Veinal chlorosis was induced in the lower leaves of young maize plants following treatment with atrazine (2-chloro-4-ethylamino-6- isopropylamino-s-triazine). An ultrastructural investigation indicated that mesophyll chloroplasts, consisting of grana, thylakoids and stroma showed severe disintegration. The seemingly unaffected bundle sheath chloroplasts with stroma, thylakoids and sometimes a few rudimentary grana which normally only show PS I activity, appeared to transform to mesophyll-type chloroplasts with formation of small granai stacks. The mesophyll chloroplasts became more spherical and various ultrastructural changes became visible. The thylakoid system was destroyed, the granai compartments became swollen and the double-layered membrane broke down, leading to disintegration of the chloroplast. Atrazine had little effect on the bundle sheath chloroplasts except in a few cases where the chloroplast membrane exhibited swellings. At rates of 6 ppm atrazine, or prolonged treatment with 3 ppm atrazine, bundle sheath chloroplasts were devoid of starch granules.
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