Lancaster Environment Centre, Lancaster University, Lancaster, UK.
Ann Bot. 2021 Jul 28;128(1):45-57. doi: 10.1093/aob/mcab029.
Rhizosheaths are defined as the soil adhering to the root system after it is extracted from the ground. Root hairs and mucilage (root exudates) are key root traits involved in rhizosheath formation, but to better understand the mechanisms involved their relative contributions should be distinguished.
The ability of three species [barley (Hordeum vulgare), maize (Zea mays) and Lotus japonicus (Gifu)] to form a rhizosheath in a sandy loam soil was compared with that of their root-hairless mutants [bald root barley (brb), maize root hairless 3 (rth3) and root hairless 1 (Ljrhl1)]. Root hair traits (length and density) of wild-type (WT) barley and maize were compared along with exudate adhesiveness of both barley and maize genotypes. Furthermore, root hair traits and exudate adhesiveness from different root types (axile versus lateral) were compared within the cereal species.
Per unit root length, rhizosheath size diminished in the order of barley > L. japonicus > maize in WT plants. Root hairs significantly increased rhizosheath formation of all species (3.9-, 3.2- and 1.8-fold for barley, L. japonicus and maize, respectively) but there was no consistent genotypic effect on exudate adhesiveness in the cereals. While brb exudates were more and rth3 exudates were less adhesive than their respective WTs, maize rth3 bound more soil than barley brb. Although both maize genotypes produced significantly more adhesive exudate than the barley genotypes, root hair development of WT barley was more extensive than that of WT maize. Thus, the greater density of longer root hairs in WT barley bound more soil than WT maize. Root type did not seem to affect rhizosheath formation, unless these types differed in root length.
When root hairs were present, greater root hair development better facilitated rhizosheath formation than root exudate adhesiveness. However, when root hairs were absent root exudate adhesiveness was a more dominant trait.
根套被定义为根系从土壤中取出后附着在根系上的土壤。根毛和黏液（根分泌物）是参与根套形成的关键根特性，但为了更好地理解所涉及的机制，应该区分它们的相对贡献。
比较了三种物种[大麦（Hordeum vulgare）、玉米（Zea mays）和日本百脉根（Gifu）]在沙壤土中形成根套的能力与其根毛缺失突变体[无发根大麦（brb）、玉米根毛缺失 3 型（rth3）和根毛缺失 1 型（Ljrhl1）]的能力。比较了野生型（WT）大麦和玉米的根毛特性（长度和密度），以及大麦和玉米基因型的分泌物粘附性。此外，还比较了禾本科不同根型（轴向与侧向）的根毛特性和分泌物粘附性。
单位根长，WT 植物中根套大小的顺序为大麦>日本百脉根>玉米。根毛显著增加了所有物种的根套形成（大麦、日本百脉根和玉米分别增加了 3.9 倍、3.2 倍和 1.8 倍），但禾本科植物的分泌物粘附性没有一致的基因型效应。brb 分泌物比其相应的 WT 更具粘性，rth3 分泌物比其相应的 WT 更不具粘性，但玉米 rth3 比大麦 brb 结合更多的土壤。尽管两种玉米基因型产生的粘性分泌物都明显多于大麦基因型，但 WT 大麦的根毛发育比 WT 玉米更广泛。因此，WT 大麦更长的根毛密度比 WT 玉米结合更多的土壤。除非这些根型在根长上存在差异，否则根型似乎不会影响根套的形成。
当存在根毛时，更发达的根毛发育比根分泌物的粘附性更有利于根套的形成。然而，当根毛缺失时，根分泌物的粘附性是一个更占主导地位的特性。
