d13C levels in shoot and root tissue of pot-grown plants averaged 6% greater for C4plants and 9% greater for rice in the field than in the greenhouse.. Corrections derived from inputs in
Trang 1BioOne sees sustainable scholarly publishing as an inherently collaborative enterprise connecting authors, nonprofit publishers, academic institutions, research libraries, and research funders in the common goal of maximizing access to critical research.
13
Carbon Isotope Discrimination in Roots and Shoots of Major Weed Species of Southern U.S Rice Fields and Its Potential Use for Analysis of Rice–Weed Root Interactions
Author(s): David R Gealy and Glenn S Gealy
Source: Weed Science, 59(4):587-600 2011.
Published By: Weed Science Society of America
DOI: http://dx.doi.org/10.1614/WS-D-10-00140.1
URL: http://www.bioone.org/doi/full/10.1614/WS-D-10-00140.1
BioOne (www.bioone.org) is a nonprofit, online aggregation of core research in the biological, ecological, and environmental sciences BioOne provides a sustainable online platform for over 170 journals and books published
by nonprofit societies, associations, museums, institutions, and presses
Your use of this PDF, the BioOne Web site, and all posted and associated content indicates your acceptance of BioOne’s Terms of Use, available at www.bioone.org/page/terms_of_use
Usage of BioOne content is strictly limited to personal, educational, and non-commercial use Commercial inquiries
or rights and permissions requests should be directed to the individual publisher as copyright holder
Trang 2Carbon Isotope Discrimination in Roots and Shoots of Major Weed Species
of Southern U.S Rice Fields and Its Potential Use for Analysis of Rice–Weed
Root Interactions David R Gealy and Glenn S Gealy*
Assessing belowground plant interference in rice has been difficult in the past because intertwined weed and crop roots
cannot be readily separated A13C discrimination method has been developed to assess distribution of intermixed roots of
barnyardgrass and rice in field soils, but the suitability of this approach for other rice weeds is not known.13C depletion
levels in roots and leaves of rice were compared with those of 10 troublesome weed species grown in monoculture in the
greenhouse or field Included were C4tropical grasses: barnyardgrass, bearded sprangletop, Amazon sprangletop, broadleaf
signalgrass, fall panicum, and large crabgrass; C4sedge, yellow nutsedge; and C3species: red rice, gooseweed, and redstem
Rice root d13C levels averaged , 228%, indicating that these roots are highly13C-depleted Root d13C levels ranged
from 212% to 217% among the tropical grasses, and were 210% in yellow nutsedge, indicating that these species were
less13C depleted than rice, and were C4plants suitable for13C discrimination studies with rice Among the C4species,
bearded sprangletop and yellow nutsedge were most and least13C depleted, respectively d13C levels in shoot and root
tissue of pot-grown plants averaged 6% greater for C4plants and 9% greater for rice in the field than in the greenhouse In
pots, shoots of rice typically were slightly more13C depleted than roots A reverse trend was seen in most C4species,
particularly for broadleaf signalgrass and plants sampled from field plots Corrections derived from inputs including the
total mass, carbon mass, carbon fraction, and d13C levels of roots and soil increased greatly the accuracy of root mass
estimates and increased slightly the accuracy of root d13C estimates (, 0.6 to 0.9%) in samples containing soil Similar
corrective equations were derived for mixtures of rice and C4weed roots and soil, and are proposed as a labor-saving option
in13C discrimination root studies
Nomenclature: Barnyardgrass, Echinochloa crus-galli (L.) Beauv.; bearded sprangletop, Leptochloa fusca (L.) Kunth var
fascicularis (Lam.) N Snow; Amazon sprangletop, Leptochloa panicoides (J Presl) A S Hitchc.; broadleaf signalgrass,
Urochloa platyphylla (Nash) R D Webster; fall panicum, Panicum dichotomiflorum Michx.; large crabgrass, Digitaria
sanguinalis (L.) Scop.; yellow nutsedge, Cyperus esculentus L.; gooseweed, Sphenoclea zeylanica Gaertn.; redstem, Ammannia
coccinea Rottb.; red rice, Oryza sativa L.; rice, Oryza sativa L
Key words: Stable carbon isotope, 13C/12C isotope ratio, d13C, 13C depletion, C3 photosynthetic pathway, C4
photosynthetic pathway, crop–weed root interference, tropical japonica rice, indica rice
13C isotope discrimination analysis was recently used to
determine the levels and distribution of roots of
weed-suppressive rice and barnyardgrass in soil (Gealy and Fischer
2010) Barnyardgrass is an aggressive tropical grass that
greatly affects rice production worldwide 13C is a naturally
occurring, stable isotope that is present in about 1.1% of the
atmospheric CO2 (West et al 2006) The availability of this
technique for rice–weed root interaction studies represents a
significant step forward because of the inherent complexities
and difficulties in sampling, extricating, and quantifying
intermixed rice and weed roots under flooded field conditions
This isotope analysis approach is feasible because
barnyard-grass uses the C4 photosynthetic pathway (Giussani et al
2001; Sage 2004; Smith and Brown 1973), whereas rice uses
the C3pathway C3plants fix a lower percentage of13C, and
therefore are13C-depleted in all plant organs compared with
C4plants because of inherent differences in the photosynthesis
processes and anatomy of these two plant types (Ehleringer
1991; Farquhar et al 1989) C4photosynthesis occurs in three
monocot families, including the Poaceae (Giussani et al
2001; Waller and Lewis 1979) and the Cyperaceae (Muasya
et al 2002), and in 16 dicot families including Amaranthaceae
and Portulacaceae (Sage 2004)
Factors that change stomatal conductance or photosynthetic capacity (e.g., light, water deficit, vapor pressure deficit) in typical C3plants can alter the ratio of the CO2partial pressures
in the leaf interior substomatal cavities and the ambient air surrounding the leaf (i.e., Pi/Pa), which alters discrimination against13C (Badeck et al 2005; Dingkuhn et al 1991) Thus, lower discrimination against 13C can result from lower leaf CO2conductance, greater CO2incorporation capacity, or both (Farquhar et al 1982) Changes in leaf CO2conductance due
to stress typically affects 13C discrimination differently in C4 plants from that in C3plants C4plants concentrate CO2into bundle sheath cells even when stomata are partially closed and shade (as well as water or nutrient stress, and genetic variation) can induce leakiness of the bundle sheath cells to CO2(Clay
et al 2009; Farquhar et al 1982; Pansak et al 2007)
13C isotope discrimination analysis, often measured as
d13C, an expression of the13C/12C isotope ratio relative to a fixed standard, has been used previously to determine the proportions of roots of C3and C4species in a number of field systems (Derner et al 2003; Eleki et al 2005; Gealy and Fischer 2010; Svejcar and Boutton 1985; Svejcar et al 1988)
In other applications, 13C discrimination analysis has been used in rice to improve water use efficiency or transpiration efficiency (Dingkuhn et al 1991; Impa et al 2005; Kondo
