Response of millet and sorghum to a varying water supply around the primary and nodal roots

Abstract

Background and aims: Cereals have two root systems. The primary system originates from the embryo when the seed germinates and can support the plant until it produces grain. The nodal system can emerge from stem nodes throughout the plant's life; its value for yield is unclear and depends on the environment. The aim of this study was to test the role of nodal roots of sorghum and millet in plant growth in response to variation in soil moisture. Sorghum and millet were chosen as both are adapted to dry conditions.

Methods: Sorghum and millet were grown in a split-pot system that allowed the primary and nodal roots to be watered separately.

Key results: When primary and nodal roots were watered (12 % soil water content; SWC), millet nodal roots were seven times longer than those of sorghum and six times longer than millet plants in dry treatments, mainly from an 8-fold increase in branch root length. When soil was allowed to dry in both compartments, millet nodal roots responded and grew 20 % longer branch roots than in the well-watered control. Sorghum nodal roots were unchanged. When only primary roots received water, nodal roots of both species emerged and elongated into extremely dry soil (0.6-1.5 % SWC), possibly with phloem-delivered water from the primary roots in the moist inner pot. Nodal roots were thick, short, branchless and vertical, indicating a tropism that was more pronounced in millet. Total nodal root length increased in both species when the dry soil was covered with plastic, suggesting that stubble retention or leaf mulching could facilitate nodal roots reaching deeper moist layers in dry climates. Greater nodal root length in millet than in sorghum was associated with increased shoot biomass, water uptake and water use efficiency (shoot mass per water). Millet had a more plastic response than sorghum to moisture around the nodal roots due to (1) faster growth and progression through ontogeny for earlier nodal root branch length and (2) partitioning to nodal root length from primary roots, independent of shoot size.

Conclusions: Nodal and primary roots have distinct responses to soil moisture that depend on species. They can be selected independently in a breeding programme to shape root architecture. A rapid rate of plant development and enhanced responsiveness to local moisture may be traits that favour nodal roots and water use efficiency at no cost to shoot growth.

Keywords: Millet; Pennisetum glaucum; Sorghum bicolor; adventitious roots; crown roots; lateral roots; plasticity; soil moisture; sorghum; split-root system.

Fig. 1.

Split-pot system used in experiments to separate seminal and nodal roots, adapted from Volkmar (1997). The inner pot (25 cm long × 9 cm diameter) is within the outer pot (30 cm long × 25 cm diameter) and is watered through straws that reach the soil surface of the outer pot. The seed is placed just inside the inner pot and the seminal roots are allowed to grow within the inner pot. The nodal roots are constrained within the soil of the outer pot and prevented from penetrating the inner pot by plastic.

Fig. 2.

Mean of (A) total seminal root length and (B) total nodal root length in the inner and outer pots, respectively, of the split-pot system shown in Fig. 1. Values with different letters are significantly different at P < 0·05.

Fig. 3.

(A) Nodal root number, (B) total axile length and (C) total branching of sorghum and millet measured after growing in the outer pot of the split-pot shown in Fig. 1. Values with different letters are signficantly different at P < 0·05.

Fig. 4.

(A) Nodal root diameter and (B) spread angle (shown in Fig. 5) of sorghum and millet measured after growing in the outer pot of the split-pot shown in Fig. 1. Values with different letters are significantly different at P < 0·05.

Fig. 5.

The nodal roots for (A, C, E) sorghum and (B, D, F) pearl millet after growth in different soil moisture contents in the split-pot system in Fig. 1. Roots in (A) and (B) grew in moist soil, roots in (C) and (D) grew in dry soil covered in plastic, and roots in (E) and (F) grew in dry soil without plastic. The white lines drawn on the outside of the nodal roots were used to measure the angle of spread (indicated by green lines).

Fig. 6.

Root length of millet and sorghum (as indicated in key) plotted against shoot total dry biomass (leaves and stems). All plants from the four treatments are presented to illustrate the relationships between shoot size and root length for the two species. The dashed ovals enclose millet plants in treatment 2, where water was withheld from the inner and outer pots.