Effects Of Low Light,Heat Acclimation And Relative Air Humidity On Thermotolerance Of Rhodondendron

Rhododendron is an important ornamental plant for landscaping and has high application value.Most cultivars used in urban gardens have an optimum growth temperature of 18 to 25?and grow slowly if the temperature exceeds 30?,requiring a warm,humid,cool and well-ventilated shade environment.But the continuous high temperature in summer can easily cause Rhododendron to be damaged,which seriously affects the application of Rhododendron in urban gardens.High temperature stress has become the most important environmental limiting factor for the introduction of wild plants in high mountains and the application of horticultural cultivars.In this experiment,two Rhododendron cultivars?Rhododendron'Fen Zhenzhu'and Rhododendron'Zhuangyuan Hong'?were selected,and effects of low light,heat acclimation and different air humidity treatments on the growth,physiological characteristics,photosynthesis and leaf structure of Rhododendron under high temperature stress were investigated by ways of artificial climate control method,providing theoretical basis for the production and application of Rhododendron in summer.By measuring the changes of Rhododendron gas exchange parameters,leaf anatomical parameters and physiological parameters?including relative water content,relative conductivity,contents of chlorophyll,osmoregulation substance,MDA and H₂O₂,and enzyme activity?,the physiological mechanism of effects of different treatments on the thermostability of Rhododendron was explored,the heat resistance mechanism was clarified,and a new way to effectively improve heat resistance was found.The experimental results were as follows.?1?In this study,two Rhododendron cultivars were respectively exposed at 45%,55%,65%,75%and 85%relative humidity,constant temperature treatments 38?,resulting in a vapor pressure deficit of 3.64,2.98,2.32,1.66,0.99 kPa for 14 d.Results showed that there were the least reduction of Pn under 75%RH treatment in R.‘Fen Zhenzhu'?decreased by79.8%?while under 85%RH treatment in R.‘Zhuangyuan Hong'?decreased by 75.4%?.RWC had less declined under 75%and 85%RH treatments and electrolyte leakage showed slightly decreasing under 75%RH treatment in two Rhododendron cultivars.Plants had less damage on appearance of two cultivars under higher RH,probably because plants can avoid damage by increasing total chlorophyll content and decreasing stomatal aperture,stomatal density and opened stomata ratio,enhancing enzymatic activity and osmoregulation substances,and also improving leaf structure.This study showed that to some degree higher relative air humidity can alleviate damage caused by heat stress and improve thermostability.?2?Two Rhododendron cultivars,Rhododendron‘Fen Zhenzhu'and Rhododendron‘Zhuangyuan Hong',were selected.Plants of Rhododendron were subjected to 14 days of stress in an artificial climate box with light intensity of 3000 lx,6000 lx,9000 lx and 12000lx at 38?by artificial climate control method.The results showed that combined stress had effects on the growth performance of Rhododendron plants,and with the lower light intensity,the relative water content of leaves was lower and the relative conductivity was higher.The net photosynthetic rate,chlorophyll a and b and the total chlorophyll content decreased with the decrease of light intensity.At the same time,content of soluble sugar and proline in R.‘Fen Zhenzhu'increased significantly under the combined stress of extremely low light for 3000 lx.It mainly depends on soluble sugar and soluble protein to regulate cell penetration in R.‘Zhuangyuan Hong'.The tightness and looseness of leaf structure,the stomatal density and opened stomata ratio of the upper epidermis in two Rhododendron cultivars decreased under stress.The ratio of cuticle thickness and leaves thickness of R.‘Fen Zhenzhu'decreased obviously under the combined stress of 3000 lx,and the ratio of cuticle thickness and leaves thickness of the R.‘Zhuangyuan Hong'was significantly increased under combined stress.In addition,increase of MDA was the largest under the combined stress of 3000 lx in R.‘Fen Zhenzhu',which indicated that under this treatment cells of Rhododendron leaf were damaged seriously.H₂O₂ content of R.‘Fen Zhenzhu'under 6000 lx,9000 lx and 12000 lx was lower than that of the control.This may be attributed to the high activity of POD and CAT under these treatments,resulting in the removal of most of the H₂O₂ transformed by SOD.Under the combined stress of high temperature and low light,the lower the light intensity,the greater the damage to Rhododendron.?3?Two Rhododendron cultivars were exposed to two heat stress levels?38?and 42??for 14 days,after heat acclimation at 30?for 7 days.The results showed as follow.Heat acclimation enhanced Pn of two Rhododendron cultivars by different changes of Gs,Ci and Ts.Under heat stress,heat acclimation enhanced ratio of palisade to spongy and it can ensure anatomy stabilization.Osmotic regulation substances like soluble sugar,soluble protein and Pro declined after heat acclimation in two Rhododendron cultivars.The decline of content of H₂O₂ and MDA being significantly different after heat acclimation were the results of different changes in SOD,POD and CAT activity in two Rhododendron cultivars.And the content of MDA changed significantly in the leaves of R.'Zhuangyuan Hong'.The findings suggested that heat acclimation can improve thermostability of Rhododendron,and probably the mechanism of alleviating the damage is related to cultivars.