et al 2004; Scartazza et al 1998; Xu et al 2009) and to explain suppression of a weed species under temporary water stress (Fischer et al 2010) Examination of genetic associa-tions of d13C levels with crop productivity traits in mapping populations of rice have indicated quantitative trait loci for
d13C on five (Xu et al 2009) or on six (Laza et al 2006) of
DOI: 10.1614/WS-D-10-00140.1
* Plant Physiologist, U.S Department of Agriculture Agricultural Research
Service, Dale Bumpers National Rice Research Center, 2890 Highway 130 East,
Stuttgart, AR 72160 Second author: Principal Professional Staff, Johns Hopkins
University Applied Physics Laboratory, Laurel, MD Corresponding author’s
E-mail: david.gealy@ars.usda.gov
Weed Science 2011 59:587–600
Trang 3the 12 rice chromosomes C discrimination analysis has also
been used to explain the effects of stress on grain crop yield
loss (Clay et al 2001, 2005)
Numerous weed species including barnyardgrass are
prob-lematic in rice fields in the southern United States (Smith
1988), but the prospects of using 13C isotope analysis to
evaluate their root interactions with rice have not been explored
in detail (Gealy et al 2005; Gealy and Fischer 2010) Among
these species are other C4grasses (Giussani et al 2001; Sage
2004; Smith and Brown 1973; Waller and Lewis 1979) such as
bearded sprangletop, Amazon sprangletop, broadleaf
signal-grass, fall panicum, and large crabgrass Additional common or
troublesome weed species in rice include biotopes of red rice,
gooseweed, redstem, and yellow nutsedge Although these
species can be controlled to some degree in rice using registered
herbicides (Scott et al 2010), they are among the most
common and troublesome weeds in this crop in the southern
United States (Smith 1988; Webster 2008)
Simple extrapolations from standard concentration curves
can provide good estimates of intermixed rice and C4 weed
root quantities using 13C isotope discrimination analysis
(Gealy and Fischer 2010) Inconsistent or incomplete soil
removal from roots during processing, however, can introduce
unpredictable errors Vigorous, extended washing/rinsing
action usually removes most of the soil residue, but potentially
increases time and resource requirements Further, the precise
point at which soil has been adequately and uniformly
removed for optimum results is difficult to determine in real
time Thus, even after extensive washing procedures are
completed, roots may retain unpredictable and sizeable levels
of soil Evidence of this soil residue phenomenon is apparent
in analyses of carbon content that show that carbon fraction
(C fraction) levels in root samples tend to be much more
variable and lower than those in shoot samples (Gealy and
Fischer 2010) These observations suggest that derivations of
soil correction calculations for root mass and d13C values
might be developed on the basis of knowledge of the carbon
composition of the plants and soil
With the exception of barnyardgrass, little is known of the
suitability of major weed species to13C isotope depletion root
interaction techniques in flooded rice systems Thus, the
objectives of this research were to: (1) quantify d13C levels in
roots of numerous troublesome weed species and rice cultivars
grown as monocultures in flooded soil in field and greenhouse
environments; (2) compare d13C levels in roots with those in
shoots; and (3) develop mathematical corrections for d13C
and root mass values in soil-contaminated samples
Materials and Methods Pot Study in Greenhouse and Field Barnyardgrass, bearded
sprangletop, broadleaf signalgrass, and fall panicum were chosen
for a pot study to determine their d13C levels and assess the
feasibility of using these grass weeds in13C discrimination/root
interaction studies with rice The rice cultivars ‘Lemont’ (Bollich
et al 1985), a tropical japonica southern long grain, and ‘PI
312777’ (T65*2/TN 1; ‘WC 4644’), a weed-suppressive Asian
indica (Gealy et al 2003; Gealy and Fischer 2010), were
included as standards
Seedlings of these weed and rice species were selected from
natural stands and drilled rows, respectively, in rice research
field plots that had been planted May 24, 2007 and emerged
June 4 Plants in the four- to six-leaf stage were transplanted on June 18 to individual pots (, 20-cm diameter and , 24-cm depth) filled to , 83% capacity (, 4 cm below the rim) with DeWitt silt loam soil (fine smectitic, thermic, Typic Albaqualfs) having a pH of 5.8 and an organic matter content
of 1.2% Pots containing individual weed species or rice cultivar were randomly assigned to one of two groups placed under substantially different environmental conditions: ‘‘field envi-ronment’’ and ‘‘greenhouse environment.’’ The field pots were placed in bar ditches on the interior perimeter of the rice research field at the University of Arkansas Division of Agriculture Rice Research and Extension Center (RREC) (34u28980N, 91u259120W) near Stuttgart, AR On June 25, nitrogen fertilizer was applied to each pot as urea at a rate of , 110 kg N/ha On June 25, a permanent flood of 8- to 10-cm depth was established and maintained for the remainder of the growing period The upper rim of each pot was placed at approximately the level of the soil surface in the research plots, allowing water to flow naturally into and submerge the pots while plots were flooded Unwanted weeds were removed by hand At harvest, the aboveground portion of each plant (the shoots) was cut from roots at the soil surface
The greenhouse pots were placed in a greenhouse equipped with a multistage evaporative cooling system that was thermostatically controlled to maintain minimum night temperatures above 21 C and maximum daytime tempera-tures below 35 C Daytime temperatempera-tures, however, sometimes exceeded 38 C during the hottest periods of the summer Midday irradiance levels in the greenhouse (photosynthetic photon flux density max , 400mEm22s21) were only about one-quarter to one-third of the ambient levels in the field primarily because of deployment of ceiling shades intended to maintain greenhouse temperatures within tolerable limits No supplemental lighting was used; thus, day lengths were the same as ambient in the field A constant flood (, 4 cm) was maintained in pots by adding deionized water as needed All other aspects of plant growth, culture, and sampling were the same as for the field pots Similar to the methods described by Gealy and Fischer (2010), roots from the entire soil/root mass
in each pot were extracted and cleaned thoroughly
Expanded Species Survey in Field In 2007 and 2008, an expanded group of weed species was sampled from natural stands present in drill-seeded, irrigated weed research plots at the RREC These areas were managed using the same general practices described previously for weed-suppressive rice experiments (Gealy and Fischer 2010) The species consisted
of the original C4 tropical grass weed species used in the pot study and six additional weed species that have typically been among the most common and troublesome weed species in rice
