Effects of photoperiod on kinetin-induced tuberization of isolated potato stolons culturedin vitro

Are short-day (SD) conditions inductive or permissive for potato tuberization? This question was addressed using anin vitro culture system. Kinetin induction ofin vitro tuberization in potato stolons cultured in the dark can be partially inhibited by light treatments. Photoperiod required for a major inhibition varied with cultivars: 8 hours for cv. Red Pontiac and 16 hours for cv. Kennebec. Short photoperiods decreased kinetininduced tuberization for all cultivars. Stolons cultured on kinetin-free medium generally did not tuberize, regardless of light treatment. Effects of kinetin and photoperiod on stolon and tuber development are observed. Relationships among kinetin, photoperiod and tuber-inducing stimulus produced under short days are discussed.     

Polygene mapping as a tool to study the physiology of potato tuberization and dormancy

Efforts to breed for the ability to tuberize under long days or for the length of tuber dormancy should benefit not only from information on the location of genes associated with these traits, but also from an understanding of the hormones the genes control. Understanding the role of the respective genes is a logical step in developing the best breeding strategy, both for conventional breeding and for gene transfer using molecular techniques. Polygene mapping affords a way to achieve such understanding and is also a tool to study the physiology of potato tuberization and potato tuber dormancy. The addition of DNA markers to a plant population facilitates the mapping of polygenes that control quantitatively inherited traits segregating in the population. A quantitative trait locus denotes a region of chromosome that is linked to the marker gene and which has a significant effect on the quantitative trait under study. Our approach has been to find the quantitative trait loci associated with tuberization and tuber dormancy in a segregating diploid population, and then to map the population for quantitative trait loci associated with levels of hormones implicated in the control of these two traits. We are using a population derived from a hybrid between haploidSolanum tuberosum andS. berthaultii that was backcrossed to a different haploidS. tuberosum. We have found ten quantitative trait loci for the ability to tuberize under long days and eight quantitative trait loci for tuber dormancy. In the same population we have found one or more quantitative trait loci for polyamines, abscisic acid, tuberonic acid, tuberonic acid glucoside, zeatin riboside, and gibberellin A₁. Some of the hormone quantitative trait loci have coincided with quantitative trait loci for tuberization or dormancy. Implications of such commonality are discussed, along with the usefulness and limitations of the methods.     

Germplasm enhancement with wild tuber-bearing species: introgression of PVY resistance and high dry matter content fromSolanum berthaultii, S. gourlayi, S. tarijense andS. vernei

Summary The narrow genetic base of the cultivated potato is a severe constraint for potato breeding. Wild tuber-bearing species constitute an important source of genes for resistance to diseases and pests as well as abiotic factors. Seven interspecific hybrid populations were generated from crosses between diploidTuberosum clones and four wild species:Solanum berthaultii, S. gourlayi, S. tarijense andS. vernei, and evaluated in a field experiment in Burgos (Spain) as a way to broaden the genetic base of the cultivated potato. Good tuberization ability and great variability for yield within the different families were detected. Hybridization with the wild species resulted in high levels of PVY resistance inS. berthaultii andS. vernei hybrids and a large increase of dry matter content in all populations except theS. berthaultii hybrids.     

The Effect of Photoperiod on Tuberisation in Cultivated × Wild Potato Species Hybrids

Wild Solanum species offer a valuable source of genetic diversity for potato improvement. Most of these species are found in equatorial South and Central America and they do not tuberise under long-day photoperiods typical of those in the major potato production areas of North America, Europe and Asia. Crosses between two haploids of Solanum tuberosum Chilotanum Group as females and two wild Solanum species clones generated four hybrid families. The parents and progeny were subjected to two greenhouse trials to evaluate tuberisation at 20-, 14-, and 8-h photoperiods. Parents and offspring tuberised most readily at the 8-h photoperiod, and poorly or not at all at the 20-h photoperiod. Segregation for tuberisation in hybrids was apparent at the 14-h photoperiod and depended on the cultivated parent, but not the wild species parent. The data support previous studies and best fit a model in which tuber production under long photoperiods is controlled by two dominant genes in cultivated × wild species hybrids.     

