Changes Of Unsaturated Fatty Acid Content And Photosystem Activity In Chrysanthemum Leaves Under Low Temperature Stress

In this study,the early chrysanthemum variety ’chaziyanhong’ and later chrysanthemum variety ’mobao’ were treated at different temperatures of 16℃(contrast group)、4℃ and-4℃ for 0h,3h,6h,9h and 12h respectively.The fluorescence parameters,fatty acid content and expression of fatty acid desaturase gene were measured.The changes of unsaturated fatty acid content and photosystem activity in Chrysanthemum leaves under low temperature stress were analyzed,and the physiological and biochemical response mechanism of chrysanthemum to low temperature was discussed to provide scientific basis for research and breeding of cold resistance of chrysanthemum.The specific research results are as follows:(1)Under 4℃ low temperature treatment,the activity ratio parameters of PSⅡreaction center and energy distribution ratio parameters of the two chrysanthemum varieties had no significant change,4℃ low temperature did not damage the PSⅡreaction center.At-4℃,with the extension of low temperature stress time,φPo decreased,while φDo do increased,which resulted in the decrease of electron transfer energy ratio φEo of QA-,but low temperature stress increased ABS/RC slightly,increased DIo/RC greatly,decreased TRo/RC and ETo/RC,The decrease of φPo in’chaziyanhong’ and ’mobao’ decreased with the increasing time,and the change trend of φPo,ψo and φEo in ’mobao’ was stable.Under low temperature stress,chrysanthemum leaves lost more light energy through heat dissipation,so as to protect the photosynthetic electron transport chain from being damaged by excessive excitation energy,and then reduce the damage of excessive excitation energy to PSⅡ reaction center.-4℃ low temperature had little effect on the photosynthetic reaction center of’mobao’.During 4℃ low temperature treating time,the parameters of PSⅠ such as REo/RC,REo/CSo,REo/CSm,φRo、φRo、and δRo increased as a whole.The changes of electron transfer between PSⅠ and PSⅡ and on the receptor side of PSⅠ in Chrysanthemum leaves were positively affected by 4℃ low temperature.under-4℃low temperature,with the extension of time,three parameters of PSⅠ capacity utilizationφRo、ψRo and δRo in ’mobao’ overall although have change but in the dynamic stability.The value of φRo、ψRo and δRo in ’chaziyanhong’ changed with time,meanwhile the MR/MRo value which reflect the activity of PSⅠ curve remain the state of decline in 4℃low temperature,as time going on,MR/MRo value of ’mobao’ decline slowed.The above results reflected that low temperature stress caused damage to PSI in chrysanthemum leaves,resulting in a decrease in PSI capacity utilization.However,with the extension of stress time,the PSI damage caused by low temperature stress recovered to some extent,and the PSI recovery capacity of ’mobao’ was significantly higher than that of’chaziyanhong’.The overall performance parameters of PItotal,PIabs and PIcs increased with the extension of time at 4℃ low temperature.The 4℃ low temperature stress improved the performance of photosynthetic mechanism in leaves,which was conducive to the more efficient synthesis of nutrients needed for plant growth and development.The overall performance parameters of PItotal,PIabs and PIcs of the two varieties of chrysanthemum leaves showed a downward trend at the low temperature of-4℃,but the decreasing range decreased with the extension of the treatment time.It can be found that although the low temperature caused the overall performance of the leaf photo synthetic mechanism to decline,the damage degree of the photosynthetic mechanism decreased with time.(2)The expression of fatty acid desaturated gene in chrysanthemum leaves showed an overall up-regulation trend,At low temperature of 4℃,the expression of the three fatty acid desaturase gene increased with the extension of time,and the content of linoleic acid and linolenic acid presented a consistent trend.4℃Low temperature stress caused the up-regulation of the expression of fatty acid desaturase gene in chrysanthemum,which promoted the increase of the content of unsaturated fatty acids,thus responding to low temperature stress.The expression level of CmFAD2 in’chaziyanhong’ and ’mobao’ were firstly down-regulated and then up-regulated with treated for-4℃ while the C18:2 value gradually increased during low temperature stress time,indicating that there was a certain lag in the response of CmFAD2 gene to low temperature in the early stage of stress.Compared with the control group,the expression level of CmSAD and CmFAD7 genes in ’chaziyanhong’ and ’mobao’ rose first and then decreased in the process of-4℃ low temperature treatment.The content of C18:3 increased in the process of low temperature stress,indicating that chrysanthemum respond to low temperature stress by upregulation of CmSAD and CmFAD7 gene expression,increasing the content of unsaturated fatty acids,reducing the phase change temperature,and changing the biofilm fluidity at low temperature.After 9h of low temperature stress,the expression of CmFAD2 and CmFAD7 gene expression decreased,but the content of C18:2 and C18:3 increased,which may be related to the response of other members of FAD gene family to low temperature stress in chrysanthemum.(3)There were significantly negative correlation about DBI、USFA/SFA and C 18:3 with φPo,φEo,PItotal,PIabs and PIcs,meanwhile the value of φPo,ψo,φEo which characterization of the parameters of the energy distribution ratio and PItotai,PIabs and PIcs that on behalf of chrysanthemum leaf photosystem performance parameters were decreased treated with-4℃,but with the extension of low temperature processing time,the value of φPo,φEo,PItotal,PIabs and PIcs slowed down.the value of REo/RC,REo/CSo which reflect the PSI and PSII unobstructed degree and coordination of the parameters of the energy transfer changed consistent with PIabs,PIcs.And unsaturated fatty acid content in chrysanthemum leaves with the extended time of low temperature showed obvious increasing trend,and The above results showed that although low temperature stress caused damage to photosystem of chrysanthemum leaves,the increase of unsaturated fatty acid content alleviated the damage and protected the structure and function of photosystem to a certain extent.(4)Under low temperature stress of 4℃ and-4℃,the expression level of three fatty acid desaturase gene in the leaves of the two varieties was up-regulated,which promoted the increase of the content of unsaturated fatty acids,protected the biofilm,especially the photosynthetic membrane,from the damage of low temperature,protected the stability of the structure and function of the optical system,and improved the performance of the photosynthetic mechanism in the leaves treated with 4℃.The low temperature of-4℃ had different effects on optical system and thylakoid of the two varieties,and the increase of unsaturated fatty acid content slowed down the effect.There was a strong resistance to low temperature in ’mobao’.Under low temperature stress,’mobao’ leaves had higher content of unsaturated fatty acids,and the content of unsaturated fatty acids increases significantly with the extension of time,responding to low temperature.