Mark A. Czarnota

Evaluation of root exudates of seven sorghum accessions

Seven sorghum accessions were evaluated quantitatively and qualitatively for the composition of their root exudates. Utilizing a unique capillary mat growing system, root exudates were collected from all sorghum accessions. Exudates were subjected to TLC and HPLC analysis to evaluate their chemical composition. Within each sorghum accession, variation existed in the amount of exudate produced and the chemical constituents of each exudate. Sorgoleone was the predominant constituent identified in each accessions exudate. Other closely related compounds, including 5-ethoxysorgoleone, 2,5-dimethoxysorgoleone, three other minor components (MW = 364, 388, and 402), and one unidentified component comprised the minor constituents of the root exudate. Our past work has shown that several of these compounds have potent phytotoxic activity as photosystem II inhibitors, thereby lending further support to the concept that Sorghum spp. are allelopathic and weed suppressive.

ANATOMY OF SORGOLEONE-SECRETING ROOT HAIRS OF SORGHUM SPECIES

Johnsongrass (Sorghum halepense [L.] Pers.) and SX‐17 (Sorghum bicolor × Sorghum sudanese) were investigated microscopically to identify specifically the location of root exudate production. Light, cryoscanning electron, and transmission electron microscopy were used to determine the area of exudate secretion. Light micrographs indicated that the exudate is solely produced by the root hairs. Scanning electron microscopy supported this conclusion. Transmission electron microscopy studies of root hairs support the hypothesis that root exudates are manufactured in the cytoplasmically dense root hair cell in association with smooth endoplasmic reticulum and possibly Golgi bodies. Ultrastructure studies indicated that small globules of cytoplasmic exudate are deposited between the cell wall and the plasma membrane, where they coalesce into larger globules that pass through the cell wall to form droplets near the tip of root hairs.

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’.

ALS–Resistant Annual Sedge (Cyperus compressus) Confirmed in Turfgrass

Abstract Acetolactate synthase (ALS) inhibitors are widely used for POST control of sedges in turfgrass. A suspected resistant (R) biotype of annual sedge was collected from a bermudagrass turf in Georgia with a history of exclusive use of halosulfuron. Research was conducted to evaluate the resistance level of this biotype to halosulfuron, efficacy of ALS-inhibiting herbicides and other mechanisms of action for control, and the molecular and physiological basis for resistance. In greenhouse experiments, the halosulfuron rate required to reduce shoot biomass 50% in comparison with the nontreated at 8 wk after treatment (WAT) were 8 and > 1,120 g ai ha−1 for the S (susceptible) and R biotypes, respectively. Imazapic, sulfosulfuron, and trifloxysulfuron reduced biomass of the S biotype greater than 60% at 8 WAT, but biomass was reduced less than 20% for the R biotype. Glufosinate, glyphosate, MSMA, and sulfentrazone reduced shoot biomass of the R biotype by 93, 86, 97, and 45%, respectively. In laboratory experiments, the halosulfuron concentration required to inhibit ALS activity by 50% in excised leaf tissues was 5.8 and > 1,000 &mgr;M for the S and R biotypes, respectively. Gene sequencing of the R biotype revealed a Pro-197-Ser substitution that confers resistance to ALS inhibitors. This is the first report of ALS-inhibitor resistance in annual sedge and herbicide resistance in a sedge species from a turfgrass system. Nomenclature: Glufosinate; glyphosate; halosulfuron; imazapic; MSMA; sulfentrazone; sulfosulfuron; trifloxysulfuron; annual sedge, Cyperus compressus L.; bermudagrass, Cynodon dactylon L. (Pers.) × Cynodon transvaalensis Burtt-Davy.

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.

First Report of Acetyl-CoA Carboxylase—Resistant Southern Crabgrass (Digitaria ciliaris) in the United States

Two populations of southern crabgrass identified in Georgia turfgrass were suspected to be resistant to sethoxydim. The objectives of this research were to evaluate the resistance levels of these biotypes to acetyl-CoA carboxylase (ACCase) inhibitors and alternative herbicides for control. From dose response experiments, the sethoxydim rate required to reduce shoot biomass 50% from the nontreated measured >64-times greater for both resistant (R) biotypes compared to the susceptible (S) biotype. Both R biotypes were cross-resistant to fenoxaprop and fluazifop. Clethodim at 290 g ai ha-1 reduced dry shoot biomass of the R biotypes and the S biotype to 47 and 21% of the nontreated, respectively. The R biotypes were equally susceptible to MSMA at 2240 g ai ha-1, quinclorac at 840 g ai ha-1, and topramezone at 37 g ai ha-1 as compared to the S biotype. Sethoxydim at 315 and 945 g ha-1 provided <20% control of the southern crabgrass populations in four field experiments. However, clethodim and topramezone provided 83% and 76% control at 4 wks after treatment, respectively. These southern crabgrass biotypes are resistant to sethoxydim and aryloxyphenoxypropionate herbicides. Although the R biotypes were less susceptible to clethodim than the S biotype, treatments provided acceptable control in the field. This is the first report of ACCase-resistant southern crabgrass in the United States. Nomenclature: Acetyl-CoA carboxylase; clethodim; fenoxaprop; fluazifop; sethoxydim; southern crabgrass, Digitaria ciliaris (Retz.) Koel.