Gelling agent and light effects onin vitro tuberization of potato cultivars

The effects of Gelrite and agar onin vitro tuberization of six potato cultivars, Atlantic, Chaleur, Cherokee, Desiree, Kennebec and Red Pontiac under low light intensity and in the dark were examined. The medium consisted of MS salts, vitamins, 6% sucrose, 2.5 mg/l kinetin and either 2 g/l Gelrite or 6 g/l agar. The cultures were kept either under diffused light with a 16 h photoperiod or in darkness at 22/19 C day/night temperatures for eight weeks. Tuberization was earlier and more uniform (a higher proportion of tubers ≥ 5 mm) on Gelrite than on agar solidified medium and earlier in the darkness than in the light. Light effects varied from cultivar to cultivar. The slowest to tuberize in light were Atlantic and Red Pontiac, the fastest, Chaleur and Cherokee. Chaleur, Cherokee and Kennebec produced significantly more large tubers and higher total tuber mass under low light than in darkness. Atlantic and Red Pontiac reacted in the opposite way. Desiree produced significantly more large tubers in the dark, but the differences in tuber mass between the dark and light conditions were not significant. Light also stimulated shoot and root growth in most of the cultures. No relation was observed between the relative maturities of the tested cultivars and their tuberization responsesin vitro.     

Male fertility and 2n pollen production in haploid-wild species hybrids

Hybrids (2n = 2x = 24) from crosses between Group Tuberosum haploids (2n = 2x = 24) and 24-chromosome, wild species must be fertile and produce 2n gametes if they are to be used in breeding schemes involving sexual polyploidization. Male fertility of the 2x hybrids depends primarily on the species parent. Hybrids involvingS. berthaultii, S. chacoense, S. kurtzianum, S. spegazzinii, andS. tarijense as parents are male fertile. The haploid parents are male sterile, so the species not only contribute genetic diversity and desirable traits, but also genes necessary for male fertility.S. infundibuliforme, S. raphanifolium, andS. sanctae-rosae parents produced male sterile hybrid progeny, while use ofS. boliviense, S. bukasovii, S. canasense, S. microdontum, andS. sparsipilum results in families containing both fertile and sterile plants. Sterility may be due to interactions between Tuberosum cytoplasm and dominant nuclear genes from the species. Genetic male sterility and environmental conditions may also be responsible for low pollen stainability in some hybrids. Many fertile, 2x hybrids produce 2n pollen. Selection for 2n pollen in the species is fast and efficient, but meiotic analysis is required in male sterile haploids. Fortunately, selection in the species parent alone results in a large number of 2n pollen-producing hybrids. These 2x hybrids have been used to generate tetraploids via 4x × 2x crosses.     

Localization of the photoperiodic perception in potatoes

Summary It was found that probably all the leaves of young potato plants played a part in perceiving the photoperiodic stimulus which induces tuberization. The importance of the mature leaves in the photoperiodic perception is emphasized by the large tuber yield which could be obtained by subjecting these leaves to inductive photoperiods while the growing tip and young leaves were subjected to non-inductive photoperiods. Tubers were produced under non-inductive photoperiods when the young leaves and meristematic stem apices were removed. Tuberization in these plants could be inhibited by application of gibberellic acid. The developing leaves, therefore, seem to produce a substance, probably gibberellin, which prevents tuberization under long-day conditions. If a specific tuberforming substance is involved it is present in active concentrations even under non-inductive photoperiods. The results are in agreement with the theory that a balance between endogenous gibberellins and another substance(s) controls tuberization.     

The effect of nitrogen on the coumarin-induced tuberization of potato axillary shoots culturedin vitro

Coumarin-induced tuberization was studied on axillary shoots excised from etiolated potato sprouts each of which was cultured in vitro. Sprouts grown on culture medium with high nitrogen concentrations produced axillary shoots which elongated and would not readily tuberize. Sprouts cultured on medium low in nitrogen produced axillary shoots which tuberized with limited elongation. Carbohydrate concentration of the culture medium for the excised axillary shoot could significantly modify the nitrogen effect of the potato sprout medium. Axillary shoots growing on high nitrogen medium did not tuberize except in cases where the carbohydrate concentration was increased.     

