Decoding the Developmental Complexities of Perennial Ryegrass Root Systems.

Mature root systems are exceedingly complex structures that have resulted from the sequential development of roots in response to genetic and environmental imperatives. Physio-genetic analyses of root systems routinely lead to conclusions of multi- or poly-genetic control of most characteristics including nutrient uptake.  The greatest difficulty in studying root system function is the apparent simplicity of the root itself.  This is exacerbated by the lack of a clear morphological differentiation between roots of different sizes and origins.  Seedling root systems are known to have at least three different physio-genetic root classes: tap root, basal root and lateral root.  There is one additional physio-genetic root class: shoot-borne roots.  Is the complexity of mature root systems due to the presence of additional physio-genetic root classes, or is there a simpler explanation?  Our analysis of perennial ryegrass root development suggests that, until contradictory evidence is discovered, a simple explanation can suffice to explain the complexity of mature root systems.  If it is assumed that root diameter predicates the physiological complexity of lateral roots, i.e. smaller diameter equals less complex, the following scenario obtains.  In Lolium perenne L., perennial ryegrass, younger (later) shoot-borne (tiller) roots are larger.  Roots smaller than 0.2 mm diameter are determinate in growth habit and unbranched.  Larger roots, which are smaller than 0.4 mm diameter, are indeterminate and unbranched.  Roots larger than 0.4 mm diameter are indeterminate and can branch.  We have observed that typical lateral roots are 25-30% of the diameter of the parent root.  The value of this latter characteristic can differ between clones derived from different seedling parents.  Environmental conditions can stimulate occasional lateral roots that have diameters up to that of their parent root.  The in-vivo (in-vitro) operation of these rules will be demonstrated.