棉花（Gossypium hirsutum L.）是一种重要的纤维经济作物，以往对于棉花获取磷（P）相关的根系性状（包括菌根根系性状）的研究较少。我们采用1950年至2013年在中国西北地区使用的8个棉花品种在3种磷供应水平下（分别为0、50和300 mg KH₂PO₄ kg^-1），研究了接种或者不接种丛枝菌根真菌Funneliformis mosseae棉花的生长和11个根系性状特征。结果表明：与老品种相比，新品种的根系直径更细，地上部吸收的磷更少，苗期生物量更低。这表明育种过程选择了更细的根系，菌根真菌定殖的皮层空间更少，因而增加了对供磷强度的更高需求。在两个低磷水平下，菌根植物比非菌根植物吸收更多的磷，产生更多的生物量（在P0时，3.2 mg对0.9 mg，1.8 g对0.9 g；在P50时，14.5 mg对1.7 mg， 4.7 g对1.6 g）。在最高磷水平下，菌根植物比非菌根植物获得更多的磷（18.8 mg对13.4 mg ），但生物量没有差异（6.2 g对6.3 g）。在中等供磷（P50）水平下，根系直径与菌根植物地上部生物量、磷浓度和磷含量呈显著正相关。我们的结果支持了植物获取磷的外包模式（即借助菌根途径）在根系经济学空间框架中的重要性。在过去的几十年里，育种使得棉花的根系更细、从菌根途径获益更低，这导致了在中等磷供应并有菌根侵染条件下，老品种的生物量显著大于新品种的生物量。未来棉花品种选育的策略要考虑到在根系自身吸收磷的能力和发挥菌根吸磷能力之间的权衡（即根系性状与菌根性状的权衡），以便选育出在中等磷肥投入条件下实现高产的品种。

Cotton (Gossypium hirsutum L.) is an important fiber cash crop, but its root traits related to phosphorus (P) acquisition, including mycorrhizal root traits, are poorly understood.  Eight cotton varieties bred in northwestern China that were released between 1950 and 2013 were grown in pots with or without one arbuscular mycorrhizal fungal (AMF) species (Funneliformis mosseae) at three P supply levels (0, 50 and 300 mg P as KH₂PO₄ kg^–1).  Eleven root traits were measured and calculated after 7 wk of growth.  The more recent accessions had smaller root diameters, acquired less P and produced less biomass, indicating an (inadvertent) varietal selection for thinner roots that provided less cortical space for AMF, which then increased the need for a high P fertilizer level.  At the two lower P levels, the mycorrhizal plants acquired more P and produced more biomass than non-mycorrhizal plants (3.2 vs. 0.9 mg P per plant; 1.8 vs. 0.9 g biomass per plant at P₀; 14.5 vs. 1.7 mg P per plant; and 4.7 vs. 1.6 g biomass per plant at P₅₀).  At the highest P level, the mycorrhizal plants acquired more P than non-mycorrhizal plants (18.8 vs. 13.4 mg per P plant), but there was no difference in biomass (6.2 vs. 6.3 g per plant).  At the intermediate P level, root diameter was significantly positively correlated with shoot biomass, P concentration and the P content of mycorrhizal plants.  The results of our study support the importance of the outsourcing model of P acquisition in the root economics space framework.  Inadvertent varietal selection in the last decades, resulting in thinner roots and a lower benefit from AMF, has led to a lower productivity of cotton varieties at moderate P supply (i.e., when mycorrhizal, the average biomass of older varieties 5.0 g per plant vs. biomass of newer varieties 4.4 g per plant), indicating the need to rethink cotton breeding efforts in order to achieve high yields without very high P input.  One feasible way to solve the problem of inadvertent varietal selection for cotton is to be aware of the trade-offs between the root do-it-yourself strategy and the outsourcing towards AMF strategy, and to consider both morphological and mycorrhizal root traits when breeding cotton varieties.