Utilizing the frost resistance of diploid solanum species

Introductions of 13 frost-resistant diploid (2n=2x=24)Solanum species (2 non-tuberous) were inter-crossed to produce F₁ hybrids involving two frost resistance sources. Successful F₁ combinations were: (i) inter-crossed to produce hybrids involving 3 to 4 frost resistance sources, (F₁×F₁); and (ii) crossed with haploids (2n=24) of frost-susceptible tetraploid (2n=4x=48)S. tuberosum subsp.tuberosum, (F₁×tbr). Thirty-five of the 75 crosses between the 13 species produced viable seed. The F₁ and F₁×F₁ progenies segregated to yield plants more resistant to freezing temperatures than the parental species. All attempts to cross these hybrids with 24-chromosometbr were successful. Although tuber production improved following the first cross totbr, the frequency of resistant plants decreased approximately 50%. Thus far, all first back-cross attempts, (F₁×tbr)×tbr, have succeeded. Utilizing hybrids synthesized from several sources of frost resistance as the donor parents, and clonal selections of 24-chromosometbr as the recurring parents, may provide a high order of resistance throughout the backcrossing program and implement selection of frost-resistant, long-day tuberizing parental stocks acceptable to the potato breeder.     

Aneuploid hybrids from 5x − 4x crosses in potato: Chromosome number, fertility, morphology and yield

The objective of this study was to characterize sixS. commersonii - S. tuberosum progenies deriving from 5x ? 4x crosses to provide evidence that they can be used in potato breeding. Hybrids analyzed (coded PTH) had a chromosome number between the 4x and the 5x level. In particular, progeny means for chromosome number ranged from 50±0.5 to 54±0.7, with most (70%) genotypes exhibiting a low aneuploid level of 48 to 53 chromosomes. Despite being aneuploid, the hybrids did not generally show phenotypic aberrations or vigor reduction common to aneuploids of other species. Most genotypes resembledS. tuberosum in growth habit, whereas corolla type, eye depth, and stolon length varied between and within progenies. Aneuploid hybrids produced tubers under long-day conditions. However, tuber yields were not as high as expected, probably due to lack of adaptation of theS. commersonii genome to long-day conditions. Although aneuploidy has often been associated with reduced male and female fertility, many 5x ? 4x hybrids were fertile in crosses withS. tuberosum. The average berry set and number of seeds/berry were 33% and 34.7, respectively, following PTH ×S. tuberosum crosses, and 40% and 51.7, respectively, forS. tuberosum × PTH crosses. The useful variation for fertility and tuber traits found in this material is being used for further breeding efforts.     

Potato minitubers: Technology validation in Mexico

The effects of four previously reportedin vitro tuber-inducing modified culture media on the potato cultivars Atzimba and Juanita were studied under three illumination regimes. Tuberization from stem segments started 7–18 days after transfer for all the cultivar-light-medium combinations, but this early start was not related to tuber yield. Atzimba tuberized more than Juanita (1.0 and 0.8 tubers/plant respectively), and 8 h light was optimal. The medium by Wang Hu (WH) promoted the greatest tuberization for cultivar Atzimba (1,198 mg/plant) and that by Palmer-Smith (PS) for Juanita (290 mg/plant). The modified Stallknecht’s (ST), on the contrary, slightly stimulated tuberization under all illumination conditions. The presence of cytokinins and light favored the tuberization process. Dormancy was not observed in any of the microtubers, regardless of the tuber-inducing treatment.     

