Non-gravitropism Respond Of Pea （Pisum Sativum L.）Seedlings’Primary Roots To Exogenous H₂O₂Stress

As a signal molecule, H2O2has become a research hotspot and gained widely attentionbecause involved in a wide range of physiological functions in plant cells, but there were fewreports on the effects of H2O2on the early stage of germination in plant roots. Recently, a fewreports indicated that H2O2could induce horizontal bending of primary roots during grasspea’s germination. In order to find the physiological mechanism of non-geotropism bendingof primary roots and alleviating effects of Ca2+, this paper used pea seeds as the experimentalmaterial, measured the effects of H2O2on pea primary roots and the alleviating effects of Ca2+on oxidative damage of plant cells under H2O2stress through a variety of physiological andbiochemical methods. Test results were as follows:1. Horizontal bending occurred during pea primary roots germination induced byexogenous H2O2Exogenous H2O2inhibited the normal growth and induced horizontal curvature ofprimary roots during pea seeds germination. Moreover, the longer processing time and higherH2O2concentration, the more serious this phenomenon. In addition, this phenomenon onlyoccurred before radicals break through the seed coats, H2O2failed to induce horizontalcurvature of roots when applied after radicals broke through the seed coats. At the same time,the curvature phenomenon was also found in the Trib. Vicieae Bronn., Glycininae Benth. andPhaseolinae Benth., all of which belong to the Papilionoideae, but not observed in Zea maysL., Triticum aestivum L., Citrullus lanatus L. and Cucumis melo L.. All above suggested thatthese species might have a unique physiological mechanism to regulate the gravitropismgrowth and respond to H2O2stress.2. Changes have taken place in cell structure of roots induced by exogenous H2O2The results of scanning electron microscopy (SEM) showed that epidermal cells of rootstreated with H2O had uniform size, arranged regularly and closely.But cells gap of roots withcurvature increased, and cells in elongation zones (EZ) exhibited asymmetric growth. Outsidecells were columnar and full, arranged closely into columnar, while inside cells werefilamentous and arranged loosely. These kinds of asymmetrical cells growth might cause the tension of outside cells to increase, resulted in a squeeze on the inside cells, and finally leadto primary roots horizontal bending.3. Root-cap starch hydrolyzed in primary roots induced by exogenous H2O2Experimental results showed that content of GA3increased and a-amylase activityenhanced in the process of exogenous H2O2-induced bending of pea primary roots, whichcontributing to the root-cap starch hydrolysis and causing the soluble sugar content increase.To infer from starch-statolith hypothesis that maybe the hydrolysis of root-cap starch madethe roots lose gravity perception and produce the horizontal bending growth.4. Horizontal bending roots induced by H2O2can be relieved by exogenous Ca2+Exogenous H2O2induced pea primary roots to occur horizontal bending, reduced rootsactivity and inhibited the normal geotropism growth. But exogenous Ca2+could mitigate thisphenomenon, and the curvature rates of primary roots reduced significantly with increasing ofexogenous Ca2+concentration. The curvature rates were only one-fifth of those under theexogenous H2O2stress When Ca2+concentration increased to10mM, and therefore thephenomenon of primary roots horizontal curvature could be relieved through applying Ca2+with appropriate concentration so that roots could tend to normal growth.5. Oxidative stress on roots caused by stress factors could alleviate by exogenous Ca2+Exogenous H2O2was used to cause oxidative stress on plant directly in this test. Theresults showed that MDA content of pea primary roots treated with H2O2enhanced, relativemembrane permeability increased, and associated enzyme activity of antioxidant enzymesystems reduced significantly; whereas increase of primary roots MDA content slowed downeffectively, relative permeability of cell membrane reduced, and activities of CAT, SOD, PODand APX continued to rise after treating with exogenous Ca2+. All of these proved thatpowerful scavenging system alleviated the oxidative stress caused by excess ROS producedfrom environmental stress and reduced the level of membrane lipid peroxidation in primaryroots, which had good restoration and protection to cell membrane and thereby mitigated thedamage caused by oxidative stress on cell membrane system.