The effects of hexazinone [3-cyclohexyl-6-(dimethylamino)-1-methyl-1,3,5-triazine-2,4 (1H, 3H)-dione] and its metabolites, A [3-(4-hydroxycyclohexyl)-6-(dimethylamino)-1-methyl-1,3,5-triazine-2,4 (1H,3H)-dione], B [3-cyclohexyl-6-(methylamino)-1-methyl-1,3,5-triazine-2,4 (1H,3H)-dione], C [3-(4-hydroxycyclohexyl)-6-(methylamino)-1-methyl-1,3,5-triazine-2,4 (1H,3H)-dione], D [3-cyclohexyl-1-methyl-1,3,5-triazine-2,4,6 (1H, 3H,5H)-trione], and E [3-(4-hydroxycy clohexyl)-1-methyl-1,3,5-triazine-2,4,6 (1H,3H,5H)-trione], on photosynthesis of loblolly pine primary needle segments were studied using a bioassay technique. Hexazinone, at concentrations equal to or greater than 1.0 × 10-6 M, inhibited photosynthesis, while 1.0 × 10-8 M and 1.0 × 10-7 M concentrations increased photosynthesis. Metabolite B at 1.0 × 10-4 M also inhibited photosynthesis. Metabolites A, C, D, and E did not affect photosynthesis at 1.0 × 10-4 in pine needle segments. The bioassay proved simple and useful for determining effects of certain herbicides and metabolites on photosynthesis.

The influence of metribuzin [4-ammo-6-tert-butyl-3-(methylthio)-as-triazin-5(4H)-one] on net photosynthesis of goosegrass [Eleusine indica (L.) Gaertn. ♯3 ELEIN] and six cultivars of bermudagrass [Cynodon dactylon (L.) Pers. ♯ CYNDA cvs. ‘Common’, ‘Tufcote’, ‘Vamont’, and ‘Midiron’, Cynodon transvaalensis Davy., and Cynodon dactylon X Cynodon transvaalensis ‘Tifway’] was evaluated. Metribuzin rapidly inhibited carbon dioxide (CO2) uptake within 1 h after application in both goosegeass and bermudagrasses. Very often, net photosynthesis was completely inhibited. Net photosynthesis in goosegrass was inhibited by lower rates of metribuzin than those required for similar reduction in bermudagrasses. Common, Tufcote, Tifway, and C. transvaalensis bermudagrasses were less sensitive to metribuzin than Midiron and Vamont based upon the degree of inhibition of net photosynthesis, the rate of its recovery, and the level of foliar injury. Recovery from inhibition of net photosynthesis was observed in both species 24 h after treatment with low metribuzin rates. High rates caused several days lag period before net photosynthesis resumed. After 10 days, 95% of the goosegrass foliage was injured (either chlorosis or necrosis) from 2 kg ai/ha metribuzin treatment. The threshold concentration for inducing leaf injury in bermudagrasses was 1 kg ai/ha. Inhibition of photosynthesis was initially rapid in both grasses; however, recovery was evident earlier and there was less foliar injury in bermudagrasses than in goosegrass. Selective control of goosegrass in bermudagrass may therefore be partially dependent upon rapid metabolism and/or inactivation of metribuzin in the bermudagrass.