Comparison of Photosynthetic Performance in Triazine-Resistant and Susceptible Biotypes of Amaranthus hybridus

Abstract

The rate of CO2 reduction in the S-triazine-resistant biotype of smooth pigweed (Amaranthus hybridus L.) was lower at all levels of irradiance than the rate of CO2 reduction in the susceptible biotype. The intent of this study was to determine whether or not the lower rates of CO2 reduction are a direct consequence of the same factors which confer triazine resistance. The quantum yield of CO2 reduction was 23 ± 2% lower in the resistant biotype of pigweed and the resistant biotype of pigweed had about 25% fewer active photosystem II centers on both a chlorophylH and leaf area basis. This quantum inefficiency of the resistant biotype can be accounted for by a decrease in the equilibrium constant between the primary and secondary quinone acceptors of the photosystem II reaction centers which in turn would lead to a higher average level of reduced primary quinone acceptor in the resistant biotype. Thus, the photosystem II quantum inefficiency of the resistant biotype appears to be a direct consequence of those factors responsible for triazine resistance but a caveat to this conclusion is discussed. The effects of the quantum inefficiency of photosystem II on CO2 reduction should be overcome at high light and therefore cannot account for the lower light-saturated rate of CO2 reduction in the resistant biotype. Chloroplast lameliar membranes isolated from both triazine-resistant and triazine-susceptible pigweed support equivalent rates of whole chain electron transfer and these rates are sufficient to account for the rate of light-saturated CO2 reduction. This observation shows that the slower transfer of electrons from the primary to the secondary quinone acceptor of photosystem II, a trait which is characteristic of the resistant biotype, is nevertheless still more rapid than subsequent reactions of photosynthetic CO2 reduction. Thus, it appears that the lower rate of light-saturated CO2 reduction of the resistant biotype is not limited by electron transfer capacity and therefore is not a direct consequence of those factors which confer triazine resistance. As many as one-half of all commercially available herbicides used in agriculture act by interfering with photosynthetic electron transfer reactions. The margin of selectivity of many of these herbicides between the crop and unwanted plant species is disap-pointingly low, although the metabolic detoxification of triazines by corn is an exception of immense economic importance. In recent years, dramatically lower sensitivities of photosynthesis to S-triazine herbicides have appeared in populations of numerous ' weed species growing on agricultural lands (for review, see 16). Investigations into the biochemical basis for the lower sensitivity of these weeds to triazine herbicides have clearly established that the mode of resistance is at the level of the interaction of the herbicide with the photosynthetic electron transport chain (16). Maternal inheritance (9, 12) of triazine resistance indicates control by a chloroplast rather than a nuclear gene(s). The implication to agriculture of herbicide selectivity based on a seemingly minor alteration in an intrinsic chloroplast membrane protein is truly exciting. The emerging technology of molecular genetics makes plausible the prospect of designed alterations in the chloroplast genome of crop plants to establish desired herbicide resistance. Unfortunately, the weed biotypes resistant to triazine herbicides often display other characteristics absent from their triazine-susceptible counterparts, characteristics which are decidedly not advantageous. These resistant biotypes are reported to be competitively less successful (1, 7, 24) with a rate of light-and C02-saturated photosynthesis which is significantly depressed (1, 20). Arntzen and colleagues (16, 17) demonstrated that herbicide resistance was manifested by a markedly decreased binding of the inhibitor molecule to a 32 to 34 kD intrinsic membrane protein of the PSII reaction center complex. Subsequently, Bowes et al. (5) demonstrated that electron transfer from the primary quinone acceptor (QA3) of the PSII reaction center to the secondary quinone acceptor (QB) was as much as 10-fold slower in chloroplasts from triazine-resistant pigweed. These data are consistent with the notion that the 32 to 34 kD protein is the apoprotein of QB (3). It might be thought, based on these and related observations, that the lower rate of CO2 fixation observed in resistant biotypes is a consequence of the increase in QA-* QB electron transfer time and consequently a trait inseparable from the trait of herbicide resistance. If so, the impact on agriculture of engineering a tria-zine-resistant crop employing this mode of resistance would be significantly diminished. In this paper, we report on an investigation of photosynthesis in triazine-susceptible and triazine-resistant biotypes of Amaranthus hybridus. Based on the measurements of the quantum yield of CO2 reduction by attached leaves and the flash-induced turnover of PSII in isolated chloroplasts, we conclude that (a) the resistant biotype has about 25% of its PSII reaction centers in a photochem-ically inactive state and (b) that this quantum inefficiency can be 3 Abbreviations used: QA, primary quinone acceptor of PSII: QB, secondary quinone acceptor of PSII; PQ, plastoquinone/