Preliminary Research On Physiological Mechanism Of Thermostability In Herbaceous Peony

Herbaceous peony (Paeonia lactiflora Pall.), one of the traditional famous flowers, has more than4000years of cultivation history in China. It is widely used in landscape construction because of its various cultivars, big and colourful flowers, and high ornamental values. Although herbaceous peony likes warm climate, it is afraid of heat condition.In summer at south areas of China, it is usually exposed to strong light and high temperature, and thus its growth is severely affected, and finally the plant wilts to death gradually. Therefore, it is extremely necessary to identify and evaluate heat tolerance of different germplasms, to screen heat-disistent cultivars, to study the mechanism of heat tolerance, apply effective measures to improve heat tolerance. This paperinvestigated the diversity of heat resistance of30herbaceous peony cultivars, analyzed the correlation between11morphological and physiological indexes, including relative electrolytic leakage, SPAD, photosynthetic rate, transpiration rate, leaf thickness, thickness rate of palisade and sponge tissue, stomatal density, plant height, width, the maximum leaf length and width to hot injury index. We discussed The internal mechanism of heat resistance diversity between different cultivars and the influencing mechanism of shade treatment delayed heat damage were discussed at morphology, physiology and molecular biology levels. The results of this paper are as follows:1Through investigation of30herbaceous peonys cultivars, we find that’Honglingzan’,’Bitianqingkong’,’Zifengyu’ and ’Fenlanlou’ are much stronger in heat tolerance, their hot injury index are0.20,0.22,0.24and0.30, respectively; while’Fenhepiaoxiang’,’Hewanglan’,’Fenyulong’ and ’Dafugui’ are relative weaker in heat tolerance, their hot injury index are0.97,0.95,0.92and0.92, respectively. Then stepwise regression analysis was used to predict the best group of factors for hot injury index and main morphological and physiological indexes, the results indicated that relative electrolytic leakage, SPAD and photosynthetic rate had highly significant correlations with hot injury index, and the adjusted determination coefficient was0.84.2The cell of heat-resistance ’Zifengyu’ had some big plastoglobulis, some chloroplast envelope were indistinct under high temperature, but its cellular structure was relatively complete. While the heat-susceptible ’Hongyanzhenghui’ had barely entire cellular structure with a large number of plastoglobulis diffused in the cell, and its grana lamella were disorganized, some chloroplast envelope were decomposed, g rana lamella had escaped. Meanwhile, the malondialdehyde (MDA) content of ’Zifengyu’increased slowly, which was only37%of that in’Hongyanzhenghui’, and the superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) activities were higher than those of the latter, being1.08,1.08,1.71times, respectively. The photosynthetic pigment content of ’Zifengyu’ was significantly higher than ’Hongyanzhenghui’, chlorophyll and carotenoid content were respective1.98,1.61times of’Hongyanzhenghui’, the photosynthetic rate was also higher than the latter. The relative expression level of HSP60, HSP70, HSP90of ’Zifengyu’ were all higher than ’Hongyanzhenghui’, being1.46,1.17,2.22times of the latter, respectively. All the results indicated that heat-resistance cultivars can regulate its high protective system activity and HSPs expression level, to clear away reactive oxygen species, reduce membrane lipid peroxidation, and decrease high temperature injury under high temperature condition.3The MDA content of heat-sensitive cultivar’Dafugui’increased slowly under shading treatments, which was only53%of control (exposed to full sunlight); SOD, POD activities were significantly higher than the control, being1.62,1.61times of the control, respectively. The reduction of plant photosynthetic pigment content under shading treatments were significantly less, and the contents of chlorophyll a, chlorophyll b, total chlorophyll and carotenoid were3.28,3.25,3.21,1.21times of the check’Dafugui’, respectively. The influence of shading treatment on different HSPs expression were different shading treatment let the relative expression level of HSP60be significantly lower than the control, which was only47%of the control. But the relative expression level of HSP7and HSP90were higher thanthe control, which were2.63,1.26times of the latter, respectively. The cellular structure of herbaceous peony under shading treatment was less damaged, though its chloroplast trended toward round, some chloroplast envelopes were indistinct, but the number and size of plastoglobulis were smaller than the control, and grana lamella were in order. Shading treatment enhanced protective enzyme activity such as SOD and POD, and relative expression level of HSPs, reduced the accumulation of reactive oxygen species and the damage to cellular structure and photosynthetic pigments. So shading treatment for herbaceous peony after flower season can reduce heat damage and delay plant senescence and death.