| Water deficit is one of the main abiotic stress which seriously affects the development of the pear industry in Chinese arid and semi-arid regions.At present,pear seedling production is mainly propagated by grafting on the rootstock.Pyrus betulaefolia Bunge and Pyrus calleryana Decne are used as grafting rootstocks in the northern and southern Chinese distribution area of pear,but their ability to resist and adapt to drought vary.Research suggests that rootstocks are essential for cultivar resistance.Therefore,the twoyear-old seedlings of Pyrus betulaefolia and Pyrus calleryana were used as materials to carry out potted water control tests,the change characteristics of growth,photosynthesis,stomatal morphology,the content of chlorophyll and carotenoid and chlorophyll fluorescence parameters were monitored to analysis both the ability and counter measures to adapt to drought.The water conductivity characteristics and stem flow rate of plants were analyzed by HPFM high-pressure water conductivity meter and Flow 32 wrapped stem flow meter respectively.The changes of gene response of pear rootstocks' roots under severe drought stress were analyzed by transcriptomics.The molecular mechanism of the difference in drought resistance between the two pear rootstocks was discussed in order to provide a theoretical basis for the further study of fine water management and drought tolerance mechanism of pear rootstock at seedling stage.The main findings are as follows:1.The results showed the growth of P.betulaefolia was significantly higher than that of P.calleryana under the same environmental conditions.The relative growth and the growth rate of pear rootstocks new shoot decreased under drought stress,P.betulaefolia was less affected by stress than P.calleryana.2.The diurnal variation of net photosynthetic rate(Pn)between the two pear rootstocks was significantly different,P.betulaefolia was single-peak curves while P.calleryana was bimodal curves,and the Pn value of P.betulaefolia was higher than P.calleryana.The stomatal conductance(Gs)and transpiration rate(Tr)of P.betulaefolia was significantly higher than that of P.calleryana as the temperature rises,but the RWUE of P.betulaefolia was still at a high level.The stomatal density of lower epidermis of P.calleryana was 1.5 times that of P.betulaefolia.Both the daily variation of stomatal length and stomatal opening of two pear rootstocks were similar,but the minimum stomatal opening of P.betulaefolia appeared at 14 o 'clock in the daytime,while that of P.calleryana appeared at 12 o 'clock in the daytime.The results of correlation analysis showed that there was a significant negative correlation between RWUE of P.betulaefolia and diurnal environment temperature change(P <0.05),and a significant positive correlation between RWUE and relative humidity of the leaves(P <0.05),while there was a highly significant negative correlation between RWUE of P.calleryana and the diurnal environment temperature(P < 0.01),and a significant positive correlation between RWUE and leaf surface relative humidity(P < 0.05).Therefore,stomatal of P.calleryana more sensitively and sharply responsed to environment such as high temperature and high light.With daily temperature increasing,the phenomenon of stomatal closure appeared earlier,which weakened photosynthesis,and P.betulaefolia were able to maintain the normal stomatal morphology,higher stomatal conductance and transpiration rate,stronger photosynthetic ability,and higher water use efficiency,so as to showed stronger environment adaptability.3.The results showed that the net photosynthetic rate(Pn)of the two kinds of rootstocks gradually decreased under drought stress,and there were significant differences among different drought treatments.The Pn value of P.calleryana decreased by 44.81% and 78.04% at 45 days under treatment of moderate and severe drought respectively,while that of P.betulaefolia Bunge decreased by 36.8% and 74.5% respectively.There was a consistency between the change trend of chlorophyll content and net photosynthetic rate.The maximum photochemical efficiency(Fv/Fm),actual photochemical quantum efficiency(?PSII),photochemical quenching coefficient(q P)and electron transport efficiency(ETR)of both P.betulaefolia and P.calleryana decreased,the minimum initial fluorescence(F0)and non-photochemical quenching coefficient(NPQ)were significantly increased.At 45 days,the NPQ value of P.betulaefolia increased by 21.7%?42.7%?68.9%,while P.calleryana increased by 12.0%?42.4%?53.0% under the mild,moderate and severe drought stress respectively.Therefore,the chlorophyll content reduced,the leaves of the pear rootstocks were photoinhibited and the photosystem II(PSII)was destroyed in case of water deficit,the photosynthesis of both the leaves of Pyrus betulaefolia and Pyrus calleryana seedlings were obviously weakene,and the shoot growth were inhibited.However,the photosynthetic apparatus in P.betulaefolia Bunge can be protected more effectively than that in Pyrus calleryana to deal with the damage caused by stress.4.The leaf special hydraulic conductance of Pyrus calleryana canopy were higher than P.betulaefolia's with normal water supply,but the differences were not significant.The decrease amplitude of the leaf special canopy hydraulic conductance of Pyrus calleryana canopy decreased more greatly under drought stress.The leaf special hydraulic conductance of P.betulaefolia stem decreased with the increasing of stress degree,while the changing trend of Pyrus calleryana is HD > CK > MD > LD,that was,decreasing firstly and then increasing with the decrease of soil water content.The stem special hydraulic conductance of stem of two pear rootstocks was similar.5.Under normal water supply conditions,both P.calleryana and P.betulaefolia's diurnal variation of flow rate were "wide peak type" curve.P.betulaefolia could start the water flow activity earlier,however,its peak of the water flow rate was slightly lower than that of P.calleryana.Under light drought stress,the flow rate of P.betulaefolia increased sharply,reaching a peak of 18.29 g/h at 14:55,which was 1.64 times of that under normal water supply conditions.The peak flow rate under moderate and severe drought stress decreased to 72.6 % and 42.2% of the control respectively.Under light,moderate and severe drought stress,the peak flow rate of P.calleryana decreased to 80.67%,53.52% and 6.63% of the control respectively,that was,if the similar drought stress was suffered,the descent range of P.calleryana' flow rate had been significantly higher than that of P.betulaefolia.6.DEseq was used for different expression analysis,and it was found that under severe drought stress,there were much differentially expressed genes in both P.betulaefolia and P.calleryana.The up-regulated genes and down-regulated genes of P.betulaefolia were higher than that of P.calleryana,and the difference in down-regulated genes was more significant.According to the analysis of 2585 DEGs specifically ecpressed in root of P.calleryana,72.73% of all were up-regulated,while the rest were down-regulated.GO enrichment analysis showed that there are respectively 114,22 and 64 different expressed genes enriched in the oxidation-reduction process,response to stimulus and response to stress.The KEGG enrichment analysis found that the different expressed genes uniques in P.betulaefolia are rich in DEGs which related to redoxrelated process,secondary metabolites such as phenylpropanoids,and hormone signaling pathways such as ABA may be involved in the improvement of drought-resistance and adaptability under the drought condition. |
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