Two 70‐day‐old cotton plants (Gossypium hirsutum L. ‘Auburn 7‐683’) were subjected to a 26‐day drying cycle at the Auburn rhizotron in order to quantitatively study water relations and growth of both root and shoot as the soil dried. Measurements were made of rooting density changes; stem diameter and height increase; and soil water content (neutron meter), soil water potential (thermocouple psychrometer), and plant water potential (pressure chamber). Marked diurnal flurtuations in plant hydration and soil water potential were observed, especially during the middle of the drying cycle. Plant height increase and stem diameter growth slowed drastically after 17 days even though 35% of the root system was in soil wetter than −1 bar and the plant was rehydrating to a water potential of −3 to −5 bars. Plant water potential in the early morning did not equilibrate with the water potential of the wettest horizon of soil. The pattern of rooting with depth shifted during drying; initially, there were more roots in the upper layers of soil but, as a result of death of old roots in the top layers and production of new roots in the lower horizons of soil, rooting density increased with depth by the end of the drought. Cotton plants grown at the same time in a similar profile that was maintained moist did not show this reversal in rooting density.

Deeper rooting should improve soybean [Glycine max (L.) Merr.] water stress avoidance by increasing water uptake from deep soil strata. Because taproots are initially the deepest segment of soybean root systems, soybean genotypes with rapidly elongating taproots may have deeper root systems than genotypes with slowly elongating taproots. Taproot‐elongation rates of 105 soybean cultivars from Maturity Groups I, II, and III were measured in a glasshouse using clear plastic tubes filled with vermiculite and then inclined 15°. Taproot‐elongation rates within a maturity group differed among cultivars by as much as 1.3 cm/day. Seed weight within a seedlot and seed source also influenced the taproot‐elongation rate of a cultivar. Selected Maturity Group II cultivars with either faster or slower taproot‐elongation rates in the glasshouse were grown in the Ames rhizotron and in field plots. Excavation of rhizotron compartments 49 days after planting revealed that roots of faster elongators were 10 cm deeper than those of slower elongators. In field plots 56 days after planting, the faster elongating group was 9 cm deeper and had more roots at 150 cm than did the slower elongating group. The two groups did not differ significantly, however, 72 days after planting. The group of cultivars with faster elongation rates also depleted soil‐water below 120 cm slightly more than did the slower group.

Measurements were made of both shoot and root growth on a corn (Zea mays L.) and a tomato (Lycopersicon esculentum Mill.) plant in a rhizotron. Root intensity at the transparent panel was estimated by two methods. It increased during the growing season for both species, but was always greater for corn. Estimates of root density and total root length were three times greater for corn than for tomato at the end of the growing season. Side walls and glass panels showed no concentrating effect on root growth.

Because they occur at greater distances from the plant stem, roots deep within the profile often are considered less effective than those near the soil surface. An experiment was conducted to compare water‐absorbing efficiency, per centimeter of root, of corn (Zea mays L.) roots deep in the profile with that of roots near the soil surface. Plants were grown in a rhizotron compartment with rainfall excluded by a metal cover over the soil. Soil water content was determined with a neutron probe; rooting density, from measurements of roots on the glass viewing surface of the compartment. Leaf area was calculated by a length‐width method and plant height was measured daily. Stomatal aperture was estimated with a pressure drop promoter twice daily. With few exceptions, soil water content decreased and rooting density increased during the experiment. For the first weeks, transpiration exceeded pan evaporation, but toward the end of the experiment it was about half as much as pan evaporation. Water uptake per centimeter of root was affected most by soil hydraulic, conductivity, and at a given conductivity, it was greater at lower root densities. This effect of root density probably occurred because the roots were younger and more permeable at low root densities than at high root densities. Thus, for the conditions of these experiments, roots deep in the profile were probably more effective per centimeter of root for water uptake than shallow roots because they were younger and were in wetter soil.

Two-month-old cotton plants, growing in a rhizotron compartment filled with loamy fine sand surface soil, were subjected to an irrigation cycle. Estimates of rooting density, soil water content, soil water potential, water extraction per unit length of root, plant height, and leaf water potential were made throughout this cycle.