Regulation of respiration in roots of barley

The regulation of respiration in roots of barley [ Hordeum distichum (L.) Lam. cv. Maris Mink] was examined in terms of the supply of carbohydrate and control by adenylates. The carbohydrate content of roots was modified by selective pruning and feeding sucrose (25 m M ) and rates of O 2 uptake were measured 16–24 h later. Pruning roots to a single seminal axis increased the soluble sugar content of the sole remaining root by 30% and its rate of O 2 uptake by 24%: pruning shoots so that all but the second leaf was removed lowered the soluble sugar content of the roots by 39% and their rate of O 2 uptake by 34%. Changes in the rate of O 2 uptake were due largely to adjustments in activity of the cytochrome path (v cyt ): these changes were accompanied by comparable adjustments in capacity of the path (V cyt ) so that it always operated close to full engagement (=90%). The capacity of the alternative path was unchanged and in no case was there any engagement. Since the alternative path failed to engage over a wide range of carbohydrate contents, we conclude that this path does not function as a simple energy overflow in barley roots. Irrespective of pruning treatment respiration appeared to be limited directly by the turnover of ATP since rates of O 2 uptake were stimulated by the uncoupler FCCP, but not by short‐term (< 1 h) feeding of sucrose. However, long‐term feeding of sucrose (16–24 h) to roots of leaf‐pruned and control plants resulted in an increase in both v cyt and V cyt . It is proposed that whilst the rate of O 2 uptake is regulated directly by the turnover of ATP, some energy‐consuming processes might be sensitive to the carbohydrate concentration: changes in supply of carbohydrate with selective pruning or sucrose feeding may alter rates of these processes and regulate v cyt through changes in energy demand. The nature of adjustments in V cyt and the possible role of the alternative path are discussed.