Starch-sugar interconversion inSolanum tuberosum L. II. Influence of membrane permeability and fluidity

A strong relationship was found between membrane permeability and starch to sugar conversion in stored potatoes. Tissue from tubers stored at low temperature had higher sugar content and more permeable membranes than from that of tubers stored at high temperature. Tubers, moisture stressed during growth and stored at 5.5 C, accumulated more reducing sugars and had more permeable membranes than tubers from normally irrigated plants when stored at the same temperature. The basal portion of Russet Burbank tubers accumulated more sugars and was also found to have significantly more permeable membranes than the apical portion. Examination of membrane order with spin-labeled lipid probes showed that the order parameter of amyloplast membranes was less at low temperatures than when measured at higher temperatures. Membranes examined with spin-labeled lipid probes from tubers stored at 5.5 C and 15.5 C showed similar order parameters at all temperatures measured between 0 and 25 C with a linear decrease in S values at higher temperatures. These results strongly indicate a physical change of membranes with changing temperature, thus influencing sugar accumulation, which is also reversible, as indicated by loss of sugars during the reconditioning process.     

Starch to sugar interconversion inSolanum tuberosum L. I. Influence of inorganic ions

No relationship could be found between calcium, magnesium, potassium, total phosphorous contents, and reducing sugar accumulation in apical and basal portions of Russet Burbank tubers. However, a highly significant positive correlation was found between inorganic phosphorous content of tubers and reducing sugar accumulation. Inorganic phosphorous and sugar levels were highest in the basal portion of moisture stressed and low temperature stored tubers, suggesting a relationship between inorganic phosphorous and enzymatic activity in the interconversion of starch to sugars in potatoes. Tubers stored at 5.5 C and 15.5 C had higher contents of calcium and magnesium in the basal portion and significantly higher potassium content in the apical portion. Storage temperatures or moisture stress during growth appeared to have no significant influence on calcium, magnesium, potassium or total phosphorus contents of the tubers.     

An evaluation of bromoethane for breaking tuber dormancy inSolanum tuberosum L.

Bromoethane was evaluated for its suitability to induce dormant tubers to sprout. Bromoethane immediately broke dormancy and caused multiple sprouting when applied to whole tubers as a vapor at 0.1 or 0.2 ml/L for 24 h at room temperature. Superficial and localized necrosis developed on the tubers if proper skin set had not occurred either in the field or after a curing period. In fall greenhouse trials, bromoethane treatment of whole tubers of the cultivars Red Pontiac, Russet Burbank, Bintje, Kennebec and Caribe induced them to sprout 40 to 70% earlier than untreated controls. Application of bromoethane significantly increased the mean rate of shoot elongation by 2.8 mm/day although there was no significant difference in its effect among the cultivars.     

Changes in endogenous abscisic acid,soluble sugars and proline levels during tuber dormancy inSolanum tuberosum L.

Abscisic acid (ABA), sucrose, reducing sugars and proline contents were monitored over an eleven-month period in tubers of three potato cultivars that varied in tuber dormancy. ABA and proline levels increased after top pulling the haulms and were not affected by harvest date. Proline did not change in either a storage temperature or cultivar-specific manner. The highest concentrations of ABA were found in tubers stored at 2°C while the lowest concentrations occurred in tubers stored at 20°C. At 10°C, the end of tuber dormancy in the cultivars Kennebec and Nooksack (but not Sebago) coincided with the decline in ABA content. There was no evidence of a threshold concentration of ABA below which sprouting would occur. Tuber samples of 10 different cultivars were removed from 10°C and placed in 20°C storage (in darkness). Initial ABA concentrations (i.e., at the time of removal from 10°C storage) were positively correlated with duration of dormancy and negatively correlated with subsequent rates of sprout elongation at 20°C. Sucrose content was negatively correlated with duration of dormancy. Reducing sugars responded primarily to storage temperature and did not appear to be related to dormancy or sprouting.     

Silencing of G1-1 and A2-1 genes. Effects on general plant phenotype and on tuber dormancy inSolanum tuberosum L.

