Factors affecting electrical impedance of internodal stem sections

A sample holder was designed and built to facilitate measuring the magnitude and phase angle of the electrical impedance of internodal stem sections from Cornus stolonifera Michx. A nonpolarizing, electrically conducting manganese dioxidecarbon paste used between the stem sample and the electrodes of the sample holder allowed measurement of impedance at frequencies from 50 hertz to 500 kilohertz without electrode polarization or electrical interference. The impedance magnitude was linearly dependent on the sample length, but this dependence was minimized by computing a normalized impedance magnitude. The normalized impedance magnitude (Z_nf) was calculated using the impedance magnitude (Z) at any specified frequency (f) and the impedance magnitude at 500 kilohertz (Z_{500 khz}) in the following formula: Z_nf = (Z - Z_{500 khz})/Z_{500 khz}. The normalized impedance magnitude was sensitive to injury produced by boiling and peeling the sample. Electrical impedance measurements on the bark and wood separately demonstrated that they have different electrical properties.     

Effekt und Risiko des Knöllchentests auf Braunfäuleresistenz bei Kartoffelsämlingen im System der Auslese in der Phytophthora-Resistenzzüchtung

The effects and risks involved with the small tuber assay in plant breeding programmes with potato seedlings for resistance to Phytophthora infestans In an assay which involved 838 potato seedlings, the relative resistance to leaf-Phytophthora (KR_G) was tested at the 4–6 leaf growth stage, and the relative tuber resistance to infection was determined on small pot-grown tubers (B_K) and, in the following 2 years, in slices from field-grown potatoes (B_s). The small tuber assay is described and its value as a measure of disease resistance, compared with the potato slice assay, discussed. Among 357 leaf-Phytophthora susceptible clones (KR_G≤ 5) were 10 clones with a good to very good tuber resistance. 18 of 204 leaf-Phytophthora highly resistant idiotypes were extremely susceptible to tuber rot. After discarding those seedlings which showed KR_G-values ≤ 5 and B_X-values ≤ 5, 208 of the remaining 241 (= 86 %) tuber rot resistance carrying seedlings were selected. 14 % of the clones which showed adequate resistance in the potato slice assay were falsely discarded according to the small tuber assay. 80 clones remained as ballast in the field trials (B_K≤ 5, B_s≤ 6.4), 160 (33 % of the idiotypes tested) had been correctly eliminated. The work-intensive small tuber assay presents a possibility by which breeding material can already be reduced in the seedling stage. In this respect there must be a pronounced genetic differentiation present in the potato population and the selection procedure carried out must not be too harsh.     

Effects of Gamma-irradiation on Lipid Metabolic Changes of Potato Tubers in Response to Mechanical Injury

Linolenic acid contents of glycolipids increased in irradiated potatoes during storage, accompanied by a decrease of linoleic acid. The puncturing of a potato tuber with a needle of a microsyringe caused the similar changes; the elevation of linolenic acid level and decline of linoleic acid were observed within 24 h after puncturing. Irradiation before the puncturing reduced the degree of the increase of linolenic acid in response to the mechanical injury. The rate of [¹³C]acetate incorporation into lipid fractions of irradiated tubers was smaller than that of unirradiated tubers, and polyunsaturated fatty acids (linoleic and linolenic acids) of lipid fractions were weakly labeled in irradiated tubers as compared with unirradiated ones. The results in this study indicate that irradiation retards lipid metabolism in response to mechanical injury.     

