Effects of storage conditions on sprouting of microtubers of yam (Dioscorea cayenensis–D. rotundata complex)

The control of field tuber dormancy in the yam (Dioscorea cayenensis–D. rotundata complex) is poorly understood. Although studies have examined single environmental factors and chemical treatments that might prolong tuber dormancy and storage, only a few were focused on further tuber sprouting. The present study concerns microtubers obtained by in vitro culture. When microtubers were harvested (after 9 months of culture) and directly transferred on a new medium without hormones, the tubers rapidly sprouted in in vitro conditions. No dormancy was observed in this case. Harvested microtubers were also stored dry in jars in sterile conditions during 2 to 18 weeks before in vitro sprouting. In this case, microtubers stored during 18 weeks sprouted more rapidly than those stored 8 weeks. A constant “dormancy-like period” (storage duration + sprouting delay) was observed, between 20 and 28 weeks respectively for the more rapid and the slower microtubers. The size of the tubers used for the storage had great influence on further sprouting. The larger they were, the better they sprouted. Light during storage had no effect on the sprouting delay while a temperature of 25 °C permit a quicker sprouting than 18 °C. The medium used to obtain microtubers could also have an effect on sprouting rate.     

Control of potato tuber dormancy and sprouting by expression of sense and antisense genes of pyrophosphatase in potato

Inorganic pyrophosphate (PPi) is an enzyme involved in sugar metabolism in potato tubers. In our previous study, we isolated an inorganic pyrophosphatase (PPase) gene from potato and obtained the transgenic potato plants transformed with the sense and antisense PPase genes respectively. In the present experiment, the physiological indexes, tuber dormancy, and sprouting characteristics of the transgenic potatoes were analyzed and evaluated. The result showed that the PPase activity and the inorganic phosphate content of tubers were lower in the antisense transgenic plant lines but were higher in the sense transgenic plant lines, compared with wild-type tubers. Soluble sugars, such as glucose, fructose and sucrose increased in transgenic plants that had overexpression of the sense PPase gene, but decreased in the antisense transgenic plant lines, compared with wild-type tubers. Tuber sprouting time of the antisense transgenic plants were delayed for 2 and 3 weeks and reached the 100 % sprouting rate only after 14 and 16 weeks storage compared with the wild-type when tubers are stored under 25 and 4 °C, respectively. In contrast, tuber sprouting time of the sense transgenic plants was earlier by approximately 2 weeks than that of wild-type tubers under these storage temperatures.     

Effect of photoperiod and thermoperiod on microtuberization and carbohydrate levels in Cocoyam (Xanthosoma sagittifolium L. Schott)

Cocoyam (Xanthosoma sagittifolium L. Schott) is an important staple tuber crop for tropic populations and consists of three cultivars with different productivity. In vitro tuber induction of a broad range of genotypes could be a very useful way to propagate and increase valuable cocoyam. Shoot tips were cultured on a multiplication medium containing MS mineral salts, Morel and Wetmore (Ann J Bot 38:141–143, 1951) vitamins, 3% sucrose and 6% agar. Multiple shoots induced with 6-benzylaminopurine (BAP) were allowed to develop on the basal medium without plant growth regulators (PGRs). The subsequent explants obtained were used for the production of microtubers on MS-based PGRs free medium supplemented with 8% sucrose. Microtuberization was evaluated after 60 days of culture under different photo- and thermoperiod regimes. Changes in carbohydrates were also examined enzymatically during this process. Microtuberization response of three cocoyam cultivars (White, Red and Yellow) was influenced by photoperiod and thermoperiod regimes. A short photoperiod (8 h lighting), combined with an application of a day/night temperature regime 25/20°C for 10 days, followed by a continuous darkness at 20°C for 50 days, was the most effective treatment. The best response was observed with White followed by Red and Yellow cultivar. The sprouting frequency of microtubers varied significantly with cvs. after 15 days of culture (56% for White, 50% for Red and 40% for Yellow). Under tuber promoting condition, the onset of the process was characterized by an accumulation of starch and hexoses in leaves. The glucose:fructose ratio changed in favour of glucose earlier in White cultivar leaves where it doubled in 10 days. Moreover, in young developing tubers, the sucrose content increased concomitantly with starch.     

