Bioresidual activity of photosynthesis - inhibiting herbicides in water and soil

Salah El-Deen Abdel-Aty Selim, Purdue University


Eurasian watermilfoil (Myriophyllum spicatum L.) and hydrilla (Hydrilla verticillata Royle) response to the photosynthesis-inhibiting herbicide simazine (2-chloro-4,6-bis(ethylamino)-5-triazine) was estimated in culture solution. Simazine concentrations of 2.0 and 3.0 mg $\sp.$ L$\sp{-1}$ (corresponding with field rates of 5.4 and 8.1 lb/acre-foot of water, were tested. Exposure time (6, 12, 18, 24, and 30 days) required for total kill (complete loss of chlorophyll) was affected by light irradiance (50, 200, and 30 $\mu$E $\sp.$ m$\sp{-2\ .}$ s$\sp{-1}$). Higher light irradiance and simazine concentrations resulted in faster development of chlorosis and necrosis of plant tissues. Treated plant shoots were transferred to a simazine-free medium. Least recovery was at the highest light irradiance and dose for both. No recovery was observed after 30 days when watermilfoil plants were exposed to 2.0 mg $\sp.$ L$\sp{-1}$ at 300 $\mu$E $\sp.$ m$\sp{-2\ .}$ s$\sp{-1}$ for 30 days. Milfoil plants developed severe chlorosis at 50 $\mu$E $\sp.$ m$\sp{-2\ .}$ s$\sp{-1}$ at 2 mg $\sp.$ L$\sp{-1}$, but recovered from 30-day exposure. Watermilfoil didn't recover at 3 mg $\sp.$ L$\sp{-1}$ at 18 and 24 days at 300 and 200 $\mu$E $\sp.$ m$\sp{-2\ .}$ s$\sp{-1}$, respectively. Hydrilla did not recover after 18, 24, and 30 days of exposure to 3 mg $\sp.$ L$\sp{-1}$ at 300, 200, and 50 $\mu$E $\sp.$ m$\sp{-2\ .}$ s$\sp{-1}$, respectively. After 24 days of exposure to 2 mg $\sp.$ L$\sp{-1}$ simazine, hydrilla was not recovered at 200 and 300 $\mu$E $\sp.$ m$\sp{-2\ .}$ s$\sp{-1}$. Both species developed phytotoxic symptoms at the higher simazine dose. Direct measurement of herbicide induced inhibition of photosynthetic oxygen production using a simple oxygen electrode was developed for symmetrical and asymmetrical triazines, substituted ureas, and uracils. Detectable concentrations for terbacil and diuron were 5.9 $\times$ 10$\sp{-8}$M and 6.4 $\times$ 10$\sp{-8}$M. Simazine, atrazine, and metribuzin were detected at 1 $\times$ 10$\sp{-7}$M. Aqueous extracts taken from samples of a clay loam soil treated with a terbacil formulation were planted with soybeans (Glycine max L. var. Century) and oats (Avena sativa L. var. Noble) to compare the sensitivity of the watermilfoil with the growth of oats and soybeans bioassay. Terbacil detected by the watermilfoil bioassay was 0.05 kg terbacil/ha (0.015 mg terbacil relative to a 250 g soil sample). Soybeans nor oats showed injury symptoms. Atrazine was detected at 0.07, 0.56, and 0.14 kg/ha using watermilfoil, soybean, and oat bioassays, respectively. (Abstract shortened with permission of author.)




Ross, Purdue University.

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