Dynamics of flower, achene and trichome development governs the accumulation of pyrethrins in pyrethrum (Tanacetum cinerariifolium) under irrigated and dryland conditions

Pyrethrum is a perennial crop commercially grown for extraction of natural insecticidal compounds (pyrethrins), which accumulate in the achenes of the flower heads. Pyrethrum achenes are densely covered by glandular trichomes and these play a vital role in biosynthesis of pyrethrins. Pyrethrum is a fairly drought tolerant crop, but irrigation is still important to obtain maximum yield of pyrethrins from this crop. No data exist in the literature on developmental changes in achenes and trichomes during pyrethrum flowering in relation to accumulation of pyrethrins. This experiment was conducted to understand the dynamics of flower, achene and trichome development governs the accumulation of pyrethrins under irrigated and dryland conditions. Plants were subjected to irrigate and dryland conditions throughout the flowering period and examined the dynamics of achenes and trichomes using light microscopy and scanning electron microscopy. Analysis and quantification of pyrethrins were performed by High-performance liquid chromatography. According to the results of this experiment, application of irrigation throughout the flowering period significantly increased flower, achene and trichome sizes and densities at all flower developmental stages. In dryland treatment, flower, achene and trichome sizes and densities were lower due to absence of adequate soil moisture to complete the development processes and accelerated flower senescence. Application of irrigation throughout the flowering period significantly increased all pyrethrin yield responses compared to the dryland treatment. Overall, irrigation provides adequate soil moisture for flower, achene and trichome development which slows the rate of flower development and increases the duration of the flowering period, resulting in higher pyrethrin yield per flower at physiological maturity. Flower size, rate of flower development, duration of flowering period, achene size, achene density per flower, trichome size and trichome density per achene are the major dynamics govern the pyrethrin yield per flower under irrigated and dryland conditions.