Laticifers and Latex in Papaver somniferum L.; Capacity, development and translocation
Creators: name: Harvest, Tammy
Publisher: name: University Of Tasmania
Year: 2023

description: The opium poppy, Papaver somniferum L., is a pharmaceutically important species that is of commercial significance in the state of Tasmania, Australia. Poppies are grown for seed and notably for their alkaloids, including morphine, codeine and thebaine. Alkaloids accumulate in the capsule during plant development, sequestered to vesicles within the latex of the reticulated network of laticifer cells. Laticifers, as the repository for alkaloids in poppies, are potentially an important component in the yield of alkaloids, and there are aspects of these cells that have not been fully investigated. The studies herein examined the volumetric capacity of capsule laticifers; the allocation of biomass to poppy capsules; and the capability of laticifers to deliver alkaloids to the capsule through the bulk movement of latex. Capsule laticifer capacity is of relevance in the cropping system used in Australia, where capsules are not lanced and remain intact until harvesting, which occurs after plant senescence. Variations in the volumetric capacity may represent a component of the alkaloid yield from capsules, and the first investigation assessed the quantity and variability of this parameter in field grown poppies. The latex volume was calculated for capsules that were harvested at one week, and at two weeks after flowering. It was demonstrated that the volume of laticifers per unit area of tissue was established early in capsule development (before the first week of flowering) and did not increase thereafter, yet capsule morphine content continued to rise. The capacity determined at either stage did not predict the end-yield of morphine, but there was a positive linear relationship with capsule mass, and capsule mass was the most important factor in capsule morphine yield. It was concluded that the volumetric capacity of capsule laticifers was established early in capsule development, and therefore has limited utility as a component that may be manipulated to increase capsule alkaloid yields. Factors influencing capsule mass were further investigated. The within-plant allocation of biomass to capsules was investigated in a field study of P. somniferum plant morphology. Greater than 80% of the capsule biomass was allocated to the upper three capsules, with the senescence of reproductive structures more prevalent on lower branches. The findings indicated that in the development of this commercial line of poppies there was sequential regulation of the sinks (fruit), and that the majority of reproductive biomass was allocated to the early developing capsules in terms of both fruit and seed. An argument was presented to suggest that developmental processes and within-plant resource limitation may influence the allocation of biomass to reproductive structures in poppies. Due to the complex interplay of sink strength and feedback mechanisms, and with so little modelling of within-plant biomass allocation in poppies, additional studies were suggested. The capability of laticifers to deliver alkaloids to the capsule through the bulk movement of latex was conducted through an investigation of the vesicle populations and osmotic potential of both capsule latex and stem latex in P. somniferum. Vesicle populations were examined during plant development, at localised positions and in terms of their volume. Capsule latex contained significantly greater vesicle numbers, packed vesicle volume (PVV), and osmotic potential than stem latex, and these significant differences occurred as a steep gradient across the thalamus of the poppy capsule. Latex from three sampled positions on plants of P. somniferum also differed in the development of vesicle populations, with capsule latex exhibiting a steep increase in vesicle number after flowering. The findings indicated that the development of latex (within laticifers) was localised to separate organs, and that in toto latex movement to the capsule was unlikely. In summary, the research in this thesis reveals that laticifers represent a consistent volume in capsule tissue; that capsule mass is important for alkaloid yield and that the upper capsules have an advantage in the development of that mass; and that the in toto movement of latex through laticifers into the capsule is improbable.