The novel, major locus Rpi-phu1 for late blight resistance maps to potato chromosome IX and is not correlated with long vegetation period

Despite the long history of breeding potatoes resistant to Phytophthora infestans, this oomycete is still economically the most important pathogen of potato worldwide. The correlation of high levels of resistance to late blight with a long vegetation period is one of the bottlenecks for progress in breeding resistant cultivars of various maturity types. Solanum phureja was identified as a source of effective late blight resistance, which was transferred to the cultivated gene pool by interspecific crosses with dihaploids of Solanum tuberosum. A novel major resistance locus, Rpi-phu1, derived most likely from S. phureja and conferring broad-spectrum resistance to late blight, was mapped to potato chromosome IX, 6.4 cM proximal to the marker GP94. Rpi-phu1 was highly effective in detached leaflet, tuber slice and whole tuber tests during 5 years of quantitative phenotypic assessment. The resistance did not show significant correlation with vegetation period length. Our findings provide a well-characterized new source of resistance for breeding early and resistant-to-P. infestans potatoes.Electronic Supplementary Material Supplementary material is available to authorised users in the online version of this article at .     

Tagging QTLs for late blight resistance and plant maturity from diploid wild relatives in a cultivated potato (Solanum tuberosum) background

Phytophthora infestans causes an economically important disease of potato called late blight. The epidemic is controlled chemically but resistant potatoes can become an environment-friendly and financially justified alternative solution. The use of diploid Solanum tuberosum derived from European tetraploid cultivars enabled the introgression of novel genes encoding foliage resistance and tuber resistance from other species into the modern cultivated potato gene pool. This study evaluated the resistance of the obtained hybrids, its quality, expression in leaflets and tubers and its relation to the length of vegetation period. We also identified genetic loci involved in late blight resistance and the length of vegetation period. A family of 156 individuals segregating for resistance to late blight was assessed by three laboratory methods: detached leaflet, tuber slice and whole tuber test, repeatedly over 5 years. Length of vegetation period was estimated by a field test over 2 years. The phenotypic distributions of all traits were close to normal. Using sequence-specific PCR markers of known chromosomal position on the potato genetic map, six quantitative trait loci (QTLs) for resistance and length of vegetation period were identified. The most significant and robust QTL were located on chromosomes III (explaining 17.3% of variance observed in whole tuber tests), IV (15.5% of variance observed in slice tests), X (15.6% of variance observed in leaflet tests) and V (19.9% of variance observed in length of vegetation period). Genetic characterization of these novel resistance sources can be valuable for potato breeders and the knowledge that the most prominent QTLs for resistance and vegetation period length do not overlap in this material is promising with respect to breeding early potatoes resistant to P. infestans. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Variations in resistance against Phthorimaea operculella in wild potato tubers

Tuber resistance can contribute to current management strategies against the potato tuber moth, Phthorimaea operculella Zeller (Lepidoptera: Gelechiidae), in field and stored potatoes. Wild potatoes represent a potential source of novel resistance traits against the moth. We assessed resistance in three wild potato species, Solanum multiinterruptum Bitt., Solanum sparsipilum (Bitt.) Juz. & Buk., and Solanum wittmackii Bitt. against neonate and developing tuber moth larvae. All three species had high levels of resistance but accessions of S. sparsipilum and S. wittmackii were significantly more resistant. Resistance in S. multiinterruptum was generally concentrated in the tuber periderm, whereas in S. sparsipilum and S. wittmackii resistance was mainly cortex‐based. Unidentified cortex‐resistance factors in all three species reduced survival and increased larval and pupal development times, but had no apparent effects on the pupal weights of survivors. A high proportion of larvae abandoned or died within tubers of S. wittmackii, which has particularly high levels of unidentified cortex‐based defenses. Resistance decreased in S. multiinterruptum and S. sparsipilum as tubers sprouted but was more stable in S. wittmackii. Periderm‐based resistance was more stable than cortex‐based resistance in S. multiinterruptum during sprouting. In contrast, cortex‐based resistance was stable in tubers of S. wittmackii as these sprouted, and resistance may have increased on some older sprouting tubers. Solanum multiinterruptum and S. sparsipilum are proposed as potential sources of resistance against the potato tuber moth.     

