Download Derek J. Herman, PhD candidate Department of Horticulture

Derek J. Herman, PhD candidate
Department of Horticulture, Washington State University
Modifying tuber size distribution, low temperature sweetening and tolerance to heat stress in
processing potatoes (Solanum tuberosum L.)
Controlling tuber size distribution and the propensity for low temperature sweetening (LTS) of processing
potatoes (French fries) are key to optimizing quality and maximizing crop value. My research focused on
(1) developing methods to increase tuber set and shift tuber size distribution in cv. Bondi, (2)
investigating how low O2 storage modulates LTS of cvs. Innovator and Russet Burbank (3) screening
cultivars for tolerance to heat stress for retention of LTS-resistant phenotype and (4) determining the
mechanism by which heat stress abolishes resistance to cold sweetening.
The cv. 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. The relative efficacies of gibberellin (GA), cytokinin (CK) and age priming seed
treatments for manipulating apical dominance, tuber set and size distribution of these genetically related
cultivars was investigated. GA hastened emergence, reduced apical dominance, increased tuber set and
decreased average tuber size; however, the optimal concentration was 4-5-fold greater for Bondi than
Ranger. CK 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 32oC 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. Bondi, however, exhibited an even greater shift toward
smaller tubers with no reduction in yield with a combined 700-DD/GA (2 mg L-1) treatment, reflecting its
decreased sensitivity to GA. The shift in tuber size distribution induced by age priming Bondi seed was
approximately equal to an 8 mg L-1 GA treatment (optimal concentration). Tuber set and size distribution
of Bondi can be effectively managed with GA and age priming treatments.
Russet Burbank and Innovator are mid- to late-season frozen-processing cultivars with inherently
different susceptibilities to LTS. In contrast to Russet Burbank, which is highly prone to accumulation of
reducing sugars (Glc + Fru) when stored below 8-9oC, Innovator tubers exhibit moderate resistance to
LTS and retain process quality longer at lower temperatures (4-6oC). However, Innovator’s LTS
resistance is not robust and often varies across production regions. Low O2 storage modulated the
sweetening responses of these cultivars to reveal metabolic differences intrinsic to their LTS
resistance/susceptibility. Changes in tuber respiration, process quality, reducing sugars, sucrose, starch
phosphorylase and invertase activities were compared at 4 and 8oC in 2.5 and 21% O2 over a 212-day
storage period. While reducing sugars increased most rapidly in tubers over the first 30 days at 4oC,
Innovator tubers had lower invertase activity and sweetened less than Russet Burbank tubers,
characterizing its LTS-resistant phenotype. Low O2 greatly attenuated this initial LTS response for both
cultivars; however, the effect was only temporary in Innovator. LTS resumed in Innovator tubers from 93
to 212 days with reducing sugars increasing to levels apparent in Russet Burbank tubers stored at 4oC and
21% O2. The low O2-mediated inhibition of LTS was largely a consequence of reduced invertase
activities. While low O2 storage is not commercially feasible, it is an effective technique for resolving
and defining differences in LTS metabolism intrinsic to susceptible and resistant cultivars.
Resistance to LTS is a major goal in breeding chip and frozen processing cultivars. In-season heat stress,
however, can undermine LTS resistance in many conventionally bred clones/cultivars. A postharvest
screening protocol was developed to select for tolerance to heat stress for retention of LTS-resistant
phenotype. Heat stress abolished the inherent resistance to LTS in Sage Russet, POR06V12-3 and
AO2138-2 tubers, and exacerbated LTS of Ranger Russet and Russet Burbank tubers (LTS-susceptible).
Payette Russet, however, had robust tolerance to heat stress for retention of its LTS-resistant phenotype.
In contrast to the non-heat tolerant clones, heat primed Payette Russet tubers accumulated 4-fold more
sucrose at 4oC, while Glc and Fru concentrations remained low, resulting in light colored fries. In the
non-heat-tolerant clones, heat stress stimulated catabolism of starch to sucrose, which was subsequently
hydrolyzed to Glc and Fru by acid invertase during cold storage. However, acid invertase activity
remained low in heat-stressed Payette tubers during LTS, effectively limiting the buildup of reducing
sugars. InnateTM tubers with silenced invertase also showed robust tolerance to heat stress for retention of
LTS-resistant phenotype. Therefore, tolerance to heat stress for retention of LTS-resistant phenotype in
Payette appears manifested by its ability to maintain low invertase activity during cold storage.