In vitro micro-tuber initiation and dormancy in yam

Dormancy is a mechanism that regulates the timing of sprouting (germination) of affected plant parts as well as ensures that the food quality of edible parts is maintained in storage until the following growing season. In yam, however, little is known about the control of tuber initiation or tuber dormancy. The objective of this study was to determine the effects of selected plant growth regulators (PGRs) on tuber initiation and dormancy, using an in vitro system. In two replicated experiments, 2-chloroethylphosphonic acid (ethephon, an ethylene source), abscisic acid (ABA) and gibberellin (GA₃) – and their inhibitors silver nitrate, fluridone and 2-chloroethyl-trimethylammonium chloride, respectively – were added at two concentrations to the culture medium prior to explant culture. Dates of micro-tuber initiation and sprouting (end of dormancy) and tuber number were recorded. In the control (no PGR) in Experiment 1, micro-tubers were initiated at the base of the stem after 176 days and sprouted 235 days later, that is 411 days after culturing. Most PGR treatments had only small effects (±30 days) on the duration of dormancy and the time of micro-tuber initiation. However, in GA₃ micro-tuber initiation occurred after 76 days, about 100 days earlier than in the control, whereas fluridone affected the position of micro-tubers and duration of dormancy. With fluridone treatments, tubers were found at the base of the stem (normal position) and on lower and upper nodes. Lower node tubers sprouted within 225 days of culturing compared with about 420 days after culturing at other nodal positions and in other PGR treatments. These data suggest an important role for ABA and gibberellic acid in yam micro-tuber initiation and the induction of dormancy.     

Changes in cis-zeatin and cis-zeatin riboside levels and biological activity during potato tuber dormancy

The effects of postharvest storage duration and temperature on endogenous cis -zeatin ( cis -Z) and cis -zeatin riboside ( cis -ZR) levels in potato ( Solanum tuberosum L.) tubers were determined in relation to tuber bud dormancy. The tubers used in these studies were completely dormant for at least 81 days of storage. Thereafter, tuber bud dormancy diminished gradually and after 165 days of postharvest storage, the tubers were completely non-dormant. Immediately after harvest, endogenous levels of cis- Z and cis -ZR were approximately 25 pmol (g fresh weight)⁻¹ and 8 pmol (g fresh weight)⁻¹, respectively. In tubers exiting dormancy but stored at a growth-inhibiting temperature (3°C), endogenous levels of cis -Z rose over threefold after 25 days of storage and remained elevated for the duration of the study. Levels of cis -ZR remained essentially constant during this same period. In tubers transferred to a growth permissive temperature (20°C) prior to use, the rise in endogenous cis -Z was less dramatic and more protracted; increasing twofold after 53 days of storage. No change in cis -Z riboside content was observed in these tubers during this period. Dose-response studies using either cis -Z or trans -Z demonstrated a time-dependent increase in cytokinin sensitivity during postharvest storage. Immediately after harvest, dormant tubers were insensitive to both zeatin isomers. Thereafter, tubers exhibited a dose-dependent increase in premature sprouting following injection with either cytokinin isomer. After injection into dormant tubers, cis -[8-¹⁴C]-zeatin was metabolized primarily to adenine/adenosine and cis -Z riboside. Seven days after injection, less than 10% of the recovered radioactivity was associated with trans -ZR. These results are consistent with a role for endogenous cis -Z (and its derivatives) in the regulation of potato tuber dormancy.     

Can loss of apical dominance in potato tuber serve as a marker of physiological age?

