Agronomic and seed quality studies in pyrethrum Tanacetum cinerariaefolium Sch. Bip./
Creators: name: Fulton, David
Publisher: name: University Of Tasmania
Year: 2023

description: At the time this research project was initiated, the Tasmanian pyrethrum industry was attempting to establish crops by sowing rather than by planting of 'splits' or seedling plugs. This thesis investigated plant density and sowing times required for maximum yield. Studies were also conducted to improve chances for successful crop establishment from seed. That work investigated environmental requirements for germination and aspects of seed quality and seed production. Previous studies have examined the influence of density on yield of pyrethrins, but none had been conducted in sown trials, in cool temperate environments, over a wide range of plant densities. Furthermore, the influence of plant density on components of yield had not been intensively investigated. This work identified that maximum yield was achieved in the first season at between 16 and 39 plants/m2 and at or above 16 plants/m2 in the second season following establishment. Yield was a function of dry flower yield rather than due to change in percentage of pyrethrins in the flowers. Higher flower yield was associated with greater above- ground dry matter production. The yield component which increased with plant density was number of flowering tillers/m 2 . Yield components decreasing with density included number of flowering tillers/plant, flowers/tiller and dry weight/flower. Other aspects of changing plant morphology and development with density investigated included, crop height, mean flower maturity and plant survival. The recommendation to industry was to aim for a plant population of above 16 and below 39 plants/m2 . Yields achieved in this work were unprecedented in the pyrethrum agronomy literature. No reported studies have examined the influence of time of sowing on pyrethrins yield. Field studies showed that sowings later than mid-November led to significant reductions in yield in the first flower harvest. Yield reductions were associated with decreased dry matter production/plant and flowering tillers/plant. Later sowings resulted in plants failing to flower or in significant reductions in the extent of flowering. Sowing earlier than mid-November resulted in no significant increases in yield. As pyrethrum crops have not been traditionally established by sowing, only scant information was available on requirements for germination, or seed quality. A field study and three laboratory trials investigated the influence of temperature and seed quality on germination and emergence. Results demonstrated that rate and final emergence varied significantly at different times of year. In general, the proportion of viable seed sown that emerged and survived was very low. Both rate of emergence and final emergence percentage were associated with temperature. Laboratory investigation of germination percentage, rate and uniformity of germination of a seed lot under a range of constant temperatures confirmed the previously reported findings relating to germination characteristics of this temperate species. Unexpected though was the high proportion of dead seed found at temperatures only several degrees higher than the temperature for optimal germination rate. Six seed lots were subsequently germinated at low, medium and high temperatures which provided some insight into the seed death phenomenon as well as documenting the range of behaviours from different seed lots. An explanation for differences in germination behaviour involving seed maturity was proposed for different seed lots and this was tested in a subsequent study. Finally, the effects of an 18 month storage period on the germination characteristics seed lots was investigated in a laboratory trial. There was little change in most germination parameters except for time to complete germination which increased in all seed lots after storage and uniformity of germination which decreased. Changes were assumed to be due to satisfaction of an after ripening requirement. Laboratory studies investigated the influence of seed mass on various germination and seedling development characteristics. Variation in seed mass within seed lots was identified but this had little impact on rate of germination or other germination parameters. However, a following, study revealed that heavier seedlings emerged from seeds that germinated earliest within seed lots. Furthermore, heavier seedlings demonstrated faster development than did lighter seedlings. Although seedling mass was found to be associated with rate of germination within seed lots, this factor failed ‚Äö to explain significant differences in mean time to germinate between seed lots. It was subsequently recommended to industry that cleaning on a size or mass basis could improve seed quality. The influence of harvest date on seed quality and quantity were investigated. Results indicated that losses of larger achenes from the harvested flowers were occurring with lateness of harvest. Characteristics of the seed including mean seed mass, proportion of viable seed in the sample, germination percentage and rate of germination were found to vary considerably with harvest date. Data generated in this work were found to be consistent with the model which proposed that variability in germination parameters was due largely due to maturity of seed at harvest. A following study revealed that relative position on the capitulum also had a profound influence on germination parameters of the seed. The recommendation for industry emanating from the harvest date work was that flowers should be cut at a field capitulum moisture of 25%. Finally, variation in seed quality and quantity was evaluated both within and between capitula. This study revealed that small flower heads produced fewer and smaller seed than larger heads. Outer achenes, regardless of whether they came from large or small capitula were found to germinate more rapidly than inner achenes. Peripheral achenes weighed more than central achenes and gave rise to heavier seedlings. The methods and results in this study will serve as a valuable source of information for agronomists and plant breeders working on improvement of pyrethrum production. The investigations on seed quality and seed production provide a sound base for future efforts to improve seed quality and crop establishment. The findings presented provide the Tasmanian pyrethrum industry with critical information with respect to target plant densities, sowing times, seed quality and seed production. It is expected that implementation of findings from the study will prove to be pivotal in continued industry profitability and expansion.