Summary We have evaluated the effect that the, silencing of two genes specifically expressed in conditions of dormancy (A2-1) and sprouting (G1-1) had on tuber dormancy. For this purpose potato plants (Solanum tuberosum L. cv. Désirée) were transformed with the antisense of the genes G1-1 and A2-1 under the control of constitutive 35S CaMV promoter. A first generation of transgenic plants was propagated from axenic stem cuttings and a second generation by tuber planting. The plants obtined were analyzed for the length of dormancy, plant morphology and agronomic characteristics. Statistical analysis of dormancy in lines obtained from the original transformants for the antisense of G1-1 gene showed a significant increase in length as compared with different types of control plants, with few effects on plant vegetative habit and tuber production. In contrast, results obtained on A2-1 antisense transformed plants did not reveal any significant change on the length of dormancy. Here we report small-scale field trials performed with the aim to select and regenerate commercially exploitable potato plants with a stable transgene-dependent phenotype, affecting the length of dormancy.     

Effect of cold storage on proteinase and chorismate mutase activities inSolanum tuberosum L. Genotypes differing in blackspot susceptibility

Potato tubers (Solanum tuberosum) from genotypes differing in chemical susceptibility to the physiological disorder blackspot bruise (TXA 763-5, Atlantic, Ranger Russet, Russet Burbank, and Lemhi Russet) were stored for eight months at 3 C. Protein extracts from each cultivar were assayed for proteinase and chorismate mutase (CM) activities. During cold storage, three of five genotypes tested increased in their ability to develop melanintype pigments responsible for blackspot. Concurrent with this were increases in either endopeptidase, aminopeptidase, or CM activities or combinations of enzyme activities specific to genotypes. These increases in enzyme activity may be responsible for the increase in levels of the melanin precursor tyrosine, associated with cold storage of potato tubers.     

Changes in malic and citric acid contents during growth and storage ofSolanum tuberosum L.

Changes in malic and citric acid contents of apical and basal portions of tubers were followed during growth and development and in storage. Malic acid content was highest in the early harvested potatoes and decreased as the growing season progressed until a minimum level was reached approximately the first to the middle of September, and then started to increase again as temperatures got cooler in the fall. Malic acid content closely paralleled sucrose content during growth and development. The period at which low malic acid content was observed coincided with optimum time of harvest. Citric acid content did not show a definite pattern of behavior with increase in maturity. Low temperature storage resulted in decreased pH and an increase in malic and citric acid contents of tubers. Both acid contents were higher in the apical portion, although the pH was lower in the basal portion. No significant correlation was found between pH and content of either acid.     

Association between trichome characters and agronomic traits inSolanum tuberosum (L.)XS. berthaultii (hawkes) hybrids

In 1977, a program to develop potato cultivars with insect resistance based on the glandular trichomes found onSolanum berthaultii was initiated at Cornell University. Associations between agronomic and trichome characters were suspected due to difficulties in selecting hybrid plants with both insect resistance and acceptable agronomic characteristics. Plants from backcrosses and intercrosses were studied. A strong association between the presence of B trichome droplets and undesirable agronomic characteristics was established in backcrossed generations. The backcross plants bearing droplets produced significantly lower yields, fewer tubers, later maturing plants, and poorer foliage and tuber appearance than their sibs without the droplets. In intercrosses, no associations were found. Good MEBA scores were seldom found in backcrossed plants without B droplets. The associations observed are speculated to be due to linkage or structural genomic differentiation between the genomes of the species involved in these populations.     

Effect of kinetin and morphactin on growth pattern of Solanum tuberosum L. leaves

Summary Kinetin and morphactin changed the growth pattern of the top leaves ofSolanum tuberosum L. to a form similar to that normally found at the basal nodes of this plant. The size of leaflets was reduced and the rachis developed lateral extensions resulting in the formation of simple leaf-like structures in place of the pinnately compound leaves produced by water-treated plants. The significance of these induced effects is discussed.     

Restriction fragment length polymorphism inSolanum spp for the construction of a genetic map ofSolanum tuberosum L.: a preliminary study

Summary When using two restriction enzymes and DNA probes from different sources, high levels of restriction fragment length polymorphism (RFLP) were observed in Southern blot hybridisation experiments, among 6 di(ha)ploid genotypes ofSolanum tuberosum and some closely related species. In 3 F1 sample populations of 14 individuals each, both heterozygosity and segregation of hybridisation patterns were observed. The non-radioactive biotin-dUTP/SA-AP method for DNA detection on Southern blots proved to be satisfactory and reliable.     