The potato tuber constitutes a model system for the study of dormancy release and sprouting, suggested to be regulated by endogenous plant hormones and their balance inside the tuber. During dormancy, potato tubers cannot be induced to sprout without some form of stress or exogenous hormone treatment. When dormancy is released, sprouting of the apical bud may be inhibited by sprout control agents or cold temperature. Dominance of the growing apical bud over other lateral buds decreases during storage and is one of the earliest morphophysiological indicators of the tuber's physiological age. Three main types of loss of apical dominance (AD) affect sprouting shape. Hallmarks of programmed cell death (PCD) have been identified in the tuber apical bud meristem (TAB-meristem) during normal growth, and are more extensive when AD is lost following extended cold storage or chemical stress. Nevertheless, the role of hormonal regulation in TAB-meristem PCD remains unclear.     

Response of potato grown under non-inductive condition paclobutrazol: shoot growth, chlorophyll content, net photosynthesis, assimilate partitioning, tuber yield, quality, and dormancy

The effect of foliar and soil applied paclobutrazol on potato were examined under non-inductive condition in a greenhouse. Single stemmed plants of the cultivar BP1 were grown at 35(±2)/20(±2) °C day/night temperatures, relative humidity of 58%, and a 16 h photoperiod. Twenty-eight days after transplanting paclobutrazol was applied as a foliar spray or soil drench at rates of 0, 45.0, 67.5, and 90.0 mg active ingredient paclobutrazol per plant. Regardless of the method of application paclobutrazol increased chlorophyll a and b contents of the leaf tissue, delayed physiological maturity, and increased tuber fresh mass, dry matter content, specific gravity, dormancy period of the tubers. Paclobutrazol reduced the number of tubers per plant. A significant interaction between rates and methods of paclobutrazol application were observed with respect to plant height and tuber crude protein content. Foliar application gave a higher rate of net photosynthesis than the soil drench. Paclobutrazol significantly reduced total leaf area and increased assimilate partitioning to the tubers. The study clearly showed that paclobutrazol is effective to suppress excessive vegetative growth, favor assimilation to the tubers, increase tuber yield, improve tuber quality and extend tuber dormancy of potato grown in high temperatures and long photoperiods.     

Nature of enhanced respiration during sprouting of aged potato seed-tubers

Respiration of 18-month-old Solarium tuberosum L. tubers was about 53% greater than that of 6-month-old tubers during sprouting at 23°C; yet, a significant loss of sprout vigor in the older tubers was apparent. Involvement of alternative oxidase (AO) in the age-induced difference in tuber respiration was assessed. AO was only detected in immunoblots if tissue disks from tubers were pre-incubated for 24 h prior to isolation of submitochondrial membrane particles (SMPs). No AO¹ was detected in SMPs from nonincubated tuber tissue of either age, indicating that it was not contributing to tuber respiration during sprouting as previously thought. Respiratory control and ADP/O ratios indicated that oxidative phosphorylation was fully coupled to electron transport in mitochondria isolated from 6- and 18-month-old tubers. Cytochrome c oxidase (EC 1.9.3.1) activities of intact mitochondria were also not affected by tuber age. The difference in respiration during sprouting was unique to whole tubers, as oxygen consumption by mitochondria from young and oid tubers was equal on a milligram protein basis. Sprouting 18-month-old tubers had 15% more mitochondrial protein per gram fresh weight than did 6-month-old tubers. Older tubers also produced more ATP than younger tubers prior to and during sprouting, through a fully coupled, Cyt-mediated respiratory pathway, reduced sprout vigor notwithstanding. From 5 to 10 days of sprouting, coinciding with development of the age-induced difference in whole-tuber respiration, ATP concentration 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 SDS-PAGE and immunoblots of β- and oligomycin-sensitivity conferring protein-subunits, also increased as a proportion of SMP protein in older tubers during this period. Relative to 6-month-old tubers, the increased respiration and associated oxidative phosphorylation of 18-rnonth-old tubers during sprouting were probably in response to a lower adenylate energy charge (AEC) prior to sprouting (from 0 fo 5 days). From 5 to 10 days of sprouting, AEC of 18-rnonth-old tubers increased to equal that of 6-month-old tubers and the two tuber ages maintained the same AEC for the remainder of the 20-day sprouting interval. Higher respiration and lower AEC of older tubers in storage at 4°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 utilization of ATP by older tubers.     

