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 disappointingly 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 implic...

Pleiotropy in Triazine-Resistant Brassica napus

“…The dynamic nature of these responses has led several to conclude that the primary effect of R is complex, involves more than one aspect of photosynthesis, and can be mitigated by other processes in the system (17,23). For example, it has been pointed out that decreased QA to QB electron transport in R plants is more rapid than the normally rate-limiting oxidation of plastoquinol (4,24), whereas other studies indicate that this step may be rate limiting (20).…”

“…Several studies have shown lower A rates in R Amaranthus hybridus (1) and Senecio vulgaris (15,24) relative to S plants. Beversdorf et al (6) found lower R plant whole plant yields in field evaluations of Brassica napus.…”

“…The dynamic nature of these responses has led several to conclude that the primary effect of R is complex, involves more than one aspect of photosynthesis, and can be mitigated by other processes in the system (17, 23). For example, it has been pointed out that decreased QA to QB electron transport in R plants is more rapid than the normally rate-limiting oxidation of plastoquinol (4, 24), whereas other studies indicate that this step may be rate limiting (20).Dekker and Sharkey (9) have shown that the primary limitation to photosynthesis changes with changes in leaf temperature, and that electron transport limitations in R plants may be significant only at higher temperatures.Several studies have shown lower A rates in R Amaranthus hybridus (1) and Senecio vulgaris (15,24) …”

Triazine Resistance in Senecio vulgaris Parental and Nearly Isonuclear Backcrossed Biotypes Is Correlated with Reduced Productivity

“…The photosynthetic rate and biomass accumulation of greenhouse grown susceptible and resistant S. vulgaris biotypes were measured 28, 35, 42, 50, 57, and (17). PSII efficiency in the use of separated charge for oxygen evolution and electron transport is lower in resistant biotypes (12, 17) which explains the lower quantum yields characteristic of triazine resistant biotypes (5,13,15,17,19).As expected from the reduced photosynthetic potential of triazine resistant biotypes, several studies have shown that in the absence of triazine herbicides, resistant biotypes produce less biomass than susceptible biotypes. In Amaranthus spp., B. napus, Chenopodium album, and Senecio vulgaris, resistant biotypes accumulated less biomass and produced less seed than susceptible biotypes in competitive and noncompetitive conditons (7,8,11,29).…”

Similar Photosynthetic Performance in Low Light-Grown Isonuclear Triazine-Resistant and -Susceptible Brassica napus L

“…However, the binding site alteration also slows by 10-fold the normal electron transfer between Qa and Qb (5). Slow electron flow between Qa and Qb has been suggested as the cause of the reduction in photon yield observed in resistant plants (14,15,19), and has been cited as the ultimate cause of decreased maximum photosynthesis in resistant plants ( 16).…”