Relationship between the Electrical and Rheological Properties of Potato Tuber Tissue after Various Forms of Processing

The impedance at frequencies of 1-1000 kHz and dynamic bending storage modulus measured by the vibrating reed method were compared for potato tuber tissue, which had been processed by various methods. Raw potato tuber tissue strips were either heated for 30 min up to 100°C or frozen-thawed. Some samples were osmotically dehydrated in a mannitol solution up to a concentration of 0.7 mol/l. The electrical reactance correlated well with the storage modulus of heated or frozen-thawed potato tissues, but not with the storage modulus of the mannitol-treated tissue. The storage modulus appeared to be strongly dependent on the turgor pressure of the cells which was drastically decreased by the heating, freezing-thawing, and osmotic treatments. The electrical properties reflect the cell integrity, and a large difference was observed between the change in impedance after heating or freezing-thawing, and that after the osmotic treatment. A significant change in the electric properties was also observed for a starch suspension at the gelatinization temperature. However, the contribution due to gelatinization did not appear to play an important role in the change of electrical properties of potato tissue by heating.     

Transfer of tuber soft rot and early blight resistances from<Emphasis Type="Italic"> Solanum brevidens</Emphasis> into cultivated potato

Tuber soft rot and early blight are serious potato diseases. Development of potato varieties resistant to these diseases has been hindered by the scarcity of resistant germplasm. A diploid wild species, Solanum brevidens, shows significant resistance to both diseases. Numerous potato breeding lines have been developed from a potato-S. brevidens somatic hybrid, A206. A BC₃ clone, C75-5+297, derived from this somatic hybrid as well as its BC₁ and BC₂ parental lines showed resistance to both tuber soft rot and early blight. Clone C75-5+297 has consistently out-yielded common varieties under disease stress. Using both molecular and cytogenetic approaches we demonstrated that a single copy of chromosome 8 from S. brevidens replaced a potato chromosome 8 in C75-5+297. Thus, C75-5+297 represents a potato-S. brevidens chromosome substitution line. Our results suggest that the presence of a single chromosome from S. brevidens may significantly impact the resistance to multiple potato diseases. The high yield potential of C75-5+297 makes it an excellent parent for developing potato varieties with resistances to both tuber soft rot and early blight.Communicated by J.S. Heslop-Harrison     

Relationship between the electrical and rheological properties of potato tuber tissue after various forms of processing

The impedance at frequencies of 1-1000 kHz and dynamic bending storage modulus measured by the vibrating reed method were compared for potato tuber tissue, which had been processed by various methods. Raw potato tuber tissue strips were either heated for 30 min up to 100 degrees C or frozen-thawed. Some samples were osmotically dehydrated in a mannitol solution up to a concentration of 0.7 mol/l. The electrical reactance correlated well with the storage modulus of heated or frozen-thawed potato tissues, but not with the storage modulus of the mannitol-treated tissue. The storage modulus appeared to be strongly dependent on the turgor pressure of the cells which was drastically decreased by the heating, freezing-thawing, and osmotic treatments. The electrical properties reflect the cell integrity, and a large difference was observed between the change in impedance after heating or freezing-thawing, and that after the osmotic treatment. A significant change in the electric properties was also observed for a starch suspension at the gelatinization temperature. However, the contribution due to gelatinization did not appear to play an important role in the change of electrical properties of potato tissue by heating.     

Periderm- and cortex-based resistance to tuber-feeding Phthorimaea operculella in two wild potato species