in the southern United States (Webster 2008) These included Amazon sprangletop, large crabgrass, yellow nutsedge, goose-weed, redstem, and the red rice biotypes AR-1995-StgB (PI653422) awned blackhull, AR-1994-8 (PI653425) awned blackhull, AR-1994-11D (PI653417) awned, LA-1995-LA3 (PI653420) awned brownhull, and AR-1995-StgS (PI653423) awnless strawhull (Gealy et al 2009; GRIN 2010) Rice entries included those from the pot study, the additional tropical japonicas ‘Wells’ (Moldenhauer et al 2007; long grain), ‘CL 141’ (imidazolinone-resistant, proprietary BASF cultivar; long grain), and ‘Bengal’ (Linscombe et al 1993; medium grain), the indica accession ‘4593’ (PI 615031; GRIN 2010), and
Trang 4‘XL723’ (proprietary RiceTec hybrid) Individual rice, red rice,
gooseweed, and redstem plants were obtained from areas
receiving a season-long flood, while the other species were
obtained from intermittently flooded or wet areas near levees
adjacent to these areas Mature plants were collected, typically
after rice harvest Although the intent was to retrieve the
complete root systems, very long or fine roots could not always
be extricated completely Plants were divided into roots and
shoots The roots were washed and rinsed to remove soil as
described by Gealy and Fischer (2010) After discovering that
the root-cleaning procedures used in field and pot studies in
2007 sometimes failed to remove soil completely from plants,
additional time and vigor of agitation were used to clean roots
in 2008
Plant Tissue Analysis Roots and shoots from all experiments
were dried to constant mass at 60 C and weighed to the
nearest 0.1 g All shoots and the largest root masses were then
ground in a large Wiley mill1with 2-mm screen openings to
produce coarsely ground tissue This material was mixed
thoroughly and a total of , 30 g of representative tissue was
removed in numerous subsamples, combined, and reground
using a smaller Wiley mill2 with 1-mm screen openings,
resulting in powdered tissue Root samples weighing less than
30 g were ground only in the smaller Wiley mill
The 13C and C fraction levels in these plant tissues were
quantified at the University of Arkansas Stable Isotope
Laboratory using the procedure described by Gealy and Fischer
(2010) Briefly, subsamples were weighed to an accuracy of
0.0001 mg, combusted in an elemental analyzer in a stream of
helium, and resultant CO2gas was analyzed by an isotope ratio
mass spectrometer Raw13C/12C isotope ratios were acquired by
comparison with a reference gas injection and were normalized
by comparison with in-house isotope standards traceable to
international references The C fractions of samples were
determined via instrument response to known standards One
third to one half of the samples processed through the
combustion/mass spectrometer procedure consisted of isotope
standards to ensure proper calibration (Gealy and Fischer 2010)
13C/12C isotope ratios were expressed relative to the
international Pee Dee Belemnite (PDB) limestone fossil
standard as d13C (Farquhar and Lloyd 1993; O’Leary 1993):
d13Csample (0=00)~ Rsample{Rstandard
=Rstandard
|1,000 ½1
where d13Csampleis the isotope ratio (in parts per thousand; %)
relative to the PDB standard Rsample and Rstandard are the
13C/12C molar abundance ratios of the plant sample and
the PDB standard (Rpd; 0.0112372), respectively (Eleki et al
2005) Average d13C values for C3and C4plants were reported
to be approximately 227% and 213%, respectively (Boutton
1996) The negative value indicates a lower 13C/12C ratio in
plants than in the PDB standard Vogel (1980) considered
d13C values for C4plants to range within 29% to 216% and
C3plants to range within 222% to 234% For classification
purposes in the present study, plants with d13C values 217%
were included with the C4plants
Statistical Design and Analysis The experimental design for
the pot study was a randomized complete block with four
replications and the two experiments were considered to be
locations d C data were analyzed using the SAS Proc Mixed procedure The shoot–root difference in d13C levels in each plant sample was compared by subtracting the root value from the shoot value A value , 0 indicates that the root value is higher (less13C-depleted) than the shoot value An LSmeans test (P 5 0.05) was performed to determine which shoot and root values were significantly different from one another (i.e., shoot–root differences not equal to zero) Shoot–root differences for C fraction and mass values were similarly calculated and analyzed statistically
The experimental design for the expanded species survey was a randomized complete block with the 2 yr of the study serving as blocks with four subsampled plants per block d13C data were analyzed using the SAS Proc Mixed procedure An LSmeans test of the root–shoot difference that yielded a value different from zero at P 5 0.05 indicated that d13C values in roots and shoots were different for a particular species C fraction and mass data were analyzed using the SAS Proc GLM procedure and the mean differences were determined using Duncan’s multiple range test at P 5 0.05
Corrections of Root d13C Values and Root Mass for Soil Contamination A mathematical expression to correct for the effect of soil contamination on estimated root mass of
a single plant species was derived from a mixing equation describing the C fractions of the sample, root, and soil components A related expression that corrects for the effect of soil contamination on the sample root d13C level was derived independently
Carbon Fraction Mixing Equation Root samples obtained from field soils contain carbon from the root tissues and from the soil that remained after washing These carbon masses can
be expressed as follows:
where Mcis the total carbon mass in the root sample, Mc1is the carbon mass of the root, and Mcsis the carbon mass of the soil These carbon masses also can be expressed as the product
of (total mass of each component in the mixture) 3 (C fraction of that component) Thus:
f M ~f1M1zfsMs ½3 where f, f1, and fsare the respective C fractions, and M, M1, and Ms are the respective masses of the total sample, root component, and soil component (g) in the sample mixture Note that for simplicity and internal consistency with variable names that were used in separately derived Equations 10–25,
we used the suffix ‘‘s’’ to designate soil and the number 1 or 2
to designate a plant species A variable name without one of these suffixes indicates that it refers to the sample mixture Substituting (M 2 M1) for Msand rearranging produces the corrected value for root mass (M1) expressed in terms of M and the component C fractions
M1~M f {fs
f1{fs
½4 And by definition:
Trang 5Using methods described in the ‘‘Plant tissue analysis’’ section,
M and f were determined for each root sample, and the fsvalue
that was obtained from samples of root-free field soil was
determined to be 0.008335 (considered a constant in this
study) An approximation was used to determine the f1values
In the context of this correction procedure, f1was set equal to
the value of the shoot C fraction (f1shoot) from the same plant
This approximation was reasonable, because root and shoot C
fractions were nearly equal in a subset of rice and C4 plant
samples that had been vigorously rewashed to remove soil from
roots In the cases in which f was f1 shoot, the f value was
typically substituted for f1, forcing M 5 M1(via Equation 4)
An alternative approach not used here would be to define f1as
the largest f value for that species, assuming that well-cleaned
