Study On The Mechanism Of Improved Drought Toleration In Hangzhou White Chrysanthemum By Grafting Onto Artemisia Annua

Drought is the one of common abiotic stress,and it is of great significance to resisting drought and saving water of plants by grafting.Chrysanthemum is susceptible to drought stress,which seriously affects the yield and quality of chrysanthemum.We studied the effects of grafting with Artemisia annua as rootstock on the growth,injury degree,photosynthetic activity,osmotic regulation and related gene expression of chrysanthemum under natural drought stress condition.The main results were as follows:1.Under drought stress,the relative water content?RWC?of chrysanthemums leaves decreased and the growth was inhibited,but leaf RWC was significantly higher in grafted plants.On day 6 and 8,compared to cutting plants,the leaf RWC of the grafted plants had increased by 53.54%and 63.62%,respectively.Grafted plants could maintain higher growth mass and lower inhibition degree of root growth.It was found that the leaf thickness,upper epidermis thickness,palisade tissue thickness and pal.and sp.ratio were significantly higher in grafted plants.The leaf structure of grafted plants was more favorable to the water retention,which accounted for higher RWC and dry weight of leaves.2.The damage of the membrane caused by drought stress in plants could relieve with grafting.The relative conductivity,the content of malondialdehyde?MDA?,the rate of superoxide anion(O₂^·-)and hydrogen peroxide?H₂O₂?increased in plants under sress.On day8,these indexes in cutting plants were 1.27?1.39?1.36 and 1.30 times higher than in the grafted plants.The production of lipid peroxidation was reduced,and the damage of cell membrane was relieved with grafting.3.The chlorophyll degradation,the activity of photosynthetic enzymes,net photosynthetic rate?P_n?and transpiration rate?T_r?decreased in plants under drought stress,and stomatal conductance?G_s?increased firstly and then decreased,which resulted in the damage of photosynthetic apparatus and the decrease of water use efficiency?WUE?.On day6 and 8,the WUE in the grafted plants was 6.00 and 6.25 times higher than in the cutting plants.The decrease of P_n was ascribed to stomatal limitation,but with the prolongation of stress,it was switched to be inhibited by non-stomatal restriction.The decrease of chlorophyll and Rub activity were slower in grafted plants,which resulted in higher P_n and T_r,and postponed occurrence of non-stomatal limitation.4.The transcripts of photosynthetic-related genes under drought stress were higher in grafted plants,which partially contributed to higher photosynthetic capability of plants.The expression of CmrbcL was inhibited in both plants under stress,while the mRNA levels of CmrbcL in grafted plants was was 2.28 and 3.91 times higher than in the cutting plants at 2and 6 d.The expression of CmrbcS in grafted plants was induced by light drought,while the CmrbcS in cutting plants maintained at a lower level.A higher Rub activity was observed in grafted plants,which was resulted from higher expression of CmrbcL and CmrbcS.At 2-8days,the relative expression of CmpsaB in grafted plants was significantly higher,which could speed up of photosynthetic electron transfer.The expression of CmCab in grafted plants was always higher than that in cutting plants,which was partially contributed to higher chlorophyll content in grafted plants.5.It was capable to improve the osmotic adjustment ability of plants under drought stress with grafting.The soluble sugar and proline content was significantly increased by drought stress.These substance in cutting plants was higher and earlier accumulated,which was indicative of more sensitivity to drought stress.At 8 d,the soluble osmotic regulators were accumulated in grafted plants with higher level,which has a higher osmotic adjustment capability.The correlation analysis suggested that proline was the major osmotic regulator,glutamate pathway was account for the proline accumulation at the early stage,while both ornithine and glutamate pathway were responsible for the proline at later stage.6.At the early stage of stress,the expression of CmP5CR,CmP5CS,Cm?-OAT,CmNI,CmAI and CmSPS in cutting plants were higher which prompted the early accumulation of osmoregulation substance contents.On day 8,the transcript levels of the CmNI and CmSPS,were respectively 1.93 and 2.20 times higher than in the non-grafted plants.CmP5CS gene expression in the grafted plants differed significantly from the cutting plants,being 1.66 times higher.the expression of CmProDH in the grafted plants was 61.99%of that in the cutting plants.It is inferred that higher osmotic regulation in the grafted plants owed to the higher soluble sugar and proline synthesis capacity.