Regulation of potato tuber dormancy and sprouting

Dormancy is the final stage of tuber life serving to preserve tubers as organs of vegetative reproduction under unfavorable growth conditions. Since the duration of potato tuber dormancy and their sprouting time have significant economic importance, much attention is given to the study of the regulation of these processes. This review considers metabolite, genetic, and hormonal aspects of regulation of potato (Solanum tuberosum L.) tuber dormancy and sprouting. Particular attention is paid to the relationship between processes occurring in different parts of the tuber: its storage tissues and buds. The interaction of hormonal and metabolite (carbohydrate) regulation of dormancy and sprouting is discussed.     

Changes in the levels of free IAA and cytokinins in potato tubers during dormancy and sprouting

Changes in the levels of free indol-3-ylacetic acid (IAA) and free cytokinins were determined in the course of dormancy and sprouting period in potato tubers(Solanum tuberosum L., cv. Nevskii) stored at 4 °C. The same analyses were performed in potato tubers after Ethrel application, which prolongs dormancy. No significant changes were found in free IAA level during dormancy followed by a rapid decrease during sprouting. After Ethrel application a significant lower IAA level was found 3 weeks after application, but further on no changes in free IAA level between treated and non-treated tubers were detected. Cytokinin level was relatively low and constant till sprouting and increases then by about 55 %, mainly due to an increase in the level of zeatin riboside and isopentenyladenosine. Ethrel application decreased cytokinin level during dormancy slightly, but postpones the increase coupled with sprouting by about 1 month. Thus, IAA does not seem to have a significant effect on tuber dormancy, while cytokinins are probably necessary for sprouting initiation.     

Novel bioreactor technology for mass propagation of potato microtubers

Potato (Solanum tuberosum L. ssp. tuberosum) microtubers were produced in vitro with Liquid Lab™ Rocker system. A thin-layer liquid culture was applied together with a regular pitch in autoclavable simple plastic vessels. All cultures were carried out at room temperature without contamination problems. Each cultivar tested (Asterix, Timo, Van Gogh, Velox) formed microtubers in the Liquid Lab system. The mean number of microtubers per vessel (50 explants) varied between 30 (cv. Asterix in 8 weeks tuber induction) and 75 microtubers (cv. Velox in 11 weeks tuber induction). Majority (63%) of the microtubers was sufficient by size and weight (above 200 mg) for further storage at dormancy (4°C). The cv. Velox yielded the highest number of microtubers with cultivation capacity. As a result of prolonged microtuber induction of 2–3 weeks, more microtubers with competence for cultivation were obtained per cultivar, except for cv. Van Gogh. Still, the mean weight of Van Gogh microtubers was significantly higher after prolonged microtuber induction (0.67 g) than after short induction (0.51 g). In conclusion, Liquid Lab™ Rocker system is a novel, efficient and rapid system for mass propagation of potato.     

Involvement of endogenous gibberellins in potato tuber dormancy and early sprout growth: a critical assessment

