Responses of maize roots to drying-limits of viability

Turgid pieces of mature maize roots were dried in air and progressive changes in their relative water content (RWC) determined. Viability was tested by reproducibility of the drying curves after dehydration to successively lower RWCs. After reaching a chosen RWC, the pieces were rehydrated (approximately 2 h), and a 2nd and 3rd dehydration curve measured. Each drying curve was characterized by two parameters (a scale parameter γ, and a shape parameter β) of a survivorship function, which is a linear function of time. The parameter γ is more informative, and does not change in successive dehydrations for RWC > 0-4, suggesting no irreversible damage to the roots. Damage and death were indicated by divergences of γ in successive dehydrations to RWC = 0.35-0.15. Cryo-analytical microscopy confirmed these data while indicating specifically death of 50 and 100% of cortical cells at RWC 0.30 and 0.15, respectively, and survival of 50% or more of sieve tubes, pericycle and vascular parenchyma cells at root RWC as low as 0.15. This pattern of stelar cell survival may allow roots to preserve their capacity for renewal of axial conductivity and branch root development following periods of severe water stress.