monoculture root samples were available for comparison
Simple d13C Mixing Equation A d13C mixing equation was
derived using carbon mass inputs along the lines of those used
for C fraction mixing above (Equations 2 and 3) Thus:
d~d1 Mc1
Mc
zds Mcs Mc
~d1 f1M1 fM
zds fsMs fM
½6
where d, d1, and ds, are d13C levels in the carbon present in
the total sample, root component (unknown), and soil
component, respectively Using methods from the ‘‘tissue
analysis’’ section above, d values were determined for each
sample The dsof root-free soil samples was determined to be
221.27% (considered a constant in this study) Other
variables were as defined previously
Substituting M 2 M1 for Ms as before, and rearranging,
yields a second expression of M1:
M1~M df {dsfs
d1f1{dsfs
½7
Combine Equations 4 and 7 by factoring out M1, rearrange,
and solve for d1, which is the soil-corrected d13C value for
root tissue in the sample mixture:
d1~ df (f1{fs){dsfs(f1{f )
f1(f {fs)
½8
This expression for d1 in Equation 8 was derived using a
simple d13C mixing analogy and is a close approximation to
the exact expression derived from a true mixing analogy on
the basis of actual 13C/12C isotope ratios instead of the
relative d13C values Over the broad range of input values
used in the present studies, this approximation yielded d13C
values nearly identical (to at least three decimal places) to
those calculated using a more complex exact derivation on
the basis of the actual carbon isotope ratios (Supplemental
Appendix 1A) A spreadsheet containing the formulas for
these equations can be accessed from Supplemental
Appendix 2
The approximation in Equation 8 produces soil-corrected
d13C values (d1) very close to those from the exact expression
because the value of the Rpd standard and all other R values
used in the exact expression are very small (i.e., 0.0112372
or less) We calculated this error to be in the range of
, 0.0180 to 0.0281% (for d12dsdifferences of 8 and 10%,
respectively; data not shown)
A more simplified approximation of d1can be derived from
Equation 8:
d1~dz (d{ds)(fs)(f1{f )
f1f
½9 This equation generally yielded the same result as Equation 8 when f 0.1 and fs of contaminating soil ,, f1 For instance, the fs50.008335 for the low-organic-matter soil in the present study will produce acceptable results over a wide range of conditions However, an fs50.05, as may occur in higher-organic-matter soils, could result in significant errors when using Equation 9
Soil Supplementing Experiment To demonstrate the effect
of soil contamination on the measured levels of d13C and C fraction in roots, root samples of monoculture Wells rice or barnyardgrass from the 2008 field study that had previously been shown to be nearly soil-free (i.e., similar C fractions in roots and shoots) were mixed with soil Pure soil used for supplementing experiments had a C fraction (fs) of 0.008335 and a d13C of 221.27% Root samples were ground to a powder using the small Wiley mill with a 1-mm screen (as described above) and supplemented with pulverized, dry field soil (described above) at planned levels of 0, 12.5, 25, 50, 75, and 87.5% (g/g; soil/[roots+soil]) As actually prepared, the rice mixtures contained 0, 12.2, 26.3, 49.2, 75.1, and 87.3% soil (one subsample) and the barnyardgrass mixtures con-tained 0, 12.5, 25.4, 50.0, 73.6, and 87.0% soil (average
of two subsamples) Samples were mixed thoroughly and submitted to the University of Arkansas Stable Isotope Laboratory for d13C and carbon content analysis as described
in the ‘‘tissue analysis’’ section above Equations 4 and 8 were used to compare the mathematically corrected values for root mass and d13C, respectively, with those obtained for samples via laboratory analysis Expected d13C values for soil levels higher than those measured experimentally (i.e., 88, 94,
97, 98.5, 99.25, 99.625, and 99.81% soil) were simulated by solving Equation 8 (after rearrangement) for d13C of the sample (d) at the sample C fractions (f ) equivalent to the respective soil% values above (all other variables held constant)
General Correction Equations for Estimation of Rice and
C4Weed Root Mass in Samples with Soil Contamination Extending the logic we had used previously to produce corrections for the mass (Equation 4) and d13C (Equation 8) values of single-species root samples containing soil, we developed another set of mathematical expressions to correct for soil in sample mixtures containing unknown amounts of
C3 rice and C4 weed roots To ensure the highest level of accuracy for results across the broadest range of variable inputs, we derived the relevant equations for this C3–C4–soil mixture from the exact expressions of the carbon isotope ratios (i.e., not d13C values) In a more complex system, attempting
to simultaneously distinguish among three different species in root mixtures, Polley et al (1992) used a mixing approach to account for inherent species differences in C fraction Definitions for the variable names are similar to those for Equations 2–9 above: f and fs are the respective C fractions and M and Ms are the respective masses of the total sample and soil component (g) in the sample mixture; f1and f2are the respective C fractions and M1 and M2are the respective masses of the C3 root component and C4 root component
in the sample mixture Similarly, d, d1, d2, and ds are d13C
Trang 6levels in the carbon contained in the total sample, the C3and
C4root components, and the soil component, respectively
We derived the appropriate expressions for M1and M2on
the basis of two basic equations The first equation expresses
the sample C fraction in terms of its component C fractions,
similar to the approach used in Equation 3 Thus:
f M ~f1M1zf2M2zfsMs ½10
Expressing Msin terms of the other mass components:
Ms~M {M1{M2 ½11
Thus:
f M ~f1M1zf2M2zfsðM {M1{M2Þ ½12
f ~f1 M1
M
zf2 M2 M
zfs M {M1{M2
M
½13
Simplifying and rearranging yields:
M1~ M f {fsð ÞzM2ðfs{f2Þ
f1{fs
½14
This is an independent expression for M1based on a mixing
equation for C fraction
The second basic equation is the expression of the ratio (R)
of 13C/12C in the sample (i.e., the 13C/12C mass fraction
ratio)
R~ C 131zC 132zC 13s
C 121zC 122zC 12s
½15
By definition, for any carbon-containing component of type i:
Ri~C 13i
where i 5 1 refers to plant type 1, i 5 2 refers to plant
type 2, and i 5 3 refers to soil We know that12C and 13C
isotopes comprise essentially 100% of the carbon mass in our
samples Thus:
fi~ C 12izC 13i
Mi
½17
which applies to plant type 1, plant type 2, and soil
Combining Equations 16 and 17, we obtain:
fi~
C 13i 1
Riz1
Mi
0 B
@
1 C
Rearranging Equation 18 yields an expression for the 13C
mass:
C 13i~ fiMi
1
Riz1
0 B
@
1 C
Substituting C12iRi (as rearranged from Equation 16) for
C13i in Equation 19, and rearranging, yields the equivalent
expression for12C mass:
C 12i~ fiMi
Riz1
½20 Substituting expressions for 13C components from Equation
19 into the numerator of Equation 15, and the expressions for
12C components from Equation 20 into the denominator of Equation 15, produces Equation 21, which is an expression of the sample R value as a function of the component R values (i.e., it is a mixing equation for the13C/12C ratios)
R~
f1M1 1zR1 1
!
z f2M2 1zR1 2
!
z fsðM {M1{M2Þ
1zR1 s
!