The role of endogenous gibberellins (GAs) in the regulation of potato (Solanum tuberosum) tuber dormancy was examined by determining: 1. changes in endogenous GA levels during natural dormancy progression, 2. the effects of GA biosynthesis inhibitors on tuber dormancy duration and 3. the dormancy status and tuber GA levels in a dwarf mutant of potato. The tubers (cv. Russet Burbank) used in these studies were still completely dormant after 98 days of storage. Between 98 and 134 days of storage, dormancy began to end and tubers exhibited limited (< 2 mm) sprout growth. Tuber dormancy weakened with further storage and tubers exhibited greater rates of sprout growth after 187 days of storage. Tubers stored for 212 days or longer were completely non-dormant and exhibited vigorous sprout growth. Immediately after harvest, the endogenous contents of GA19, GA20, and GA1 were relatively high (0.48-0.62 ng g fresh weight(-1)). The content of these GAs declined between 33 and 93 days of storage. Internal levels of GA19, GA20, and GA, rose slightly between 93 and 135 days of storage reaching levels comparable to those found in highly dormant tubers immediately after harvest. Levels of GA19, GA20, and GA1 continued to increase as sprout growth became more vigorous. Neither GA4 nor GA8 was detected in any tuber sample regardless of dormancy status. Dormant tubers exhibited a time-dependent increase in apparent GA sensitivity. Freshly harvested tubers were completely insensitive to exogenous GAs. As postharvest storage continued, exogenous GAs promoted premature dormancy release with GA1 and GA20 eliciting the greatest response. Injection of up to 5 microg tuber(-1) of kaurene, GA12, GA19 or GA8 had no effect on dormancy release. Sprout growth from non-dormant tubers was also promoted by exogenous GA in the following sequence of activity: GA1 = GA20 > GA19. Kaurene, GA12, and GA8 were inactive. Continuous exposure of developing tubers to inhibitors of GA biosynthesis (AMO-1618, ancymidol, or tetcyclasis) did not extend tuber dormancy but rather hastened dormancy release. Comparison of tuber dormancy and GA1 content in tubers of a wild-type and dwarf mutant of S. tuberosum ssp. andigena revealed a near-identical pattern of dormancy progression in spite of the absence of detectable levels of GA1 in tubers of the dwarf sibling at any time during dormancy progression. Collectively, these results do not support a role for endogenous GA in potato tuber dormancy release but are consistent with a role for GAs in the regulation of subsequent sprout growth.     

Variation in Endogenous Gibberellins,Abscisic Acid,and Carbohydrate Content During the Growth Cycle of Colored <Emphasis Type="Italic">Zantedeschia</Emphasis> spp., a Tuberous Geophyte

Phenologic changes and variation in the level of endogenous gibberellins (GAs), abscisic acid (ABA), carbohydrate content, and α-amylase activity were examined in colored Zantedeschia spp. cv. Cala Gold. These changes were examined in the primary bud tissues and in the attached tuber tissue during the growth cycle. Dormant tubers were dry-stored at 20°C for 3 months, planted in a phytotron, and grown under 22/16 ± 1°C. Plant development was monitored under continued irrigation until leaf senescence and tuber dormancy. GAs and ABA were extracted from the primary bud tissues, fractionated by HPLC, and analyzed using GC-SIM. Starch, glucose, soluble protein, and α-amylase activity were monitored in the tuber tissue attached to the primary bud. Endogenous changes in GAs and ABA in the primary bud were correlated with endogenous changes in carbohydrate content and α-amylase activity in the attached tuber tissue. These correlations were observed during the rest and the growth periods and were associated with developmental changes in the plant, that is, bud dormancy relaxation, bud growth, and inflorescence differentiation. ABA content decreased and a transient pulse of GA was measured in the primary bud concomitantly with the onset of shoot elongation in dry tubers during storage, before planting. The sharp increase of GAs in the bud preceded inflorescence differentiation as observed in dissected apices by about 15 days, as well as the increase in α-amylase activity in the attached tuber tissue. A steep decrease in starch level was measured in the tuber after planting, concomitantly with massive plant growth. These findings suggest a possible involvement of gibberellin in the initiation of α-amylase activity during dormancy relaxation in colored Zantedeschia and in the autonomous induction of flowering.     

