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Inhibition and subsequent recovery of net CO2exchange (NCE) by single leaves of 53 grass species of the subfamilies Festucoideae, Panicoideae, and Eragrostoideae was monitored following limited root uptake of atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine]. Rates of NCE recovery were correlated with rates of14C-atrazine metabolism in leaf sections of festucoid and panicoid grasses but not eragrostoid grasses. Rates of NCE recovery among the nine festucoid and 12 eragrostoid species tested did not exceed 0.2 and 1.1 mg CO2per dm2per hr/hr, respectively, whereas for the 31 panicoid species, it ranged from 0.2 to 2.4 mg CO2per dm2per hr/hr. Selected species exhibiting NCE recovery rates exceeding 1.2 mg CO2per dm2per hr/hr were tolerant to 1.0 kg/ha preemergence and 1.25 kg/ha postemergence atrazine applications. These panicoid species included a number of common weed species belonging to the generaDigitaria, Panicum, andSetaria, as well as species belonging toBracharia, Pennisetum, Sorghum, andZea.Unchanged14C-atrazine accounted for 11.3 to 92.7% of the total14C-radioactivity extracted from14C-atrazine infiltrated leaf sections following an 8-hr incubation period. N-dealkylation, hydroxylation, and peptide conjugation occurred in all species, although rates of these metabolic pathways varied widely among species. Recovery of NCE was correlated with formation on the glutathione-atrazine conjugate, and conjugation was the major detoxification pathway in species exhibiting tolerance to atrazine.

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75: 923-926. Photosensitization of livestock in Nova Scotia has often been attributed to St. John's wort (Hypericum perforatum L.), even though the weed generally occurs at low densities. This study's objective was to determine whether levels of hypericin, the photodynamic toxin causing photosensitization, were unusually high in Nova Scotia biotypes and whether levels increased in late spring, when photosensitization most commonly occurs. The mean hypericin content in vegetative growth of I I Nova Scotia 1L perforatum biotypes was 195 pg g-l dry wt, whereas levels in biotypes from British Columbia and Australia were at least two-and ttneefold higher, respectiveiy. HvetJ in field-collected plants were lowest in May (60 lrg g-l dry wt) and peaked in midsummer (160-280 pg g-l dry wt). In a controlled-environment study, hypericin levels increased linearly with increasing temperatures. Furthermore, hypericin levels were not affected by infection by Colletotrichum gloeosporioi'des.Thtts, the hypericin content of Nova Scotia biotypes is too low to be the cause of photosensitization in livestock.