The trichome‐bearing wild potatoes Solanum berthaultii (Hawkes) and Solanum tarijense (Hawkes) (Solanaceae) have noted resistance to leaf‐feeding insect herbivores; however, little is known about their resistance to tuber‐feeding herbivores. This study evaluates resistance in tubers of these two species to attack by the potato tuber moth, Phthorimaea operculella (Zeller) (Lepidoptera: Gelechiidae). Tubers from a range of accessions were presented to recently emerged neonate tuber moth larvae. Resistance to neonates varied between accessions and was generally higher in accessions of S. tarijense. The contribution to observed resistance of periderm vs. cortex factors was assessed by perforating tuber periderm in paired‐tuber experiments. Across species and accessions, an average of 62% of resistance was attributed to periderm‐related factors. All larvae entered tubers through the eyes. Unidentified cortex‐related factors affected larval development time and pupal weight. Sprouting compromised resistance by reducing the protective value of the periderm. The degree of sprouting also decreased larval development times and increased pupal weights in one of two accessions examined. These results demonstrate the potential of S. berthaultii and S. tarijense as sources of tuber‐resistance and identify accessions of both species with notably high periderm‐based protection. Levels of periderm resistance were not correlated to levels of cortex resistance and represent an independent resistance source compatible with the food value of crop potato.     

Resistance and tolerance of potato varieties to potato rot nematode (Ditylenchus destructor) and stem nematode (Ditylenchus dipsaci)

Ditylenchus destructor and Ditylenchus dipsaci are economically important plant‐parasitic nematodes, affecting potato production mostly in temperate climates. Management through crop rotation is not feasible because of their wide host range. These nematodes are listed as quarantine pests in many countries. Limited information exists on the resistance and tolerance of currently cultivated potatoes to D. destructor and D. dipsaci. Two greenhouse experiments were conducted to screen 25 potato varieties for resistance to and tolerance for D. destructor and D. dipsaci infections. Reproduction factor (RF) and relative susceptibility (RS) were used to evaluate resistance, while potato tuber damage and tuber weight reduction was used to evaluate tolerance. Based on the RF, 16 varieties were evaluated as susceptible (S) while 5 varieties were evaluated as resistant (R) to D. destructor; varieties ‘Innovator’, ‘Aveka’ and ‘Spunta’ were resistant to D. dipsaci based on RF. ‘Désirée’ was observed to be highly susceptible to D. destructor and D. dipsaci in both experiments and was used as the standard susceptible control variety for the calculation of RS. A scale of 1–9 was used to classify RS of the potato varieties to D. destructor and D. dipsaci, where 9 indicated the highest level of resistance. All classes of resistance to D. destructor and D. dipsaci were observed in the potato varieties tested in the experiments. Six varieties had significantly lower RS to D. dipsaci than the standard susceptible control variety. Tolerant to highly sensitive potato varieties to both nematodes were also observed. RS and external potato tuber damage were identified as suitable methods for resistance and tolerance determination, respectively. This study provides essential information on the status of resistance and tolerance in potato varieties against D. destructor and D. dipsaci but needs to be confirmed under field conditions.     

Efficacy of fungicide combinations,phosphoric acid and plant extract from stinging nettle on potato late blight management and tuber yield

Late blight caused by Phytophthora infestans is a major constraint to potato production. Inadequate control of the disease has often resulted in potato yield losses. We assessed the efficacy of fungicides, phosphoric acid and stinging nettle extract combinations for late blight control at two locations in Kenya. Disease severity, relative area under disease progress curves (RAUDPC), pathogen lesions and tuber yield were quantified during the 2008 and 2009 cropping cycles. The application of metalaxyl alternated with phosphate resulted in the greatest suppressive effects on late blight. The average late blight severity ranged from 3.5 to 34% in 2008 and 4.7 to 50% in 2009 at Tigoni location. RAUDPC for the same location ranged from 5 to 40% and 5 to 50% in 2008 and 2009, respectively. Similar levels of late blight severity were recorded at Marimba location in both years. Lesion growth and pathogen lesion numbers on potato plants differed significantly (p < 0.05) among treatments. Fungicides, phosphoric acid and stinging nettle extract varied in late blight control. Potato tuber yield varied among treatments. Phosphoric acid treatment had significantly (p < 0.05) greater tuber yield compared to metalaxyl at both locations. Field plots treated with plant extracts from stinging nettle resulted in the lowest tuber yield compared to other treatments with the exception of the untreated control. Fungicides, phosphoric acid, stinging nettle extract and their combinations can be readily effective in the suppression of late blight severity and pathogen lesions with moderate increases in tuber yield.     