The Effect of Plant Polygalaturonase-Inhibiting Protein on the Rate of Oligouronide Formation

The effects of the polygalacturonase-inhibiting protein (PGIP) on the rate of oligouronide formation were studied in a model system containing polygalacturonic acid and polygalacturonase (PG) from the culture medium of phytopathogenic fungi. PGIP preparations were prepared from stored potato tubers and sprouts and also from apple fruits. The PGIP effects on oligouronide synthesis depended markedly on the physiological state of the source plant. Apple cultivars differing in their earliness differed in PGIP effects as well. The PGIP from potato tubers, which were in deep dormancy, suppressed oligouronide formation. The inhibitory PGIP action was decreased after dormancy release and tuber sprouting, which resulted in the oligouronide accumulation. The effects of PGIP from apple fruits on the oligouronide synthesis in the system containing PG from various phytopathogenic fungi were not correlated with tissue damage induced by these fungi. The PGIP effects on oligouronide formation are evident; however, their role in plant-cell processes related to the pectin compound conversions and plant resistance to diseases remains to be elucidated.     

Evidence that Tuber Respiration is the Pacemaker of Physiological Aging in Seed Potatoes (Solanum tuberosum L.)

Storage temperatures greater than 4 °C (that is, heat-unit accumulation) increase respiration and accelerate physiological aging of seed tubers. The degree of apical dominance is a good indicator of physiological age (PAGE). As seed age advances, apical dominance decreases, resulting in more stems, greater tuber set, and shifts in tuber size distribution. Herein we provide evidence that tuber respiration rate may constitute the “pacemaker” of aging. Tubers exposed to a brief high-temperature age-priming treatment initially in storage, followed by holding at 4 °C for the remainder of a 190–200-day storage period, maintained a higher basal metabolic (respiration) rate throughout storage compared with tubers stored the entire season at 4 °C. Tubers thus “remembered” the age-priming treatment as reflected by their elevated respiration rate. Moreover, reducing the respiration rate of age-primed seed by subsequently storing it at 3.5 % O₂ (4 °C) until planting significantly attenuated the effects of the aging treatment on apical dominance, tuber set, and size distribution. The effect of the age-priming treatment on the magnitude of the respiratory response was the same whether given at the beginning or toward the end of storage. However, moving the age-priming treatment progressively later in the storage season effectively decreased its impact on plant growth and development. These results underscore the importance of time in the aging process. Exposure of seed to a high-temperature age-priming treatment at the beginning or end of storage elevated respiration (the pacemaker) to the same extent; however, the timing of these treatments resulted in vastly different physiological ages. The longer the respiration rate of tubers remains at an elevated level, the greater their PAGE at planting. Thus, an accurate but impractical measure of PAGE may be the respiratory output from vine kill to subsequent planting. Respiration appears to be the pacemaker of PAGE and production, and storage conditions that affect respiration may “set the clock speed” that will ultimately determine the PAGE at planting.     

Combining conventional QTL analysis and whole-exome capture-based bulk-segregant analysis provides new genetic insights into tuber sprout elongation and dormancy release in a diploid potato population

Tuber dormancy and sprouting are commercially important potato traits as long-term tuber storage is necessary to ensure year-round availability. Premature dormancy release and sprout growth in tubers during storage can result in a significant deterioration in product quality. In addition, the main chemical sprout suppressant chlorpropham has been withdrawn in Europe, necessitating alternative approaches for controlling sprouting. Breeding potato cultivars with longer dormancy and slower sprout growth is a desirable goal, although this must be tempered by the needs of the seed potato industry, where dormancy break and sprout vigour are required for rapid emergence. We have performed a detailed genetic analysis of tuber sprout growth using a diploid potato population derived from two highly heterozygous parents. A dual approach employing conventional QTL analysis allied to a combined bulk-segregant analysis (BSA) using a novel potato whole-exome capture (WEC) platform was evaluated. Tubers were assessed for sprout growth in storage at six time-points over two consecutive growing seasons. Genetic analysis revealed the presence of main QTL on five chromosomes, several of which were consistent across two growing seasons. In addition, phenotypic bulks displaying extreme sprout growth phenotypes were subjected to WEC sequencing for performing BSA. The combined BSA and WEC approach corroborated QTL locations and served to narrow the associated genomic regions, while also identifying new QTL for further investigation. Overall, our findings reveal a very complex genetic architecture for tuber sprouting and sprout growth, which has implications both for potato and other root, bulb and tuber crops where long-term storage is essential.Subject terms: Genetic markers, Next-generation sequencing, Plant breeding, Agricultural genetics, Genetic mapping     