f1M1
R1z1
z f2M2
R2z1
z fsðM {M1{M2Þ
Rsz1
0 B B
@
1 C C
A ½21
This can be rearranged to produce a second, independent expression for M1:
M1~
M2 f2ðR2{RÞ
R2z1 z
fsðR{RsÞ
Rsz1
zM fsðRs{RÞ
Rsz1
fsðRs{RÞ
Rsz1 z
f1ðR{R1Þ
R1z1
0 B B
1 C C
A ½22
The two expressions for M1 (Equations 22 and 14) are set equal, M1 is factored out, and the equation solved for M2, yielding the soil-corrected root mass of plant type 2 (i.e., C4):
fsð Rs{R Þ
R s z1 zf1 ð R{R1Þ
R 1 z1
f {f s
ð Þ{ fs ð Rs{R Þ
R s z1
f 1 {f s
f2ð R2{R Þ
R 2 z1 zfs ð R{RsÞ
R s z1
f 1 {f s
ð Þ{ fs ð Rs{R Þ
R s z1 zf1 ð R{R1Þ
R 1 z1
fs{f 2
0
@
1 A
This equation can be rearranged by grouping common terms, and further simplified to the following form:
M2~M
f s ð R s {R Þ
R s z1
f {f1
ð Þ{ f1 ð R 1 {R Þ
R 1 z1
f {fs
f 2 ð R 2 {R Þ
R 2 z1
f1{fs
ð Þz fs ð R s {R Þ
R s z1
f2{f1
ð Þ{ f1 ð R 1 {R Þ
R 1 z1
f2{fs
0
@
1 A
For any13C/12C ratio (Ri), its d13C value (di) can be expressed relative to the R value of the PDB standard (Rpd) according to the definition:
Ri:Rpd½1z dð i=1,000Þ ½25 Note that this is a generalized rearrangement of Equation 1 Substituting this definition for the R values in Equation 23
or Equation 24 yields equations that express13C/12C ratios in terms of Rpd, a fixed constant, and the familiar d13C term The mass of rice roots (M1) can be obtained from the same general equations (Equation 23 or Equation 24) after exchanging the fiand Riindices for plant 1 and plant 2 (i.e., the original f1 and R1values become f2 and R2, respectively, whereas the original f2 and R2 values become f1 and R1, respectively) The fsand f values and the Rsand R values are left unchanged This maneuver temporarily redefines plant 1
as plant 2 and vice versa, which facilitates the calculation of the root mass of the other species (M1) Soil mass (Ms) was calculated as before using Equation 11
Equations 10–25 were derived with Rpdand other R values expressed on both a molar abundance ratio basis and a mass
[23]
[24]
Trang 7fraction basis Rpd molar abundance ratio 5 13C/12C 5
0.0112372 (Eleki et al 2005), and Rpdmass fraction ratio 5
(Rpd molar abundance ratio)(13/12) Because calculated
results were , identical to four decimal places, both models
were considered equally acceptable
A more complete explanation of the various steps used to
derive the equations in this section is presented in Supplemental
Appendix 1B A spreadsheet containing formulas to calculate
results for Equation 23 can be accessed from Supplemental
Appendix 2 A less cumbersome and simplified approximation
of the exact expression of soil-corrected root masses shown in
Equation 23 was developed using a simple mixing model for
d13C values It is presented as Equation 14c.12 in Supplemental
Appendix 1C
Results and Discussion
d13C Levels in Roots and Shoots of Rice and Weeds:
Pot Study The d13C levels in roots of the tropical C4grass
weeds were readily distinguished from those in rice cultivars in
both the field and greenhouse (Table 1) Soil-corrected d13C
values for the C4grass roots ranged from 212.4% to 216.7%
and the uncorrected values were slightly lower, ranging from
212.9% to 216.8% Among these four weed species, root
and shoot d13C levels in barnyardgrass and fall panicum were
highest, whereas those in bearded sprangletop were lowest
Soil-corrected values for rice roots averaged , 228.5% and the
uncorrected sample averages were slightly greater at 228.1%
Root d13C levels in tropical japonica Lemont rice were usually
similar to those in indica PI 312777 rice, but shoot d13C levels
in the greenhouse were 4% (or 1.2%) lower in PI 312777 than
in Lemont The d13C levels in rice in the field and greenhouse
in the present studies generally were similar to those in
nonstressed rice described earlier (Scartazza et al 1998; Zhao
et al 2004) Our data clearly confirm these four grass weeds and rice to be C4and C3plants, respectively
Root d13C soil-corrected values averaged up to 2.1% higher
in C4 grasses and 2.5% lower in C3 rice compared with noncorrected values (Table 1) These divergent trends for the corrected values of C4grasses and rice are consistent with the fact that the d13C level in our soil (, 21.27%; as described in Materials and Methods) was between that of the two plant types These results for C4 and C3 plants were generally consistent with those estimated previously (Gealy and Fischer 2010), where root d13C levels in monoculture C4 barnyard-grass and rice averaged 213.1% and 228.5%, respectively Similarly, d13C levels in nonstressed Leptochloa fusca (L.) Kunth (Kallar grass), a bearded sprangletop C4relative, were 214.7% (Akhter et al 2003)
d13C levels in both root and shoot tissues were greater (i.e., less 13C-depleted) in the field than in the greenhouse, exhibiting increases of about 6% for C4 plants and 9% for rice (Table 1) On calm, sunny days, air in the rice field canopy may have become CO2depleted (Gealy, unpublished data) compared with the well-mixed ambient air introduced into the greenhouse Plants were probably not fully light saturated in the greenhouse where they were at O lower irradiance levels than in the field Corn under low light conditions has been reported to have greater13C discrimina-tion levels than if grown under full sun (Clay et al 2009) Therefore, low light conditions in the greenhouse may have contributed to discrimination differences seen in our plants when compared with field values
In both pot environments, shoot d13C levels closely mirrored those in the roots The 13C levels in C4 grass species (Table 1) generally were similar or lower (more 13
C-Table 1 d C levels in weed and rice samples from pots maintained in greenhouse or field environments, and the application of a mathematical correction for soil contamination in roots a,b
Species
Growth environment
d 13 C
Shoot Root Corrected root c Shoot minus
corrected root c,d Corrected root c,d
(species main effect)
Shoot minus corrected root c,d,e (species main effect) -(%) -Barnyardgrass Greenhouse 213.8 ab 213.8 a 213.8 0.05 213.1 a 20.23 ab
Field 213.0 a 212.9 a 212.4 20.51 Bearded sprangletop Greenhouse 215.8 c 216.8 b 216.7 0.94 215.8 c 0.58 a
Field 214.7 b 215.0 ab 214.9 0.23 Broadleaf signalgrass Greenhouse 213.8 ab 215.1 ab 215.0 1.23* 215.0 bc 1.56* a
Field 213.0 a 215.0 ab 214.9 1.90*
Fall panicum Greenhouse 213.4 a 213.7 a 213.6 0.21 213.8 ab 0.55 a
Field 213.2 a 214.3 ab 214.0 0.89 Lemont rice Greenhouse 230.9 e 228.6 c 228.8 22.12* 228.1 d 21.76* b
Field 228.8 d 226.9 c 227.4 21.40*
PI 312777 rice Greenhouse 232.1 f 228.7 c 229.0 23.02* 229.0 d 21.71* b
Field 229.4 d 228.2 c 228.9 20.39
Species 3 environment interaction not significant (ns)
at P 5 0.05
Species 3 environment interaction ns
at P 5 0.05
a
Plants were grown in flooded pots in soil in the field or greenhouse during 2007.