Bisanthraquinones,Inhibitors of Plant Transformation

A potato tuber disk assay of culture broths which had been confirmed to show neither antibacterial nor phytotoxic activity resulted in the discovery of a novel type of crown gall formation inhibitors, julimycin B-II and julichrome Q_1.3 These two bisanthraquinones, products of Streptomyces sp. TM-71, inhibited crown gall formation on potato tuber disks at a minimum inhibitory dose of 1.6 μg/disk without affecting the growth of Agrobacterium tumefaciens or the germination of alfalfa seeds.     

Expression of an Arabidopsis CAX2 variant in potato tubers increases calcium levels with no accumulation of manganese

Previously, we made a chimeric Arabidopsis thaliana vacuolar transporter CAX2B [a variant of N-terminus truncated form of CAX2 (sCAX2) containing the “B” domain from CAX1] that has enhanced calcium (Ca²⁺) substrate specificity and lost the manganese (Mn²⁺) transport capability of sCAX2. Here, we demonstrate that potato (Solanum tuberosum L.) tubers expressing the CAX2B contain 50–65% more calcium (Ca²⁺) than wild-type tubers. Moreover, expression of CAX2B in potatoes did not show any significant increase of the four metals tested, particularly manganese (Mn²⁺). The CAX2B-expressing potatoes have normally undergone the tuber/plant/tuber cycle for three generations; the trait appeared stable through the successive generations and showed no deleterious alternations on plant growth and development. These results demonstrate the enhanced substrate specificity of CAX2B in potato. Therefore, CAX2B can be a valuable tool for Ca²⁺ nutrient enrichment of potatoes with reduced accumulation of undesirable metals.     

The effect of formalin on potato cyst-nematode, Heterodera rostochiensis

Applying 5 ml of a 38% aqueous solution of formaldehyde to 1500 ml of sandy loam prevented potato cyst-nematode, Heterodera rostochiensis, from increasing on the roots of Arran Banner potatoes planted in the soil and increased the weight of tubers produced. A comparable dosage applied to plots of sandy loam and sandy clay in winter did not prevent the nematode from multiplying on Pentland Crown potatoes but increased tuber yields. Smaller amounts of formalin had less effect on tuber yields and drenching dilute formalin solutions onto the soil was no more effective than incorporating undiluted formalin into the soil by rotavation 15 cm deep.     

Alterations in Solanum tuberosum polyphenol oxidase activity induced by gamma irradiation

On gamma irradiation of potato tubers at sprout-inhibiting dose (10 krad) the cresolase activity showed a 45% increase while catecholase was reduced by 25%. This reduced the ratio of catecholase to cresolase from 11–12 in unirradiated to 5–6 in irradiated potatoes. Chlorogenic acid oxidation was enhanced by about 25% on irradiation. The increase in the oxidation of p-cresol corresponded with the production of diphenolic compounds. The process of activation of cresolase was slow, reversible and oxygen dependent. A comparative study of the isoenzyme pattern suggested that this activation was due to conformational change, rather than synthesis of new protein.     

Life Histories and Fitness of Two Tuber Moth Species Feeding on Native Andean Potatoes

