Effects Of Heat And Drought Stress On Physiology And Biochemistry Of Heimia Myrtifolia

Heimia myrtifolia is a branch of Lythraceae subordinate to Heimia.It is an adaptable shrub with abundant golden yellow flowers and long flowering period.H.myrtifolia is easy to reproduce?manage and shape.It grows rapidly and has good plantation coverage.Soil moisture loss with high natural temperature and drought conditions,without replenishment,were artificially simulated.We considered four levels of drought stress,no stress(control),light,moderate,and heavy,and three levels of heat stress,30?(control),36?,and 42?.Malondialdehyde content,ascorbate-glutathione cycle related enzyme activity and related substance content,osmotic adjustment substance content,gas exchange parameters and chlorophyll fluorescence parameters were measured.The results are as follows:(1)With drought only or high temperature only stress,the activity of superoxide dismutase(SOD),peroxidase(POD)and catalase(CAT)each increased,and the increase with high temperature stress was higher than that under drought stress,which indicated that H.myrtifolia was more sensitive to high temperature stress.As A-GSH cycle-related enzymes and related reducing substances increased with both drought and high temperature stress,but the cycle efficiency began to decrease with the deepening of stress.With the simultaneous stress of high temperature and drought,the activity of each of the three aforementioned enzymes were higher than when with a single stress and the cycle efficiency was significantly lower than that under either single stress.(2)The content of malondialdehyde(MDA)increased significantly with each incremental increase of stress degree,and the simultaneous stress of drought and heat was significantly higher than each as a single stress.The permeability of cell membrane increased and the function of the cell membrane was inhibited.With both drought only and high temperature stress,water loss was reduced by increasing the contents of soluble sugar,proline and soluble protein.With simultaneous stress,H.myrtifolia maintained water by increasing soluble sugar and proline content,but soluble protein content decreased under the influence of temperature.(3)The decrease of Pn in the early and middle stages of drought stress was mainly restricted by stomatal factors,while at the end of drought stress it was restricted by non-stomatal factors.The decrease of Pn was mainly limited by non-stomatal factors with high temperature stress.The maximum photochemical efficiency Fv/Fm and potential photochemical efficiency Fv/Fo of PSII both significantly decreased,indicating that H.myrtifolia had been restrained by light with drought stress.Fv'/Fm',the activity of PSII reaction center and the electron transfer efficiency ETR decreased significantly with increasing levels of stress.With simultaneous stress,the photosynthesis was more affected under simultaneous stress.(4)With drought stress,except POD activity,CAT activity and individual photosynthetic index,the variation range of other indexes was significantly higher than that with high temperature stress,which indicated that H.myrtifolia was more sensitive to drought stress.