Aluminium-phosphorus interactions in plants growing on acid soils: does phosphorus always alleviate aluminium toxicity?

Aluminium (Al) toxicity and phosphorus (P) deficiency are considered to be the main constraints for crop production in acid soils, which are widely distributed in tropical and subtropical regions. Conventionally, P addition is regarded as capable of alleviating Al toxicity in plants. However, this field is still rife with unsubstantiated theories, especially for different plant species growing on acid soils. In this review, the responses of plants to different methods of Al-P treatments are briefly summarized, and possible reasons are proposed by considering recent results from our laboratory. It is shown that: (1) long-term Al-P alternate treatment is advantageous for studying Al-P interactions in plants; (2) under the long-term Al-P alternate treatment, the roles of P in Al phytotoxicity might be associated with the Al resistance capability and P use efficiency of the plant, and a P/Al molar ratio exceeding 5 in roots may be the threshold of P alleviating Al toxicity based on the calculation of the tested plants; (3) in acid soils, P application may be effective only after Al stress is overcome for Al-sensitive species. Thus it is concluded that P application does not always alleviate Al toxicity under long-term Al-P alternate treatment.

Phosphorus enhances Al resistance in Al-resistant Lespedeza bicolor but not in Al-sensitive L. cuneata under relatively high Al stress.

BACKGROUND AND AIMS: Aluminium (Al) toxicity and phosphorus (P) deficiency often co-exist in acidic soils and limit crop production worldwide. Lespedeza bicolor is a leguminous forage species that grows very well in infertile, acidic soils. The objective of this study was to investigate the effects of Al and P interactions on growth of Lespedeza and the distributions of Al and P in two different Al-resistant species, and to explore whether P can ameliorate the toxic effect of Al in the two species. METHODS: Two species, Lespedeza bicolor and L. cuneata, were grown for 30 d with alternate Al and P treatments in a hydroponics system. Harvested roots were examined using a root-system scanner, and the contents of Al, P and other nutrient elements in the plants were determined using inductively coupled plasma-atomic emission spectroscopy (ICP-AES). Haematoxylin staining was used to observe the distribution of Al in the roots of seedlings. After pre-culture with or without P application, organic acids in the exudates of roots exposed to Al were held in an anion-exchange resin, eluted with 2 m HCl and then analysed using high-performance liquid chromatography (HPLC). KEY RESULTS: Lespedeza bicolor exhibited a stronger Al resistance than did L. cuneata; Al exclusion mechanisms may mainly be responsible for resistance. P application alleviated the toxic effect of Al on root growth in L. bicolor, while no obvious effects were observed in L. cuneata. Much less Al was accumulated in roots of L. bicolor than in L. cuneata after P application, and the P contents in both roots and shoots increased much more for L. bicolor than for L. cuneata. Lespedeza bicolor showed a higher P/Al ratio in roots and shoots than did L. cuneata. P application decreased the Al accumulation in root tips of L. bicolor but not in L. cuneata. The amount of Al-induced organic acid (citrate and malate) exudation from roots pre-cultured with P was much less than from roots without P application; no malate and citrate exudation was detected in L. cuneata. CONCLUSIONS: P enhanced Al resistance in the Al-resistant L. bicolor species but not in the Al-sensitive L. cuneata under relatively high Al stress, although P in L. cuneata might also possess an alleviative potential. Enhancement of Al resistance by P in the resistant species might be associated with its more efficient P accumulation and translocation to shoots and greater Al exclusion from root tips after P application, but not with an increased exudation of organic acids from roots.