Christopher R. Johnston

Seed Priming with Polyethylene Glycol Induces Physiological Changes in Sorghum (Sorghum bicolor L. Moench) Seedlings under Suboptimal Soil Moisture Environments.

Osmopriming with PEG has potential to improve seed germination, seedling emergence, and establishment, especially under stress conditions. This research investigated germination performance, seedling establishment, and effects of osmopriming with PEG on physiology in sorghum seedlings and their association with post-priming stress tolerance under various soil moisture stress conditions. Results showed that seed priming increased the environmental range suitable for sorghum germination and has potential to provide more uniform and synchronous emergence. Physiologically, seed priming strengthened the antioxidant activities of APX, CAT, POD, and SOD, as well as compatible solutes including free amino acid, reducing sugar, proline, soluble sugar, and soluble protein contents. As a result, seed priming reduced lipid peroxidation and stabilized the cell membrane, resulting in increased stress tolerance under drought or excessive soil moisture environments. Overall, results suggested that seed priming with PEG was effective in improving seed germination and seedling establishment of sorghum under adverse soil moisture conditions. Osmopriming effectively strengthened the antioxidant system and increased osmotic adjustment, likely resulting in increased stress tolerance.

Primisulfuron-Methyl Efficacy and Fate in Annual Bluegrass (Poa annua) and Kentucky Bluegrass

Abstract Annual bluegrass is a problematic weed of Kentucky bluegrass turf that can be selectively controlled with POST applications of primisulfuron-methyl. The objective of this research was to evaluate physiological behavior of primisulfuron-methyl attributed to selectivity in these species. In application placement experiments, annual bluegrass shoot weight reductions from the nontreated from high to low were treatments including: foliar + soil ≥ soil only ≥ foliar. Annual bluegrass averaged 33 and 52% shoot weight reductions from the nontreated after 4 wk from primisulfuron-methyl at 40 and 80 g ha−1, respectively. Kentucky bluegrass shoot weight was not reduced from the nontreated, and application placements were similar. From five harvests ranging 1 to 168 h after treatment (HAT), annual and Kentucky bluegrass absorbed up to 25 and 32% of foliar applied 14C-primisulfuron-methyl, respectively. Both grasses distributed 15% of foliar absorbed 14C to nontreated shoots with minimal translocation (≤ 2%) to roots after 168 h. Annual bluegrass translocated 2 times more root-absorbed 14C to shoots than Kentucky bluegrass at 24, 72, and 168 HAT. From foliar uptake, the time required for annual and Kentucky bluegrass to degrade 50% of the absorbed herbicide to the major metabolite detected (Rf 0.1) measured > 168 and 93 h, respectively. In root metabolism experiments, annual bluegrass had ≈ 3 times and 2 times more primisulfuron acid in roots and shoots, respectively, than Kentucky bluegrass at 24, 72, and 168 HAT. The isolated acetolactate synthase (ALS) enzymes from the two grasses were equally susceptible to inhibition by primisulfuron-methyl. Overall, selectivity of primisulfuron-methyl for annual bluegrass control in Kentucky bluegrass is attributed to differential translocation and metabolism between species. Nomenclature: Annual bluegrass (Poa annua L.); Kentucky bluegrass (Poa pratensis L.) ‘Midnight’.

Indaziflam Enhances Buckhorn Plantain (Plantago lanceolata) Control from Postemergence Herbicides