b Values in columns are the estimated means according to an LSmeans test Values followed by the same letter were not different according to LSmeans (P 5 0.05) c
Corrected root d13C values were calculated using Equation 8.
d For the difference ‘‘shoot minus corrected root,’’ a value 0 indicates that root value is lower (d 13 C is more negative; tissue is more 13 C-depleted) than shoot value.
* indicates that the ‘‘shoot-corrected root’’ difference within that species is different from zero (i.e., shoot and root values are different from one another) according to
an LSmeans test (P 5 0.05).
e
Main effect means for growth environment ‘‘Corrected root’’; field 5 218.8% and greenhouse 5 219.5% (P ,, 0.05) ‘‘Shoot-corrected root;’’ field 5 0.12% and greenhouse 5 20.45% (P 5 0.118).
Trang 8depleted) in roots than in shoots, and this difference averaged
, 12% (1.6%) in broadleaf signalgrass Data from Badeck
et al (2005) indicated that d13C levels in roots were
sometimes greater and sometimes less than in shoots of C4
species (n 5 10) In contrast to the C4weeds, rice d13C levels
averaged , 6% (1.7%) greater (less 13C-depleted) in roots
than in shoots (Table 1) Previous reports have also indicated
that rice leaves and shoots generally were more13C-depleted
than roots (Badeck et al 2005; Klumpp et al 2005; Scartazza
et al 1998; Zhao et al 2004) A compilation of , 400
comparisons of 13C depletion in numerous species showed
that roots of C3 plants were, on average, 1.08% less 13
C-depleted compared with leaves (Badeck et al 2005)
The corrected root d13C values in rice were at least 78%
(12.3%) lower than in the four weed species (on the basis of
species main effect means), whereas noncorrected root d13C values
in rice were at least 70% (11.8%) lower than in these weeds (on
the basis of the species 3 environment interaction means)
(Table 1) Clearly, C3and C4plant roots can be distinguished in
our rice field soils containing low levels of organic carbon
d13C Levels in Roots and Shoots of Rice and Weeds: Expanded Species Field Survey In the species common to both experiments, d13C levels in the expanded field survey generally followed trends similar to those in the pot study d13C levels for roots of C4plants were lowest in bearded sprangletop (217.1%), followed by Amazon sprangletop, intermediate in broadleaf signalgrass, fall panicum, and barnyardgrass, and greatest in crabgrass and yellow nutsedge (210.3%) (Table 2) Rajagopalan et al (1999) reported similar high d13C levels (28.2% to 211.5%) in the cellulose of relatives of yellow nutsedge (Cyperus spp.) growing in peat bogs
In contrast to the pot study, root d13C levels in nearly all of the C4grass species in the expanded species survey were lower than in shoots, ranging from 8.7% (1.1%) for barnyardgrass
to 16.7% (2.5%) for bearded sprangletop Yellow nutsedge differed from most of the other C4 weed species in that its root d13C levels were 15.8% (1.9%) higher than in shoots, which was similar to the trend for rice (Table 2)
Both root and shoot d13C levels were similar among the four rice cultivars in the field plots (Table 2) Root d13C levels
Table 2 d C levels in additional weed species and rice cultivars growing in or near rice field plots in 2007 and 2008, and the application of a mathematical correction for soil contamination in roots a,b
Speciesc
d 13 C Shoot Root Corrected rootd Shoot minus corrected rootd,e - (%)
Broadleaf signalgrass 212.9 ab 214.2 bc 214.2 bc 1.30* ab
AR-1995-StgB awned red rice 229.1 f 228.9 e 229.0 e 20.12 bc
Bengal medium-grain rice 227.6 de 226.8 e 227.3 e 20.31 bc
Additional O sativa entries from same field location c
AR-1995-StgS awnless red rice
(2007 only)
228.1 6 0.1 (n 5 4)
228.0 6 0.7 (n 5 4)
228.2 6 0.7 (n 5 4)
0.03 6 0.72 (n 5 4) AR-1994-8 awned red rice
(2008 only)
229.4 6 0.8 (n 5 4)
227.8 6 0.4 (n 5 4)
227.8 6 0.4 (n 5 4)
21.63 6 0.63 (n 5 4) AR-1994-11D awned red rice
(2008 only)
229.6 6 0.6 (n 5 4)
228.6 6 0.1 (n 5 4)
228.7 6 0.1 (n 5 4)
20.97 6 0.67 (n 5 4) LA-1995-LA3 awned red rice
(2008 only)
229.8 6 0.9 (n 5 4)
228.7 6 0.3 (n 5 4)
228.7 6 0.3 (n 5 4)
21.15 6 1.08 Lemont rice (2008 only) 227.9 6 0.5
(n 5 4)
227.9 6 1.2 (n 5 3)
228.0 6 1.2 (n 5 3)
20.10 6 0.84 (n 5 3)
CL 141 rice (2007 only) 227.5 6 0.1
(n 5 4)
227.1 6 0.5 (n 5 4)
227.8 6 0.5 (n 5 4)
0.37 6 0.55 (n 5 4)
PI 312777 rice (2008 only) 227.9 6 0.1
(n 5 4)
225.9 6 1.3 (n 5 4)
226.0 6 1.3 (n 5 4)
21.90 6 1.33 (n 5 4)
a Plants were grown in or near flooded rice field plots in 2007 or 2008 (or both years).
b
Values in columns are the estimated means according to an LSmeans test in Proc Mixed Values followed by the same letter were not different according to LSmeans (P 5 0.05) The additional O sativa entries (bottom section of table) that were evaluated in 1 yr only were not included in the statistical analysis with other data Only the means and standard deviations of subsamples were calculated.
c The d 13 C of yellow nutsedge nutlets averaged 211.53% (data from 2008 only; not included in statistical analysis; not corrected for soil contamination) The additional O sativa entries were obtained from same field location as species above, evaluated 1 yr only, and were not included in statistical analysis Plant growth environment: rice cultivars, red rice lines, redstem, and gooseweed obtained from flooded rice fields; bearded sprangletop from flooded rice fields or area adjacent to rice field levees; all other plant species from areas adjacent to rice field levees.
d
Corrected root d13C values were calculated using Equation 8.
e For the difference, ‘‘shoot minus corrected root,’’ a value 0 indicates that root value is lower (d 13 C is more negative; tissue is more 13 C-depleted) than shoot value.