Effects of reducing sugar concentration on in vitro tuber formation and sprouting in yam (<Emphasis Type="Italic">Dioscorea cayenensis–D. rotundata</Emphasis> complex)

The effects of reducing sucrose level on tuber formation (% of cultures with microtubers), development (length and fresh weight of microtubers) and sprouting in yam Dioscorea cayenensis–D. rotundata complex in vitro were investigated. Only 29% of the explants showed tuber formation after 3 weeks in the presence of 1% sucrose in contrast to 100% with 3%. After 120 days of culture, the length and the weight of the tubers obtained in the presence of 1% sucrose were less than with 3% sucrose. Addition of sorbitol to keep osmolarity at the same level did not restore normal rate of tuber formation. Similar results were obtained with the use of reduced fructose or glucose level. Microtuber sprouting was also affected by sucrose level incorporated into the tuberisation medium. Tubers obtained on reduced sucrose level sprouted later and the increase of osmolarity with sorbitol did not restore normal sprouting. The bigger tubers obtained on high sucrose media could contain more carbohydrate reserves that could partially explain a higher sprouting rate. These results can be used for optimising in vitro conditions for mass production of microtubers in yam and especially in Dioscorea cayenensis–D. rotundata complex, a very important species in West Africa. They specially showed the importance of tuberisation conditions on precocity of tuberisation, on tuber length and weight and on their further sprouting.     

In-season heat stress compromises postharvest quality and low-temperature sweetening resistance in potato (Solanum tuberosum L.)

Key message

High soil temperature during bulking and maturation of potatoes alters postharvest carbohydrate metabolism to attenuate genotypic resistance to cold-induced sweetening and accelerate loss of process quality.

Abstract

The effects of soil temperature during tuber development on physiological processes affecting retention of postharvest quality in low-temperature sweetening (LTS) resistant and susceptible potato cultivars were investigated. ‘Premier Russet’ (LTS resistant), AO02183-2 (LTS resistant) and ‘Ranger Russet’ (LTS susceptible) tubers were grown at 16 (ambient), 23 and 29 °C during bulking (111–164 DAP) and maturation (151–180 DAP). Bulking at 29 °C virtually eliminated yield despite vigorous vine growth. Tuber specific gravity decreased as soil temperature increased during bulking, but was not affected by temperature during maturation. Bulking at 23 °C and maturation at 29 °C induced higher reducing sugar levels in the proximal (basal) ends of tubers, resulting in non-uniform fry color at harvest, and abolished the LTS-resistant phenotype of ‘Premier Russet’ tubers. AO02183-2 tubers were more tolerant of heat for retention of LTS resistance. Higher bulking and maturation temperatures also accelerated LTS and loss of process quality of ‘Ranger Russet’ tubers, consistent with increased invertase and lower invertase inhibitor activities. During LTS, tuber respiration fell rapidly to a minimum as temperature decreased from 9 to 4 °C, followed by an increase to a maximum as tubers acclimated to 4 °C; respiration then declined over the remaining storage period. The magnitude of this cold-induced acclimation response correlated directly with the extent of buildup in sugars over the 24-day LTS period and thus reflected the effects of in-season heat stress on propensity of tubers to sweeten and lose process quality at 4 °C. While morphologically indistinguishable from control tubers, tubers grown at elevated temperature had different basal metabolic (respiration) rates at harvest and during cold acclimation, reduced dormancy during storage, greater increases in sucrose and reducing sugars and associated loss of process quality during LTS, and reduced ability to improve process quality through reconditioning. Breeding for retention of postharvest quality and LTS resistance should consider strategies for incorporating more robust tolerance to in-season heat stress.