Effects Of Heat Stress On Photosynthetic Characteristics And Antioxidant Capacities In Wucai Seedlings

Wucai(Brassica campestris L.ssp.chinensis var.rosularis Tsen)is a type of Chinese cabbage with a beautiful shape and high nutritional value.This crop is a cruciferous,biennial herb,which originated in China and distributed mainly in Yangtze-Huaihe River basin.Because it prefers the cool climate,wucai is suitable to be cultivated in open fields in cold weather of late autumn and winter.However,in southern China,it usually suffers from extreme hot weather(> 30?)for relatively short time,which is called 'old wives' summer',severely limiting the yield and quality of wucai.As the cultivation area of wucai is increasing gradually each year,it is important for variety breeding and facility cultivation to evaluate its heat tolerance systematically and study its heat tolerance mechanisms deeply.In this study,two wucai cultivars,‘WS-6'(heat-sensitive)and ‘WS-1'(heat-tolerant),were selected as representative varieties with different sensitivities to heat stress in a preliminary experiment(the calculation of semi-lethal temperature).The seedlings were subjected to three heat-stress treatments for 5 days.The day/night temperatures were as follows:25 ?/18?(control),30 ?/26?,35 ?/28?,and 40 ?/30 ?.Plant growth,photosynthetic and fluorescent characteristics,oxidative damage,osmotic adjustment substances,antioxidant enzymes activities and antioxidants of the two cultivars were evaluated.The main results are as follows:1.We choosed six representative wucai cultivars according to the previous experience of planting,and measured their semi lethal temperatures.The results showed that the semi lethal temperature of ‘WS-6' is lowest,suggesting that it is the heat-sensitive material in the study.In the same way,‘WS-1' was selectd as the heat-tolerant material for its highest semi lethal temperature.2.High temperature(>30?)stress resulted in spindly growth of seedlings in wucai.The plant height was increased while stem diameter,single weight,leaf area and leaf relative water content were significantly decreased after 5 days of exposition to 35 ? and 40 ? heat stress.However,less increase in plant height and less decreases in stem diameter,single weight,leaf area and leaf relative water content were showed under all the stress conditions in the heat-tolerant ‘WS-1',as compared to the heat-sensitive ‘WS-6'.This indicated that the extent of heat stress injury in seedling growth of wucai was cultivar-dependent.3.As the stress temperature increased,the PN,Fv/Fm,?PSII and chlorophyllcontent of seedlings in wucai were all gradually decreased,indicating that high stress might cause the splintering of photosynthetic pigment,reduce the photosynthetic efficiency and inhibit photochemical activity.However,all the data showed that the decreases in above parameters were more serious in heat-sensitive ‘WS-6' than in heat-tolerant ‘WS-1'.In addition,microscopy analyses also revealed that the damage degree of ‘WS-6' was higher in the structures of photosynthetic apparatus and membrane system than that of ‘WS-1'.4.There was substantial accumulation of MDA and ROS(including H2O2 and O2.-)under heat stress,with greater accumulation under higher temperature stess condition.The generation of MDA and ROS were shown to induced membrane lipid peroxidation in wucai seedlings,which was aggravated at higher temperature.In addition,the obvious greater accumulation of MDA and ROS was observed in ‘WS-6'and led to more serious oxidative damage as compared with ‘WS-1'.5.As the stress temperature increased,the osmotic adjustment substances(including soluble sugar,soluble protein and proline)were generally increased,SOD and POD activities both were firstly increased and then decreased in both cultivars,and COD activity was decreased gradually in the two wucai cultivars.In the cycle of As A-GSH,APX and MDAR activities were increased firstly and then decreased with increasing stress temperature,GR and DHAR activities were both increased generally,As A and GSH also exhibited increases first and then decreases,while AsA/DHA and GSH/GSSG were both decreased generally.Furthermore,higher levels in terms of osmotic adjustment substances,antioxidant enzymes activities and reduced antioxidants were observed in ‘WS-1' as compared to heat-sensitive ‘WS-6'.These results fully illustrated that wucai seedlings could start their own osmotic adjustment and antioxidant system against oxidative stress caused by high temperatures.The heat-tolerant cultivar ‘WS-1' was better at scavenging MDA and ROS,reducing the membrane lipid peroxidation and for its greater osmotic adjustment function and stronger antioxidant defense capability under heat stress,as compared to the heat-resensitive ‘WS-6'.It confirmed that the heat-tolerant cultivar had a greater heat stability in terms of osmotic adjustment and antioxidant defense system,which facilitated a stronger self-regulation and self-protection capacity,as compared with the heat-sensitive cultivar under heat stress in wucai.