In the inter-Andean valleys of central Perú, two species of tuber moth, Phthorimaea operculella (Zeller) and Symmetrischema tangolias (Gyen), often occur simultaneously in stored potatoes. Traditional farming communities in the region produce a variety of native potatoes for local consumption. These include Solanum tuberosum subsp. andigena, the presumed predecessor of commercial potatoes, S. tuberosum subsp. tuberosum. In this study, we examined resistance against P. operculella in ten native Peruvian potato varieties (Casa blanca, Chispiadita, Madre de vaca, Mamaco negro, Misha, Chorisa, Mamaco rosado, Occa papa, Vacapa jayllo, and Yana tornasol). We also compared resistance in the first five of these varieties against S. tangolias. Varieties with pigmented periderms showed moderate resistance (30?C40% against P. operculella in Mamaco negro, Mamaco rosado, and Yana tornasol and 55% against S. tangolias in Mamaco negro). All the other varieties were susceptible to both moth species. Small tubers tended to be the most resistant to the attack by both moths; however, this was not related to the availability of food for developing larvae, since pupal weight and development time were unaffected by the size of tubers. Similar responses by the two moths to native potatoes indicate that tuber resistance could be used to control the complex of tuber moths that damage potatoes in the Andes. We suggest that native potatoes, which are often easily introgressed with commercial potatoes, are a potential source of resistance against tuber moths.     

Overcompensating plants: their expression of resistance traits and effects on herbivore preference and performance

Overcompensation is a plant tolerance response in which plants have higher fitness after herbivory than without damage. Although it has been demonstrated that plants are able to simultaneously express resistance and tolerance traits, it remains unclear whether overcompensating plants are also inducing resistance‐mediating secondary metabolite production and how herbivores perform on plants that overcompensate. Our previous work has shown that a potato variety [Solanum tuberosum L. cv. Pastusa Suprema (Solanaceae)] from Colombia can express overcompensatory responses to damage by larvae of the Guatemalan potato moth, Tecia solanivora Povolny (Lepidoptera: Gelechiidae). Here we investigated (1) whether potatoes that express overcompensatory responses also induce resistance traits and (2) how the previous damage affects Guatemalan potato moth preference and performance. Our results show that larval feeding not only systemically induces higher tuber biomass but also an increased production of resistance‐related compounds, such as phenolics and proteinase inhibitors. Pupal mass increased with increasing tuber size, whereas changes in tuber secondary metabolism did not correlate with any metric of larval performance. Oviposition preference did not change between induced and undamaged plants. Our data show that potato plants expressing overcompensatory responses also induce secondary compounds known to increase resistance against herbivores. However, the induced response was relatively small, reducing the opportunities for a negative effect on the herbivore. Hypotheses for why larvae perform better in larger tubers and are not affected by the secondary metabolism are discussed. From an ecological and agricultural point of view, our results suggest that the expression of overcompensatory traits could have positive effects on herbivore performance.     

Effects of rotation length, fungicide treatment of seed tubers and nematicide on diseases and the quality of potato tubers

In 1982 – 88, potatoes were grown in 2-, 4- and 6-course rotations with spring barley on a field infested with Globodera rostochiensis. Severity of stem canker and black scurf increased with increasing frequency of previous potato crops, and seed tuber treatment with tolclofos-methyl became less effective in controlling diseases. This suggested that previous crops had increased the amounts of soil-borne inoculum of Rhizoctonia solani. Oxamyl soil treatment increased stem canker in one year and decreased black scurf in four years. Seed tuber treatment with imazalil or prochloraz decreased stem base infection by Polyscytalum pustulans and skin spot and silver scurf on tubers. Black dot was prevalent on tubers in all years and was not affected by seed tuber treatment or previous cropping. Oxamyl increased black dot and common scab in five years and decreased % tuber dry matter in six years. Cysts of G. rostochiensis were found attached to Désirée but not to Maris Piper tubers in August. At harvest tubers of both cultivars were affected by superficial pitting and its severity was related to soil populations of G. rostochiensis at planting. This damage was controlled by oxamyl. It is suggested that the pitting developed from holes made in the tuber skin at larval invasion. In 1989, Désirée seed tubers and healthy mini tubers were planted in all plots and severity of stem canker and black scurf increased with increasing proximity of previous potato crops and with the number of previous crops. Black dot on stems and tubers was not affected by previous cropping but was much less severe in a plot that had not grown potatoes during the seven years of the experiment. The severity of common scab generally decreased as the number of preceding potato crops increased.