Detection of ribosomal RNA genes by chemiluminescence inSolanum tuberosum L.: a rapid and non-radioactive technique for the characterisation of potato germplasm

Summary The characterisation of potato germplasm is important for the exploitation of this crop, and DNA techniques are playing an increasingly important role as valuable tools in the evaluation of agronomic characters. The rapid, non-radioactive detection of the ribosomal RNA gene fragments which identify restriction fragment length polymorphisms (RFLPs) in two cultivars ofSolanum tuberosum L. is described.     

Influence of plant spacing on potato (Solanum tuberosum L.) morphology,growth and yield under two contrasting environments

Summary In experiments at a temperate (43^oN) and a tropical (14^oN) location, closer plant spacing decreased stem branching, root growth, and mean tuber size but increased tuber yields per ha. Closer spacing increased plant height at the temperate site but decreased it at the tropical site where canopy cover did not reach 100%. The increased branching at the wider spacing did not compensate for fewer plants/m². Total and tuber weight per plant increased with wider spacing and was much greater under temperate than tropical conditions. Tuber yield/stem also increased with lower stem densities. Although these results provide initial data for modelling, the contrast between the two environments and interactions between spacing and cultivar suggest that further experiments need to be done with a range of cultivars at intermediate locations.     

Influence of high temperature and terminal moisture stress on dormancy in wheat (Triticum aestivum L.)

Preharvest sprouting is common in cereals that lack grain dormancy if maturing grain is exposed to rain. Over three successive seasons wheat genotypes with a range of dormancy levels, were exposed to moisture stress and periods of high temperature stress (>30 °C) in controlled field trials. Dormancy assessments were based on a germination index of hand threshed grain throughout grain filling. There were three main results. First, moisture stress combined with consistently high temperature during grain filling was associated with induced dormancy in Cunderdin, (germination index of 0.41) in a normally non-dormant genotype (germination index normally >0.80), but no additional dormancy in DM 2001, a dormant genotype (germination index normally <0.10). In contrast sudden heat shocks (>30 °C max. for >12 days) at 30–50 days post-anthesis reduced dormancy, germination index increase of 0.42 on average across five genotypes. Secondly, whilst dormancy was affected by moisture and heat stress, genotypes maintained their relative rankings across environments and genotype had the most effect on dormancy (70–92% of the variation in germination index) with DM 2001 and DH 22 more dormant than DH 56, DH 45 and Cunderdin. Finally, the effect of environment was different for different genotypes; those with partial dormancy (germination index usually 0.20–0.50, DH 56 and DH 45) were most influenced by the environmental conditions with germination indexes ranging from 0.06 to 0.85 depending on environment. Consequently avoidance of high temperatures, moisture stress, and maturity × stress interactions, are important prerequisites in screening for genotypes with genetic differences in dormancy.     

Changes in ascorbic acid content during growth and short-term storage of potato tubers (Solanum tuberosum L.)

Changes in ascorbic acid levels were followed in apical and basal portions of Russet Burbank and Kennebec tubers during growth and development. Influence of nitrogen fertilization during growth and short-term storage on ascorbic acid level of the tuber was also studied. Changes in ascorbic acid content during growth showed two distinct phases, the first phase being characterized by an increase in ascorbic acid content with growth and development of the tuber, followed by a second phase which showed a decrease in ascorbic acid content with increasing maturity. The shift from the first phase to the second phase occurred earlier for Kennebec than for Russet Burbank. Increased nitrogen fertilization resulted in a delay of this shift from the first to the second phase. The apical portion of the tuber consistently showed higher ascorbic acid content than the basal portion. Kennebec tubers were higher in vitamin C content than tubers of Russet Burbank cultivar. High nitrogen fertilization resulted in less ascorbic acid content during growth. Storage of tubers for four weeks at 5.5°C also resulted in a marked decrease in ascorbic acid content of tubers. Nitrogen fertilizer rate during growth had no influence on the decrease in ascorbic acid content of tubers in storage.     

The effect of growth regulators on meristem tip development and in vitro multiplication ofSolanum tuberosum L. plants

Summary We studied the effect of β-indoleacetic acid (IAA), α-naphthylacetic acid (NAA), kinetin (K), 6-benzylaminopurine (BAP) and gibberellic acid (GA₃) on development of isolated meristems of four potato cultivars (subsp. tuberosum). GA₃ had a stimulating effect on in vitro meristem growth. The differences among the cultivars are presented. K+IAA+GA₃ induced formation of multiple shoots. We describe a method of continual plant multiplication under in vitro conditions and propose the produce for maintenance, multiplication and virus eradication of potato germplasm and cultivars.