Effect of three anti-gibberellin growth retardants on tuberization of induced and non-induced Katahdin potato leaf-bud cuttings

Katahdin potato plants were grown in a growth chamber undernoninducing conditions and treated with three anti-gibberellin growth retardants. Other plants were sprayed with gibberellic acid and placed in aninducing growth chamber adjusted to promote tuber induction. Treatments were repeated the following week. After final treatment, apical, sub-apical, medial and basal leaf-bud cuttings were taken from each plant and placed in a mist chamber. After two weeks, cuttings were examined for tuberization. Two of the retardants were associated with approximately 3 and 2 fold increases in tuberization over the non-induced control. Although induced control cuttings exhibited almost complete tuberization, application of gibberellic acid to plants grown under identical conditions, reduced tuberization 14 fold. Tuberization response of induced control cuttings was unaffected by stem location. In non-induced control and quaternary ammonium compound-treated cuttings, basal cuttings tuberized significantly better than those taken from higher on the stem. This pattern was eliminated for plants treated with the triazole compound. Below-ground portions of donor plants were also examined and showed a significant increase in average rhizome number and total rhizome length associated with inducing conditions. Tuber number of donor plants was also increased for those plants grown under inducing conditions or treated with the triazole compound under noninducing conditions.     

Effect of photoperiod on tuberization in the Livingstone potato (Plectranthus esculentus N.E.Br. Lamiaceae)

The Livingstone potato (Plectranthus esculentus N.E.Br.) produces a cluster of edible tubers at the base of the stem that are used as a potato substitute in many areas of Africa. No information is available on factors affecting tuberization in this species. Photoperiod is known to influence storage organ formation in a wide range of crops, and pilot trials indicated that it also played a role in tuberization in P. esculentus. The critical photoperiod for tuber induction, as well as the number of short-day cycles required to induce tubers, were investigated in glasshouse trials using in vitro plants. Nine photoperiod treatments (30-min intervals from 10 to 14 h of light) were applied in the critical photoperiod experiment. To determine the number of short-day cycles required to induce tuberization, plants were exposed to short days (10 h light) for periods ranging from 0 to 20 days at 2-day intervals. Data were collected from both intact plants and sub-apical two-node cuttings made after the experimental treatments were completed. The critical photoperiod for the specific genotype used in this study was between 12.5 and 13 h, with cuttings giving a slightly exaggerated result over that of intact plants. It was established that exposure to four short-day cycles was sufficient to induce tuberization in this species.     

Identification of late blight, Colorado potato beetle, and blackleg resistance in three Mexican and two South American wild 2x (1EBN)Solanum species

Wild potatoes are important sources of genes for resistance to disease and insect pests. A collection of wild Mexican and South AmericanSolarium species from the US potato Genebank was evaluated under laboratory and/or field conditions for their reaction to late blight (Phytophthora infestans), Colorado potato beetle (CPB,Leptinotarsa decemlineata Say), and blackleg (Erwinia carotovora subsp.atroseptica (van Hall) Dye) in order to identify individual genotypes with multiple resistance genes. Late blight inoculations using aggressive isolates (US-8/A2 and US-11/A1 mating types) of P.infestans revealed a wide range of variation for resistance between and within the accessions of the wild species tested. For late blight, susceptible as well as moderately to highly resistant genotypes were observed in all the species tested. However, at least one accession from the three Mexican and one South American wild diploid species tested showed a relatively uniform high level of resistance toP. infestans. These includedS. bulbocastanum, S. pinnatisectum, S. cardiophyllum, andS. circaeifolium. Two accessions from South American speciesS. commersonii were highly susceptible to late blight. For the Colorado potato beetle test, only one species,S. pinnatisectum appeared uniformly resistant to CPB under field conditions. Results of screening for blackleg resistance showed that there were major differences between genotypes in the wild species. Accessions ofS. circaeifolium PI 498119 andS. bulbocastanum PI 243504 were identified as having significantly higher blackleg resistance than cultivated potato and the other wild species tested. However, genotypes from these two accessions were more susceptible to late blight and CPB. Characterization of theP. infestans isolate P1801C.16 used for late blight evaluation and multi-locus isolate tests using US-8/A2 and US-11/A1 races revealed that the resistance inS. pinnatisectum genotypes tested corresponded to a race-non-specific genetic system, which was different from any existing R genes.Solanum pin-natisectum genotypes with both high levels of late blight and CPB resistance as well as blackleg resistance genotypes identified in the present study represent a diverse gene pool that may be useful for development of new potato cultivars with multiple disease and insect resistance. The potential utilization of these valuable sources for improvement of cultivated potato is discussed.     

