| Rhododendron is one of the famous ornamental flower in China,which has great varieties,rich colors,and high ornamental and economic value.The most suitable habits for Rhododendron is widely distributed in high altitude areas,and the most moderate growth temperature for it is around 12-25?.It will go into the dormant state and will inhibit the growth of blade tips and new leaves.At present,most valuable varieties of rhododendrons are poor in heat resistance,and summer high temperature has become the main environmental factor that restricts their growth and development.In summer Fujian Province is more hot and humid,the maximum temperature can up to 40?,limiting many excellent varieties of Rhododendron in landscaping applications.To find the most suitable cultivars planted in Fujian province,plant physiological and biochemistry indexes,the semi-lethal temperatures,browning indexes and chlorophyll fluorescence of nine Rhododendron cultivars.1.The changes of rate of injured cells and the high semi-lethal temperatures(LT50)were studied with four-year-old nine Rhododendron cultivars by using conductance method.At the same time observed the morphological characteristics and calculated the browning index of 9 Rhododendron cultivars under heat treatment by using plant growth chamber to study on the relationship of Rhododendron cultivars between the high semi-lethal temperatures and the heat resistance and the heat resistance level.The results showed that the rate of injured cells on leaves of Rhododendron increased like "S" curve;the high semi-lethal temperatures for Rhododendron cultivars was:'shiyan':53.84?,'liuqiuhong':52.71?,'baidanmai':52.35?,'tianzhang':51.62?,'chunshi':51.33?,'daqinglian':50.67?,'fenwubao':48.40?,'yudaizhirong':47.40?and 'xiahong':46.92?.The conclusion showed that the heat resistance of Rhododendron cultivars could be accurately reflected by the high semi-lethal temperature and the order of heat resistance for Rhododendron cultivars was:'shiyan'>'liuqiuhong'>'baidanmai'>'tianzhang'>'chunshi'>'daqinglian'>'fenwubao'>'yudaizhirong'>'xiahong'.Nine Rhododendron cultivars were to be divided into three classes via dendrogram for the cluster analysis of LT50 and browning index:? includes:'shiyan' and 'liuqiuhong' ? includes 'tianzhang','chunshi','baidanmai' and'daqinglian';? includes 'fenwubao','yudaizhirong' and 'xiahong'.2.Seven physiological indexes of nine Rhododendron cultivars were measured and the results showed that:after 35? heat treatment,the SOD activity of nine cultivars increased than that of control,but after 40?,the activity of SOD declined.The POD activity and proline contents of nine cultivars all strengthened as temperature increasing,and for proline contents it they had negative correlation with the heat resistance of different cultivars after 35? but had positive correlation at 40?.The contents of MDA and H2O2 increased with the stress temperature growing,and the contents of soluble protein showed negative trends,with the increasing of the treatment temperature the contents of soluble protein declined.The result of component analysis of showed that Pro,MDA,chlorophyll contents and H2O2 were important in high temperature stress evaluation of Rhododendron plants.The order of heat resistance for Rhododendron cultivars was:'shiyan'>'liuqiuhong'>'baidanmai'>'tianzhang'>'daqinglian'>'chunshi'>'fenwubao'>'yudaizhirong'>'xiahong'.3.The effects on chlorophyll fluorescence leaves of Rhododendron.L in nine cultivars were studied.The results showed that for nine varieties,after 3 different heat stresses,maximum efficiency of PSII(Fv/Fm),the photochemical quenching coefficient(qP),actual PSII efficiency(Y(?)),electron flow through PSII(ETR(?))decreased.Fv/Fm of 'tianzhang','fenwubao','chunshi','yudaizhirong','xiahong','baidanmai' and'daqinglian' decreased significantly after 40? and 50?,but for 'shiyan'and 'liuqiuhong' Fv/Fm decreased significantly only after 50? heat treatment.Y(?)and qP of 'tianzhang','fenwubao','chunshi','yudaizhirong','xiahong','baidanmai' and 'daqinglian'showed significant reduction after 3 different heat stresses.For 'shiyan' and 'liuqiuhong' qP decreased significantly after 50? and Y(?)reduced evidently after 40?and 50?.The changes for value of non-photochemical quenching(NPQ)showed irregular for the nine cultivars.High temperature showed significant influence on the distribution of energy on PSII,with the increase of the heat stress,the value of Y(?)decreased,Y(NO)increased sharply,and Y(NPQ)increased firstly and then decreased.After 40? 'tianzhang','fenwubao','chunshi','yudaizhirong','xiahong','baidanmai' and 'daqinglian'all showed photo inhibition on PSII,but for 'shiyan' and 'liuqiuhong',the PSII showed photo inhibition after 50?.The results showed that PS? had higher conversion efficiency of heat-tolerant cultivars compared with non-heat-tolerant cultivars under high temperature.Excess excitation energy of photosynthetic apparatus was lower,the chances of generating reactive oxygen species was less,PS? reflected more opening of the center,the influence of electron transfer rate is small,effectively protecting photosynthetic apparatus. |
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