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Zusammenfassung Von vier Kartoffelsorten (Blanik, Cira, Nora und Radka) wurden die Meristemspitzen entnommen. Der N?hrboden enthielt Makround Mikroelemente gem?ss Murashige & Skoog (1962), Vitamine entsprechend B-5 (Gamborg et al., 1968), 100mg/l Inositol, 30 g/l Saccharose und 8 g/l Agar. Wir untersuchten die Wirkung von β-Indolessigs?ure (IAA), α-Naphytlessigs?ure (NAA), Kinetin (K), 6-Benzylaminopurin (BAP) und Gibberellins?ure (GA₃). Auf den N?hrb?den mit IAA und NAA wurde mangelhaftes Triebwachstum und unvollst?ndige Blattentwicklung (Abb. 1) festgestellt; die Pflanzen konnten nicht in Erde versetzt werden. K (0.1 μM) induzierte vollst?ndige Pflanzenentwicklung (Tabelle 1), w?hrend BAP (0,1μM) die Triebenentwicklung anregte, worauf die Bewurzelung auf N?hrboden mit 0,1 μM IAA (Abb. 2) erfolgte. Alle GA₃-Konzentrationen hatten einen vorteilhaften Einfluss auf die Entwicklung von Pfl?nzchen aus Meristemspitzen (Abb. 1 und Tabelle 1). Optimales Wachstum und rasche Entwicklung der Pflanzen aller vier Sorten wurde mit dem N?hrboden mit IAA+K+GA₃ erreicht. Mehrfache Triebbildung erfolgte auf den Medien mit erh?hten GA₃-Konzentrationen (5 und 10 μM), speziell bei den Sorten Cira und Blanik (Abb. 3). Die Pflanzen aus der Meristemspitzenkultur konnten unter sterilen Bedingungen in Perlit, das mit der Hoagland's-L?sung und mit einem Zusatz von 0,1 μM IAA ges?ttigt war, fortlaufend vermehrt werden. Die Pflanzen entwickeln sich aus Achselknospen. Ein Steckling aus einem Meristem ist f?hig, 5–8 Pflanzen in zwei Monaten hervorzubringen. Der Artikel beschreibt das System der mehrfachen Triebbildung und die laufende Fortpflanzung in vitro zur Erhaltung und Vermehrung von Kartoffel-Keimplasma (Westcott et al., 1977) und virusfreien Züchtungen (Abb. 4).

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Résumé Des méristèmes apicaux ont été prélevés sur quatre variétés de pommes de terre: Blanik, Cira, Nora et Radka. Le milieu de culture contenait des macro-et microéléments d'après les indications de Murashige & Skoog (1962), des vitamines B-5 (d'après Gamborg et al., 1968), 100 mg/l d'inositol, 30 g/l de sucrose et 8 g/l d'agar. Nous avons étudié l'action de l'acide β-indoleacetic (IAA), de l'acide α-naphtylacétique (NAA), de la kinétine (K), de la 6-benzylaminopurine (BAP) et de l'acide gibberellic (GA₃). Sur les milieux contenant de l'IAA et du NAA, nous avons observé un développement imparfait des pousses, et un développement partiel des feuilles (fig. l); les plantes ne pouvaient pas être transplantées en terre. K (0,1 μM) a induit un développement complet des plantes (tableau 1) tandis que le BAP (0,1 μM) a stimulé le développement des pousses suivi de leur enracinement dans le milieu contenant 0,1 μM d'IAA (fig. 2). Toutes les concentrations de GA₃ ont eu un effet bénéfique sur le développement des plantules issues de méristèmes apicaux (fig. 1 et tableau 1). La croissance optimale et le développement rapide des plantes des 4 variétés ont été observés sur le milieu contenant IAA+K+GA₃. Des pousses multiples se sont formées sur les milieux contenant des concentrations élevées en GA₃ (5 et 10 μM), notamment sur les variétés Cira et Blanik (fig. 3). Les plantes obtenues à partir de culture de méristèmes apicaux ont p? être continuellement propagées en conditions stériles dans de la perlite saturée d'une solution de Hoagland avec addition de 0,1 μM de IAA. Les plantes se développent à partir des bourgeons axillaires; une plantule d'origine méristématique est capable de donner naissance à 5 à 8 plantes en 2 mois. L'article décrit le système de formation des pousses multiples et la propagation continue in vitro pour la maintenance et la multiplication de variétés de pommes de terre indemnes de maladies (Westcott et al., 1977) et de virus (fig. 4).

