Screening tuber-bearingSolanum (potato) germplasm for efficient accumulation of tuber calcium

Resistance to several pathological and physiological tuber defects has been correlated with calcium level in the peel. In this study, a representative sample of potato species (including cultivated), was grown in the greenhouse. Plants were watered with low and high calcium solutions, and the resulting tubers analyzed. Thus, potato germplasm was screened for ability to accumulate tuber calcium when a control level of calcium is available, and ability to respond when higher levels are supplied. Some wild species significantly exceeded cultivated materials in one or both of these measures. Such species may provide a resource for breeding varieties which are more efficient calcium accumulators.     

Fine screening potato (Solanum) species germplasm for tuber calcium

A representative sample of the US potato germplasm collection was screened in the greenhouse for ability to accumulate tuber calcium. Germplasm with very high and very low calcium accumulation capacity has been identified by progressively screening for extremes among species, among populations (accessions), and finally among genotypes (i.e., fine screening). Among accessions, the bestS. gourlayi andmicrodontum populations averaged more than six-fold greater tuber calcium than the poorestS. kurtzianum population. When screening was performed at the genotype level, the best individuals ofgourlayi andmicrodontum had over eight-fold greater calcium than the poorestkurtzianum individual. When grown in the field, calcium levels ofS. microdontum selections were confirmed to be significantly higher thantuberosum cultivars. When selected genotypes were intercrossed and their progeny were compared to their original populations, mean tuber calcium was significantly changed in the intended direction of selection forS. kurtzianum (made lower) andS. microdontum (made higher). These stocks are expected to be useful tools for study of the genetic and physiological bases of tuber calcium accumulation, refining screening methods, and breeding improved cultivars.     

Extreme resistance to infection by potato virus X in genotypes of wild tuber-bearingSolanum species

Summary Clones derived from thirty-one different accessions (nineteen of Argentine origin) belonging to eightSolanum species were screened for resistance to infection by potato virus X strain cp (PVX cp) by mechanical inoculation of plantlets that had been micropropagated in vitro. Estimates of PVX multiplication obtained by enzyme linked immunosorbent assay and slot blot nucleic acid hybridization allowed the identification of resistant clones derived from five accessions belonging toS. commersonii S. oplocense, S. sparsipilum andS. tuberosum andigena. Resistant genotypes supported PVX concentrations 5 to 15 times smaller than did the susceptible control cultivar Spunta. Graft inoculation test confirmed the presence of extreme resistance similar to that conferred by the ‘immunity’ gene X¹ (also called RX_act).     

Evaluation of tuber-bearingSolanum species for symptomology,as diagnostic hosts,and sources of immunityto potato virus Y necrotic strain (PVYN)

A total of 258 Plant Introductions (PI) belonging to 69Solarium species were evaluated in the greenhouse for their reaction to the tobacco veinal necrosis strain of potato virus Y (PVY N). One hundred and thirty-one (50.7%) of the PI accessions produced mosaic symptoms ranging from mild to severe. Local lesion and veinal necrosis symptoms were observed in 19 PI accessions (7.3%) and a variety of other symptoms were observed in another 11 PI accessions (4.2%). Only 97 PI accessions (37.5%) were symptomless carriers of PVY^N. PI accession 473505 ofS. sparsipilum and PI accession 498021 ofS. brachycarpum developed local lesions and veinal necrosis with PVY^N, but necrotic spots and mosaic with PVY^o. Common mechanically-transmitted potato viruses A, S, M, and X did not interfere with PVY symptom development inS. sparsipilum andS. brachycarpum. Thus, PI 473505 and PI 498021 can be used as indicator plants for specific identification of PVY^N. PI accession 472819 ofS. chacoense developed local lesions with systemic spread in PVY^o, but without systemic spread in PVY^N. Thus, this can be used as a differential host plant for PVY strains. Two PI accessions ofS. stoloniferum, PI 160372 and 161171 were immune to PVY^N.     

Resistance to potato viruses M,S, X and the spindle tuber virus in tuber-bearingSolanum species

On two separate occasions, some available accessions of tuber-bearingSolanum species were tested for resistance to potato viruses. Challenge inoculations were made mechanically with infective sap; and assessment was by sub-inoculation to differential host plants, supplemented by serological tests. In 1956–57, four accessions remained free from potato virus S (PVS), and nine remained free from PVM. In 1968–70, one or more clones of 11 accessions showed resistance to PVS—one of them,S. megistacrolobum, a diploid, apparently being hypersensitive. Resistance to PVX was found in seven accessions, and to PVM in two. There appeared to be resistance to the potato spindley tuber virus in five accessions.     

