School of Plant Biology, University of Western Australia, 35 Stirling Hwy, Crawley, WA 6009, Australia.
Ann Bot. 2013 May;111(5):801-9. doi: 10.1093/aob/mct035. Epub 2013 Mar 1.
The Australian legume species Viminaria juncea forms both cluster roots and mycorrhizal associations. The aim of this study was to identify if these root specializations are expressed at differential supplies of phosphorus (P) and at different shoot P concentrations [P].
Seedlings were planted in sand and provided with a mycorrhizal inoculum and basal nutrients plus one of 21 P treatments, ranging from 0 to 50 mg P kg(-1) dry soil. Plants were harvested after 12 weeks, and roots, shoots and cluster roots were measured for length and fresh and dry weight. The number of cluster roots, the percentage of mycorrhizal colonization, and shoot [P] were determined.
Shoot biomass accumulation increased with increasing P supply until a shoot dry weight of 3 g was reached at a P supply of approx. 27·5 mg P kg(-1) dry soil. Neither cluster-root formation nor mycorrhizal colonization was fully suppressed at the highest P supply. Most intriguingly, shoot [P] did not differ across treatments, with an average of 1·4 mg P kg(-1) shoot dry weight.
The almost constant shoot [P] in V. juncea over the very wide range of P supplies is, to our knowledge, unprecedented. To maintain these stable values, this species down-regulates its growth rate when no P is supplied; conversely, it down-regulates its P-uptake capacity very tightly at the highest P supplies, when its maximum growth rate has been reached. It is proposed that the persistence of cluster roots and mycorrhizal colonization up to the highest P treatments is a consequence of its tightly controlled shoot [P]. This unusual P physiology of V. juncea is surmised to be related to the habitat of this N2-fixing species. Water and nutrients are available at a low but steady supply for most of the year, negating the need for storage of P which would be metabolically costly and be at the expense of metabolic energy and P available for symbiotic N2 fixation.
澳大利亚豆科植物 Viminaria juncea 既形成簇状根又形成菌根共生体。本研究的目的是确定这些根系特化是否在不同的磷（P）供应和不同的地上部 P 浓度 [P]下表达。
将幼苗种植在沙中，并接种菌根共生体和基本养分，再加上 21 种 P 处理中的一种，P 处理范围从 0 到 50mg P kg(-1)干土。12 周后收获植物，测量根、茎和簇状根的长度以及鲜重和干重。确定簇状根的数量、菌根侵染率和地上部[P]。
地上部生物量的积累随着 P 供应的增加而增加，直到在 P 供应约 27.5mg P kg(-1)干土时达到 3g 的地上部干重。在最高 P 供应下，簇状根的形成和菌根侵染都没有完全受到抑制。最引人注目的是，在整个处理中，地上部[P]没有差异，平均为 1.4mg P kg(-1)地上部干重。
在非常广泛的 P 供应范围内，V. juncea 的地上部[P]几乎保持不变，据我们所知，这是前所未有的。为了保持这些稳定的值，当没有 P 供应时，该物种会降低其生长速度；相反，当达到最大生长速度时，在最高 P 供应下，它会非常严格地降低其 P 吸收能力。据推测，簇状根和菌根共生体在最高 P 处理下的持续存在是其地上部[P]受到严格控制的结果。V. juncea 这种不寻常的 P 生理学被认为与其固氮物种的栖息地有关。在大多数时间里，水和养分的供应都是低但稳定的，这否定了 P 储存的必要性，因为 P 储存会代谢成本高，并且会消耗代谢能量和用于共生固氮的 P。