* indicates that the shoot-corrected root difference within that species is different from zero (i.e., shoot and root values are different from one another) according to an LSmeans test (P 5 0.05).
Trang 9were greater than shoot d C levels in 4593 indica rice only,
but a similar trend was noticed in PI 312777 indica rice (2008
only) (Table 2) This tendency toward greater d13C levels in
roots than in shoots of rice was even more pronounced in the
pot experiments (Table 1)
Earlier studies have also reported a tendency toward lower
shoot d13C levels (greater13C discrimination) in indica than
in japonica rices (Dingkuhn et al 1991; Kondo et al 2004;
Peng et al 1998) By contrast, screening of 57 3- to 4-wk-old
rice cultivars showed that13C discrimination averaged about
1.7% lower in indica types compared with tropical japonica
types (Xu et al 2009) In the present studies, d13C levels
differed between tropical japonica and indica rice only in
greenhouse pots where the 13C discrimination was 3.8%
greater in shoots of PI 312777 indica compared with Lemont
(Table 1) Similar but nonsignificant trends were observed for
root d13C levels in pots in both greenhouse and field
environments Under environments that may be particularly
stressful to one of these rice types and not the other (e.g., cool
early-season conditions that are more stressful to indicas than
japonicas), the d13C signatures of these two rice types could
change slightly These differences would be expected to be no
more than a few percent, however, and are not likely to
contribute substantially to errors in d13C root analysis studies
Separate monoculture standards of tropical japonica and
indica cultivars could be grown if greater precision for rice
d13C values is desired
Overall, the d13C levels in rice in the field and greenhouse
environments in the present study ranged from about 227%
to 232% A d13C of 232% equates to about the greatest
13
C discrimination reported by Xu et al (2009) in a
comparison of 116 accessions from seven different Oryza
species in well-watered greenhouse pots The lowest 13C
discrimination level in our test (d13C 5 227%) equates to
about 12% lower than the minimum reported by Xu et al
(2009) This may be attributable to our later growth stage of
sampling (mature vs 3- to 4-wk-old plants), and the
possibility that as plants matured in pots, they experienced
additional stress due to pot-bound roots (Comstock et al
2005) This type of stress was apparently avoided in the Xu et
al (2009) study Scartazza et al (1998) have reported 13C
discrimination levels in potted rice plants similar to those in
the present study and showed that the discrimination
decreased by , 10% in 170-d-old plants compared with
20-d-old plants
Corrected root d13C levels in C3 plants averaged 100%
(14%) lower, and all rice cultivars were at least 58% (9.9%)
lower compared with the C4 plants grown in field plots in
2007 and 2008 (Table 2) These contrasts between C3and C4
plants are similar to those observed in the pot experiment,
again confirming our C4weeds to be ideal for d13C rice root
interaction studies in field soils The d13C values for
barnyardgrass and fall panicum roots were statistically
indistinguishable in all of the environments/years evaluated
in this study (Tables 1 and 2) Thus, 13C discrimination
methods potentially could be used to evaluate the combined/
average effects of these two common C4 weeds in mixtures
with rice in the field Because of their distinctively high d13C
levels and the resulting large d13C differential with rice, yellow
nutsedge, and perhaps crabgrass may be especially well suited
to13C discrimination studies, and potentially could yield root
mixture data that are more accurate than those of the other C4
weed species (Table 2)
Root d C levels in gooseweed, redstem, AR-1995-StgB red rice, and all rice cultivars were similar, averaging , 227.9% (Table 2) These data confirmed that these three weed species are C3 plants similar to rice, and thus unsuitable for 13C discrimination studies with rice–weed root mixtures United States red rice types often share key genetic traits with indica rice (Gealy et al 2009; Londo and Schaal 2007; Vaughan et al 2001) Generally consistent trends between d13C levels of roots and shoots of key C4weed species and rice were observed in these studies (Tables 1 and 2) d13C levels in roots of C4 weeds (except for crabgrass and yellow nutsedge) and rice were 1.0 to 1.1 times and 0.9 to 1.0 times the respective levels in shoots Such trends between the d13C levels in these plant organs have been observed in numerous species (Badeck et al 2005; Klumpp et al 2005; Scartazza et al 1998) If monoculture root samples were unreliable or unavailable for some reason, shoot d13C values potentially could be substituted for root d13C values, or used as an internal standard check for d13C levels within the same plant
Carbon Content and Mass of Roots and Shoots The shoot
C fraction of C4weed species ranged from 41 to 44% in pot studies (Table 3) and from 39 to 42% in the field survey (Table 4) The shoot C fraction of rice in pot studies (Table 3) ranged between 41 and 42%, and in the expanded field survey (Table 4) was more variable and slightly lower, ranging from 34 to 39% These rice shoot C fraction levels are similar to those reported for rice in an earlier study where the foliar C fraction of an indica subgroup, aus (0.392), was lower (P , 0.01) than for indica (0.402) or tropical japonica (0.404) groups (Dingkuhn et al 1991)
In the pot studies, C fraction of root samples was generally greatest in barnyardgrass and broad-leaved signalgrass in the greenhouse and lowest in rice in the field (Table 3) The C fraction of root samples, particularly for rice, was often much lower than in shoots (Tables 3 and 4), a phenomenon that has been attributed primarily to presence of difficult-to-remove soil residue (Gealy and Fischer 2010) In the pot and survey studies conducted in the field in 2007, C fraction of root samples of some rice cultivars (e.g., PI 312777 and CL 141) averaged as much as 80% lower than the levels in shoots (Tables 3 and 4) Implementation of a more vigorous root cleaning/extraction process in the 2008 field study resulted in substantially greater C fraction levels in roots that often approached those in shoots (data not shown), and a trend toward higher root C fraction values that year (Table 4) This improvement was also evident in the additional rice and red rice entries sampled from field plots in 2008 compared with
