,resulting in the improvement of photosynthetic rate.In addition,population density and growth rate were increased,proline content of shoot and fine root were significantly improved,and soluble sugar remobilized from rhizome and stem base were distributed to shoot and fine root.These responses could offset the carbon loss due to clipping and ensure the substrate supply in resources absorption organs.Therefore,total biomass of L.chinensis showed over-compensatory growth(with the average compensation index of 1.36),similarly,different organs(e.g.shoot,steam base,rhizome,fine root)also presented over-compensatory growth or equal-compensatory growth.(4)Severe clipping significantly decreased total 15 N recovery ratio by 29.1 %(foliar labelling)and 20.1%(underground labelling)under saline-alkali conditions.L.chinensis showed a conservative nitrogen allocation strategy under dual stresses: 15 N allocation to stem base was significantly increased by 32%(foliar labelling)and 120%(underground labelling),respectively.The stem base is called the "blind area" of stress,which is not directly exposed to clipping of aboveground,and is not easy to be affected by the saline-alkali stress of belowground,so increasing the nitrogen allocation into stem base could be an effective way to reduce the risk of resources loss and improve the regrowth potential of L.chinensis.As a result,the stem base biomass had over-compensation or equal-compensation,with the compensation index of 1.33(foliar labelling))and 1.03(underground labelling),while the total biomass,shoot,rhizome and fine root biomass of L.chinensis showed under-compensatory growth in the face of those two stresses.In conclusion,L.chinensis will not take a completely passive strategy in the face of environmental stresses,the response way to clipping disturbance and saline-alkali stress is the result of trade-off between benefit and cost.The nitrogen allocation strategy under saline-alkali stress is influenced by the clipping levels: moderate clipping can increase the allocation of newly absorbed nitrogen in shoot,to provide the nutrition supply for shoot regrowth and total biomass over-compensation;however,under severe clipping,L.chinensis took the conservative nutrition allocation strategy which more nitrogen was allocated into stem base,by means,it can reduce the risk of sustained loss of restricted resources,and this strategy could be better for its survival and reproduction. |
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