An investigation into accelerating the rate of reproductive cycling in Humulus Lupulus : A thesis submitted in fulfilment of the requirements for the Degree of Master of Science at Lincoln University

Abstract

Humulus lupulus is an economically important horticultural crop grown globally in narrow bands of latitude and climate. The H. lupulus female inflorescence are used in brewing to add flavours, aromas and bitterness to beer. Within the New Zealand context, the majority of H. lupulus are grown in the Nelson and Tasman regions, however there is increasing interest to expand the growing region further south. New Zealand Humulus breeders also face unique challenges, with the inability to easily breed from the majority of current elite cultivars, limited genetic diversity available in the country and limited information available of the drivers of most aspects of cone quality and agronomic traits. A rapid cycling breeding program has the potential to aid some of these unique challenges and accelerate the potential rate of genetic gain for the New Zealand Humulus industry. A rapid cycling breeding program aims to maximise the potential rate of gain within a given timeframe by reducing the length of time between generations, increasing the number of generations within that given time. This can be achieved by manipulating plant growth and development by artificially replicating natural seasonal cues that plants utilise to trigger key development growth phases, for example, photoperiod and temperature. While some research has been completed on the flowering controls and triggers in H. lupulus there are still a number of unknowns that need to be addressed in order to establish a rapid cycling breeding program, especially within the context of the New Zealand industry and cultivars. The initial purpose of this research program was to explore the possibilities of growing H. lupulus, indoors in a highly controlled environment, and describe what a potential rapid cycling breeding program could look like. While this research indicated that it would be possible to grow H. lupulus from root straps indoors, out of season with a mean reproductive cycle of 95 days, limited potential genetic gain would have been possible through this form of propagation. Theoretically, the largest potential rate of change in genetic gain can be made through seed to seed (seed germination to seed harvest) propagation. However, plants grown from seedlings in the highly controlled environment did not flower, which was believed to be related to the low level of available photosynthetic active radiation in the growth room. Subsequently, the potential to manipulate plant growth and development with plant growth regulators in order to accelerate generational cycling was investigated. The use of plant growth regulators had no effect on plant growth and development and were deemed not feasible for use to accelerate breeding cycles. Throughout this research it became clear that even if a potential rapid cycling breeding program was possible through seed to seed propagation the ability to complete two cycles within a calendar year would be reliant on the ability to germinate the seed in a timely manner. A seed germination methodology was developed, which maximised the germination percentage of the seed, in the shortest period of time. This methodology involved the use of 70 percent sulphuric acid to scarify, and remove the outer layer of lupulin from the seed, a 15 day chilling period at four degree Celsius and the addition of one millimolar of gibberellic acid into the imbibition solution. Further work was initiated to explore the potential to further reduce the chilling time required to germinate a significant amount of the seed, using the phytoene desaturase inhibitor fluridone, which showed promise to increase the germination percentage of H. lupulus seed with less chilling. However, this work was discontinued due to unavoidable and irremediable fungal issues. In conclusion, Humulus lupulus is an important horticultural crop with expanding growing regions. New Zealand H. lupulus breeders face unique challenges such as limited genetic diversity and limited information on agronomic and cone quality traits. This research program has shown that while it is possible to grow H. lupulus indoors as apart of a rapid cycling breeding program, only a limited increase in potential genetic gain would be possible. The use of plant growth regulators was also found to be not feasible for accelerating breeding cycles. Germinating seed in a timely manner is crucial for a potential rapid cycling breeding program, and a seed germination methodology was developed to maximize germination percentage in the shortest period of time. However, further research on the use of phytoene desaturase inhibitors to reduce the chilling time required for germination was discontinued due to fungal issues.