Interspecific crossability and cytogenetic analysis of sexual progenies of Mexican wild diploid 1EBN speciesSolanum pinnatisectum andS. cardiophyllum

Mexican wild diploid species,Solanum pinnatisectum (S. pnt) (2n=2x=24, 1EBN: endosperm balance number) is a useful germplasm source of late blight and Colorado potato beetle (CPB) resistance in potato improvement. However, it is very difficult to cross this species with other 1EBNSolanum species. Sexual hybrids among three accessions ofS. pnt and two accessions ofS. cardiophyllum (S. cph) (2n=2x=24, 1EBN) were studied. There were large differences in the cross-compatibility among the genotypes and accessions from these two 2x-1EBN species. Interspecific incompatibility existed in the crosses betweenS. pnt andS. cph, in whichS. cph functioned only as the male parent. The crosses with accessions PI 275236 ofS. pnt produced 35 hybrids following the conventional propagation procedures. Seven hybrids were obtained with theS. pnt accession PI 275233 with the aid of embryo rescue, while hybridization involving accession PI 253214 resulted in no fruit. The cytogenetic analysis indicated that all of the hybrids derived from crosses of accession PI 275233 ofS. pnt withS. cph had 2n = 24 chromosomes and were pollen fertile. Crosses involving accession PI 275236 ofS. pnt produced both 2x (2n=24) and 3x (2n=36) hybrids. Meiotic analysis at metaphase I of pollen mother cells (PMCs) showed a fairly high rate of chromosome pairing that averaged between 10.10 and 11.10 bivalents per cell in most of the 2x hybrids, indicatingS. pnt andS. cph have the similar genome. A high frequency of trivalents and quadrivalents were observed in 3x hybrids derived fromS. pnt 2 ×S. cph 1, indicating homology exists among these chromosomes. These results suggest that specific gene(s) and not either differences between genomes or EBN controlled interspecific crossability and embryo development. The disease and insect tests on the 2x and 3x hybrids revealed that all have high levels of resistance to both late blight and CPB. Thus it could appear that selection of genotypes is a key for successful interspecific hybridization when using Mexican wild diploid species as a source of economic important traits.     

Germplasm transfer from the wild tetraploid speciesSolanum acaule bitt. to the cultivated potato,S. tuberosum L. using 2N eggs

Solanum acaule Bitt. (acl ¹, 2n=4x=48 and 2n=6x=72) is a wild potato species in which resistance to adverse biotic and abiotic agents has been found. The tetraploid subspeciesacaule andaemulans cannot be easily crossed with the common potato,S. tuberosum L. Grouptuberosum (tbr, 2n=4x=48) because the development of the endosperm in the hybrid seed is abnormal. Since 4xacl behaves as a diploid in intra and interploidy crosses, it was postulated that the hybridization barrier withtbr could be overcome if the wild species produced 2n gametes. One hundred and ninety-seven plants of 4xacl (22 introductions) were screened for pollen of heterogeneous size; only two plants produced this type of pollen, with 3% and 5% of pollen in the large class, respectively. On the other hand, 427 plants (47 introductions) were screened for 2n eggs through controlled crosses withtbr. Forty-one plants (13 introductions) produced 219 berries of which 46 contained at least one plump seed. Seed from berries with low seed set gave rise to hexaploid hybrids possibly through the functioning of 2n eggs from 4xacl. A sample of these hybrids was successfully backcrossed as female parents totbr, giving rise to 5x BC₁ progenies. These BC₁ progenies could be easily back-crossed totbr, yielding tetraploid or near tetraploid BC₂ plants.     