     

The Combined Effect of CO2 and Ethylene Sprout Inhibitor on the Fry Colour of Stored Potatoes (Solanum tuberosum L.)

Recently, it has been shown that the darkening of potato processing colour attributable to a trace concentration of ethylene gas is more severe when CO₂ is also elevated. In view of the increasing use of ethylene gas for sprout suppression in potato storage facilities, it was considered important to determine whether this effect also occurs at the much higher ethylene concentration used in commercial practice. Sprouting and processing colour of the French fry cultivars Russet Burbank, Shepody and Innovator and the potato crisp cultivar Dakota Pearl were tested during the November to June storage season of two consecutive years. Treatments were 0 or 2 kPa CO₂ and 0 or 10 μL?L^?1 ethylene in a factorial design, plus a chlorpropham check. The 0 CO₂?+?0 ethylene treatment constituted an untreated control. The ethylene exposure was commenced abruptly to maximize its effect on colour. The main effect of ethylene resulted in darker processing colour in all cultivars, whereas darkening attributable to the main effect of CO₂ was observed only in Innovator and Dakota Pearl. The statistical interaction of the CO₂ and ethylene was not significant except in Dakota Pearl Hunter a (redness) scores, although a tendency to darker colour when both gases were present was seen in Russet Burbank and Innovator at all evaluation dates. The results indicate that both gases can affect processing colour when ethylene is used to control sprouting, although considerable variability in the response exists among cultivars. This variability in combination with management of storage conditions such as temperature and CO₂ can be utilized to minimize the impact of these gases on the processing colour of stored potatoes.     

Effects of a mixture of NAA+BA on numbers and growth rates of tubers ofSolanum tuberosum L.

Summary A mixture of NAA at 10 mg 1¹+BA at 1 mg 1¹ caused increases in growth rates of treated tubers of potato. Sub-irrigation and foliage sprays with the same mixture caused tuber initiation and increased the growth rates of some of the tubers. In a field study substantial increases in tuber number and final tuber volume per plant were recorded.

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Zusammenfassung Die Faktoren, die die Knollenbildung und Wachstumsrate bestimmen, sind wenig bekannt. Die ?usserliche Anwendung verschiedener Wachstumsregulatoren, die getrennt auf Einzelknollen von Pflanzen, die in Spezialbeh?ltern wuchsen (Abb. 1), aufgetragen wurden, brachte wenig Ergebnisse, die kommerziell nützlich sein konnten. Eine Mischung aus NAA (10 mg/l) und BA (1 mg/l), in der selben Weise angewendet, schien jedoch das Wachstum der meisten behandelten Knollen zu f?rdern (Abb. 2). In einem 2. Versuch wurde die gleiche Mischung direkt auf ausgesuchte Knollen gegeben und es zeigte sich wieder eine Wachstumssteigerung bei einigen (Abb. 3d-f). Wurden die Wurzeln mit dieser Mischung behandelt, so beschleunigte sich das Wachstum einiger Knollen und in zwei von drei Pflanzen wurde das Wachstum des Knollenansatzes gef?rdert (Abb. 3g-j). Blattspritzungen führten zu einer starken Knollenbildung (Abb. 3k-m). In einem dritten Versuch wurden pro Woche 3 oder 7 Mal die Stengel gespritzt und das führte zu einem signifikanten Anstieg in der Knollenzahl und im Gesamtknollenvolumen/Pflanze (Tab. 2). Abweichungen von den genannten Konzentrationen oder im Verh?ltnis der beiden Wachstumsregulatoren ergaben einen Wirkungsverlust. Der Wirkungsmechanismus ist noch nicht gekl?rt.

