Development of Water Conducting Capacity in the Root Systems of Young Plants of Corn and Some Other C4 Grasses

Abstract

Development of the primary and early nodal roots was studied in Zea mays L., Zea mexlcana (Schrad.) Soil or sand sheaths encasing roots of grasses have been described as features found in desert and dry environments (13). More recently they have been described on corn (12) and mesophytic grasses in humid environments (3). Striking illustrations of such sheaths will be found in these three papers. The sheathed regions of corn roots coincide with the regions of immature xylem where LMX2 elements are still alive (1 1). This region stretches much further back (up to 50 cm from the apex) than has been supposed. The present paper extends the association of soil sheaths with the immaturity of the LMX to four other C4 grasses, and explores the likely effect of the nonfunctioning xylem on water supply to the shoot.It is widely believed that the permeability of roots to water is much greater in the axial direction (P,) than in the radial direction (P,). The finding (11) that, in axile roots of field-grown corn, maturation of LMX is long-delayed, raised the question when in the development of the seedling the full axial permeability became operative. Each nodal root goes through a stage when it bears a coherent soil sheath for up to half a meter proximal to the bare elongating tip (2-5 mm), and has inside immature LMX (with cross walls and cytoplasm) which cannot engage in transporting water more efficiently than any other file of living cells (7). The longitudinal water permeability of such roots is thus limited to the outer ring of small (20 ,um diameter) EMX elements, which are open vessels to within 10 cm from the tip (2). When the LMX elements lose their crosswalls and become open, and the soil sheath disperses leaving the root bare, the longitudinal permeability ofthe root will increase by a factor of at least 100 (1 1). If the first formed seedling roots showed a similar slow maturation oftheir LMX there could be a stage when the primary and early nodal roots all had only their EMX vessels open. The time at which one or more of these early roots opened their LMX would herald a dramatic change in the availability ofwater to the developing shoot. The sequence and numbers ofdeveloping roots in corn have been described (4) for the variety Seneca Chief. The primary root is followed closely by a tier of four roots from the coleoptilar node, and a little later by four to five roots from the second node. Roots ofeach tier are even-aged. Roots of successive tiers (usually seven tiers) go through similar stages ofdevelopment to the earlier lower ones, from sheathed to bare. We determined to follow the development of the primary and early tiers of nodal roots, recording the presence and extent of soil sheaths, the presence or absence of crosswalls and cytoplasm in the LMX and the stage oflignification of the vessel sidewalls. These measurements would be related to the number and size of the leaves. Measurements of stomatal resistance on days of high evaporative demand might reveal whether at any stage the young seedl...

How Do Real Roots Work? (Some New Views of Root Structure)

“…One result of this isolation from the transpiration tension is that immature roots may conserve water in young and older seedlings until an extensive root system is established. Wenzel et al (1989) showed that in corn no LMX is open in the roots of soil-grown seedlings up to as many as 14 d after germination and that plants of corn and three other C 4 grasses as large as that shown in Figure 1A have as little as 14% and rarely more than 50% of total root length with open LMX conducting tubes. Another result of the closed LMX may be that the meristematic tissues are protected from the transpiration tension.…”

Xylem development in relation to water uptake by roots of grapevine (Vitis vinifera L.)

“…Ho\ve\er, recent studies in corn {Zea mays) ha\e shown that despite eotnplete ligtiificatioti, late tnetaxylem eletnents (ofteti very large) ha\ e dela> ed tnatitratioti with persistetit cross-walls atid protoplasm present up to 20-30 cm from the root tip (St Aubiti et al, 1986). Sitnilar finditigs were reported for teosinte {Zea mexicana), sorghum {Sorghntti bicolor) and sudati grass {Sorghu»i Sudanese) where all roots reached lengths of about 30 cm before the late metaxylem elements matured (Wenzel, McCully & Canny, 1989).…”

Soil exploration and resource acquisition by plant roots: an architectural and modelling point of view

“…Water uptake along roots is related, for the radial pathway (water transport from soil to xylem vessels), to the differentiation of relatively impermeable structures (suberization) and for the axial pathway (water transport along roots), to xylem maturation [78]. In some monocot species, late metaxylem vessels (able to carry much of the water) were shown to open at large distances from the root tip (around 20-30 cm for main axes of maize [92]). Based on experimental data on water flow in maize roots [84] and architectural/ water transport modelling of the root system, Doussan et al [18,19] derived the variations of hydraulic conductance along maize primary and lateral roots, and showed the impact of these variations on the distribution of water uptake within the maize root system for the case of a uniform water avaiblity (Fig.…”