Factors affecting dry matter yield and pyrethrin content in pyrethum in Tasmania

Pyrethrum is a major crop in Tasmania, Australia. The lack of predictability in flower dry matter (DM) production and therefore crop yield is one of the biggest challenges for the industry, with large variation in dry matter yield currently occurring between production sites and seasons. If the industry is to expand, consistency in production is required to ensure crop production matches the volumes contracted for delivery to the market. Knowledge of the main factors affecting DM accumulation under commercial production conditions is required to develop crop management strategies capable of delivering consistent, high yields. The specific objectives of the studies reported in this thesis were to investigate the effect of: (i) light interception; (ii) plant density; and (iii) water stress on DM yield and pyrethrin content of pyrethrum. The indirect measurement of light interception in pyrethrum using crop green leaf area and volume was used to assess the relationship with flower DM yield and pyrethrin content. Strong relationships were identified within sites and seasons, but not across all seasons and sites assessed in the study. As there was no clear relationship between light intercepted and pyrethrin yield, other factors could be involved. It was hypothesised that differences could be related to variation in rate of crop development, plant density and drought stress, which can occur during the later stages of flower development in Tasmania. Plant density was shown to have a large effect on DM partitioning in a field trail conducted in north-western Tasmania in 2008-09. The relatively large leaf, stem and flower DM yield per plant at low (6 plants/m2) plant densities was compensated by the increase in plant numbers at high (44 plants/m2) densities. Variation in plant density is therefore likely to affect light interception. There was, however, no difference in the concentration of pyrethrin, the active ingredient extracted from the pyrethrum flowers, with plant density. No significant differences in DM accumulation were found between pyrethrum genotypes and there were no density x genotype interactions. Drought stress was investigated by conducting pot trials under controlled conditions. Treatments included 3, 4 and 5 day watering intervals applied at early flower maturity stage 2 (FMS 2), mid (FMS 4) and late (FMS 6) flowering stages for either short (10 days) and long (20 days) durations. Plants watered on a 3 day interval tended to have lower (less negative) leaf water potential and displayed no visual signs of water stress. The FMS stage that water stress treatment was applied affected plant and flower dry matter yield, but watering interval or duration of stress treatment had no significant effect. Plants produced greater DM when drought stress was imposed during early compared with late flowering. Plant pyrethrin content, measured as pyrethrin concentration in the harvested flowers, was not affected by any of the drought treatments. Consequently, it was concluded that plants grown in field conditions are likely to be able to recover from mild drought stress, particularly during early flowering, but yield will be reduced if plants are stressed at later flowering stages. This thesis has provided information to the pyrethrum industry on plant growth in response to crop management strategies and has highlighted the complexity of processes determining crop dry matter and pyrethrin yields.