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Résumé Les facteurs qui contr?lent la tubérisation et la croissance des tubercules ne sont pas très bien compris. Plusieurs substances régulatrices de la croissance ont été appliquées seules sur des tubercules de plantes cultivées dans des unités spéciales (fig. 1), mais peu d'information commercialement utilisable n'a p? en être tiré. Toutefois, un mélange de NAA (10 mg 1¹) plus BA (1 mg 1¹) appliqué de la même fa?on semble stimuler la croissance de la plupart des tubercules traités (fig. 2). Dans une seconde expérimentation, des applications du même mélange, faites encore directement sur les tubercules sélectionnés, favorisent la croissance de certains d'entre eux. Appliqué sur les racines, le mélange favorise la croissance de quelques tubercules ainsi que le début de tubérisation sur deux ou trois plantes (fig. 3g-j). Pulvérisé sur le feuillage, le mélange provoque une importante tubérisation (fig. 3k-m). Dans une troisième expérimentation en plein champ, la pulvérisation sur le feuillage trois à des accroissements significatifs du nombre de tubercules (tableau 2). Lorsqu'on s'écarte des concentrations établies ou du rapport des deux substances de croissance, ces effets disparaissent. Les mécanismes impliqués ne sont pas compris.

     

Segregation of leptines and other glycoalkaloids inSolanum tuberosum (4x) ×S. chacoense (4x) crosses

A clone, 8380-1, selected fromSolanum chacoense (PI 458310) for its high foliage content of the leptine glycoalkaloids, a factor in resistance to Colorado potato beetle, was doubled in chromosome number from 2n=2x=24 to 2n=4x=48. Three 4x clones were crossed with sixS. tuberosum (4x) clones. Foliage glycoalkaloid contents were measured for 452 F1 hybrids from 15 crosses. The 4xchacoense parental clones were not different in respect to glycoalkaloid contents and were similar to the original 2x clone. All F1 hybrids synthesized foliage leptines ranging from 9 to 369 mg/100 g fresh weight (fw) with a mean content of 113 mg/100 g fw. The proportion of leptines in the total glycoalkaloid content ranged from 1% to 62% with a mean of 25%. The 4xchacoense parent mean leptine content was 1482 mg/ 100 g fw which was 90% of the total glycoalkaloid content. Tubers from 136 hybrids and the threechacoense parental clones were tested for glycoalkaloid contents. The tuber solanine + chaconine contents of the 136 hybrids ranged from 30 to 180 mg/100 g fw with a mean of 79. The mean tuber content of the threechacoense parental clones was 157 mg/100 g fw. Leptines were not found in any of the tubers.     

Effects of growth regulators on the growth and yield of detached and transplanted potato (Solanum tuberosum L.) sprouts

Summary Roots of 15-day-old sprouts detached from tubers of potato (Solanum tuberosum L.) cv., Local Red, were dipped for 1 min in a 0.25% solution of Ethrel (ethephon), Rootone-F (RF), or Transplantone (TP) or in water and then planted in the field. Plants treated with RF and TP were significantly (P<0.05) taller, had heavier haulm, produced more leaves, and yielded more tubers than those of the control treatment with water, ethephon had no effect. RF was significantly (P<0.05) superior to TP for all measured components except leaf number.     

Effects of sulphur dioxide and nitrogen dioxide on fourSolanum tuberosum L. cultivars

The effects of sulphur dioxide (SO₂) and nitrogen dioxide (NO₂), singly and in combination, on the shoot growth of four potato cultivars differing in maturity classification (Solanum tuberosum L. ‘Superior,’ ‘Norchip,’ ‘Kennebec, rs ‘Russet Burbank’) were determined in controlled conditions. Plants were exposed to 0.11 ppm SO₂ and/or 0.11 ppm NO₂ for 24 hours a day for 7 or 14 days. There were no significant differences in leaf growth of ‘Superior’ or ‘Norchip’ plants at 7 or 14 days. Stem dry weight was significantly reduced in ‘Superior’ only after 14 days in the mixture. ‘Kennebec’ and ‘Russet Burbank’ plants had significantly less leaf area, leaf fresh weight, and leaf water content after 14 days of exposure to the mixture, but there was no significant change in dry weight. Specific leaf weight was significantly increased in the mixture treated plants of both cultivars. The two earlier maturing cultivars, ‘Superior’ and ‘Norchip,’ were less sensitive to the SO₂ and NO₂ combination than the two later maturing cultivars, ‘Kennebec’ and ‘Russet Burbank’.