Genetic and physiological aspects of postharvest flower longevity in Asiatic hybrid lilies (Lilium L.)

In The Netherlands Lilium is economically the fourth overall flower crop for cut flower production. Longevity is a main quality characteristic for cut flowers. During postharvest handling of Asiatic hybrid lilies pretreatment with chemical solutions containing silver is carried out to ensure a satisfactory longevity at the consumers'. However, the extent to which such a treatment can delay senescence is limited and is dependent on the genotype. Developing cultivars with genetically improved longevity may provide the consumer with a more reliable expectation for postharvest quality in a less environment polluting way. Flower longevity is a particularly difficult genetic characteristic to assess, since it is markedly affected by growing conditions prior to harvest, stage of flowering at harvest and environmental conditions during distribution and after sale. Additionally, longevity of lily is complex because of its inflorescence-type flower. Longevity is determined by two conflicting processes: (1) promotion of flower bud growth and anthesis; (2) retardation of metabolic processes leading to senescence. Therefore, besides knowledge of genetic aspects of flower longevity also insight in the physiological regulation is needed for its improvement. The objectives of the experiments described in this thesis were to obtain knowledge of (1) methods for longevity determination, (2) available genotypic variation, (3) mode of inheritance and (4) physiological regulation of postharvest flower longevity in Asiatic hybrid lilies.The development of a standardized method for the determination of flower longevity in lily is described. To optimize the screening procedure, the main sources of environmental variance (parameter for screening, harvest stage) were located and standardized, whereas conditions improving the degree of variation among genotypes (evaluation temperature) were determined and optimized (Chapter 2 and 3). By using standardized conditions during forcing, harvest and postharvest evaluation measurements became discriminative and repeatable. Standardization of the forcing method is usually not possible in practice. The ranking of the genotypes based on their longevity levels using standardized conditions during forcing was comparable with the ranking found when forcing conditions in practice were used (Chapter 4). This makes screening results highly reliable for practical use. Because of the large influence of temperature on flower longevity postharvest evaluation should preferably be carried out under controlled temperature conditions.For breeding purposes adequate genotypic variation in the breeding material is necessary. Large differences in individual flower longevity were found screening a wide range of genetic material of Asiatic hybrid origin (Chapter 4). Interspecific hybridization research showed that crosses between genotypes from different taxonomic sections is possible (Chapter 1 and 8). So, enhancement of the exploitable genetic variation may be obtained by screening also other lily groups. The in potential reachable individual flower longevity, not preceded by stress, appeared to be a reliable overall parameter for screening as postharvest stress conditions (storage, ethylene) introduced no large differences in the ranking of the genotypes based on their longevity levels (Chapter 8).Segregation after crossbreeding makes selection possible. Large variation in flower longevity within and among progenies was found. Selection at seedling level appeared to be possible because of a high broad-sense heritability based on one individual plant per genotype and because of an equal expression of individual flower longevity in plants obtained from both seedling bulbs and scale propagated bulbs (Chapter 5). This can considerably speed up the breeding process.For an efficient selection of genotypes with improved flower longevity, knowledge is required of the inheritance of flower longevity. Genetic analysis of the results of the individual plant test showed that additive genetic variance is important in the inheritance of individual flower longevity as the general combining ability (GCA) was the most important genetic component and the estimate for narrow- sense heritability was high. Consequently, individual flower longevity of the genotype can be used as an indication for its breeding value in practical breeding (Chapter 5). A more specific statement on the inheritance of flower longevity can be obtained by indirect selection using genetic markers as discussed in Chapter 8.The physiological regulation of postharvest longevity was investigated by studying the role of tepal carbohydrate content (Chapter 6 and 7). The importance of tepal carbohydrate content and distribution in lily postharvest performance (bud-growth, anthesis, longevity) was revealed by comparing the development of inflorescence-attached and detached buds and flowers. Genotypic differences in the availability and use of carbohydrate were found. Per genotype tepal carbohydrate and individual flower longevity of detached flowers were well correlated. Blocking carbohydrate export by detaching the flowers from the inflorescence increased their longevity compared to the longevity of inflorescence-attached flowers. The complexity of the carbohydrate economy within lily inflorescences hampered the detection of a causal relationship among genotypes. However, the data presented suggest that genotypic differences in flower longevity might at least partially be due to genotypic differences in postharvest carbohydrate utilization (Chapter 7).In conclusion, the prospects for improving flower longevity in lily by breeding and selection are promising. The methods described for screening and selection, the genotypic variation in flower longevity found in this study and the way of inheritance as discussed in Chapter 5 make it possible to obtain new lily cultivars with an improved flower longevity in a relatively short period of time. Furthermore, a high broad-sense heritability in combination with a large segregation of individual flower longevity are of great benefit in linkage studies and future breeding programs based on indirect selection using genetic markers. The physiological regulation of lily flower longevity and the important but partially unknown role of carbohydrates in this complex process needs still further research.