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, 166 (4), 1943-55

Reduced Root Cortical Cell File Number Improves Drought Tolerance in Maize

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Reduced Root Cortical Cell File Number Improves Drought Tolerance in Maize

Joseph G Chimungu et al. Plant Physiol.

Abstract

We tested the hypothesis that reduced root cortical cell file number (CCFN) would improve drought tolerance in maize (Zea mays) by reducing the metabolic costs of soil exploration. Maize genotypes with contrasting CCFN were grown under well-watered and water-stressed conditions in greenhouse mesocosms and in the field in the United States and Malawi. CCFN ranged from six to 19 among maize genotypes. In mesocosms, reduced CCFN was correlated with 57% reduction of root respiration per unit of root length. Under water stress in the mesocosms, genotypes with reduced CCFN had between 15% and 60% deeper rooting, 78% greater stomatal conductance, 36% greater leaf CO2 assimilation, and between 52% to 139% greater shoot biomass than genotypes with many cell files. Under water stress in the field, genotypes with reduced CCFN had between 33% and 40% deeper rooting, 28% lighter stem water oxygen isotope enrichment (δ(18)O) signature signifying deeper water capture, between 10% and 35% greater leaf relative water content, between 35% and 70% greater shoot biomass at flowering, and between 33% and 114% greater yield than genotypes with many cell files. These results support the hypothesis that reduced CCFN improves drought tolerance by reducing the metabolic costs of soil exploration, enabling deeper soil exploration, greater water acquisition, and improved growth and yield under water stress. The large genetic variation for CCFN in maize germplasm suggests that CCFN merits attention as a breeding target to improve the drought tolerance of maize and possibly other cereal crops.

Figures

Figure 1.
Figure 1.
Cross-section images showing genotypic differences in root CCFN in maize: eight cell files (A) and 14 cell files (B). Cross sections are from standard reference tissue collected 10 to 20 cm from the base of the second nodal crown root at 70 d after planting from field-grown plants. Images were obtained from laser ablation tomography.
Figure 2.
Figure 2.
Genetic variation for root CCFN in maize selected IBM lines (GH1; A) and RILs from the Malawi maize breeding program (MW2011; B). The data shown are from standard reference tissue collected 10 to 20 cm from the base of the second nodal crown root. In the greenhouse, roots were sampled 30 d after planting, and in the field, roots were sampled 70 d after planting.
Figure 3.
Figure 3.
Correlation of root respiration per unit of length and CCFN for GH1-NyH (y = 1.7x − 0.31, r2 = 0.46, P = 0.009), GH1-IBM (y = 1.9x − 0.49, r2 = 0.46, P = 0.009), GH2 (y = 0.8x − 4.32, r2 = 0.59, P = 0.001), and GH3 (y = 2.11x − 3.09, r2 = 0.52, P = 0.018) in the mesocosms 30 d after planting. Each point is the mean of at least three measurements of respiration from the second nodal crown root per genotype. [See online article for color version of this figure.]
Figure 4.
Figure 4.
Correlation of root depth (D95) and CCFN for GH2WS (y = 113.4 – 2.4x, r2 = 0.57, P < 0.001), GH2WW (y = 120.6 + 0.003x, r2 = 0.003, P not significant), GH3WS (y = 124.9 – 4.2x, r2 = 0.41, P < 0.01), and GHWW (y = 138.6 – 2.9x, r2 = 0.10, P not significant) in greenhouse mesocosms 30 d after planting. Data include water-stressed (WS) and well-watered (WW) conditions. [See online article for color version of this figure.]
Figure 5.
Figure 5.
Carbon dioxide exchange rate (A) and stomatal conductance (B) of six genotypes with contrasting CCFN 28 d after planting in well-watered (WW) and water-stressed (WS) conditions in greenhouse mesocosms (GH3). Bars represent means ± se of four replicates per treatment. Bars with the same letters are not significantly different (P < 0.05).
Figure 6.
Figure 6.
Shoot dry weight of genotypes contrasting in CCFN at 30 d after planting in well-watered (WW) and water-stressed (WS) conditions in mesocosms (A) and GH2 and GH3 (B). Bars show means ± se of four replicates per treatment. Bars with the same letters are not significantly different within the same frame (P < 0.05).
Figure 7.
Figure 7.
Correlation of root depth (D95) and root CCFN in rainout shelters at Rock Springs, Pennsylvania, for PA2011WS (y = 66.7 – 1.59x, r2 = 0.59, P < 0.01), PA2012WW (y = 49.89 + 0.02x, r2 = 0.002, P not significant), PA2012WS (r2 = 0.42, P < 0.05), and PA2012WW (y = 47.4 + 0.02x, r2 = 0.05, P not significant) 80 d after planting. Data include water-stressed (WS) and well-watered (WW) conditions. [See online article for color version of this figure.]
Figure 8.
Figure 8.
Performance of maize lines contrasting in CCFN in water-stressed (WS) and well-watered (WW) conditions in rainout shelters at Rock Springs, Pennsylvania. Leaf RWC is shown at 60 d after planting for PA2011 (A) and PA2012 (B); shoot biomass per plant is shown at 70 d after planting for PA2011 (C) and PA2012 (D); and yield per plant is shown for PA2011 (E) and PA2012 (F). Bars show means ± se of four replicates per treatment. Bars with the same letters are not significantly different within the same frame (P < 0.05).
Figure 9.
Figure 9.
Mean oxygen isotope composition ± se of soil water along the soil profile in the rainout shelters (PA2011). Sampling was done 65 d after planting. Values are means ± se of three observation points in the rainout shelters.
Figure 10.
Figure 10.
Performance of maize lines contrasting in CCFN in the field in water-stressed (WS) and well-watered (WW) conditions at two field sites in Malawi. Leaf RWC is shown at 60 d after planting for Bunda (A) and Chitala (B), shoot biomass per plant is shown at 70 d after planting for Bunda (C) and Chitala (D), and yield per plant is shown for Bunda (E) and Chitala (F). Bars show means ± se (n = 16–18) of four replicates per treatment and trait. Bars with the same letters are not significantly different within the same frame (P < 0.05).
Figure 11.
Figure 11.
Effect of drought treatment on soil volumetric water content at 15-, 30-, and 50-cm depths both in well-watered (WW) and water-stressed (WS) conditions in the rainout shelters at Rock Springs, Pennsylvania (PA2012). Points are means ± se of six measurements in the rainout shelter and three measurements in well-watered plots. Terminal drought was imposed in water-stressed plots beginning at 30 d after planting. [See online article for color version of this figure.]

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