Studies were conducted that supported the hypothesis that the mutation to the psbA plastid gene that confers S-triazine resistance (R) in Brassica napus also results in an altered diurnal pattern of photosynthetic carbon assimilation (A) relative to that of the susceptible (S) wild type, and that these patterns change over the ontogeny of a plant. Photosynthetic photon flux density, under closely controlled environmental conditions, was incrementally increased and decreased on either side of the midday maxima of 1150 to 1300 ;tmol quanta m-2 s-'. In all experiments, A approximately tracked the increasing and decreasing diurnal light levels. Younger (3-to 4-leaf) R plants had greater photosynthetic rates early and late in the diurnal light period, whereas those of S plants were greater during midday as well as during the photoperiod as a whole. These relative photosynthetic characteristics of R and S plants changed in several ways with ontogeny. As the plants aged during the vegetative phase of development, S plants gradually assimilated more carbon in the early, and then in the late, part of the day. At the end of the vegetative phase of development, R plant carbon assimilation was less relative to S plants at most times of the day, and was never greater. This relationship between the two biotypes dramatically changed with the onset of the reproductive phase (81/2 to 91/2 leaf) of plant development: R plants assimilated more carbon than S plants during all periods of the diurnal light period with the exception of the late part of the day. In addition to these differences in A, R plant stomatal function differed from that in S plants. R plant leaves were always cooler than S plant leaves under the same environmental and diurnal conditions. Correlated with this difference in leaf temperature were equal or greater total conductances to water vapor and intercellular CO2 partial pressures in R compared to S leaves in most instances. These studies indicate a more complex pattern of photosynthetic carbon assimilation than previously observed. The photosynthetic superiority of one biotype relative to the other was a function of the time of day and the age of the plant. These thus, the codon 264 change in the psbA gene caused a change in its product, the Dl protein, a key functional element in PSII electron transport (28). R2 plants have a decreased quantum efficiency of CO2 assimilation compared to S plants (15). This decreased efficiency is due to the altered redox state of PSII quinone acceptors and a shift in the equilibrium constant between QA-and QB in favor of QA-(2). What is less clear in the literature is whether this change in Dl structure and electron transport function directly modifies whole-leaf photosynthesis and plant productivity or only indirectly influences these functions (14).The genetic change in R plants leads to a profound reorganization of functional units in the chloroplast. This pleiotropic cascade includes both structural (25) and functional changes (2). Many of these changes in R ar...

A fluorometric method to determine the kinetics of the uptake and metabolism of a p-halohydroxybenzonitrile herbicide has been developed using variable chlorophyll a fluorescence.Bromoxynil inhibits photosynthetic electron transport and results in large variable chlorophyll -a fluorescence yields.Lethal and sub-lethal doses of bromoxynil were applied to two maize genotypes. Measurement of variable chlorophyll a fluorescence over several hours revealed the uptake and detoxification of bromoxynil by the appearance and 2077 2078 DEKKER AND BURMESTER disappearance of inhibited fluorescence transients. A difference in bromoxynil susceptibility was observed between inbreds B73 and OH43 in these preliminary studies. Also observed was a difference in susceptibility between the oldest and second oldest leaves of these maize seedlings. Variable chlorophyll5 fluorescence has the potential t o be a rapid, non-daetructive analytical method for absetvfqg the b2 metabolism of herbicides in plants.

Suspected triazine resistant Pennsylvania smartweed plants were discovered near Holy Cross, Dubuque County, IA. Subsequent variable chlorophyllafluorescence assays confirmed these selections to be triazine resistant. This is the first reported evidence ofs-triazine resistance in Pennsylvania smartweed.

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