Evaluation of several wild tuber-bearingSolanum species for scab resistance

One hundred accessions of 18 wild diploid potato species were screened for resistance to scab through three cycles of selection, from which a total of 322 clones were selected as resistant. The putative ancestral species of the cultivated potatoes (Solanum bukasovii, S. canasense andS. multidissectum) produced resistant clones with significantly higher rates. Thus, these are primary gene sources for conferring scab resistance to cultivated potatoes.     

Low temperature (−196°c) storage of true seed from the tuber-bearingSolanum species

Seeds from several of the tuber-bearingSolanum species were exposed to low temperatures using liquid nitrogen. No differences in final percent germination or in the rate of germination were observed between the control and treated seeds for any species tested. High viability after liquid nitrogen treatment suggests that long-term storage ofSolanum seed should be practical and warrants further investigation, especially for the long-term storage of valuable germplasm.     

Support of the endosperm balance number hypothesis utilizing some tuber-bearingSolanum species

The normal development of the endosperm appears to depend on a ratio of two female genomes to one male genome in most angiosperms. However, successful crosses between some tuber-bearingSolarium species do not conform to this ratio. An endosperm balance number (EBN) hypothesis has been proposed to reconcile the two female genomes: one male genome ratio proposal and the anomalous crossing behavior of some species. Crosses performed to test the EBN hypothesis are reported in this paper. The results support the hypothesis and demonstrate its usefulness in predicting the success or failure of a cross and the ploidy of the offspring. A format for conducting and reporting crossing experiments is proposed that facilitates the comparison of results of different experiments.     

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.     

Screening potato (Solanum) species for male fertility under heat stress

Heat stress limits botanical seed production by inhibiting flowering, pollen production and pollen viability. Three accessions (PIs) of each of 23 diverseSolanum species were screened for stability of fertility parameters under heat stress. Seedlings were grown to flowering in temperate conditions (16–25 C), then treated with three weeks of heat stress of up to 45 C for 4–6 hours per day at midday with nighttime lows of 18 C, and compared to temperate-grown controls. A highly significant effect of species, temperature, and their interaction was detected for flowering. This means species were inherently different and heat had a general depressing effect, but the degree of that effect varied among species. While most species had little flowering in the hot house,S. commersonii andmicrodontum flowered significantly more there than in the temperate house. Of species which flowered in hot conditions, some shed no pollen and some had good shed of mostly dead pollen, but onlyS. commersonii, jamesii, kurtzianum andmegistacrolobum had good flowering, pollen shed and viability. This work is expected to contribute to the study of the genetic and physiological bases of heat stress fertility, true potato seed (TPS) breeding, and possibly improvement of tomato production under heat stress.     

Cytology of potato triploids producing 2N pollen

Parallel spindles (ps) were found in a spontaneously occurring triploid plant ofS. chacoense and two triploids ofS. commersonii. This may be the basis for the formation of plump, stainable pollen in these triploids, since ps at Ana II could confer male fertility to odd-ploids by forming pollen with the 2n chromosome number. The participation of 2n gametes in the origin and evolution of polyploidsolanum via sexual polyploidization is strongly implicated.     

Comparative analysis of quantitative trait loci for foliage resistance toPhytophthora infestans in tuber-bearingSolanum species

Previously published data for potato foliage resistance to late blight were examined in a combined analysis to determine how the genetic backgrounds of the host and pathogen affect the occurrence of resistance QTLs. Data from 19 diploid populations and one tetraploid population that originated from at least 12 different tuber-bearingSolanum species were included. Comparative analysis across all populations revealed three highly active genomic regions on the distal parts of chromosomes 3,4, and 5. The region most consistently detected that conferred foliage resistance inSolanum was located on chromosome 5, near marker locus GP21. A previously identified cluster of three race-specific R-genes on chromosome 11 was not associated with polygenic resistance. Statistical examination of active QTLs indicates that congruence among QTLs is significantly affected by both genetic relatedness of mapping populations and race ofPhytophthora infestans used for resistance tests. The proportion of congruent QTLs for late blight resistance decreased from 0.52 detected in relatedSolanum populations tested with the same race ofPhytophthora infestans to 0.20 found in unrelated mapping populations tested with different races ofPhytophthora infestans. Analysis of resistance mapping tests provided statistical evidence for the occurrence of race-specific QTLs in tuber-bearingSolanum. The highly conserved genomic regions identified in the comparative analysis are likely to be good candidates for gene cloning or marker assisted selection in potato breeding programs.     

