Nutrition

Plant Physiol. (1 995) 109: 7-1 3

Root Architecture and Plant Productivity’
Jonathan Lynch* Department of Horticulture, The Pennsylvania State University, University Park, Pennsylvania 16802-4201
Water and nutrient availability limit plant growth in a11 but a very few natural ecosystems. They limit yield in most agricultural ecosystems, and in the United States and other industrializednations, intensive irrigation and fertilization have generated serious environmental problems. The acquisition of soil resources by plant root systems is therefore a subject of considerable interest in agriculture and ecology, as well as a complex and challenging problem in basic plant biology. Symbioses between roots and otlier organisms (notably mycorrhizas and N-fixing bacteria), modification of therhizosphere through root exudates, and the uptake and transport characteristics of root axes are a11 important dimensions of this problem that are being actively researched by plant biologists. Another aspect of this problem that has received less attention, despite its probable importance, is root architecture. Recent methodological innovations present opportunities for improved under.standingof the functional importance of root architecture in the efficient acquisition of soil resources and plant adaptation to suboptimal soil conditions. The purpose of this Update is to briefly summarize conceptual issues and recent developments in the study of root architecture and to propose a framework for understanding its physiological basis.
WHAT IS ROOT ARCHITECTURE?

The term ”architecture”in reference to biological objects usually denotes the spatial configuration of some complex assemblage of subunits, with the implication that the overa11 configuration has some functional significance. The term “root architecture” has been used in various contexts to refer to distinct aspects of the shape of root systems. The following glossary more clearly delineates architecture from otherterms: Morphology. Root morphology refers to the surface features of a single root axis as an organ, including characteristics of the epidermis such as root hairs, root diameter, the root cap, the pattern of appearance of daughter roots, undulations of the root axis, and cortical senescence. Anatomical features of a root related to cell and tissue organization are not usually part of architecturalconsiderations. Topology. Root topology refers to how individual root axes are connected to each other through branching. As in mathFinancia1 support was provided by U.S. Department of Agriculture/National Research Initiative grant 94371000311 and National Science Foundation grant BIR-9220330. * E-mail jlynch@psupen.psu.edu;fax 1-814-863-6139.

ematical usage, root topology is stable to deformationor rotation of the axes themselves and therefore is possible to measure on excavated root systems. Distribution. Root distribution refers to the presence (rather than the orientation) of roots in a positional gradient or grid. Typically, studies of root distribution are concerned with root biomass or root length as a function of factors such as depth in the soil, distance from the stem, andposition between neighboring plants. Measurement of root distribution in agricultural and natural plant communities often includes roots of more than one plant or more than one species. Architecture. Root architecture refers to the spatial configuration of the root system, i.e. the explicit geometric deployment of root axes. Usually, studies of root architecture do not include fine structural details,such as root hairs, but are concerned with an entire root system or a large subset of the root system of an individual plant. As a descriptor of multiple root axes, architecture is senior to topology and distribution, since if root architecture is known, both topology and distribution are also known, whereas neither topology nor distribution can be used to derive the other two descriptors....