NATURE OF ENHANCED RESPIRATION DURING SPROUTING OF AGED POTATO SEED-TUBERS

Respiration of 18-month-old Solanum tuberosum L. tubers was about 53% greater than that of 6-month-old tubers during sprouting at 23 degrees C; yet, a significant loss of sprout vigor in the older tubers was ap parent. Involvement of alternative oxidase (AO) in the age-induced dif ference in tuber respiration was assessed. AO was only detected in imm unoblots if tissue disks from tubers were pre-incubated for 24 h prior to isolation of submitochondrial membrane particles (SMPs). No AO was detected in SMPs from nonincubated tuber tissue of either age, indica ting that it was not contributing to tuber respiration during sproutin g as previously thought. Respiratory control and ADP/O ratios indicate d that oxidative phosphorylation was fully coupled to electron transpo rt in mitochondria isolated from 6- and 18-month-old tubers. Cytochrom e c oxidase (EC 1.9.3.1) activities of intact mitochondria were also n ot affected by tuber age. The difference in respiration during sprouti ng was unique to whole tubers, as oxygen consumption by mitochondria f rom young and old tubers was equal on a milligram protein basis. Sprou ting 18-month-old tubers had 15% more mitochondrial protein per gram f resh weight than did 6-month-old tubers. Older tubers also produced mo re ATP than younger tubers prior to and during sprouting, through a fu lly coupled, Cyt-mediated respiratory pathway, reduced sprout vigor no twithstanding. From 5 to 10 days of sprouting, coinciding with develop ment of the age-induced difference in whole-tuber respiration, ATP con centration in 18-month-old tubers increased to become 52% higher than that in 6-month-old tubers. ATP synthase (EC 3.6.1.34), assessed by SD S-PAGE and immunoblots of beta- and oligomycin-sensitivity conferring protein-subunits, also increased as a proportion of SMP protein in old er tubers during this period. Relative to 6-month-old tubers, the incr eased respiration and associated oxidative phosphorylation of 18-month -old tubers during sprouting were probably in response to a lower aden ylate energy charge (AEC) prior to sprouting (from 0 to 5 days). From 5 to 10 days of sprouting, AEC of 18-month-old tubers increased to equ al that of 6-month-old tubers and the two tuber ages maintained the sa me AEC for the remainder of the 20-day sprouting interval. Higher resp iration and lower AEC of older tubers in storage at 4 degrees C, along with the fact that older tubers respired at a higher rate to achieve the same AEC as younger tubers during sprouting, indicate greater util ization of ATP by older tubers.