Total foliar glycoalkaloids and resistance of wild potato species toEmpoasca fabae (Harris)

Levels of total glycoalkaloids (TGA) in foliage of 10 wild, tuberbearingSolanum (Tourn.) L. species differentially resistant to infestation by the potato leafhopper,Empoasca fabae (Harris), were determined. Levels of TGA ranged from a high of 688 mg/100 g fresh wt. in a resistant species,S. polyadenium Greenm. to a low of 13 mg/100 g fresh wt. in a susceptible species,S. bulbocastanum Dun. Foliar concentration of TGA and nymphal infestation by the potato leafhopper were highly correlated (r = ?0.75, p = 0.01). The significant correlation of TGA levels and potato leafhopper resistance suggests that foliar TGA may be a significant factor in the defense of wild potato species against this pest.     

Advantages of storage of potato pollen in liquid nitrogen

Summary Potato pollen stored in liquid nitrogen (−196 C) showed no significant decrease in percentage germination after nine months, whereas pollen stored at −20 C showed a decrease in germination to <1% over the same period.     

Introgression of cold (4 C) chipping from 2x (2 endosperm balance number) potato species into 4x(4EBN) cultivated potato using sexual polyploidization

Cold-chipping cultivars could reduce microbial spoilage and chemical use due to cold storage of chipping potatoes. Sexual polyploidization breeding schemes that introgress cold chipping from diploid potato species may develop improved cultivars at an accelerated rate. The research objectives were (1) to determine the breeding behavior of cold chipping introgressed from 2× 2 Endosperm Balance Number (EBN) potato species into 4×(4EBN) progeny using sexual polyploidization, and (2) to determine if differences exist between 2n gametes and n tetrasomic gametes for transmission of cold chipping. Experimental families of 4×(4EBN) progeny were synthesized using sexual polyploidization (2× × 4× and 4× × 2×) and 4× × 4× matings using cold-chippingS. phureja and cultivated potato -wild species (C -W) hybrid parents. Control families from 4× × 4× matings using as parents current industry leading germplasm were also made. Progeny were field grown and evaluated for chip color (1 light-10 dark, ≤4.0 is industry acceptable) after 3 and 6 months storage at 4 C. A larger percentage of progeny from experimental families had acceptable chip color compared to control families, likely due to introgression of cold-chipping alleles from diploid potato species. Out of experimental families, the best chip color means, highest phenotypic variance, and greatest percentage of acceptable chipping progeny resulted in 2× × 4× and 4× × 2× matings. The ability of 2n gametes to transmit dominant alleles to high frequencies of 4× progeny may contribute to the superiority of 4× -2× crosses. Breeding schemes that use sexual polyploidization in conjunction with early generation selection should rapidly develop 4× cold-chipping germplasm.     

An effective method for culturing pollen tubes of potato

In vitro culture of pollen tubes is a convenient way to distinguish inviable pollen from stylar incompatibilities which prevent pollen tube growthin vivo. This usefulness ofin vitro pollen tube culture depends on how successful the technique is in promoting optimal pollen tube growth. Experiments were conducted to compare the effects of the type of sugar and method of aeration of cultures ofSolanum species (potato) pollen. Lactose was superior to sucrose in promoting pollen germination and pollen tube growth on hanging drop cultures. Germination and growth were greatly enhanced when pollen was cultured in actively aerated lactose medium. Since pollen growth comparable to that occurringin vivo was observed using lactose medium with active aeration, this technique may be useful in various types of research related to reproductive biology.     

Amplification of RAPD markers from single pollen grains of diploid (2N=2X=24) potato

Pollen grains from a diploidSolatium clone 9507-04 were used for RAPD analysis. Single pollen grains were isolated from pollen in liquid suspension, or from dry pollen spread on a glass slide, and collected in individual PCR tubes. Dry pollen isolation was found advantageous due to the convenience of handling and reduction in DNA cross-contamination. Primers UBC291 and UBC504 were selected for the RAPD analysis of both the pollen samples and the pollen donor.Solanum clone 9507-04 produced three RAPD markers UBC291-1.4, UBC291-0.8 and UBC291-0.4, under the selected PCR conditions. Amplification of three additional parental markers UBC504-1.17, UBC504-0.96 and UBC504-0.78 was also tested in a selected pollen sample. RAPD products were discernible from single pollen grains without requiring any DNA extraction procedures. The three UBC291 amplified RAPD markers of the parent segregated in the selected sample of 60 pollen grains. Potential applications of the RAPD analysis of single pollen grains are described.