Protoporphyrinogen oxidase (PPO)-inhibiting herbicides remain an important and useful chemistry 60 yr after their first introduction. In this review, based on topics introduced at the Weed Science Society of America 2021 symposium titled “A History, Overview, and Plan of Action on PPO Inhibiting Herbicides,” we discuss the current state of PPO-inhibiting herbicides. Renewed interest in the PPO-inhibiting herbicides in recent years, due to increased use and increased cases of resistance, has led to refinements in knowledge regarding the mechanism of action of PPO inhibitors. Herein we discuss the importance of the two isoforms of PPO in plants, compile a current knowledge of target-site resistance mechanisms, examine non–target site resistance cases, and review crop selectivity mechanisms. Consistent and reproducible greenhouse screening and target-site mutation assays are necessary to effectively study and compare PPO-inhibitor resistance cases. To this end, we cover best practices in screening to accurately identify resistance ratios and properly interpret common screens for point mutations. The future of effective and sustainable PPO-inhibitor use relies on development of new chemistries that maintain activity on resistant biotypes and the promotion of responsible stewardship of PPO inhibitors both new and old. We present the biorational design of the new PPO inhibitor trifludimoxazin to highlight the future of PPO-inhibitor development and discuss the elements of sustainable weed control programs using PPO inhibitors, as well as how responsible stewardship can be incentivized. The sustained use of PPO inhibitors in future agriculture relies on the effective and timely communication from mode of action and resistance research to agronomists, Extension workers, and farmers.

Imazethapyr at 0.05 to 0.14 kg ai/ha applied preplant incorporated, preemergence, and postemergence was evaluated alone and with complementary herbicides in the field for weed control in soybean. Imazethapyr controlled 90% or more smooth pigweed regardless of application method or herbicide rate. Imazethapyr at 0.05 kg/ha controlled jimsonweed 30% better postemergence compared to soil applications. Imazethapyr at 0.10 kg/ha controlled 90% or more velvetleaf regardless of application method. The addition of alachlor to soil-applied imazethapyr enhanced giant foxtail, jimsonweed, and velvetleaf control. Adding acifluorfen or bentazon to postemergence imazethapyr antagonized weed control. Adding sethoxydim to postemergence imazethapyr was not beneficial.

The herbicide clomazone caused ultrastructural damage to etioplasts of cotton cotyledons. Etioplast envelopes were irregular, prothylakoids were absent or irregular, stroma density was low, and there were abnormal stromal vesicles. Further damage occurred upon exposure to light. Clomazone greatly slowed the conversion of chlorophyllide to chlorophyll in cotton, suggesting that phytol synthesis was affected. Neither synthesis of protochlorophyllide nor phototransformation of protochlorophyllide to chlorophyllide was affected by clomazone. Clomazone completely inhibited carotenoid synthesis without an accumulation of phytoene. However, the sesquiterpenoids hemigossypol and the dimeric sesquiterpenoid gossypol accumulated in greater amounts in clomazone-treated than in control tissues. These results indicate that this herbicide inhibits synthesis of terpenoids after the formation of farnesyl pyrophosphate.

Glyphosate is the world's most widely used herbicide. It is nonselective and has been used to control a broad range of weed species for the past 20 yr, without the appearance of resistant weed biotypes. However, a biotype of Lolium rigidum from a field in Northern Victoria, Australia, in which glyphosate had been used for the past 15 yr, failed to be controlled by label recommended rates. Based on LD50 values from pot dose-response experiments, this biotype exhibited resistance to glyphosate and was nearly 10-fold more resistant compared to the susceptible biotypes tested. The biotype was resistant to three different salts of glyphosate. The biotype was also nearly threefold more resistant to diclofop-methyl but was susceptible to other commonly used selective and broad-spectrum herbicides. Between the two-leaf and tillering stages of development, a susceptible biotype exhibited a small but significant decrease in tolerance to glyphosate, whereas tolerance of the resistant biotype remained unchanged with age. The resistant phenotype was verified in experiments in which seed was germinated in the presence of glyphosate. Observations on shoot and root growth of seedlings in these experiments suggested that the resistance mechanism might be associated more with the shoot than with the root.

