Bicarbonate content in growth medium has been considered to be one of the major factors in inducing zinc (Zn) deficiency in lowland rice. In this study, the effects of bicarbonate on root elongation, distribution and exudation of organic acids were examined in the Zn-efficient and Zn-inefficient rice genotypes. Bicarbonate significantly increased the growth of fine roots in the Zn-efficient genotype but inhibited their growth in the Zn-inefficient genotype. In fine root zones of 2-8mm from the tip, of plants grown with bicarbonate, malate, citrate, and fumarate accumulation was greater in the Zn-inefficient genotype, but not the Zn-efficient genotype, than the control. Enhanced exudation of malate at fine root zones of 1-2 cm from the tip by bicarbonate was noted in the Zn-efficient genotype, but not in the Zn-inefficient genotype. Bicarbonate induced greater exudation of citrate at fine root zones of 1-2 cm from the tip in the Zn-efficient than in the Zn-inefficient genotypes. The increase in the concentrations of malate and citrate in xylem sap by bicarbonate were found to be greater for the Zn-efficient genotype than that for the Zn-inefficient genotype. The results indicated that bicarbonate-mediated inhibition of fine root elongation in the Zn-inefficient genotype might result from excessive accumulation of organic acids, particularly malate, in root elongation zones. In the Zn-inefficient rice genotype the increased accumulation of malate and citrate in fine roots by bicarbonate appeared to result from less exudation of them to the rhizosphere and transport up to the shoots.

植物长期生长在重金属污染的生境中，逐渐进化成不同的生态型。通过调查中国东南部古老Pb/zn矿和非矿山生境中的植物种群，发现生长在古老Pb/Zn矿的东南景天（Sedum alfredii Hance）是一种新的Zn超积累植物。在自然和控制条件下，古老Pb/Zn矿生态型比非矿山生态型植株的茎粗、叶片大、植株高。在矿山土壤Zn有效含量为105.5～325.4mg·kg-1时，矿山生态型东南景天植株地上部Zn含量为4134～5000mg·kg-1；当营养液中Zn浓度为1223.6μmol时，其Zn含量高选2%。在相同Zn浓度下，矿山生态型地上部Zn含量比非矿山生态型高30倍左右。两种生态型体内Zn分布也不同，古老铅锌矿山生态型的不同器官中Zn古量以茎>叶片>根系，而非矿山生态型则以根系>茎>叶片。古老铅锌矿山生态型地上部积累的Zn占植株总积累量的90%以上，其中叶片和茎分别占41.66%±5.46%和54.7±5.87%：非矿山生态型各器官中积累的Zn远远低于古老铅锌矿山生态型，各器官中积累的Zn以茎>根系>叶片。本研究表明,这两种生态型东南景天的发现，为今后探讨植物耐高Zn胁迫和超积累Zn的微进化过程提供了非常有价值的材料，也为Zn污染土壤的植物修复提供了一种很有潜力的候选材料。

超积累植物从根际吸收重金属，并将其转移和积累到地上部、这一过程包括跨根细胞质膜运输、从根表皮细胞向中柱的横向运输、从根系的中柱薄壁细胞装载到木质部导管、木质部长途运输，从木质部卸载到叶细胞（跨叶细胞膜运输）、跨叶细胞的液泡膜运输等主要环节和调控位点。本文就近十年来这方面的研究进展作一综述。

通过野外调查和温室试验，发现并鉴定出东南景天（Sedum alfedii H）是一种新的锌超积累植物。调查结果发现，东南景天对土壤中高含量的锌有很强的忍耐、吸收和积累能力，地上部Zn含量为4134～5000mg/kg，平均为4515mg/kg。营养液培养实验证明，东南景天对生长介质中的Zn有很强的忍耐能力，当生长介质中Zn浓度高达240mg/L时，植株仍生长正常，其生物量与对照相比无显著差异。地上部Zn含量及其积累量均随生长介质中Zn浓度的增加而增加，当生长介质中Zn浓度为80mg/L时，地上部Zn含量和积累量达到最高值，分别为19.674g/kg，19.83mg/株。结果表明，东南景天是我国发现的一种新的Zn超积累植物，为今后探明植物超积累Zn的机理和Zn污染土壤的植物修复提供了一种新的种质资源。

Lead concenrations in roots stems and leaves of accumulating and non-accumulating ecotypes of Sedum alfredii (Hance) were studied through a hydropinic experiment with different Pb concentrations supplied as Pb(NO3)2. Lead concentrations in leaves and stems of the accumulating ecotpe were 4-9time and 3-5time those of the non-accumulating ecotype, and Pb-accumlate amounts in stems and leaves of the accumulating ecotype were 4-9 time and 8-11 times higher than those of the non-accumulating ecotype, respectively. The results indicated that the accumulating ecotype had better ability to transport Pb form roots to shoots. The subcellular disributions of Pb in the root, stem and leaf tisues were sutdid using sucrose differential centrifugation. Approximately 50% of Pb contents was found to be associated with the cell wall fraction in stems of the accumulatig ecotype and the percentage in creased to 80% both i roots and leaves, no matter when plants were grown with different levels of Pb. The results indicated that the distribution of Pb on cell walls of the account for the high tolerance to Pb.