Does morphological and anatomical plasticity during the vegetative stage make wheat more tolerant of water deficit stress than rice?

Abstract

Water scarcity and the increasing severity of water deficit stress are major challenges to sustaining irrigated rice (Oryza sativa) production. Despite the technologies developed to reduce the water requirement, rice growth is seriously constrained under water deficit stress compared with other dryland cereals such as wheat (Triticum aestivum). We exposed rice cultivars with contrasting responses to water deficit stress and wheat cultivars well adapted to water-limited conditions to the same moisture stress during vegetative growth to unravel the whole-plant (shoot and root morphology) and organ/tissue (root anatomy) responses. Wheat cultivars followed a water-conserving strategy by reducing specific leaf area and developing thicker roots and moderate tillering. In contrast, rice 'IR64' and 'Apo' adopted a rapid water acquisition strategy through thinner roots under water deficit stress. Root diameter, stele and xylem diameter, and xylem number were more responsive and varied with different positions along the nodal root under water deficit stress in wheat, whereas they were relatively conserved in rice cultivars. Increased metaxylem diameter and lower metaxylem number near the root tips and exactly the opposite phenomena at the root-shoot junction facilitated the efficient use of available soil moisture in wheat. Tolerant rice 'Nagina 22' had an advantage in root morphological and anatomical attributes over cultivars IR64 and Apo but lacked plasticity, unlike wheat cultivars exposed to water deficit stress. The key traits determining the adaptation of wheat to dryland conditions have been summarized and discussed.

Figure 1.

Specific leaf area (A), whole-plant WUE (B), and Δ¹³C (C) of rice and wheat (mean ± se). White columns represent control, and black columns represent water deficit stress. Values in parentheses represent the significant percentage change (increase or decrease) over the control. The ANOVA results with lsd values are given for cultivar (C), treatment (T), and cultivar-treatment interaction (C×T). Significance levels are as follows: *P < 0.05, **P < 0.01, and ***P < 0.001.

Figure 2.

Specific root length (A), total root weight density (B), and average root thickness (C) of rice and wheat (mean ± se). White columns represent control, and black columns represent water deficit stress. Values in parentheses represent the significant percentage change (increase or decrease) over the control. The ANOVA results with lsd values are given for cultivar (C), treatment (T), and cultivar-treatment interaction (C×T). Significance levels are as follows: *P < 0.05, **P < 0.01, and ***P < 0.001.

Figure 3.

A, Root samples were collected in three different zones on nodal roots for anatomy study. B, Radial root cross sections showing anatomical variation in rice and wheat. Scale bars on root images = 10 cm (A) and 100 µm (B).

Figure 4.

Root diameter at the RSJ (A), 10 to 15 cm from the RA (B), and 6 cm from the RA (C) on nodal roots of rice and wheat cultivars (mean ± se). White columns represent control (C), and black columns represent water deficit stress (WD). Values in parentheses represent the significant percentage change (increase or decrease) over the control value. Scale bars on root images = 200 µm.

Figure 5.

Stele diameter at the RSJ (A), 10 to 15 cm from the RA (B), and 6 cm from the RA (C) on nodal roots of rice and wheat (mean ± se). White columns represent control (C), and black columns represent water deficit stress (WD). Values in parentheses represent the significant percentage change (increase or decrease) over the control value. Scale bars on root images = 50 µm.

Figure 6.

SD:RD (%) at the RSJ (A), 10 to 15 cm from the RA (B), and 6 cm from the RA (C) on nodal roots of rice and wheat (mean ± se). White columns represent control, and black columns represent water deficit stress. A pictorial representation of radial distance in wheat and rice is shown in D. Values in parentheses represent the significant percentage change (increase or decrease) over the control value. Scale bars on root images = 200 µm.

Figure 7.

LMXD and LMXN at the RSJ (A and D), 10 to 15 cm from the RA (B and E), and 6 cm from the RA (C and F) on nodal roots of rice and wheat (mean ± se). White columns represent control, and black columns represent water deficit stress. Values in parentheses represent the significant percentage change (increase or decrease) over the control value.

Figure 8.

Theoretically calculated axial hydraulic conductance at the RSJ (A), 10 to 15 cm from the RA (B), and 6 cm from the RA (C) on nodal roots of rice and wheat cultivars (mean ± se). White columns represents control, and black columns represent water deficit stress.