Effects Of Different Air Relative Humidity On The Growth And Development Of Cucumber Under High Temperature

Cucumber is the vegetable with the largest facility cultivation area in China.Under the influence of high temperature and humidity stress,cucumber is prone to overgrowth,poor differentiation of flower buds and serious meltification.In this study,the artificial environment control experiment was carried out with"Jinyou 101"cucumber as the test material.Three factors orthogonal experiment of temperature,relative humidity of air and duration was designed.Four levels of day/night temperature were set at 32℃/22℃,35/25℃,38℃/28℃and 41℃/31℃.The air relative humidity was set at（50±5）%,（70±5）%and（90±5）%levels,and the duration was designed for 3,6,9 and 12 days.The day temperature was 28℃/night temperature was 18℃and the air relative humidity was（50±5）%as the control（CK）.By measuring photosynthetic parameters,photosynthetic pigment content,rapid fluorescence kinetics curve,protective enzyme content,crown and endogenous hormone content in fruit,observation of each section of a flower,fruit transverse and longitudinal diameter,abnormal fruit rate and final yield,the effects of different air relative humidity on cucumber vegetative growth,floral differentiation and fruit development under high temperature were investigated.The comprehensive evaluation model of flowering characteristics was established,and the disaster stress levels of different high temperature,air relative humidity and duration were established.The main results are as follows:（1）The deeper the degree of high temperature stress,the lower the Chl a,Chl b,Car,LSP,AQE and Pmax of cucumber leaves,and the higher the LCP;The photosynthesis of leaves under RH70%treatments were stronger than the other two air relative humidity treatments.Under high temperature stress,the OJIP curve of the leaves decreased significantly in the J-I and I-P stages,and the activity of the PSⅡreaction center decreased,the size of the functional antenna increased,the energy transfer of the thylakoid was inhibited,the OEC was inactivated,and the reduction rate of the PSI-terminal electron library was accelerated.The contents of CAT,SOD and POD in cucumber leaves increased firstly and then decreased with the increase of temperature and air relative humidity.The content of MDA and Sp increased with temperature.（2）The contents of ZT,IAA and GA₃ in terminal buds decreased under high temperature stress,while the contents of ABA increased.The contents of ZT,GA₃ and IAA in terminal buds treated by RH70%humidity were higher than those of the other two air relative humidity treatments.Under high temperature stress,the number of female flower buds decreased,the node position of female flowers increased,and the node ratio of female flowers decreased.The comprehensive evaluation of flowering characteristics of cucumber under high temperature stress was carried out by using fuzzy mathematics method.The score of fuzzy comprehensive evaluation showed that when the relative humidity was 70%and the duration was short,the index value of flowering characteristics of cucumber was better,the score was high,and the ranking was relatively high.The R² of the comprehensive evaluation model of cucumber flowering characteristics was 0.980,which could simulate the measured data well.（3）Under the influence of high temperature,the development speed of cucumber fruit transverse diameter gradually slowed down,and the beginning point,peak point and end point of fruit diameter growth was delayed compared with control,and the rapid growth time was also increased.The development of fruit transverse and longitudinal diameter under RH70%treatment was close to CK,with low malformed fruit rate and high yield.Compared with normal fruits,malformed fruits under high temperature stress had lower ZT content and higher ABA content in the middle part,which resulted in growth retardation in the middle part.The content of IAA in the upper part was accumulated from other parts,making the upper part of the fruit enlarged.（4）Principal component analysis was conducted on physiological growth indexes,and six indexes:Chla,Pmax,AQE,MDA,IAA and Sp,were extracted to classify the grade of high temperature stress.Heat stress index I_HTS was calculated according to the corresponding weight coefficient.The mean-standard deviation method was used to classify the high temperature,air relative humidity and duration days of different gradients into five grades（Mild,Light,Moderate,Severe and Extra severe）.When the temperature reaches 38℃and the air relative humidity is lower than 50%or higher than 90%,the stress level is up to severe and extra severe.When the relative humidity of air is about 70%,the stress grade is usually mild or light.