It has become a tradition that the section Potatoes of the European Association for Research on Plant Breeding, EUCARPIA, and the section Breeding and Varietal Assessment of the European Association for Potato Research, EAPR, hold their section meetings simultaneously, thereby benefiting mutually from the experience of experts from both Associations.

The present proceedings ‘The Production of New Potato Varieties Technological Advances’, constitute however the very first publication in extenso of papers presented at joint meetings of the sections.

The book contains, among other items, papers presented at the meeting held at Cambridge, England, between December 16th – 20th, 1985, under the main theme ‘The development and Identification of Superior Potato Genotypes – limitations and Prospects for the Future’. It is a coherent publication, offering non-participants as well as participants in the section meetings a presentation of such important aspects as the current strategies employed in the breeding of new potato varieties, the achievements to date, and future prospects for varietal improvements.

The European Association for Research on Plant Breeding and the European Association for Potato Research welcome this initiative on the part of the sections, and hope that readers of the book will give it the appreciation it deserves.

INTRODUCTION

A method has been developed for growing commercial-type potatoes from true potato seed (TPS) sown directly into fields. The productivity, quality and uniformity of such crops are usually inferior to those of tubergrown crops; however, plants superior in many important characteristics can be selected for subsequent clonal evaluation. Direct-seeded TPS populations are exposed to diseases, pests and cultural or environmental stresses and selected for resistance. The full-grown, mature tubers produced can be selected for type, productivity, quality, handling, storability and processing characteristics. Tubers from selected seedlings provide normal-size seedpieces that are planted as single tubers or in replicated plots for evaluation in the first clonal generation. This first clonal generation is again exposed to a wide range of selection pressures. This contrasts with single hills normally grown from pot-grown tubers where over 99% of the genetic potential of segregating populations is discarded based on cosmetic attributes of tubers from single hills grown from tiny, variable-sized seed tubers under minimal stresses and selection pressures. This new method provides more information about each clone and increases the chances of finding and saving valuable genotypes.

PROCEDURE FOR DIRECT SEEDING TPS

During the past nine years crops have been grown annually from TPS sown directly into fields in eastern Washington and Oregon (Martin 1978a, 1983a,b). They have been grown on soils ranging from light sandy, through various loams to heavy clay, using either sprinkle or furrow irrigation.

SELECTION OF HEAT TOLERANT CLONES

The adverse effect of high temperatures on tuber yield and quality is a major obstacle to potato production in hot regions (Ewing 1981). In the Mediterranean region, as well as in subtropical parts of Asia and Africa, potatoes are exposed to high day and night temperatures and a comparatively dry atmosphere. Almost all Solanum tuberosum cultivars from Europe or North America respond to such conditions with a significant loss in tuber yield and quality (Levy 1985). Local breeding of “heat tolerant” cultivars has been necessary to improve potato production in hot regions.

The selection of heat tolerant clones is carried out in the field or in environmentally controlled glasshouses. Should controlled conditions not be available, selections can be made in aphid-proof screenhouses. Avoidance of virus infections is a major obstacle because of the high aphid populations for most of the year.

The inhibition of tuberization by high temperatures is illustrated in Figure 1. Seedlings from open pollinated seeds of Desiree were grown in the field in Israel under 40-mesh screens, seeds were sown on the first day of each month and seedlings examined for tubers 90 days later. Each point on the graphs represents results obtained from 111 to 264 seedlings (Levy 1984).

FIELD EVALUATION OF HEAT AND DROUGHT TOLERANCE

Although potatoes grown in hot, semi-arid conditions are irrigated throughout most of the growing season, they are nevertheless commonly exposed to water stress resulting from high ambient temperatures and low humidity during the spring and summer.

INTRODUCTION

Diploid potato clones which produce unreduced gametes merit particular attention because of their value in genetic research and in the breeding of new cultivars. There are various mechanisms that lead to the production of unreduced (2n) gametes; and these have diverse genetic consequences. The value of diploid clones and their utilization depends on the production mechanism (Mendiburu et al. 1970; Mendiburu & Peloquin 1979). Clones forming unreduced gametes via First Division Restitution (FDR) can be used in the following ways:-

1) To transfer genes from diploid species to tetraploid cultivars since they cross easily with both diploids and tetraploids (Hanneman & Peloquin 1967, 1968; Jakubiec & Suska 1981).

2) To study the inheritance of characters in the tetraploid progeny and the combining ability of the maternal parent (Jakubiec & Narkiewicz 1981), because they form homogenous gametes.

3) To transfer heterozygous blocks of alleles to the Fl progeny by crossing selected diploids with tetraploids, and thus ensuring a high level of heterosis (de Jong et al. 1981).

