Effect Of Mitochondrial AOX Respiratory Pathway On Photosynthetic Activity And Growth Dynamics Of Alfalfa Leaves Before And After Overwintering

Medicago sativa L.,the"king of forage grasses",is widely planted in Chian,especially,Xinjiang.As the special geographical and climatic conditions in Xinjiang,the low temperature and cold wave caused by spring cold often occurs,which seriously affects the growth and production of alfalfa and limits the development of grass animal husbandry in Xinjiang.The Alternative oxidase（AOX）respiratory pathway is unique to plants and helpful to improve the stress resistance of plants.Photosynthesis is the basis for plant growth,development and yield composition.Nevertheless,it is rarely reported how the AOX affects leaf photosynthesis in alfalfa during the returning green process and then improves the cold resistance of plants.In this study,as research materials,Medicago sativa L.cv.Xinmu No.4 and Medicago sativa L.cv.Gannong No.5 were planted in Xinjiang agricultural university sanping practice teaching base.Salicylhydroxamic acid（SHAM）,AOX breathing specific inhibitor was used to study the effect of AOX respiratory pathway on alfalfa photosynthesis and growth.The photosynthetic activity,stomatal morphology,reactive oxygen production were tested in early May after overwintering and in late October before overwintering in 2019-2021.The main study results are as follows:（1）By limiting the AOX,two varieties of alfalfa in experimental monitoring growth dynamics of 12 weeks,growth index,such as leaf length,leaf width,stem,plant height,decreased significantly,the first crop of grass production also decreased,which shows that AOX respiration pathway to maintain the growth and development of alfalfa,and has an vital contribution to the grass production.（2）After restricting the AOX,chlorophyll content,photosynthetic rate,stomatal conductivity,light saturation point and apparent quantum efficiency were all greatly decreased in two varieties of alfalfa,and the decline occured before and after overwintering.It demonstrates that AOX limits the photosynthetic rate of alfalfa leaves by affecting the light energy utilization efficiency and photosynthetic mechanism of alfalfa leaves,and the effect of AOX is more pronounced after overwintering.Further investigation of the effect of AOX on the stomatal length and width of alfalfa leaves before and after overwintering,as well as the length and width of the aperture,showed a significant decline,and the decline was more significant after overwintering than before,which shows that AOX affects stomatal development and stomatal morphology,limited CO₂mesophyll cells were entered for photosynthesis.In addition,the decline in photosynthesis of alfalfa leaves was due to stomatal limitation during during the returning green process,while that was due to non-tomatal limitation before overwintering.（3）The inhibition of the AOX,the Fv/Fm decreased significantly in alfalfa before and after overwintering,indicating that AOX affects the maximum PSII photochemical efficiency of alfalfa leaves.As the K point reflects the activity of the PSII electron transport chain from the donor side oxygenation complex（OEC）,the j point reflects the transmission of the Q_A to the Q_B on the PSII electron donor side,and the I point shows the transmission of the Q_B to the PQ,all of them showed no significant increase,and there is less variation before and after overwintering,it shows that AOX has no obviuos effect on the electron transport through PSII in alfalfa leaves.The energy distribution of PSII,including the effective quantum yield QY,non-photochemical chlorophyll fluorescence quenching NPQ,and photochemical quenching coefficient Qp,.It showed that AOX had no significant effect on the energy distribution of PSII in alfalfa leaves before and after overwintering.（4）The restriction of the AOX respiratory pathway resulted in a significant increase in H₂O₂content in alfalfa leaves and an increase in the degree of peroxidation（MDA）of the cystoid membrane,the effect of which was very pronounced after overwintering,leading to increased oxidative damage to alfalfa.In addition,the activity of reactive oxygen scavenging enzymes in alfalfa leaves decreased after AOX was inhibited,with a significant decrease after overwintering,resulting in a weakened antioxidant capacity in alfalfa.This suggests that the AOX respiratory pathway has a significant role in reducing oxidative damage in alfalfa leaves by reducing the production of reactive oxygen species and affecting the activity of reactive oxygen scavenging enzymes after overwintering.（5）The comparative study of the two varieties of alfalfa after overwintering showed that,the inhibition of the AOX lead to a greater decrease in photosynthetic rate and light energy utilization and other related indicators,and a higher accumulation of reactive oxygen species in Xinmu No.4with higher cold-resistant.The results indicated that AOX improved the cold resistance of alfalfa by affecting the light energy utilization efficiency,photosynthetic mechanism activity and stomatal development of alfalfa leaves,and maintaining the redox balance in leaves during the process of turning green after overwintering.