Effect Of Exogenous Proline On The Salt-Tolerance Of Rice (Oryza Sativa L.)

Soil salinity is one of the major abiotic stresses limiting crop growth and productivityworldwide. Thus, improving crop yield under salt stress is an important issue for nationalagricultural workers. Proline as an organic osmolyte accumulated by plants under stress, and playsan important role in the response of salt stress. It was reported that exogenous applied prolineprovides an effective method to study the salt-tolerant mechanism of plant. To discuss thephysiological mechanism of salt tolerance in rice improved by proline and explore the applicationvalue of exogenous proline in agricultural production, and to perfect the salt-tolerant mechanismand provide a theoretical basis of a salt tolerance cultivation technology system in rice,Dongnong425, a japonica rice cultivar, was used as tested material, and different concentrations ofexogenous proline were applied as presoaking agent and foliar spraying during the tillering andbooting stage, the effect of exogenous proline on long term salt-stressed rice were well studied inthis present work. The main results of this study are as follows:1. Proline effectively alleviated the inhibitory effect of salt stress on seed germination of rice.Presoaking with15~45mmol/L proline improves germination energy (GE) and relativegermination energy (RGE) of rice under normal and salt conditions. In addition, Germination rate(GR), and relative germination rate (RGR) of salt-stressed rice were also improved. The activity ofalpha-amylase, beta-amylase and total amylase in the rice cultivar under normal and salt stressenvironment was significant increased at the concentration of15mmol/L and30mmol/L proline.Salt stress inhibited the expression of β-amylase isoenzyme in a short time, without affecting thebound type of α-amylase. Presoaking seed with30mmol/L proline increased the brightness (boundsof e and f) and the width (bound of i) of α-amylase isoenzyme, but less affect the bound ofβ-amylase isoenzyme.2. Presoaking with proline effectively alleviated the inhibitory of salt stress on rice seedlingmorphology traits (e.g. height, root length and root number), increased the shoot fresh weight, dryweight of roots, and soluble sugar content in shoots and roots. Comprehensive evaluation of salttolerance at seedling stage indicated that soaking with30mmol/L proline was the best treatmentwhich effectively improved the dry weight of rice seedlings under normal and salt stress.Compared with salt-tolerance index (STI) and salt-tolerance rate (STR), salt-proline interactionindex (SPII) and salt-proline interaction rate (SPIR) were able to distinguish the differencebetween various concentrations of proline, and could be a new evaluation indicator.3. Plant height, tiller number, plant biomass, content of chlorophyll (including chlorophyll a (Chl a), chlorophyll b (Chl b), total chlorophyll) and Chl a/Chl b under long-term salt stress weresignificantly reduced. Foliar application of proline less affected plant height under salt stress, butpromoted the growth rate of tillering and delayed the demise speed at an appropriate concentrationof proline. Moreover, foliar spraying30mmol/L proline at tillering and booting stages significantincrease the shoot and root biomass of rice under moderate and severe salt stress. The content ofchlorophyll, especially Chl a, was improved by appropriate exogenous proline applied at tilleringand booting stages.4. Organic osmolyte (such as soluble sugar, soluble protein and proline) and antioxidantenzymes (including SOD, POD and CAT) in the salt-stressed rice leaves were affected by saltstress level and time. While, Na~+content in the rice shoot and root significantly increased with saltstress level intensified, while K~+content and K~+/Na~+decreased. Spraying proline at tillering stageor booting stage, endogenous proline content in rice leaves significantly increased, the content ofsoluble sugar and soluble protein that affected by salt stress level and exogenous prolineconcentration were increased at suitable proline concentration. Exogenous proline effectivelyinhibited the accumulation of Na~+content in rice shoot and improved the content of K~+in riceshoot and roots under salt stress, thus effectively maintain steady state of K~+in plant. The activityof POD during the early or later period of salt stress was improved by45mmol/L proline under allsalt levels, but the activity of SOD and CAT changed with the salt level and proline concentration.Moreover, exogenous proline effectively reduced the content of MDA in salt-stressed rice.5. Rice yield significantly decreased under salt stress, and exacerbate the decline with theincreasing of stress level. The salt-sensitivity order of rice yield components was that effectivepanicles> seed setting rate> spikelet no. per panicle>1000-kernel weight. Salt stress had greaterimpact on the spike length, secondary branch number and total grains of secondary branch.Exogenous applied proline at tillering and booting stage significantly improved rice yield undersalt stress. Application of30mmol/L proline proved to be more effective in increasing yield undersalt stress as compared to other levels. Exogenous Proline increased spikelet no. per panicle orseed setting rate under mild and moderate salt stress, but had no effect on1000-kernel weight.While spikelet no. per panicle, seed setting rate and1000-kernel weight significantly increased byexogenous proline under severe salt stress. In addition, exogenous proline effectively increasedsecondary branch number and total grains of secondary branch.6. Salt tolerance of rice was analyzed using subordinate function method. The comprehensiveevaluation results of salt tolerance index at tillering stage showed that30mmol/L prolineeffectively improved salt tolerance and alleviated the inhibitory of salt stress on rice at tilleringstage, and the comprehensive evaluation results at booting stage showed that the best treatmentwas30mmol/L proline under mild salt stress and severe salt stress, while15mmol/L prolinetreatment works best under moderate salt stress. However, the comprehensive evaluation resultsabout tillering and booting stage indicated that30mmol/L proline treatment proved to be more effectively. The correlation analysis showed that the average of subordinate function forsalt-tolerance physiological indicators and relative yield was highly significant positive correlation,indicating the analysis and evaluation method of subordinate function has strong reliability.