Controlling Influence of Thickness on Development & Type of Respiratory Activity in Potato Slices.

Early studies of the respiratory behavior of potato tuber slices showed the specific respiration rate to be inversely related to thickness in slices up to 3.0 mm thick, and to approach a constant low value with increasing thickness thereafter [(38); see (27) for a brief summary of respiratory studies relating to slices of bulky storage organs in general]. The observed relationship was explained on the basis that a shallow superficial layer of actively respiring tissue borders tissue of uniform low activity, so that with increasing slice thickness the active layer is increasingly diluted, and the specific respiration rate thereby lowered. Steward (35, 38) postulated that the elevated respiratory activity at the surface of a disk or slice is the consequence of enhanced oxygen availability, a point of view shared with some modification by Boswell and Whiting (3) and Stiles and Dent (42). Steward further specified that where respiration rate and disk thickness are inversely related, oxygen availability determines the depth of the active surface layer rather than the level of respiratory activity throughout the tissue mass (35, 38,41). Several considerations call for a reappraisal of the above-mentioned classical and generally accepted view. First, the earliest experiments dealing with the relation between thickness and respiration rate showed that the respiration of potato disks in the range where rate and thickness are reciprocally related is not enhanced by increasing the external oxygen concentration above that in air (36,41). The assumption that the active surface layer is determined by oxygen availability is thus put in question. The depth of the active surface layer was estimated by Steward (35) on the presumption that the enhanced surface respiration decreases with oxygen tension with distance from the surface, and terminates where the oxygen concentration drops to zero. In disks of 1.0 mm or less in thickness maintained in water the calculations indicated virtually the entire disk to comprise the active surface layer. However, as will be shown, the observed relationship between thickness and respiration rate [fig 1 & (38)] is inconsistent with the preceding deduction. Where the active surface layer comprises but a fraction of the disk, the foregoing view ostensibly affirms that the surface respiration is aerobic, in distinction to the subsurface, or bulk, respiration, which is anaerobic. Since respiratory rate was described solely in terms of CO2 evolution, the term respiration was presumably used generically to include both the aerobic and anaerobic production of CO2. However, since the respiratory quotient of potato tuber tissue is essentially 1.0 regardless of mass [see James (16)], it is evident that if oxygen tension in any way regulates respiratory activity in potato slices, control must relate to the magnitude or type of aerobic respiratory activity, rather than to the presence or absence thereof [see Choudhury (7)]. Some 70 years ago Devaux (9) recognized that oxygen, though diminished in the heart of bulky storage organs, is nevertheless plentiful, and recently Burton (5) has elegantly demonstrated that the po 2