1. O
Absorption, Translocation, and Metabolism of Prohexadione Calcium in Annual Bluegrass (Poa annua), and Three Turfgrass Species
Goddard, M.J., J.B. Beam, S.D. Askew, Virginia Tech, Blacksburg O * O H * O O
Partition
R2 values
Leaf
0.99
y = -0.02x x
Rinse
0.90
y = -9.76Ln(x)
Foliage
0.98
y = 2.99Ln(x)
Root
0.89
y = 1.48Ln(x)
CO2
0.82
y = 1.06Ln(x)
Introduction ______
Prohexadione Ca (PC) is an experimental turfgrass growth regulator that selectively controls or suppresses annual bluegrass in desirable turfgrass such as creeping bentgrass (Agrostis stolonifera), Kentucky bluegrass (Poa pratensis), and perennial ryegrass (Lolium perenne). PC inhibits gibberellin production in turfgrass similar to trinexapac-ethyl (Rademacher, 2000). Previous research indicates that annual and Kentucky bluegrass growth is suppressed more by prohexadione Ca than creeping bentgrass and perennial ryegrass.
Methods and Materials cont._______
Plants partitioned into treated leaf, other foliage, roots, CO2, and rinse
Radioactivity detected by LSS. Plant parts were combusted in a biological oxidizer, prior to LSS
Radioactivity (% of recovered)
Results _______
Annual and Kentucky bluegrass absorbed more prohexadione Ca than creeping bentgrass and perennial ryegrass when averaged over harvest timing and trial
Neither translocation out of the treated leaf or metabolism of prohexadione Ca differed between species
When averaged over species and trial, 22% of recovered prohexadione Ca was metabolized within 1 HAT, and plants metabolized an additional 0.7% each additional hour for a period of 48 hours
Time after treatment (h)
Figure 1: Effect of time after treatment on radioactivity distribution averaged over plant species and trial.
Objectives ______
Determine absorption, translocation and metabolism rates of 14C PC when applied to foliage to elucidate reasons for differential plant response.
Prohexadione Calcium (RF = 0.58)
Metabolite 1 (RF = 0.12)
Standard
Kentucky bluegrass* treated leaf
Radioactivity (CPS)
Metabolite 2 (RF = 0.33)
Materials and Methods ________
2 laboratory trials, RCBD with 3 replications each
Plants were field collected and thinned to one tiller
Plants placed in 0.25% Hoagland's solution and maintained at 20/30 C night/day with an average irradiance of 80 µmol/m2/s PAR
Spotted with three 1 µl drops of 14C PC with 91% purity and 2.1 kBq radioactivity, in 88% 1M HCL, 12% Acetonitrile, and 0.25% v/v nonionic surfactant, to newest fully expanded leaf
Plants were harvested at 1, 12, 24, and 48 hours after treatment (HAT)
Air derived from each plant was vacuumed through a 50 ml glass burette containing 20 g of glass beads and 20 ml of 3M NaOH and rinsed with 20 ml acetonitrile: water (3:7)
1 ml aliquot of rinse + 19 ml scintillation cocktail was analyzed with a liquid scintillation spectrometer (LSS)
Radioactivity partitioning
Species
CO2
Rinse
Treated leaf
Foliage
Roots %
Annual bluegrass 4a
17c
53b 15
11a
Creeping bentgrass 2b
31a
46b 11
9bc
Kentucky bluegrass
3ab
15c
60a 12
10ab
Perennial ryegrass
24b
54ab 13 7c
Distance (mm)
Figure 2: Radiochromatogram trace peaks of PC standard and Kentucky bluegrass 48 HAT, showing displacement of PC with polar metabolites *Kentucky bluegrass values were adjusted to meet the levels of the standard.
100
Conclusion _______
Annual and Kentucky bluegrass sensitivity to PC is partially explained by increased absorption rates but may be due to factors other than translocation and metabolism 80
y = 0.69x
R2 = 0.93 60
Prohexadione calcium
Polar metabolites
Radioactivity (% of recovered) 40
y = 0.65x
R2 = 0.93 20
Reference _______
Rademacher, W Growth retardants: effects on gibberellin biosynthesis and other metabolic pathways. Annu. Rev. Plant Physiol. Plant Mol. Biol. 51: 10 20 30 40 50
Time after treatment (h)
Figure 3: Effect of time after treatment on percent PC and polar metabolites in treated leaves, averaged over plant species and trials.
Preliminary experiment determined that appreciable PC was lost as radioactive CO2 when exposed to plants
Three drops (3 µl) of PC placed on each plant
Chambers trapping air released from plant to be collected and analyzed for 14C CO2
Air vacuumed through burette containing glass beads with 3M NaOH to trap CO2
Plant parts were clipped and separated for analysis
Treated leaves and other plant parts were cut, rinsed, dried, and combusted
Absorbed PC was separated from metabolites via thin layer chromatography