2007 (Table 4; ‘‘additional entry’’ section) In most entries collected exclusively in 2008, C fractions were only 0 to 4% less in root samples than in shoots, although in PI 312777 the
C fraction was 23% less in roots than shoots
By contrast, the C fraction of root samples collected exclusively
in 2007 averaged 63% less than in shoots The masses of the 2007 root samples were also unusually high, averaging four times greater than those in 2008, which further indicated heavy soil contamination in 2007 Similar large discrepancies in C fractions and masses between rice root and shoot samples also were observed
in the 2007 field pot study (Table 3)
Because of the variation and uncertainties in C fraction and mass of root samples caused by soil contamination, we performed a mathematical calculation that corrected root
Trang 10Table 4 Carbon content and mass of additional weed species and rice cultivars growing in or near rice field plots, and the application of a mathematical correction for soil contamination in roots.a,b
Species c
C content (C fraction 3 100) Mass Shoot Root Shoot Root Corrected root d Soil calculated d - % -g plant 21 -Bearded sprangletop 41.4 bc 29.0 b–f 89.4 bc 18.3 b–d 10.2 a–c 8.1 bc Amazon sprangletop 41.5 bc 30.5 a–e 54.0 b–d 9.0 d 5.1 b–d 3.9 c Barnyardgrass 39.6 c–e 37.0 a–c 66.3 b–d 9.6 d 8.7 a–d 0.9 c Broadleaf signalgrass 39.6 c–e 34.4 a–d 54.4 b–d 4.6 d 3.1 cd 1.5 c Fall panicum 40.6 cd 28.1 b–f 141.8 a 16.3 cd 10.9 ab 5.4 c
Yellow nutsedge c 39.4 c–e 38.9 ab 25.8 d 5.8 d 5.2 b–d 0.5 c
AR-1995-StgB awned red rice 37.7 e 26.4 c–f 101.6 ab 21.6 a–d 13.6 a 8.0 bc Wells long-grain rice 37.8 e 22.1 ef 67.9 b–d 47.5 a 14.0 a 33.5 a
4593 indica rice 37.3 e 21.3 ef 55.6 b–d 45.4 ab 8.6 a–d 17.5 a–c Bengal medium-grain rice 37.9 e 18.9 f 46.3 cd 46.2 ab 8.6 a–d 37.7 a XL723 hybrid rice 37.8 e 21.4 ef 56.7 b–d 39.6 a–c 10.7 ab 28.9 ab Additional entries from same field location c
AR-1995-StgS awnless
red rice (2007 only)
36.6 6 0.8 (n 5 4)
20.2 6 6.2 (n 5 4)
100.1 6 62.9 (n 5 4)
41.9 6 39.7 (n 5 4)
17.5 6 7.8 (n 5 4)
24.4 6 32.2 (n 5 4) AR-1994-8 awned
red rice (2008 only)
36.4 6 0.6 (n 5 4)
35.3 6 2.9 (n 5 4)
69.8 6 18.4 (n 5 4)
12.0 6 5.1 (n 5 4)
11.5 6 5.4 (n 5 4)
0.5 6 0.6 (n 5 4) AR-1994-11D awned
red rice (2008 only)
35.3 6 1.0 (n 5 4)
33.8 6 3.1 (n 5 4)
116.8 6 53.3 (n 5 4)
16.6 6 4.2 (n 5 4)
15.6 64.1 (n 5 4)
6 1.5 (n 5 4) LA-1995-LA3 awned
red rice (2008 only)
34.9 6 1.3 (n 5 4)
33.7 6 1.5 (n 5 4)
145.6 6 54.1 (n 5 4)
20.8 6 5.5 (n 5 4)
19.9 6 5.8 (n 5 4)
0.9 6 0.8 (n 5 4) Lemont (2008 only) 35.7 6 0.6
(n 5 4)
35.7 6 5.3 (n 5 3)
15.8 6 4.2 (n 5 4)
6.7 6 1.8 (n 5 4)
6.4 6 2.1 (n 5 4)
0.3 6 0.6 (n 5 4)
CL 141 (2007 only) 38.9 6 0.5
(n 5 4)
7.2 6 2.2 (n 5 4)
70.4 6 44.8 (n 5 4)
70.3 6 42.8 (n 5 4)
11.6 6 7.5 (n 5 4)
58.6 6 35.8 (n 5 4)
PI 312777 (2008 only) 34.2 6 2.5
(n 5 4)
26.4 6 3.7 (n 5 4)
24.5 6 13.7 (n 5 4)
8.4 6 4.9 (n 5 4)
6.2 6 4.9 (n 5 4)
2.1 6 1.7 (n 5 4)
a Plants were grown in or near flooded rice field plots in 2007 or 2008 (or both years).
b Values in columns are the arithmetic means Values followed by the same letter were not different according to Duncan’s multiple range test (P 5 0.05) The additional O sativa entries (bottom section of table) that were evaluated in 1 yr only were not included in the statistical analysis with other data Only the means and standard deviations of subsamples were calculated.
c
Carbon content of yellow nutsedge nutlets averaged 41.9% (data from 2008 only, and were not included in statistical analysis; values not corrected for soil contamination) The additional O sativa entries were obtained from same field location as species above, evaluated 1 yr only, and were not included in statistical analysis Plant growth environment: rice cultivars, red rice lines, redstem, and gooseweed obtained from flooded rice fields; bearded sprangletop from flooded rice fields or area adjacent to rice field levees; all other plant species from areas adjacent to rice field levees.
d
Corrected root mass was calculated using Equation 4 Soil mass was calculated using Equation 5.
Table 3 Carbon content and mass of weed and rice samples from pots maintained in greenhouse or field environments, and the application of a mathematical correction for soil contamination in roots a,b
Species
Growth environment
C content (C fraction 3 100) Mass Shoot Root Shoot Root Corrected rootc,d Soil calculatedc -% - -g plant21 -Barnyardgrass Greenhouse 40.7 f 28.9 ab 27.1 a 7.8 c 5.3 2.3 b
Field 41.5 d–f 11.5 c–e 17.2 a–c 41.2 ab 9.6 31.4 a Bearded sprangletop Greenhouse 42.5 a–d 27.2 a–c 20.2 ab 7.9 c 4.6 3.0 b
Field 43.5 ab 19.0 a–e 29.0 a 18.4 bc 6.8 9.8 b Broadleaf signalgrass Greenhouse 42.1 c–e 32.0 a 17.1 a–c 4.7 c 2.9 1.5 b
Fall panicum Greenhouse 42.9 a–c 26.9 a–c 29.3 a 12.2 c 7.0 5.1 b
Lemont rice Greenhouse 42.1 b–e 25.1 a–d 21.9 a 10.7 c 5.9 4.6 b
Field 41.5 d–f 9.8 de 19.3 ab 59.2 a 13.1 45.9 a
PI 312777 rice Greenhouse 40.9 ef 16.1 b–e 18.9 ab 14.7 c 5.4 9.1 b
Field 41.2 d–f 7.8 e 17.6 a–c 51.7 a 8.6 42.9 a
not significant at
P 5 0.05 a
Plants were grown in flooded pots in soil in the field or greenhouse during 2007.
b Values in columns are the estimated means according to an LSmeans test Values followed by the same letter were not different according to LSmeans (P 5 0.05) c
Corrected root mass was calculated using Equation 4 Soil mass was calculated using Equation 5.
d Corrected root mass Main effect means for growth environment; field 5 8.12 g and greenhouse 5 5.18 g (P 5 0.0042) Main effect means for species (P 5 0.0865) Species 3 growth environment interaction (P 5 0.1012).