| Maize is photophilous and thermophilic C4 plant,which needs high temperature and generally cannot bear cold conditions.It is a vital food crop.Cold temperature is an important meteorological disaster during the early spring season for the maize.Because of cold damage fluctuations in maize yield is greater so the quality and quantity of maize yield is greatly affected.In fact cold injury is a vital factor to restrict the development,normal growth and production of maize.The experiment was divided into two parts,in first part of research 20 varieties were used to select with the different capacity of low temperature resistance.To study the effect of low temperature on the biochemical,physiological and molecular levels five ranges of low temperatures were used which were(8 ?,6 ?,4 ?,2 ?,0 ?)the experiment was carried out in the incubator.After the cold stress the plants were kept in the normal room temperature(25 ?)for 3 days to observe the ability of plants to restore the normal conditions and then physiological and biochemical changes were also recorded to observe the tolerance of plants to cold stress.In the second part,4 varieties were used to observe the effect of salicylic acid under cold stress.As described before four varieties were involved in this experiment in which Hei 8834 and Chang 7-2 were used as cold susceptible inbred lines and KR701 and B125 were used as cold tolerant inbred lines as seed materials.5 SA concentration gradients were tested to select proper concentration on both control(25 ?)and cold temperature(4 ?).Through this study on the growth of seedlings and photosynthetic capability,to observe the SA effect on maize seedlings for cold tolerance and to study the SA mechanism of cold resistance of maize.The main findings of research are described below:1.Seed germination was severely affected by the low temperature the rate of germination decreased with the decrease of temperature.With the use of cluster analysis at germination stage and seedling stage the 20 inbred lines were divided into three kinds 5 inbred lines like Ye 478 were cold tolerant,7 like He 344 as intermediate types and 8 inbred lines like Ji 853 were cold susceptible,respectively.Under 8 ? temperature and recovery,all inbred lines was higher than the control,it indicates that growth rate of seedlings of Ye 478,K10,M8 and Nan 9808 at 6 ? temperature was higher than the control,while growth rate of He 344 and Ji853 was less than the control.Growth rate of recovery of 3 days at 4 ?,2 ? and 0 ? was limited that reveals the damage was too serious.2.Photosystem ?(PS?)of leaves was damaged by the cold stress,so initial florescence(Fo)was increased,Fv/Fo,Fv/Fm was reduced under cold stress in all of the six varieties,PS? of cold susceptible were seriously damaged under low temperature.The POD and SOD activities of protective enzymes were increased in the tolerant inbred lines.While POD and SOD activities in the cold susceptible showed downward trend at 2 ? and 0 ? but it was higher than the normal temperature(Control).Beyond the function and protection of protective enzymes cell membrane was damaged more seriously because of cold stress.The membrane permeability and electrical conductivity of maize seedlings was increase with the each cold temperature stress.MDA contents were also increased and it was higher in the cold susceptible Nan 9808 inbred line.3.After restoration to normal temperature the initial fluorescence(Fo)was dropped,Fv/Fo and Fv/Fm expressed upward trend in all three types of inbred lines.Cold resistant inbred lines required 3 days to fully restore to the normal but the cold sensitive inbred lines need more than 3 days to restore to normal condition because of more damage after cold stress.Recovery of different inbred lines under 8 ? and 6 ? temperature was same and restored to normal after 3 days but under 2 ? and 0 ? temperature the intermediate type and cold susceptible inbred lines did not returned to normal after 3 days it reveals that they need more than 3 days to recover for example Nan 9808 because membrane was damaged more seriously under cold stress.4.Data of plant height,root length,shoot length,root shoot fresh weight,root shoot dry weight,relative water content,relative conductivity and photosynthetic characteristics(chlorophyll,Fo,Fv/Fm,Ci and Pn)was recorded after using 5 SA concentrations under normal and low temperature and was found that 50 mg/L SA concentration was optimum in both temperatures conditions.5.SA treatment was useful and promoted osmotic adjustment and maintain the relatively high water contents in the plant under low temperature.Fo was increased Fv/Fo was decreased and photosynthetic rate(Pn)and chlorophyll contents were stabilized,intercellular carbon dioxide(Ci)was reduced,Fv/Fm high level was maintained and high photosynthetic capability was maintained after using the SA spray so it can be said that SA can improve the cold resistance of the maize. |
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