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Russet Burbank and Innovator are mid- to late-season frozen-processing cultivars with inherently different dormancy periods and susceptibilities to low temperature-induced sweetening (LTS). In contrast to Russet Burbank, which is highly prone to accumulation of reducing sugars (Glc + Fru) when stored below 8–9 °C, Innovator tubers exhibit moderate resistance to LTS and retain process quality longer at lower temperatures (4–6 °C). However, Innovator’s LTS resistance is not robust and often varies across production regions. Here we show that low O2 storage modulates LTS to reveal metabolic differences intrinsic to these cultivars. Changes in tuber respiration, process quality, reducing sugars, sucrose, starch phosphorylase and invertase activities were compared at 4 and 8 °C in 2.5 and 21 kPa O2 over a 212-d storage period. Tuber respiration declined rapidly as O2 level decreased from 21 to 2.5 kPa during acclimation at 8 °C. Respiration rates then fell further as the temperature was lowered from 8 to 4 °C, but this response was greatly muted for tubers at 2.5 versus 21 kPa O2. Tubers at 21 kPa O2 completed their cold-induced respiratory acclimation response (RAR) within 7 d compared with 13 d at 2.5 kPa O2, and the RAR was much greater for tubers at 2.5 kPa O2. While reducing sugars increased most rapidly in tubers over the first 30 d at 4 °C, Innovator tubers had lower invertase activity and sweetened less than Russet Burbank tubers, characterizing its LTS-resistant phenotype. Low O2 greatly attenuated these initial LTS responses for both cultivars; however, the effect was only temporary in Innovator. LTS resumed in Innovator tubers from 93 to 212 d with reducing sugar levels increasing to equal that of Russet Burbank tubers stored at 4 °C and 21 kPa O2. Activities of α-1,4 glucan phosphorylase (SP) were higher in Russet Burbank than Innovator and increased progressively over the storage period regardless of temperature and oxygen concentration. Innovator tubers stored at 2.5 kPa O2 had higher SP activities from 30 to 153 d at 4 and 8 °C compared with tubers stored at 21 kPa O2, which correlated well with increased sucrose buildup and earlier sprouting. The low O2-mediated inhibition of LTS was largely a consequence of reduced invertase activities.

The cultivar, Bondi, was selected from a cross between Ranger Russet (maternal parent) and Karaka and is currently being evaluated as a frozen processing cultivar. Relative to Ranger, Bondi produces vigorous foliar growth, low tuber set and high yields of large tubers that frequently exceed optimum size for seed and processing markets. We evaluated the relative efficacies of gibberellin (GA), cytokinin (benzyladenine (BA)) and seed aging for altering apical dominance, tuber set and size distribution of these genetically related cultivars. GA applied to cut seed prior to planting hastened emergence, reduced apical dominance, increased tuber set and decreased average tuber size; however, the optimal concentration to maximally shift tuber size distribution without decreasing marketable yield was 4–5-fold greater for Bondi than Ranger. BA marginally hastened plant emergence (Bondi) and decreased apical dominance (both cultivars) only when combined with GA, but had no further effects on tuber set, yields or tuber size distributions in either cultivar. Age-priming Ranger seed for 700 degree days (DD) at 32 °C during storage shifted the tuber size distribution to a much greater extent than 2 mg L−1 GA (optimal concentration) without reducing marketable yield. The combined age and GA treatment resulted in no further shift in size distribution for Ranger beyond that induced by the 700-DD treatment alone, but reduced the marketable yield by 9.6 MT ha−1. In contrast to GA, the mechanism by which age-priming altered tuber size distribution in Ranger could not be explained by effects on stem number/tuber set relationships alone. Bondi, however, exhibited an even greater shift toward smaller tubers with no reduction in yield with the combined 700-DD/GA (2 mg L−1) treatment, reflecting its decreased sensitivity to GA. Moreover, the shift in tuber size distribution induced by aging Bondi seed for 700 DD was approximately equal to that observed by treating seed with 8 mg L−1 GA (optimal concentration). The reduced sensitivity of Bondi to GA was likely inherited from its paternal parent Karaka, which displays similar morphological traits, including high yield of large tubers.