An analysis of the hybrid nature ofsolanum xedinense Berth, using molecular, cytological and crossability studies

Summary Solanum xedinense Berth., a pentaploid (2n=5x=60) interspecific hybrid between the cultivated potato and the wild speciesSolanum demissum Lindl., represents evidence of genetic exchange between wild and cultivated species. It has been found in Central Mexico at altitudes between 2000 and 3500 meters, growing in and along the edges of cultivated fields, roadside thickets, forest margins and along irrigation ditches. This study examined the hybrid nature of S.xedinense using molecular, cytological and crossability data. RAPD and AFLP markers were used to supportS. xedinense as a hybrid between the cultivated potato andS. demissum. Meiotic analysis and crossability studies were used to examine the potential ofS. xedinense to form offspring. Several accessions ofS. tuberosum L. subsp.tuberosum and subsp.andigena Hawkes,S. demissum and an accession ofS. xedinense were used in the study. Artificial hybrids betweenS. tuberosum subsp.tuberosum andS. demissum were included for comparison. Molecular analysis positioned the natural and artificial hybrids as an intermediate group between the parental populations, supporting their relationship with the parental populations. Meiosis was abnormal in both natural and artificial hybrids, and reduced crossability was noted forS. xedinense. Although gene flow appears to happen between wild and cultivated potato, the potential for the natural hybrids to become established may depend mostly on their ability to clonally propagate themselves.     

Evaluation of tuber-bearingSolanum species for nitrogen use efficiency and biomass partitioning

Modern potato cultivars (Solanum tuberosum L.) require high rates of fertilizer nitrogen (N). This practice is costly and can pose a serious threat to surface and groundwater. Previous evaluation of wild potato germplasm demonstrated the existence of species capable of producing high total biomass under low N conditions, with the ability to make maximum use of added N. Therefore, a two-year field experiment was conducted in 1994 and 1995 to investigate the response of selected wild potato accessions and their hybrids with the haploid USW551 (USW) to low and high N environments. The haploid USW and cultivars Russet Burbank, Red Norland, and Russet Norkotah were also included in the study. Uniform propagules and seedlings from the variousSolanum species were transplanted to a Hubbard loamy sand (Udic Haploboroll) at Becker, Minn. and were subjected to two N treatments: 0 and 225 kg N ha^-1. At harvest, total dry biomass of wild and hybrid potato germplasm was equal to or higher than that of the cultivars. However, cultivar biomass partitioning was 1% to roots, 15% to shoots, 0% to fruits, and 84% to tubers, whereas wild potato species partitioned 18% to roots plus nontuberized stolons, 52% to shoots, 23% to fruits, and only 7% to tubers. Hybrids were intermediate, allocating 9% of their biomass to roots plus nontuberized stolons, 39% to shoots, 14% to fruits, and 38% to tubers. Nitrogen use efficiencies for many of the species and crosses were comparable to that for Russet Burbank and greater than those for Red Norland and Russet Norkotah. Of the wild species tested,S. chacoense accessions had the highest biomass accumulation and N uptake efficiencies and may be the best source of germplasm for improving NUE in a potato breeding program.     

Genetic Resources (Including Wild and Cultivated Solanum Species) and Progress in their Utilisation in Potato Breeding

The genetic resources available for the improvement of the cultivated potato (Solanum tuberosum) are reviewed along with progress in their utilisation. The conclusions are as follows. The wild and cultivated species of potato have been utilised in potato breeding to good effect, but only a very small sample of the available biodiversity has been exploited. New knowledge and technology will open possibilities for much greater use of these genetic resources in breeding. The strategy for utilising the cultivars native to Latin America will either be the introgression of desirable genes or the direct use of parents from improved populations, depending on how far modern S. tuberosum cultivars have genetically diverged from them and the extent to which S. tuberosum cultivars have been improved in the process. Molecular marker-assisted selection will be used for faster introgression of desirable genes from wild species, and the possibility exists of moving genes directly from wild species to cultivated potato with transgenic methods. New cultivars will continue to come from crosses between pairs of parents with complementary features but adapted to local growing conditions. However, increasingly these parents will possess desirable genes which have been introgressed from wild species and may also be from complementary groups of cultivated germplasm to exploit hybrid vigour. Successful cultivars may be genetically modified, if consumers see benefits in the use of the technology, to introduce genes not present in cultivated potatoes and their wild relatives to achieve novel biochemistry and further desirable improvements.