Effects of L-Ethionine on Adenosine Triphosphate Levels, Respiration, and Salt Accumulation In Carrot Xylem Tissue

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Abstract
The time course during aging of carrot xylem disks for development of salt respiration and sensitivity of respiration to inhibition by L-ethionine was determined. 5mM L-ethionine has no effect on the respiration of freshly cut disks, but sensitivity developed during the aging process and was maximal from 25 to 120 hr., falling to zero at 150 hr. Maximal inhibition (35-40 [degree]/o) coincided with the period of maximum basal respiratory rate. Rb+ uptake into carrot xylem disks aged for 77 hr. was linear and at 80 % of the control rate for nearly 2 hr. in the presence of 5 mM L-ethionine, despite inhibition of salt respiration and a concomitant drop in the ATP level to 50 [degree]/o of control values during this period. 5 mM L-ethionine failed to inhibit Rb+ and Cl- uptake into disks aged for 125 hr. although the ATP level fell to 40 [degree]/b of control after 2 hr. and both basal and salt respiration were inhibited; 90 [degree]/o of the salt respiration was eliminated within 60 min. Cl- uptake from 40 mM NaCl Into disks aged for 125 hr. proceeded linearly at 85 [degree]/o of the control rate for over 2 hr. despite a 60 [degree]/o drop in ATP level. A lag in the onset of oligomycln Inhibition of K+ and Cl- uptake into aged carrot disks and of KC1 uptake into aged beetroot disks was observed. Anaerobic conditions strongly inhibited Cl- uptake into aged carrot disks over 30 min., although the ATP level fell by only 30 %. The results provide further evidence that salt accumulation into aged carrot xylem tissue is linked to electron transport rather than to hydrolysis of ATP, and indicate that a large proportion of salt respiration may be an indirect consequence of salt accumulation.