4) To produce Fl seeds which are genotypically uniform by utilizing the ability of clones to produce female and male unreduced gametes. This provides the opportunity of producing cultivars from true seed (TPS) (Anon 1979; Jackson, this volume).

In this paper the following are discussed:

1. 1) The characteristics of ‘NG’ clones which produce unreduced gametes.

2. 2) The fluctuation of big pollen production during the growing season.

3. […]

INTRODUCTION

The utilization of true potato seed (TPS) for potato production has developed rapidly in many parts of the world since 1978, when TPS was adopted as a principal research programme at the International Potato Center (CIP) in Peru. By 1984, the range of TPS activities had become extensive (International Potato Center 1984). In 34 countries, research on TPS was being conducted at the experiment station level, while 10 countries were involved in on-farm research. TPS was being used at that time by farmers in Sri Lanka, the People's Republic of China, Rwanda, Samoa and in the Philippines.

Much of the early work on TPS agronomy was carried out using open pollinated seed collected from cultivars which produced large quantities of true seed. One of the problems with this true seed was the heterogeneity of some of the progenies, because of the high heterozygosity of the mother plants, which could not be compared with clones in terms of uniformity. Initially it was felt that complete uniformity might not be necessary for potatoes from TPS. Researchers at CIP had been encouraged in this belief by the fact that they had observed farmers in Costa Rica (Central America) mixing white-skinned and red-skinned varieties at harvest. It was thought that this might also be typical of the situation in other countries. Subsequent studies have indicated however that uniformity is important, not only with regard to phenotype in the field, but also with regard to maturity, pest and disease resistance and cooking quality.

The potato is an autotetraploid plant, multiplied vegetatively in most instances, with many clones being male sterile, or even completely sterile. The potato was brought into France by only two introductions of a limited number of clones, and was subsequently severely selected to change photoperiod sensitivity. Genetic variability is thus restricted, and this is serious since there is evidence of a correlation between heterozygosity and vigour.

CURRENT SELECTION PRACTICES

Potato breeding practice is in part dictated by the peculiarities of the species. Selection is applied to the products of crosses between plants, and while there are data on the progeny, they are of limited value given the incomplete genetic information for the species. Selection is thus largely ineffective. The sheer mass of vegetative material imposes drastic selection in the early stages, with few representatives per clone. The risk of discarding worthwhile material is high. Hougas and Peloquin (1958) proposed using diploids (dihaploids) as a means of overcoming these difficulties.

USE OF DIPLOIDS IN BREEDING PROGRAMMES

Production of diploids

The first diploids were obtained by Hougas and Peloquin (1957) using Solanum phureja to provoke parthenogenesis. Dunwell and Sunderland (1973) obtained diploids by anther culture and Irikura (1972) cultured anthers of diploids to produce haploids. It is worth investigating several methods of haploid production because, as San (1977) has shown for barley, plants produced by androgenesis, gynogenesis and interspecific crosses can be different from one another.

The most intense selection in many potato breeding programmes is carried out in the early generations. The conditions under which these generations are grown are often not typical of normal agronomic practice. The large number of genotypes that are assessed usually dictates that each genotype is grown in small plots, in many cases at a single location without replication and the “better” clones are identified by visual appraisal. In this study progenies from eight potato crosses were examined. From each cross 70 clones were grown from true potato seed in a glasshouse (GH), in single plant plots in the first clonal year (FCY) and 3-plant plots in the second clonal year (SCY). A random subsample of 25 clones per cross was also grown in 5-plant plots in the third clonal year (TCY). Subject to the availability of tubers, the FCY, SCY and TCY plots were replicated twice at two locations (BB and MURR). After harvest in each year all the plots were visually assessed independently by four potato breeders on a 1–9 scale of increasing desirability. The data presented in this report are the mean scores of the four breeders.

Correlations of mean breeders' preference between the different years' assessment were all found to be significantly greater than zero (Table 1). Scores in the GH correlated most highly with those recorded in the FCY and lowest with those recorded in the TCY. Similarly, the correlation between the FCY and SCY was larger than that between the FCY and TCY.

INTRODUCTION

Peloquin (1982) proposed five breeding schemes for the utilization of primitive cultivars and wild relatives of the potato, Solanum tuberosum ssp. tuberosum (Tuberosum) (2n = 4x = 48). Of these breeding schemes, 4x x 2x crosses (unilateral sexual tetraploidization), using diploids which produce 2n gametes, allow the breeder to obtain almost entirely 4x progenies, due to the strong triploid block existing in potato and its wild relatives (Johnston et. al. 1980).