The extent of 14C-imazaquin and 14C-imazethapyr abiotic vs. biotic degradation in soil was investigated. Degradation was measured in an in vitro system which allowed 90% recovery of applied herbicide. Triallate biodegradation is well documented and therefore used as a standard. Herbicide degradation was compared in two soils, a Cisne silt loam and a Drummer silty clay loam. Herbicide degradation in gamma-irradiated soil was compared to fresh soil. Biomass quantities were measured for the duration of the experiments. 14CO2 evolution, extractable parent, metabolites, and unextractable residue were measured. After 12 weeks of incubation, 95% of the radioactivity could be extracted as parent from sterilized soil. In unsterilized soil, imazaquin and imazethapyr degraded at a similar rate which was dependent upon soil type. All herbicides degraded slower in the Drummer soil and triallate degraded two to three times faster than the imidazolinones in either soil. 14C-imazaquin degradation products included 14CO2 and unextractable residues. The major product from 14C-imazethapyr degradation was 14CO2. Evolution of 14CO2 from an imazethapyr-treated Cisne soil, containing a serial dilution of activated charcoal, demonstrated that adsorption of herbicide was negatively correlated with degradation. Therefore imidazolinone microbial degradation is regulated by the amount of herbicide in soil solution as determined by soil characteristics.

Oxadiazon (10 μM) caused rapid, light-dependent membrane damage to cucumber cotyledon discs. Electrolyte leakage was detected within 1 h of exposure to light, as were cytoplasmic vesiculation and breakage of the tonoplast and plasmalemma. The ultrastructure of chloroplasts was not affected until the cytoplasm was dispersed. Photosynthetic inhibitors had no effect on activity and, after a period of dark incubation with oxadiazon, there was little effect of temperature on the light-caused membrane destruction. Porphyrin synthesis inhibitors (gabaculine and 4,6-dioxoheptanoic acid) almost completely prevented the herbicidal activity of oxadiazon. Oxadiazon treatment caused accumulation of protoporphyrin IX, a photodynamic pigment. Oxadiazon caused physiological effects on cucumber cotyledons that were virtually identical to those of p-nitro-diphenyl ether herbicides like acifluorfen and its methyl ester, which have recently been shown to also cause protoporphyrin IX accumulation.

When the United Nations Organization was set up in 1945, one of its first intellectual tasks was to define the problem of racism which had been one of the major factors in Hitler's rise to power and the subsequent devastation of Europe. Hence UNESCO asked a group of biologists and social scientists to draw up a statement in 1949. The original statement and a further version in 1951 proved, however, to be ambiguous and unsatisfactory in certain respects and the Director-General of UNESCO therefore called upon biologists to meet separately in Moscow in 1964 and for their meeting to be followed by a meeting of biologists, social scientists and lawyers which would then address itself to the specific sociological problem of racism.

It is well known that radiocarbon years do not directly equate to calendar time. As a result, considerable effort has been devoted to generating a decadally resolved calibration curve for the Holocene and latter part of the last termination. A calibration curve that can be unambiguously attributed to changes in atmospheric 14C content has not, however, been generated beyond 26 kyr cal BP, despite the urgent need to rigorously test climatic, environmental, and archaeological models. Here, we discuss the potential of New Zealand kauri (Agathis australis) to define the structure of the 14C calibration curve using annually resolved tree rings and thereby provide an absolute measure of atmospheric 14C. We report bidecadally sampled 14C measurements obtained from a floating 1050-yr chronology, demonstrating repeatable 14C measurements near the present limits of the dating method. The results indicate that considerable scope exists for a high-resolution 14C calibration curve back through OIS-3 using subfossil wood from this source.