Pleiotropy in Triazine-Resistant Brassica napus

Abstract

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...

Environmental effects on the fitness of triazine‐resistant and triazine‐susceptible Brassica rapa and Chenopodium album in the absence of herbicide

“…It also changes the pattern of photosynthetic rate diurnally, and ontogenetically, so that R Brassica napus shows relatively high levels of photosynthesis at low light flux, at the reproductive phase, compared with S plants (Dekker & Burmester, 1992). This study also showed that R plants tended to have more efficient stomatal conductance than S plants, and, in general, R tended to be fitter than S in cool, low-light environments.…”

“…Moreover, Plowman & Richards (1997) rather unexpectedly showed that R B. rapa can outcompete S plants in some constant environments. Using the related B. napus, Dekker & Burmester (1992) showed that photosynthetically, R plants were relatively efficient at low light levels when flowering, which might in part explain these findings. However, by contrast in Amaranthus hybridus (Jordan, 1996) and Datura stramonium (Williams & Jordan, 1995) fitness penalties of triazine resistance tend to be greater in field conditions and competitive environments than in constant environment experiments.…”

“…Although R plants might have persisted more in south-facing treatments, numbers were so low that these differences were not significant. Thus, despite the relatively high photosynthetic ability of R B. napus at low light (Dekker & Burmester, 1992), there was no indication that R B. rapa outperformed S plants in low-light treatments. The different R frequencies in generations 2 and 4 were attributed to sampling effects in these small second generations of the year, and to possible selection for early maturing characteristics.…”

Herbicide-Resistant Field Crops

“…Several studies have demonstrated lower carbon assimilation efficiency, or lower productivity, in the resistant mutant compared to the wild type Beversdorf et a/., 1988;Conard and Radosevich, 1979;Holt, 1990;Holt et al, 1981;McClosky andHolt, 1989, 1991;Ort et al, 1983;Warwick, 199 1). Other studies have shown that the photosynthetic activity in these two may be similar Van Oorschot and van Leeuwen, 1984) or have found it greater in the resistant mutant relative than in the wild type under some environmental conditions (Dekker, 1993;Dekker and Burmester, 1992;Dekker and Sharkey, 1992).…”