Tetraploid offspring with high tuber yields, good vine vigour and significant heterosis have been reported from 4x x 2x crosses (Hanneman & Peloquin 1969; Mok & Peloquin 1975; Kidane-Mariam & Peloquin 1974; 1975; Mendiburu & Peloquin 1977; De Jong & Tai 1977; De Jong et al. 1981; McHale & Lauer 1981; Schroeder 1983). In their crosses the 2x clones, used as males, produced 2n pollen by first division restitution (FDR).

This paper presents results obtained from 4x progenies of 4x x 2x FDR crosses. The experiment was carried out in France during 1983 and three major economic traits were studied: total tuber yield, marketable tuber yield and total solids.

MATERIALS AND METHODS

During the spring of 1981, 4x x 2x crosses were made in the glasshouses of the GIE Germicopa Recherche et Création, at Chateauneuf du Faou, France. Six European cultivars (2n = 4x = 48) - Claustar, Spunta, BF 15, Sirtema, Charlotte and Desiree - were used as seed parents. US-W 5295.7 and five progeny clones of US-W 5295.7 x US-W 5337.3 from the University of Wisconsin potato programme were used as male parents.

INTRODUCTION

At present, potato production from true seed (TPS) has advantages in developing countries, where market quality is not too important. However, this technology might even be adopted by developing countries, if progenies with improved quality were available. At the Institute of Agronomy, University of Milan, the possibility of growing potatoes from true seed in Italy is being evaluated. It is well known that production of high quality seed tubers is difficult to achieve in Italy, because of unfavourable weather conditions; TPS technology could solve this problem, besides saving money and making crop establishment, storage and transport less of a problem.

MATERIAL AND METHODS

Four progenies selected by the International Potato Center (CIP) were studied: open pollinated DTO-28 and DTO-33, and hybrids Atzimba x R-128.6 and Atzimba x 7xy–1. A preliminary trial was carried out in the glasshouse. Seeds were sown on 4 April 1985 in two different substrates in peat pots; one substrate was prepared with peat and sand (1:1 v/v), the other one with peat and soil (1:1 v/v). Both substrates were fertilized with 100 mg N, 300 mg P2O5 and 100 mg K2O. A split-plot design with three replications was used. Seedlings at the 4 to 5 leaf stage were transplanted in the field, in ridges 75 cm apart.

INTRODUCTION

Plant breeders have been criticized for selecting varieties which respond well to increasing levels of inputs, especially fertilizers and irrigation. It is implicit in this criticism that these varieties do not perform as well as their contemporaries at lower input levels and that profitability for the farmer depends on continuing use of high-input husbandry. A trial was grown in 1985 to start to examine these assertions. Only data from early-maturing varieties are reported here although more extensive trials of maincrop varieties were also grown.

MATERIALS AND METHODS

Seed tubers of the old varieties Puritan and Early Rose (both grown before 1850), British Queen (1884), Great Scot (1909) and King George (1911) were obtained from the Department of Agriculture and Fisheries for Scotland. In 1984 the seed of these varieties was multiplied using stem cuttings in an aphid-proof glasshouse at Cambridge along with seed of the newer varieties Pentland Javelin (1968), Wilja (1972), Maris Bard (1974), Marfona (1977), Ukama (1980), Provost (1981) and a new variety from the Plant Breeding Institute, Rocket (1986).

Four replicates of a split-plot field trial were grown in 1985. The whole plots comprised three fertilizer treatments: (1) the normal fertilizer rate used on potato trials at Cambridge (1.76 t/ha of a 10:10:15 compound fertilizer), (2) half the normal rate and (3) no fertilizer at all. Because of shortage of seed, only four tubers were planted per plot. Planting was on 17 April and harvest on 1 August.

Centres of diversity (“gene-centres”) are areas where wild crop ancestors developed and adapted themselves throughout the ages to what were to become their natural habitats. In these areas, in order to survive as a crop species, coevolution between parasites and host plants and between the environment and plant species in general, took place and cultivation of the better adapted native cultivars commenced (Hawkes 1971).

The rich genetic heritage of the gene pool in these areas is of immense importance to plant scientists and plant breeders who are concerned with the incorporation of resistance to pests and diseases, the improvement of quality characters, and a wider environmental adaptability of cultivated varieties.

Plant extinction is a serious threat in many areas of diversity, due among other things to the introduction of new varieties possessing a narrow genetic base (Hawkes 1979), thus lacking the adaptability of the original flora. Fortunately this “genetic erosion” has attracted considerable interest, particularly throughout the 1960s and 1970s, not only from plant geneticists and taxonomists, but also from governments and official bodies. Increasing concern has resulted in greater efforts to preserve this “treasure of nature”.

