The Mechanisms Of Aluminum Toxixity And Tolerance In Cowpea(Vigna Unguiculata L.)

Aluminum (Al) is the most abundant metal element in the earth’s curst, usually in the form of insoluble silicate or alumina which was not toxic to plants. But in the acidic soil (pH<5), soluble A1(mainly Al3+) was of toxic to most of plant. In the acidic soil, Al toxicity inhibited the growth of plant, decrease the quality and output of crops. So the researches on Al resistant mechanism are of important theoretical and practical significance. In this research., the seed germinate, seedling growth, nutrient uptake, photosynthetic characteristics, reactive oxygen system, cell wall characteristic and the secretion of organic acid of two cowpea (Vigna unguiculata L.) cultivars ’T6’(Al-toleranct) and ’S3’(Al-sensitive) were studied, to reveal the Al resistance mechanism of cowpea. The results were reported as follows:1. There were no significant change on the germination energy, germination percentage, amylase activity and soluble sugar content of ’S3’ after exposure to10and50μmol·L-1Al, but significantly decreased after exposure to100、500、1000μmol·L-1Al. These indexes of ’T6’ significantly increased after exposure to10and50μmol·L-1Al, and significantly decreased after exposure to500、1000μmol·L-1.There was no significant change on the plant height and shoot dry weight of’T6’and ’S3’ after exposure to10and50μmol·L-1Al, and significant decrease after exposure to500μmol·L-1Al. Compared with control, there was no significant change on the root length and root dry weight of ’T6’ after exposure to10μmol·L-1Al, significantly decreased after exposure to50,100and500μmol·L-1Al. The root length and root dry weight of’S3’ decreased significantly by Al increase from10to500μmol·L-1.2. The Al content in ’T6’ and ’S3’ increased after Al stress, and Al mainly accumulated in roots, the increase of ’T6’ was lower than ’S3’; The content of N, P, K, Ca, Mg in’T6’ and’S3’decreased after exposure to500μmol·L-1Al. Besides K, the decrease of N, P, Ca, Mg in’T6’was lower than’S3’. The uptake of trace elements was inhibited in cowpea, the decrease of Cu and Fe was highest, Zn next, the decrease of Mn was lowest.Al mainly accumulated in root apex (0～10mm) after A1stress, the content of A1in ’S3’was higher than ’T6’.The root apex of’S3’was stained more deep than ’T6’ by hematoxylin after Al treatment. The results of X-ray analysis indicated that A1mainly distributed in epidermal cells and subepidermal cortical cells.3. The chlorophylla a, chlorophyll a/b, net photosynthetic rate (Pn) and non-photochemical quenching coefficient (NPQ) of ’T6’ increased and the intercellular CO2concentration (Ci) of ’T6’ decreased after exposure to10umol·L-1Al, so the photosynthetic capacity was strengthened. The structure of chloroplast, mitochondria and cell nucleus of ’T6’ and ’S3’ was destroyed; the chlorophylla a, chlorophyll a/b, Pn, stomatal conductance (Gs), plant water use efficiency (WUE), PSⅡ photosynthetic electron transport quantum efficiency (ΦPSⅡ), photochemical quenching (qP) and NPQ of ’T6’ and ’S3’ decreased; the Ci of ’T6’ and ’S3’ increased after exposure to100and500μmol·L-1Al, so the photosynthetic capacity was inhibited, and the inhibition in’S3’was larger than’T6’.4. The uronic acid and sugar of pectin and cellulose in cell wall of ’T6’ and ’S3’ root apex increased significantly with the increase of Al, and the sugar in hemicellulose (HC) increased significantly with the increase of Al, and increase of’S3’was significantly higher than ’T6’. The A1content of pectin, HC, and cellulose increase significantly after exposure to100and500μmol·L-1Al, and the increase of’S3’was significantly higher than’T6’.The cell wall PME activity of ’T6’ and ’S3’ root apex increased significantly after exposure to100,500μmol·L-1Al, the increase of ’S3’ was significantly higher than ’T6’.The degree of cell wall pectin methyl of ’T6’ and ’S3’ root apex decrease significantly after exposure to100and500μmol·L-1Al, the decrease of’S3’was significantly higher than’T6’.The superoxide anion radical, H2O2content, MDA content and plasma-membrane permeability of’T6’ and ’S3’ root tip increase significantly by A1increase from0to500μmol·L-1, and the increase range of ’S3’ was higher than ’T6’.The SOD, POD, CAT, APX, GR, AsA, GSH of ’T6’ and ’S3’ root tip increased after exposure to0to500μmol·L-1Al. The increase range of SOD in’S3’root tip was higher than ’T6’, but increase range of POD、CAT、APX in ’S3’ root tip were significant lower than ’T6’.We presume that ’S3’ root tip accumulated more H2O2and increased the oxidative stress then result in the inhibited of root growth.5. After exposure to Al, both malate and citrate were secreted from ’T6’ and ’S3’ roots. There was no significant change of citrate between’T6’and’S3’. The amount of malate secreted from ’T6’ and ’S3’ increase significantly by Al increase, the secretion of malate from ’T6’ root was higher than’S3’. The secretion type of malate and citrate from ’T6’ and ’S3’roots was similar, and both of them have a6h lag-phase. Potassium (P) starvation also induced the secretion of malate and citrate, but the secretion amount did not increase by P starvation time increase, and Al stimulate10times secretion than P starvation. Exposure to La (Lanthanum)did not induce organic acid secretion from cowpea root. The secretion site was mainly located in the0～10mm segment from root tip.The secretion of malate increase from cowpea root, and the malate content of cowpea tips decrease by Al increase, there was no significant change in enzyme activity relate to malate metabolism. The anion-channel inhibitor (A-9-C) and protein-synthesis inhibitor (CHM) completely inhibited the organic acid secretion of cowpea root under Al treatment, but the organic acid content in root tip did not changed. So we presume that there was no relative between the organic secretion and organic metabolism under A1treatment, and maybe related to the new protein synthesis.Exogenous SA and ABA did not changed the organic acid secretion in cowpea root induced by A1treatment, exogenous IAA significantly increased the malate secretion, and exogenous Ca2+channel inhibitors and chelator significantly inhibited the malate secretion. Meanwhile the organic acid content and enzyme activity relate to organic acid metabolism did not changed. So we presume that IAA and Ca2+may involved in the regulation of malate secretion induced by Al, but the regulation may be not related to malate metabolism.