Root Cortical Aerenchyma Improves the Drought Tolerance of Maize.

Root cortical aerenchyma (RCA) disproportionately reduces root respiration in maize by converting living cortical tissue to air volume.  We hypothesized that RCA increases drought tolerance by reducing root metabolic costs, permitting greater root growth and water acquisition.  Four recombinant inbred lines (RILs) of maize with high RCA and three RILs with low RCA were selected from a population derived from B73 and Mo17 for a field study examining their growth and performance under drought in rainout shelters.  Volumetric soil moisture was maintained around 30% at both 25 cm and 50 cm depths in well-watered conditions throughout the growing season, while for water-stressed plants, volumetric soil moisture progressively decreased from 30% to 10% at 25 cm depth, and to 15% at 50 cm depth over the growing season.  Shoot biomass at flowering was reduced by an average of 46% by water-stress.  Under water stress, the four lines with high RCA had 30% more shoot biomass at flowering compared to the three lines with low RCA.  The high RCA lines averaged four times the yield of the low RCA lines under water-stress.  The increased yield under water stress was caused by increases of both kernel numbers per plant and individual kernel weight.  Root length density at 40 to 50 cm depth in the soil was significantly greater in the four high RCA lines compared to the three low RCA lines at 56 day after planting (DAP).  Mid-day leaf relative water content (RWC) of plants at 42 DAP in the high RCA lines was 10% greater than the low RCA lines in water-stressed conditions, while there was no significant difference for RWC in well-watered conditions for all lines. Our results support the hypothesis that RCA improves the drought tolerance of maize by permitting greater soil exploration at depth.