Some physiological effects of the herbicide bromacil (5-bromo-3-sec-butyl-6-methyluracil) on Asparagus officinalis L. : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science at Massey University
The root-absorbed, photosynthesis-inhibiting herbicide bromacil (5-bromo-3-sec-butyl-6-methyluracil) was applied in sand culture to tissue-cultured 18-month-old Mary Washington 500W clone of Asparagus officinalis L. grown under controlled environmental conditions. Dose-response characteristics were determined and ED₂⁰ and ED⁵⁰ values computed by regression analysis for several parameters for asparagus plants exposed to a single application of 0, 2, 4, 8, 16, 32, 64, 128, 256, and 512 p.p.m. bromacil in non-draining pots. The results of this initial broad spectrum studies revealed a drastic decline in visually assessed foliage damage score, shoot growth and root fresh weight, and an increase in shoot death at relatively low concentrations. Good dose-response characteristics were obtained, and time-course data showed that the rate and severity of effects increased with increasing dose. The ED⁵⁰ values 18 days after treatment were: visually assessed damage score, 2.7 p.p.m.; shoot growth, 25 p.p.m.; shoot death, 4.6 p.p.m.; and root fresh weight, 2.1 p.p.m. A catalogue of colour plates showing visual phytotoxic effects was compiled. The injury symptoms observed were: yellowing of cladophyll tips followed by bleaching with the effects extending towards the base, cladophyll tipping and progressive cladophyll death leading to shoot death. Equal increment dose-response experiments were conducted at 0, 2, 4, 6, and 8 p.p.m. bromacil, using a portable fluorometer (Model SF-10) to obtain fluorescence emission measurements. The results showed a dramatic decline in the initial rise in fluorescence yield from the cladophyll tips 156 hours after treatment. The ED⁵⁰ value was computed to be 2.3 p.p.m. Fluorescence emission measurements from cladophyll tips from excised shoots placed in bromacil solution at the same concentrations showed a dramatic decline in fluorescence yield within 17 hours indicating that uptake and translocation was more rapid without the roots. No significant changes in chlorophyll a, chlorophyll b and total chlorophyll concentrations, as determined by 80% acetone extraction technique, were evident in the samples in which a dramatic decline in fluorescence yield occurred. The results of this study, conducted under controlled environmental conditions, showed that the asparagus clone tested readily absorbed bromacil through its roots and translocated it to the foliage causing severe initial damage to the photosynthetic apparatus followed by detrimental effects on other parameters such as shoot growth, root fresh weight and shoot death. Even at a bromacil concentration of 2 p.p.m. the asparagus plants were found to susceptible to herbicide damage.