CSIRO Plant Industry, Black Mountain Laboratories, Canberra, ACT, Australia.
Ann Bot. 2013 Jul;112(2):439-46. doi: 10.1093/aob/mct099. Epub 2013 Jun 7.
Cereals have two root systems. The primary system originates from the embryo when the seed germinates and can support the plant until it produces grain. The nodal system can emerge from stem nodes throughout the plant's life; its value for yield is unclear and depends on the environment. The aim of this study was to test the role of nodal roots of sorghum and millet in plant growth in response to variation in soil moisture. Sorghum and millet were chosen as both are adapted to dry conditions.
Sorghum and millet were grown in a split-pot system that allowed the primary and nodal roots to be watered separately.
When primary and nodal roots were watered (12 % soil water content; SWC), millet nodal roots were seven times longer than those of sorghum and six times longer than millet plants in dry treatments, mainly from an 8-fold increase in branch root length. When soil was allowed to dry in both compartments, millet nodal roots responded and grew 20 % longer branch roots than in the well-watered control. Sorghum nodal roots were unchanged. When only primary roots received water, nodal roots of both species emerged and elongated into extremely dry soil (0.6-1.5 % SWC), possibly with phloem-delivered water from the primary roots in the moist inner pot. Nodal roots were thick, short, branchless and vertical, indicating a tropism that was more pronounced in millet. Total nodal root length increased in both species when the dry soil was covered with plastic, suggesting that stubble retention or leaf mulching could facilitate nodal roots reaching deeper moist layers in dry climates. Greater nodal root length in millet than in sorghum was associated with increased shoot biomass, water uptake and water use efficiency (shoot mass per water). Millet had a more plastic response than sorghum to moisture around the nodal roots due to (1) faster growth and progression through ontogeny for earlier nodal root branch length and (2) partitioning to nodal root length from primary roots, independent of shoot size.
Nodal and primary roots have distinct responses to soil moisture that depend on species. They can be selected independently in a breeding programme to shape root architecture. A rapid rate of plant development and enhanced responsiveness to local moisture may be traits that favour nodal roots and water use efficiency at no cost to shoot growth.
谷物有两种根系。初生根系由种子萌发时的胚胎产生，可以支撑植物直到它结出果实。节根系统可以从植物的整个生命周期中的茎节点中出现；其对产量的价值尚不清楚，并且取决于环境。本研究的目的是测试高粱和谷子的节根在植物生长中对土壤水分变化的作用。选择高粱和谷子是因为它们都适应干旱条件。
在分室系统中种植高粱和谷子，允许分别为初生根系和节根系浇水。
当同时为初生根系和节根系浇水（土壤含水量为 12%，SWC）时，谷子的节根比高粱的节根长 7 倍，比干旱处理中的谷子植株长 6 倍，主要是由于分支根长度增加了 8 倍。当两个隔室的土壤都变干时，谷子的节根响应并生长出比充分浇水对照长 20%的分支根。高粱的节根没有变化。当只有初生根接收水时，两种植物的节根都会出现并延伸到极干燥的土壤中（SWC 为 0.6-1.5%），可能是从湿润的内室中的初生根输送韧皮部水。节根又粗又短，无分支且垂直，表明向性在谷子中更为明显。当用塑料覆盖干燥的土壤时，两种植物的总节根长度都增加了，这表明残茬保留或叶片覆盖可以促进节根到达干旱气候下更深的湿润层。与高粱相比，谷子的节根长度更长，这与（1）由于更早的节根分支长度的发育和通过个体发育的更快生长，以及（2）与地上部大小无关而从初生根向节根长度的分配，导致对节根周围水分的响应更具可塑性有关。
节根和初生根对土壤水分有不同的反应，这取决于物种。它们可以在一个育种计划中独立选择，以塑造根系结构。快速的植物发育速度和增强对局部水分的响应能力可能是有利于节根和水分利用效率而不影响地上部生长的特征。