Inseparably linked to the preservation of this genetic diversity should be its utilization for the continued improvement of cultivatable varieties: without preservation its utilization is impossible, and without utilization preservation becomes meaningless.

Since water availability is a major factor in the production of potatoes, it must also be a major factor in the testing of new varieties. This becomes increasingly important as the potato area under irrigation increases, and the trial system is based on local practice to ensure that results are relevant to current cultural methods. The percentages of NIAB Recommended List trials at present irrigated are: 56% first early, 38% second early and 32% maincrop trials. These closely reflect the national situation in the UK.

The usefulness of drought resistant varieties is obvious in dry situations. There is also a need to know the response of varieties to irrigation. Since irrigation is costly and sometimes limited, it is important to know the likely return of marketable yield from different varieties when planning irrigation schedules. It should be possible to consider irrigation water in the same way as other inputs like fertilizers, fungicides or aphicides.

Hitherto, the assessment of drought resistance has been based on yield results at centres affected by drought, so information depended on the seasons in which the variety was in trials. However, it is difficult to determine differential variety effects of drought and irrigation from the data. Therefore, in order to reduce dependence on standard field trial data, a two-bed system was used to develop a standard test (Flack & Richardson 1984).

The two bed system consisted of a variable depth bed lined with an impervious membrane to give a linear decrease in available soil water, and a variable irrigation bed with a sprinkler line designed to give a linear decrease in applied water.

INTRODUCTION

All breeding strategies, classic as well as unconventional, aim at the production of better varieties. Today this means in particular, higher levels of disease resistance. This aim is most easily achieved when both classical and unconventional techniques complement each other and increase the efficiency of the breeding processes, i.e. creation of variation, hybridization and selection. New variation may be obtained in vitro through somaclonal variation or by making use of natural meiotic segregation. For the production of clones with resistance to Phytophthora and Fusarium, spontaneous in vitro mutation can be used; this is, however, only applicable when the induction of variation is coupled with a powerful in vitro screening system. From abiotic cultures of the two fungi mentioned, exotoxins can be extracted and used for selection. This process may be also used for hybrid selection, when both fusion partners have a different resistance. In addition, for hybrid selection, hybrid vigour of the fusion products may be used. The regeneration of potatoes via another or isolated microspore culture (Wenzel et al. 1982; Uhrig 1985) provides the basis for the use of meiotic segregation in vitro. Haploids can be produced from tetraploids and from dihaploids. The use of such haploids should facilitate the incorporation of virus resistance, and of other characters, regardless of whether they are monogenically or polygenically inherited.

INTRODUCTION

The mandate of the International Potato Center (CIP) is to increase potato productivity by helping to develop varieties better adapted to the growing conditions of developing countries and improving agronomic, seed production, and storage technologies.

Since the foundation of CIP, its scientists have constantly visited and consulted with the research staff of national programmes to analyse biotic and abiotic yield constraints related to the performance of their commercial cultivars. Using this information, research needs for breeding programmes have been established for the developing countries associated with each of the seven regions of CIP. The regional network has been developed to coordinate CIP's research involvement through the world. Furthermore, Planning Conferences have been organized on a regular basis with the participation of breeding experts from leading institutions throughout the world, CIP breeding staff, and scientists from developing countries. At these conferences, research problems are discussed and breeding priorities defined. CIP's main contribution to potato production has been in germplasm improvement and the distribution of genetic materials. These materials are distributed as selected clones with specific adaptation and resistance to or tolerance of climatic stresses, pests and diseases. CIP also distributes advanced populations with a wide genetic diversity segregating for the same characters as the clonal materials. National programmes have to evaluate this advanced germplasm and select varieties adapted to their local conditions and suitable for their needs. They will eventually release their own varieties from CIP's genetic material.

INTRODUCTION

The breeding strategy advocated in this paper is based on genetic diversity. By diversity we mean both the many valuable traits available in the wild and cultivated relatives of the 4x potato (Solanum tuberosum ssp. tuberosum (hereafter, Tuberosum)), and the allelic diversity present in these relatives that provides the opportunity to approach maximum heterozygosity in developing new 4x cultivars. The breeding strategy contains three essential components; 1) the wild and cultivated relatives are the source of genetic diversity, 2) haploids (2n = 2x = 24) of Tuberosum (2n = 4x = 48) are effective tools in capturing the genetic diversity (putting the germplasm in a usable form), and 3) 2n gametes, gametes with the sporophytic chromosome number, are the basis of an efficient method of transmitting the genetic diversity to the cultivated 4x potato (Peloquin 1982).

