Study On Evaluation Of Heat Resistance Resources And Physiological Mechanism Of Rhododendron

Rhododendron is one of Chinese traditional ornamental flowers of the top 10 with a wide variety and high ornamental value.At present,the cultivars of R.applied in landscaping are scarce while Rhododendron pulchrum Sweet and some Rhododendron simsii Planch takes up a big share.Previous studies have shown that high temperature heat damage is one of the main factors limiting the application of fine R.cultivars in landscaping.Therefore,research on the heat-tolerant physiological mechanism of R.cultivars and the screening of heat-resistant cultivars resources have an important reference value for promoting the garden application of fine R.cultivars.In this study,the identification and evaluation of field heat tolerance were carried out on some R.species resources.The photosynthetic characteristics,antioxidant properties and expression characteristics of heat shock protein gene 20(HSP20s)of different cultivars with heat tolerance were compared.The main results of the research are following.1 Through the open field investigation of 34 R.cultivars,it is found that both the heat damage index and the SPAD value can be used as important indicators for the identification of heat resistance.Cultivars with high heat damage index,small SPAD value and fast deceleration are weak in heat resistance while those with low heat damage index,large SPAD value and slow speed have great heat resistance.Among them,cultivars with great heat resistence include’FengmeiIhao’,’Fentuan’,’Zichanghua’ and ’Paozhanghong’ while ’Yangzhizhu’,’Zijinfenyu’,’Fenghongpaopao’ and ’Xiaoying’are weak heat resistant cultivars.2 By studying the photosynthetic characteristics of six R.cultivars,we can foud that the photosynthesis rate,transpiration rate,CO2 utilization rate and light energy utilization rate of’Fengmeilhao’ and ’Paozhanghong’ were higher than other cultivars in June,which indicated that they grow better during this period.In the hot weather in July and August,the photosynthesis rate,water use efficiency,CO2 utilization rate and light energy utilization rate of’Zijinfenyu’ and ’Xiaoying’ were lower than other cultivars,which indicated that they have weak adaptability to the high temperature.Using gray correlation analysis and fuzzy comprehensive evaluation to find out the comprehensive photosynthetic capacity of six R.cultivars,we can foud that the great heat resistant cultivars ranked higher than weak heat resistant cultivars.Under the pressure of high temperature,the great heat resistant R.cultivars have stronger photosynthesis ability.3 The content of malondialdehyde and relative electrical conductivity of ’FengmeiIhao’ and’Paozhanghong’ were lower than ’Zijinfenyu’ and ’Xiaoying’ at the same period under the high temperature.The activities of antioxidant enzymes,proline and chlorophyll were higher than’Zijinfenyu’ and ’Xiaoying’ at the same period.It indicates that ’Fengmeilhao’ and’Paozhanghong’ have little damage under high temperature.Under high temperature stress,the great heat resistant R.cultivar cell membrane was relatively stable,and the antioxidant enzyme activity was high,which was beneficial to reduce membrane lipid peroxidation and the damage of high temperature to plants.4 The expressions of HSP20s gene in great heat resistance R.cultivars ’FengmeiIhao’and weak heat resistance R.cultivars ’Xiaoying’ were carried on.The results showed that under the treatment of 25,35 and 40 0C,with the change of temperature and time,the expression of HSP20I gene in ’Fengmeilhao’ increased first and then decreased.Under the treatment of 35 C,the expression of HSP20I gene in ’FengmeiIhao’ was overall higher than other moment.The expression of HSP20II gene in ’FengmeiIhao’ showed an increasing trend.Under the treatment of 40℃,the expression of HSP20II gene was overall higher than other moment.The expression of HSP201 and HSP20II genes in ’Xiaoying’ were not changed much,and the expression level was low,and the expression of HSP201 and HSP20II genes were overall lower than that in’Fengmeilhao’t the same period.Under high temperature stress,the overall trend showed that the expression of HSP20s gene in great heat resistant R.cultivar was higher than that in the weak heat resistant R.cultivar.