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 O 2 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 N 2 ‐flushed solutions (0.003 mol m −3 O 2 ) compared with growth in continuously acrated solutions (0.26 mol m −3 O 2 ). 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 N 2 ‐flushed treatments. A vernier microscope and cylindrical platinum‐electrode were used to examine the relationship between root extension and transport of O 2 from shoots to roots via the gas spaces. Measurements were made when the roots were in an anoxic medium and were dependent solely on O 2 supplied from the shoots. For seminal roots of 5–7‐d‐old seedlings raised in stagnant solutions (90–100 mm), internal O 2 transport was sufficient to support a rate of root elongation in the O 2 ‐free medium of between 0.03 and 0.17 mm h −1 . When the O 2 pressure around the shoots was increased from 20 to 100 kPa O 2 , the O 2 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 O 2 ‐free root medium, O 2 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.