First Report of Pronamide-Resistant Annual Bluegrass (Poa annua)

A biotype of annual bluegrass with suspected resistance to pronamide was collected from a golf course in Georgia. The objectives of this research were to determine the level of resistance to pronamide and the mechanisms associated with resistance. From POST applications, the pronamide rate that reduced shoot biomass 50% from the nontreated bluegrass measured >10 times higher for the resistant (R) biotype as compared with susceptible (S) biotypes. The R biotype was not controlled by PRE applications of dithiopyr or prodiamine, but was controlled >92% by PRE applications of pronamide at 0.56 and 1.68 kg ha-1. Mature plants (3- to 5-tiller) of the R biotype absorbed 32% less [14C]pronamide than the S biotype after 72 h in hydroponic culture and accumulated 39% less radioactivity per gram basis of dry shoot mass. The R biotype metabolized [14C]pronamide similar to the S biotype, averaging 16% of the extracted radioactivity. The resistance to POST pronamide applications in the R biotype is associated with reduced absorption and translocation compared with the S biotype. Nomenclature: Pronamide (propyzamide); annual bluegrass, Poa annua 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.

Physiological Basis for Tall Fescue (Festuca arundinacea) Tolerance to Florasulam

Abstract Tall fescue is susceptible to injury from many acetolactate synthase (ALS) inhibitors used for broadleaf weed control in turfgrass. Florasulam is an ALS inhibitor that selectively controls broadleaf weeds in tall fescue, but the mechanisms for selectivity are not well understood. The objective of this research was to evaluate the physiological basis of tall fescue tolerance to florasulam. In greenhouse experiments, florasulam rates required to injure tall fescue 20% (I20) and white clover 80% (I80) measured 320 and 65 g ai ha-1, respectively. The I20 and I80 values of another ALS inhibitor, flucarbazone, on these species measured 33 and 275 g ai ha-1, respectively. In laboratory experiments, the time required to reach 50% foliar uptake for 14C-florasulam and 14C-flucarbazone measured 23 and 62 h for white clover, respectively, and >72 h for both herbicides in tall fescue. The half-lives of florasulam and flucarbazone in tall fescue were 15 and 40 h, respectively, whereas the half-life in white clover was >72 h for both herbicides. The concentrations of florasulam and flucarbazone required to inhibit ALS enzymes 50% in excised leaves of tall fescue measured >1,000 and 32 µM, respectively. The selectivity of florasulam for white clover control in tall fescue is associated with differential levels of absorption and metabolism between species. Tall fescue has faster metabolism and less ALS enzyme inhibition from florasulam as compared to a more injurious ALS inhibitor, flucarbazone, which contributes to the differential tolerance levels between these herbicides. Nomenclature: Florasulam; flucarbazone-sodium; tall fescue, Festuca arundinacea Shreb.; white clover, Trifolium repens L.

Annual Bluegrass (Poa annua) Biotypes Exhibit Differential Levels of Susceptibility and Biochemical Responses to Protoporphyrinogen Oxidase Inhibitors

Abstract An annual bluegrass (Poa annua L.) biotype with limited susceptibility to POST flumioxazin applications was identified in Georgia. The objectives of this research were to quantify tolerance levels of this biotype (R-biotype) to protoporphyrinogen oxidase (PPO) inhibitors and characterize physiological responses to flumioxazin. In dose–response experiments on 3-to 5-tiller plants, flumioxazin and sulfentrazone rates required to reduce dry-shoot biomass 50% from the nontreated were >14.5 and 10.4 times greater for the R-biotype, as compared with a susceptible (S)-biotype, respectively. Establishment of the R-biotype from seed was completely controlled by PRE applications of flumioxazin and oxadiazon, similar to the S-biotype. Tank mixtures of chlorpyrifos with flumioxazin did not enhance biomass reductions of the R-biotype, suggesting that tolerance levels may not be related to cytochrome P450–associated metabolism. In laboratory experiments, the R-biotype averaged 27% less electrolyte leakage, as compared with the S-biotype, after flumioxazin treatments. Lipid peroxidation in the R-biotype, as measured by malondialdehyde levels, averaged 25% less than the S-biotype at 72 h after broadcast flumioxazin treatments at 280 and 560 g ha -1. The tolerance to POST applications of PPO inhibitors in this P. annua biotype is associated with less lipid peroxidation and electrolyte leakage as compared with the S-biotype. These biochemical differences in biotypes may contribute to erratic levels of POST control from flumioxazin and could contribute to PPO-inhibitor resistance.