Alleviating Effect Of Moderate Zn In Cd Treated Lemna Minor L. And Toxicity Of Excessive Zn In Alternanthera Philoxeroides Callus

As a kind of micro mineral elements, zinc (Zn) presents in many enzymes and plays an important role in several plant metabolic processes. Furthermore, as transition metals, Zn and Cd have same ionic charge and share many similar chemical properties. Hence, moderate Zn supply could alleviate Cd toxicity by inhibiting the absorption of Cd in plants. However, limited information is currently available regarding the mitigative effect of Zn on polyamines metabolism and proline accumulation in Lemna minor L. exposed to Cd. On the other hand, Zn also induces series of toxicity at high concentration. However, most existing studies focus on the terrestrial plants in Thlaspi for example Thlaspi caerulescens, little is known about aquatic plants used for forecasting water pollution.In view of this, Lemna minor, a floating plant and callus of Alternanthera philoxeroides were chosen as experimental materials. At absolutely artificial condition, the mitigative effect of moderate Zn on Cd toxicity and the toxic effect of excessive Zn were studied. The results are as follow:(1) L. minor was exposed to0,1,2,3,4umol/L of Cd for4days. Gradually increasing Cd induced a marked Cd accumulation and varying suppression on the absorptions of calcium (Ca), magnesium (Mg) and iron (Fe) but no significant change in Zn uptake. Furthermore, it caused a remarkable increase in free putrescine (f-Put) level and a slight increase in free spermine (f-Spm) but a distinct decrease in free spermidine (f-Spd), which in turn led to a sharp decline in the free (Spd+Spm)/Put ratio. Meanwhile, it enhanced the activities of arginine decarboxylase (ADC), ornithine decarboxylase (ODC) and polyamine oxidase (PAO). Besides, it raised the activities of pyroline-5-carboxylate synthetase (P5CS) and ornithine5-aminotransferase (8-OAT) and inhibited the proline dehydrogenase (PDH) activity. As a result, the free proline level increased. However, Zn addition suppressed the Cd accumulation remarkably. Moreover, it weakened the decrease in the free (Spd+Spm)/Put ratio by raising the levels of f-Spd and f-Spm. In addition, it suppressed the massive accumulation of free proline by keeping the activities of P5CS, OAT and PDH at the control level. In conclusion, Zn addition showed a lower accumulation of Cd and free proline as well as a milder disorder of polyamines metabolism, which indicates alleviating effect of Zn in L. minor exposed to Cd.(2) A. philoxeroides callus was exposed to0,0.2,0.4,0.8,1.6mmol/L of Zn for7days. By the gradually increase in Zn stress, Zn accumulated remarkably, Na content raised significantly, the levels of Mg, P and Fe first increased followed by a decrease. The generation rate of O2·-,the levels of H2O2and TBARs increased constantly. SOD activity was inhibited, the activities of POD, CAT, APX and GR first enhanced followed by a decline. The contents of GSH, AsA and soluble protein showed a similar trend as APX activity. SDS-PAGE analysis showed a new polypeptide with apparent molecular weights about80kDa in treated groups. The Chl alChl b ratio deceased evidently. In summary, excessive Zn caused a toxic effect by inducing a disorder of mineral elements homeostasis and the oxidative damage in A. philoxeroides callus.(3) A. philoxeroides callus was exposed to0,0.2,0.4,0.8,1.6mmol/L of Zn for7days. Under the gradient concentrations of Zn stress, levels of total Put and total Spm first decreased followed by a increase, total Spd level showed little change except appearing an increase at1.6mmol/L Zn. Free Put content changed in a similar trend as its total level and free Spd content reduced under0.8mmol/L and1.6mmol/L Zn but free Spm content had little fluctuation except a decrease at0.2mmol/L Zn. The contents of PS-conjugated Put and PS-conjugated Spm first descend at0.2mmol/L Zn followed by a rise, and the content of PS-conjugated Spd is elevated in all treated-groups. PIS-bound Put and PIS-bound Spd declined at0.2mmol/L Zn and0.4mmol/L Zn and no distinct difference was found in the later treatments. Activities of ADC and ODC increased and activities of PAO and DAO first increased followed by a decline. Activities of P5CS and δ-OAT successively increased but PDH activity showed no significant change. As a result, free proline level enhanced. Excessive Zn caused the disorder of polyamines metabolism and the accumulation of free proline.