Reduced crown root number improves water acquisition under water deficit stress in maize (Zea mays L.)

Abstract

In this study we test the hypothesis that maize genotypes with reduced crown root number (CN) will have greater root depth and improved water acquisition from drying soil. Maize recombinant inbred lines with contrasting CN were evaluated under water stress in greenhouse mesocosms and field rainout shelters. CN varied from 25 to 62 among genotypes. Under water stress in the mesocosms, genotypes with low CN had 31% fewer crown roots, 30% deeper rooting, 56% greater stomatal conductance, 45% greater leaf CO2 assimilation, 61% net canopy CO2 assimilation, and 55% greater shoot biomass than genotypes with high CN at 35 days after planting. Under water stress in the field, genotypes with low CN had 21% fewer crown roots, 41% deeper rooting, 48% lighter stem water oxygen isotope enrichment (δ(18)O) signature signifying deeper water capture, 13% greater leaf relative water content, 33% greater shoot biomass at anthesis, and 57% greater yield than genotypes with high CN. These results support the hypothesis that low CN improves drought tolerance by increasing rooting depth and water acquisition from the subsoil.

Keywords: Crown root number; drought; maize; respiration; rooting depth; water stress..

Fig. 1.

Crown root number (CN) of maize 35 DAP in greenhouse mesocosms (A) and at anthesis in the field (B) under water-stressed (WS) and well-watered (WW) conditions. Bars show means of four replicates+SE. Different letters represent significant differences between means within the same section (P<0.05).

Fig. 2.

Crown root number per whorl of maize 35 DAP in greenhouse mesocosms (A) and at anthesis in the field (B) under water-stressed (WS) and well-watered (WW) conditions. The data shown are means of four replicates of the four genotypes (+SE) in each phenotypic class of either high CN or low CN. Different letters represent significant differences (P<0.05) compared within each root whorl.

Fig. 3.

Leaf CO₂ assimilation rate (μmol CO₂ m⁻² s⁻¹), leaf stomatal conductance (mmol H₂O m⁻² s⁻¹) and leaf relative water content (% w/w) at 35 DAP in greenhouse mesocosms (A, C, E), at anthesis in the field (B, D, F) under water-stressed and well-watered conditions. The data shown are means of four replicates for each of four genotypes in each phenotype category±SE. Different letters represent significant differences within a panel at the level of α=0.05. HCN: high CN; LCN: low CN.

Fig. 4.

Net canopy CO₂ assimilation (μmol CO₂ s⁻¹ plant⁻¹) (A) and total root respiration (μmol CO₂ s⁻¹ plant⁻¹) (B) at 35 DAP in greenhouse mesocosms under water-stressed and well-watered conditions. The data shown are means of four replicates for each of four genotypes in each phenotype category±SE. Different letters represent significant differences within each panel at the level of α=0.05. HCN: high CN; LCN: low CN.

Fig. 5.

Root length density (cm cm⁻³) of maize at 35 DAP in greenhouse mesocosms under water stress (A) and well-watered (B) conditions, and at anthesis in the field under water stress (C) and well-watered (D) conditions. The data shown are the mean of four replicates of the four genotypes of high CN and low CN (±SE). The average values of D ₉₅ for four replicates of four high-CN and four low-CN genotypes are shown in each panel. *P<0.05, **P<0.001. HCN: high CN; LCN: low CN.

Fig. 6.

Correlations between crown root number and rooting depth (D ₉₅, cm) and root length density (cm cm⁻³) from 80–140cm soil depths of maize at 35 DAP in greenhouse mesocosms (A, C), and from 40–60cm soil depth at anthesis in the field (B, D) under water-stressed conditions. Each point is the mean of four replicates of each genotype (±SE).

Fig. 7.

Soil water oxygen isotope composition in six soil layers in the field under water stress conditions. Values are means±SE of four observation points. Different letters represent significant differences at α=0.05.

Fig. 8.

Correlation of δ¹⁸O of stem water and crown root number at anthesis in the rainout shelters under water stress conditions. Each point is the means ±SE of four replicates of each genotype.

Fig. 9.

Shoot biomass (dry weight) of maize 35 DAP in greenhouse mesocosms (A), and shoot biomass (dry weight) (B) and yield (C) at anthesis in the field under water-stressed (WS) and well-watered (WW) conditions. Bars shown are means of four replicates±SE. Different letters represent significant differences among means within the three panels (P<0.05).

Fig. 10.

Correlations between crown root number and shoot biomass of maize at 35 DAP in greenhouse mesocosms (A), and shoot biomass (B) and relative yield (WS:WW, %) (C) at anthesis in the field under water-stressed conditions. Each point is the mean of four replicates of each genotype±SE.