Extension rate and respiratory activity in the growth zone of wheat roots: Time‐course for adjustments after defoliation

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Abstract
The time‐course for adjustments in the rate of extension of wheat (Triticum aestivum L. cv. Alexandria) roots, and the activity and capacity of respiratory pathways in the root apex, were determined after pruning the shoot to the ligule of the first leaf. Leaf pruning reduced the extension rate of both seminal and lateral roots. The onset of the response occurred within 1 h of pruning for laterals and between 2 and 3 h for seminals. The reduction in rate appears to be the result of a decrease in carbohydrate availability because (1) in seminal roots it was preceded by a decrease in soluble sugar content of the apical part of the growth zone (0–5 mm behind the root apex) and (2) supplying glucose (50 mM) to the roots of plants defoliated 24 h earlier led to a steady increase in extension rate of both seminal and lateral roots compared to non‐fed controls. Supplying 3‐O‐methyl glucose had no effect. The reduction in extension rate of seminal roots was accompanied (or slightly preceded) by a reduction in respiratory O2 uptake in the apical part of the growth zone (0–5 mm). Changes in respiratory activity in the basal part of the growth zone (5–10 mm) only occurred several hours later. At the time root extension rate was reduced, the rate of O2 uptake could be stimulated with FCCP, which indicates that respiration was under the fine control of adenylates. From these results we suggest the following sequence of events occurs after defoliation. Firstly, defoliation reduces the supply of sugars to the root apex, this leads to a reduction in rate of extension through some form of coarse control by carbohydrates on cell division and expansion, which in turn reduces the rate of respiratory O2 uptake because of a smaller demand for ATP. The results also indicate that there is a rapid (<1.5 h) reduction in respiratory capacity in the root apex after defoliation which occurs before any change in the overall rate of respiration.