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Water Status of Cotton as Related to Taproot Length¹

In dryland crop areas it is common for plants to depend on subsoil water for some part of the growing season. In this situation, water moves through a considerable length of root axis to reach the plant top. Resistance to this movement of water (axial resistance) can become significant. This experiment was designed to assess the interaction of root geometry and plant water status and to compare values of root resistance calculated for two contrasting cotton (Gossypium hirsutum L.) root systems. Cotton plants were grown in a greenhouse either in pots (26-cm-deep) or in soil-filled tubes (120 cm-long) supported in base pots. After initial watering of the soil in tubes, only the base pots were watered, so that plants developed long taproots and depended on water in the base pots.

Except for the strong development of the taproot within the tube, little other root growth occurred there because of soil drying. The taproots of pot-grown plants were not well defined 20 cm from the plant base because of extensive branching. Rooting density in the base pot of plants with long taproots was 5.2 ± 0.7 cm cm * and compared with 13.7 — 5.6 cm cm'3 for plants grown hi pots. Plants of the two treatments had similar leaf area, rate of water uptake, and value of both covered and exposed leaf water potential. Within the long taproots many secondary xylem vessels had been laid down. Individual xylem vessel size tended to increase with distance from the plant base. Calculated values of axial resistance (0.0097 cm"3 day bar) for long taproot were less than for the taproot of pot-grown plants (0.0153 cm'3 day bar) because the increase in equivalent xylem vessel size more than offset the effect of the increased length. Values of total root resistance were calculated from the potential difference between covered leaf water potential and soil water potential divided by the water uptake rate. Total root resistance values were similar, and so, by difference, radial root resistance was larger for the long-rooted plants. This difference is explained by the difference in rooting density. We concluded that a balance was apparently maintained between the absorbing length of the root system and the longitudinal transport system.

Key Words: Axial root resistance • Radial root resistance • Xylem vessel radius • Rooting density • Water use • Leaf water potential • Gossypium hirsutum L.

² Former assistant professor, Texas Agric. Exp. Stn., presently senior plant physiologist, Soil and Irrigation Res. Inst., Private Bag X79, Pretoria 0001, Republic of South Africa; and soil scientist, USDA, SEA, AR, P. O. Box 748, Temple, TX 76501, respectively.

Received for publication August 20, 1979.