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Analysis of the Components of Area Growth of Bean Root Systems¹

Root and leaf surface areas were measured on green bean (Phaseolus vulgaris L. cv. Ouray) plants, with light intensity (photosynthetic photon flux density, PPFD) as the major growth variable, to determine how various sizes of roots are related to water transport and growth rates of whole-root systems. Plants were grown in aerated nutrient solution for 41 days in a greenhouse. Two light intensities were used: 425 and 320 µE m^–2 sec^–1 Leaf area:root area ratios, distribution of root sizes, and the fraction of the total root surface area of each root class were determined. All of these parameters remained stable for plants with leaf and root areas greater than 1,000 cm² and they were unchanged by light intensity or growth rate. On the basis of previous data the mean root system hydraulic conductance (Lp) appeared to be keyed to plant size rather than age. The conductance was very low in small plants, increased about sixfold and peaked when the root systems reached approximately 1,000 cm² surface area. For plants larger than 1,000 cm², when the root size distributions were stabilized, conductance gradually declined, probably because of suberization or some other growth-related factor. Plants grown at lower light intensity showed the same pattern of relationships between root system size, root size distribution, and hydraulic conductance except that the overall Lp was consistently lower for plants of similar sizes.

Key Words: Root water transport • Root growth model • Root conductance

² Plant physiologist, USDA-SEA-AR, Irrigation and Soil-Plant-Water Relations Research Group, Crops Research Laboratory, Colorado State Univ., Fort Collins, CO 80523.

Received for publication February 12, 1981.

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