Root morphology, water uptake, growth and survival of seedlings of ryegrass and Phalaris

The development of secondary roots was prevented or delayed in ryegrass and phalaris, and the effects on plant water relations, growth and survival were studied in a controlled environment. Delayed development of secondary roots reduced transpiration within 15 days of emergence and reduced tillering and leaf area by the 22nd day in ryegrass and the 28th (leaf area) or 42nd day (tiller number) in phalaris. These effects were apparently due to high axial resistances to water flow through the plant, rather than to an inadequate capacity of the seminal roots for water uptake. Measurements of water flow through xylem vessels agreed well with predictions from the Poiseuille equation. Secondary roots were able to support seedlings from about 20 days after sowing in the absence of seminal roots, but most seedlings survived less than 4 months in the absence of secondary roots, even when subsoil water was available to the seminal roots. Delays in the establishment of secondary roots (up to 63 days) and phosphorus deficiency both reduced the number of secondary root axes forming, but this did not affect survival because the conductance of each secondary root axis was about two orders of magnitude higher than that of the seminal axis.