Mechanisms on the Al tolerance and accumulation of plants adapted to low pH soils

研究了氮源和铝对热带木本植物马拉巴马、马占相思和彩叶千里光幼苗以及对铝敏感的大麦幼苗生长的影响。Al和N处理分别为0或0.5 mM Al和2 mM NH_4~+或NO_3~(-)。Al和NH_4的施用促进了热带植物的生长。离体根用NH_4溶解不溶性Al，主要从根尖吸收Al。除大麦外，铝对叶片氮素浓度无影响。Cajuputi根分泌大量柠檬酸，+Al处理使柠檬酸含量略有增加。在马占相思中，在+Al+NH_4处理的培养液中，可溶性Al对邻苯二酚紫(PCV)的反应活性降低，根中的Al浓度很低。木兰根系在吸收Al和NH4+的同时，也释放了H+。结果表明，Al和NH4+对热带树木幼苗的生长有一定的促进作用，0.1 mM的铝处理180min后，铝主要出现在离体质外体中，CCCP对铝的吸收没有影响，表明离体根对铝的吸收主要是通过质外体中被动积累的方式进行的。Al离子使黄瘤根质外体的羧基上的~3H~+发生位移，这在大麦中是没有观察到的。与Ca^&lt；2+&gt；或La^&lt；3+&gt；相比，该吸附中心对Al离子具有更高的亲和力。另一方面，铝处理3个月后，共生体和残余体中的铝含量很高，并且超过了质外体中的铝含量。根据现有的研究结果和前人的研究结果，提出了黑斑菌根系铝吸收机制的假说。

The effects of N-source and Al on the growth of seedlings of Melastoma malabathricum, Acacia mangium and Melaleuca cajuputi, which are tropical woody plants and are very tolerant to Al, and barley (Hordeum vulgare), which is a typical Al-sensitive plant, were investigated. The Al and N treatments consisted of the application of either 0 or 0.5 mM Al, and 2 mM NH_4^+ or NO_3^-, respectively. Growth of the tropical plants was enhanced by Al and NH_4 application. Excised roots of M.malabathricum dissolved insoluble Al with NH_4 application and absorbed Al mainly from root tips. Al did not affect the leaf N concentration except in the case of barley. Roots of M.cajuputi exuded a large amount of citrate, which slightly increased by the +Al treatment. In A.mangium, the reactivity of soluble Al to PCV (pyrocatecholviolet) decreased in the culture solution of the +Al+NH_4 treatment and Al concentration of roots in this treatment was very low. Roots of M.malabathricum released H+ along with Al uptake as well as NH_4^+ uptake. It is concluded that Al and NH_4^+ exert beneficial effects on the growth of tropical tree seedlings.Al appeared mostly in apoplastic fraction in excised melastoma roots treated with 0.1 mM Al for 180 min, and CCCP did not affect the Al uptake, indicating that Al uptake in the excised roots occurs mostly through passive accumulation in the apoplast. The Al ion displaced 3H^+ on carboxyl groups in the melastoma root apoplast when it was taken up, a finding not observed in barley. This adsorption site had a higher affinity to the Al ion compared with Ca^<2+> or La^<3+>. On the other hand, three months after Al exposure, Al concentration in the symplastic and residual fractions of the melastoma roots was very high and surpassed that in the apoplastic fraction. From the results in the present and previous studies, a hypothesis of the Al uptake mechanism in melastoma roots was proposed.