Another culture of Solanum genotypes.

Abstract

Being the third most cultivated crop in South Africa, potatoes are of great economic importance. As potatoes originated from cooler areas in the world, they do not easily adapt to South African conditions. The main objective of potato breeding is, therefore, to extend the crop's limited genetic base. Progress in crop improvement is slow due to dominance, segregation and other factors caused by the tetraploid character of cultivated potatoes. A new breeding program for rapid progress has been initiated at the Vegetable and Ornamental Plant Institute, Roodeplaat, South Africa, which comprises the combination of conventional and unconventional breeding techniques. The program is based on the reduction of the ploidy level from the tetraploid to the dihaploid level to facilitate crossings with diploid wild species. Anther culture is the preferred technique for the rapid reduction of the ploidy level and has been successfully applied on different members of the Solanaceae. Cultivated potato, Solanum tuberosum is, however, an important exception. In this study various potato genotypes (tetraploid cultivars, dihaploid breeding lines and a diploid wild species) were used in experiments concerning microtechniques, alternative culture methods and medium manipulation. The main objectives were to evaluate and compare the androgenetic ability of the various genotypes used and to try and identify the factors limiting their in vitro response. Regarding microtechnique, the study focussed on the investigation of the frequency of androgenesis - as a function of plant age - and the determination of defined flower bud lenqths representative of the correct microspore developmental stage for optimal androgenetic response. Combined with an extensive histological study on the microspore development within anthers, from the time of flower selection, after a cold-pretreatment and at various time-intervals during the culture period of 42 days, the following conclusions were reached: In vitro androgenetic response proved optimal when flowers of responsive genotypes were selected during the first seven to 21 days of the flowering period. Both microspore derived embryoid- and callus development were visible within responsive anthers after a culture period of only seven days. The flower bud length required for anthers to be in the optimal stage of microspore development, e.g. the uninucleate stage, varied between the different genotypes but could readily be determined with the DAPI (4,6-diamidino-2- phenylindole) technique. It was also concluded that anthers of the tetraploid cultivar Atzimba should be selected later, between the late-uninucleate and the early-binucleate developmental stages. This suggested a limited selection period for Atzimba anthers, as starch depositioning - which prevent embryogenesis - occurs within anthers during the binucleate stage. Histologically, Atzimba showed limited embryoid development with no embryoid release, while the diploid wild species, S. canasense, proved androgenetically unresponsive. Alternative culture methods were applied to study the effect of different culture phases (liquid, double layered and agar solidified) and anther orientations (lateral, dorsal and ventral) on the androgenetic response of the potato genotypes used. Liquid cultures, based on the so-called shed-pollen technique, enhanced the androgenetic response of the tetraploid cultivar Atzimba. Optimal embryogenesis was obtained for responsive breeding line 87.2002/3 with the utilization of agar solidified media, with maximal response when anthers were cultured in the lateral orientation. No response was observed from S. canasense. The effect of medium manipulation on the androgenetic response of the three genotypes was investigated. The utilization of various combinations of different concentrations of indole-3-acetic acid (1M) and benzyladenine (BA), the alteration of the initial time of incubation of anthers on the initiation media and the use of media without growth regulators compared to that containing gibberellic acid (GA[3]), were investigated. BA had to be present in the initiation media and had a major, though not exclusive, effect on embryogenesis compared to 1M. The optimal BA concentration varied between the two trials. IAA also had an increasing effect on anther response, both in the absence of BA and, especially, in addition with relatively high BA concentrations. In this experiment, only breeding line 87.2002/3 responded. The initial culture of anthers, during the first seven to 21 days of the culture period, on media containing growth regulators proved essential for microspore derived embryoid production in the tetraploid cultivar Atzimba. As these growth regulators are metabolized in the culture media, the regular transfer at shorter, two-weekly intervals to media containing metabolically active substances, proved important. GA[3] had no enhancing-effect on embryogenesis in any of the three tetraploid cultivars. The results obtained in this study suggest that the first 21 days is the critical stage in the anther culture period in terms of the optimal time for flower selection, embryoid induction and the increase in embryogenetic response due to growth regulator influence. It is important to pre-determine the developmental stage when most microspores were in the uninucleate stage of development and to correlate this stage with a specific flower bud length. This would assure maximum response of those genotypes amenable to anther culture. It also implies a more practical and economical starting pOint to anther culture experiments. Following the determination of microspore developmental stage and pollen fertility, flowers should be selected from the donor plants only during the first three weeks of the flowering period. The composition of the nutrient media used for potato anther cultures were sufficient with respect to growth regulators. The growth regulators SA, IAA and the amines glutamine and asparagine had to be present in the initiation media, especially during the first three weeks of the culture period. As microspore development within anyone anther was found to be asynchronous, the regular transfer of anthers to fresh media is recommended to assure proper development of all microspores. The use of a slightly higher IAA concentration could be considered, but care should be taken as too-high concentrations would induce callus production. Microspore derived embryoid production is preferred, as the ploidy level of callus derived plantlets normally varies and somaclonal variation can occur. Liquid media should be considered for anther culture of tetraploid genotypes, while embryoid production can be increased by culturing the anthers of responsive genotypes on agar solidified media on the lateral orientation. Finally, the diploid wild species S. canasense seemed androgenetically unresponsive, or the media and culture conditions used did not satisfy the specific requirements of this genotype. Androgenetic amenability should first be transferred by means of interspecific crossings with a responsive dihaploid genotype, such as the breeding line 87.2002/3.