An essential for the third component is that there are 2n pollen producing 2x hybrids from crosses between haploids of Tuberosum and the cultivated 2x species S. phureja (hereafter, Phureja) and S. stenotomum or the 2x wild species. The 2n pollen can be formed by either the genetic equivalent of first division restitution with crossing-over, FDR-CO (parallel spindles mutant, ps) or FDR without crossing-over, FDR-NCO (parallel spindles in combination with the synaptic 3 mutant, sy 3). The significance of the mode of 2n pollen formation resides in the genetic consequences.

The potato is one of the world's most important food crops, being surpassed in total production only by wheat, corn and rice. Therefore, advances in potato breeding may greatly contribute to the world's food supply.

Potato breeders are expected to produce improved cultivars that give high yields of high quality tubers. Furthermore, resistance is required to diseases and pests during growth and storage, to stress conditions and to mechanical damage. Finally, specific properties of the tubers are required for various processing industries.

A rich source of genetic variation is available in existing cultivars, in primitive forms and in wild relatives of potato. The main problem faced by potato breeders is how to exploit most efficiently this large genetic variation.

In the last 75 years there has been a considerable expansion of world potato breeding and associated research. Before World War I breeders used to grow no more than several thousand seedlings, and a few years of basic selection were sufficient to put a new cultivar on the market. Research associated with potato breeding was very limited. Nowadays breeders in many countries grow several hundreds of thousands of seedlings. The breeding cycle is usually 10–12 years and breeding often involves sophisticated selection procedures. Governments in many countries have organized an elaborate varietal assessment system to make sure that only cultivars of satisfactory quality are being released. In addition, many sophisticated research centres make available to breeders new genetic variation, new breeding methods and improved selection techniques.

INTRODUCTION

In Egypt and other countries with a similar climate, potatoes are grown as both a spring crop and an autumn crop. The spring crop is usually planted in January with imported seed tubers, while the autumn crop is planted in September using local seed tubers taken from the preceding spring crop. Attempts to grow the spring crop using local seed taken from the preceding autumn crop are usually hampered by the dormancy of the freshly harvested tubers. Potato varieties grown in Egypt show wide variation in length of dormancy and in their reaction to the different chemical agents that break dormancy.

METHOD

In the present investigation, seed tubers of five potato cultivars Alpha, Berolina, Domina, Granola and Hilta were purchased from the local market and grown on Assiut University Experimental Farm during the autumn season of 1984. After harvest in January 1985, tubers were exposed to the following treatments to break dormancy:

1. 1. Cutting of tubers (cut, whole).

2. 2. Pretreatment temperature (ambient temp. 5–22°C, high temp. 25–30°C).

3. 3. Chemical treatment: a) soak treatment: thiourea, potassium thiocyanate.

b) gas treatment; carbon disulphide (CS2), ethylene chlorohydrin, rindite. The tubers were planted on 19 February 1985 in a factorial experiment and data were recorded on rate of emergence, final plant stand, shoot length, number of stems per plant and total yield.

RESULTS

Alpha was the slowest cultivar to emerge and had the lowest number of stems per plant. Results for final plant stand (56 days after planting) are presented in Table 1.

INTRODUCTION: HISTORICAL BACKGROUND

At the turn of this century, potato cultivars were produced by interested farmers and enthusiastic amateurs. If a common breeding strategy could be perceived, it was to replace existing cultivars, whose stocks had “degenerated” during the normal course of vegetative reproduction over a number of years in agriculture (Robb 1921, 1948). As with other crop species, potato breeding gained impetus with the increase in understanding of the science of heredity and the rediscovery of Mendel's laws of inheritance. With potatoes, in particular, the practical implications of the discovery that some clones were immune to wart disease (Synchytrium endobioticum), and that this was a heritable trait, provided added impetus. In Scotland, for example, in the 1920s a group of farmers and merchants founded the Scottish Society for Research in Plant Breeding (SSRPB). In cooperation with the Department of Agriculture and Fisheries for Scotland (DAFS), the SSRPB set up what became the Scottish Plant Breeding Station and is now part of the Scottish Crop Research Institute (SCRI) (Gallie 1955; Simmonds 1968). Over the next half century, state support for this and other similar ventures increased and private sector involvement declined in proportion. Lack of legal protection for the breeders of cultivars contributed to a situation where plant breeding in general, and potato breeding in particular, became almost entirely a state funded activity. The situation is now changing and private sector involvement in plant breeding has increased substantially since the introduction of plant variety protection legislation.