Abstract Buckhorn plantain is a perennial weed in turfgrass and efficacy of POST herbicides is often inconsistent for control in spring. Indaziflam is a cellulose biosynthesis inhibitor used for PRE control of annual weeds in turf and applications have shown to be injurious to established buckhorn plantain. The objectives of this research were to evaluate (1) effects of indaziflam application rate and placement on buckhorn plantain injury; (2) effects of tank-mixing indaziflam with POST herbicides for buckhorn plantain control; and (3) physiological effects of indaziflam on absorption and translocation of 14C-2,4-D in buckhorn plantain. In greenhouse experiments, indaziflam reduced buckhorn plantain shoot mass 61 to 75% from the nontreated at 4 wk after treatment (WAT) and hierarchical rank of application placements were: foliar + soil ≥ soil ≥ foliar. Differences in biomass reduction from application rates (27.5 and 55 g ai ha−1) were not detected. In field experiments, indaziflam at 55 g ha−1 controlled buckhorn plantain 34% at 9 WAT but enhanced the speed of control from all herbicides tested in tank mixtures. Exclusive applications of 2,4-D or 2,4-D + dicamba + MCPP provided poor control (< 70%) of buckhorn plantain at 9 WAT, but tank mixtures with indaziflam provided 81 and 98% control, respectively. Fluroxypyr and simazine alone controlled buckhorn plantain < 38% but tank mixtures with indaziflam enhanced control more than twice as much from exclusive applications. Tank-mixing indaziflam with metsulfuron did not improve control from metsulfuron alone after 9 wk. Bermudagrass injury was not detected from any treatment. In laboratory experiments, 14C-2,4-D absorption and translocation in buckhorn plantain was similar with or without indaziflam tank mixtures at 72 and 168 h after treatment. Overall, indaziflam may improve buckhorn plantain control from POST herbicides by providing additive phytotoxicity in tank mixtures in spring. Nomenclature: Buckhorn plantain, Plantago lanceolata L.; bermudagrass, Cynodon dactylon L. Resumen Plantago lanceolata es una maleza perenne en céspedes y la eficacia de herbicidas POST es comúnmente inconsistente para su control en la primavera. Indaziflam es un inhibidor de la biosíntesis de cellulose usado para el control PRE de malezas anuales en césped y su aplicación ha demostrado ser dañina para plantas establecidas de P. lanceolata. Los objetivos de esta investigación fueron evaluar (1) los efectos de la dosis y lugar de aplicación de indaziflam sobre el daño de P. lanceolata; (2) los efectos de mezclar en tanque indaziflam con herbicidas POST para el control de P. lanceolata; y (3) los efectos fisiológicos de indaziflam en la absorción y translocación de 14C-2,4-D en P. lanceolata. En experimentos de invernadero, indaziflam redujo la masa de tejido aéreo de P. lanceolata 61 a 75% en comparación al testigo no-tratado a 4 semanas después del tratamiento (WAT) y el ranking jerárquico del lugar de aplicación fue: foliar + suelo ≥ suelo ≥ foliar. No se detectaron diferencias en la reducción de biomasa según la dosis de aplicación (27.5 y 44 g ai ha−1). En los experimentos de campo, indaziflam a 55 g ha−1 controló P. lanceolata 34% a 9 WAT, pero mejoró la velocidad de control de todos los herbicidas evaluados en mezclas en tanque. Aplicaciones exclusivas de 2,4-D o 2,4-D + dicamba + MCPP brindaron un control pobre (<70%) de P. lanceolata a 9 WAT, pero las mezclas en tanque con indaziflam brindaron 81 a 98% de control, respectivamente. Fluroxypyr y simazine solos controlaron P. lanceolata <38%, pero las mezclas en tanque con indaziflam mejoraron el control en más del doble en comparación con las aplicaciones solas. El mezclar en tanque indaziflam con metsulfuron no mejoró el control en comparación con metsulfuron solo a 9 WAT. No se detectó daño en Cynodon dactylon con ninguno de los tratamientos. En los experimentos de laboratorio, la absorción y translocación de 14C-2,4-D en P. lanceolata fue similar con o sin mezclas en tanque con indaziflam a 72 y 168 h después del tratamiento. En general, indaziflam mejoró el control de P. lanceolata con herbicidas POST al brindar una fitotoxicidad aditiva en las mezclas en tanque en la primavera.

Biochemical Effects of Imazapic on Bermudagrass Growth Regulation, Broomsedge (Andropogon virginicus) Control, and MSMA Antagonism

