Aerenchyma formation and associated oxygen movement in seminal and nodal roots of wheat

Abstract The present paper describes the effects of growth of roots of wheat (Triticum aestivum cv. Gamenya) in hypoxic nutrient solutions on acrenchyma formation and O₂ movement from shoots to roots. Two types of roots were investigated: (1) seminal roots of 4–7‐d‐old seedlings, and (2) seminal and nodal roots of 10–28‐d‐old plants. Gas‐filled porosity of seminal and nodal roots increased from 3 to 12% and from 5–7 to 11–15%, respectively, when the roots emerged in stagnant or N₂‐flushed solutions (0.003 mol m ⁻³ O₂) compared with growth in continuously acrated solutions (0.26 mol m ⁻³ O₂). However, neither root type increased in porosity when they were longer than 100–200 mm at the start of the exposure to these stagnant or N₂‐flushed treatments. A vernier microscope and cylindrical platinum‐electrode were used to examine the relationship between root extension and transport of O₂ from shoots to roots via the gas spaces. Measurements were made when the roots were in an anoxic medium and were dependent solely on O₂ supplied from the shoots. For seminal roots of 5–7‐d‐old seedlings raised in stagnant solutions (90–100 mm), internal O₂ transport was sufficient to support a rate of root elongation in the O₂‐free medium of between 0.03 and 0.17 mm h⁻¹. When the O₂ pressure around the shoots was increased from 20 to 100 kPa O₂, the O₂ concentrations at the walls of the expanding zone (2–7 mm from the tip) of these roots increased from 0.006 mol m⁻³ to between 0.04 and 0.26 mol m⁻³, and the rate of root extension increased five‐fold. Oxygen transport to roots grown continuously in acrated solutions was considerably less than for roots raised in stagnant solutions; this difference was greater for seminal than for nodal roots. When the acrated seminal roots were longer than 100 mm and transferred to an O₂‐free root medium, O₂ concentration became zero at the root tip causing elongation to cease. After 24 h of anoxia, none of these roots were able to resume elongation following a return to acrated solutions.