Abstract Broomsedge populations have increased substantially over the last decade on roadsides in Georgia. The invasiveness of this species might have resulted from imazapic use for bermudagrass growth regulation and the limited use of MSMA on roadsides. The objectives of this research were to evaluate (1) differential growth inhibition of bermudagrass and broomsedge to imazapic, (2) susceptibility of isolated acetolactate synthase (ALS) enzymes of bermudagrass and broomsedge to imazapic, (3) broomsedge control with tank mixtures of imazapic with MSMA, and (4) the influence of imazapic on absorption and translocation of 14C-MSMA. In greenhouse experiments, imazapic reduced bermudagrass shoot biomass ~ 2 times more from the nontreated than broomsedge. Isolated ALS enzymes of bermudagrass were ~ 100 times more susceptible to inhibition by imazapic than broomsedge. In field experiments, imazapic provided no control of broomsedge, but MSMA alone controlled broomsedge 81% at 12 mo after initial treatments (MAIT). Broomsedge control was reduced to 45% when MSMA was tank mixed with imazapic at 12 MAIT. In laboratory experiments, imazapic tank mixtures did not reduce broomsedge absorption or translocation of 14C-MSMA. Overall, bermudagrass is more susceptible to imazapic due to greater target-site inhibition than broomsedge. Results emphasize the importance of MSMA use for broomsedge control, but agronomists should avoid tank mixtures with imazapic to reduce potential antagonism. Nomenclature: Bermudagrass, Cynodon dactylon (L.) Pers × C. transvaalensis Burtt-Davy ‘Princess-77’, broomsedge, Andropogon virginicus L.

ALS-Resistant Spotted Spurge (Chamaesyce maculata) Confirmed in Georgia

Abstract Metsulfuron is used for POST control of spotted spurge in many warm-season turfgrasses. A suspected resistant (R) biotype of spotted spurge was collected from turfgrass in Georgia with a history of exclusive metsulfuron use. Research was conducted to evaluate the resistance level of this biotype to metsulfuron, efficacy of other mechanisms of action for control, and the molecular basis for resistance. Compared with a susceptible (S) biotype, the R biotype required >90 and >135 times greater metsulfuron rates to reach 50% injury and reduce biomass 50% from the nontreated, respectively. The R biotype was also resistant to trifloxysulfuron but was injured equivalent to the S biotype from dicamba, glyphosate, and triclopyr. Gene sequencing of the R biotype revealed a Trp574 to Leu substitution that has conferred resistance to acetolactate synthase (ALS) inhibitors in previous research. This is the first report of ALS resistance in spotted spurge. More importantly, this is the first report of a herbicide-resistant broadleaf weed from a turfgrass system in the United States. Nomenclature: Metsulfuron-methyl; spotted spurge, Chamaesyce maculata (L.) Small.

Time of Application Influences Translocation of Auxinic Herbicides in Palmer Amaranth (Amaranthus palmeri)

The efficacy of WSSA Group 4 herbicides has been reported to vary with dependence on the time of day the application is made, which may affect the value of this mechanism of action as a control option and resistance management tool for Palmer amaranth. The objectives of this research were to evaluate the effect of time of day for application on 2,4-D and dicamba translocation and whether or not altering translocation affected any existing variation in phytotoxicity seen across application time of day. Maximum translocation (Tmax) of [14C]2,4-D and [14C]dicamba out of the treated leaf was significantly increased 52% and 29% to 34% in one of two repeated experiments for each herbicide, respectively, with application at 7:00 AM compared with applications at 2:00 PM and/or 12:00 AM. Applications at 7:00 AM increased [14C]2,4-D distribution to roots and increased [14C]dicamba distribution above the treated leaf compared with other application timings. In phytotoxicity experiments, dicamba application at 8 h after exposure to darkness (HAED) resulted in significantly lower dry root biomass than dicamba application at 8 h after exposure to light (HAEL). Contrasts indicated that injury resulting from dicamba application at 8 HAEL, corresponding to midday, was significantly reduced with a root treatment of 5-[N-(3,4-dimethoxyphenylethyl)methylamino]-2-(3,4-dimethoxyphenyl)-2-isopropylvaleronitrile hydrochloride (verapamil) compared with injury observed with dicamba application and a root treatment of verapamil at 8 HAED, which corresponded to dawn. Overall, time of application appears to potentially influence translocation of 2,4-D and dicamba. Furthermore, inhibition of translocation appears to somewhat influence variation in phytotoxicity across times of application. Therefore, translocation may be involved in the varying efficacy of WSSA Group 4 herbicides due to application time of day, which has implications for the use of this mechanism of action for effective control and resistance management of Palmer amaranth. Nomenclature: 2,4-D; 5-[N-(3,4-dimethoxyphenylethyl)methylamino]-2-(3,4-dimethoxyphenyl)- 2-isopropylvaleronitrile hydrochloride; dicamba; Palmer amaranth, Amaranthus palmeri S. Wats.