Effects of Temperature, Water Potential, and Light onGermination Responses of Redroot Pigweed Seeds to Ethylene
Received for publication September 18, 1979 and in revised form January 11, 1980
MARK W. SCHONBECK AND GRANT H. EGLEYSouthern Weed Science Laboratory, United States Department ofAgriculture, Science and EducationAdministration, Agricultural Research, Stoneville, Mississippi 38776'
ABSTRACT
The responses of redroot pigweed (Amaranthus retrojixus L.) seeds tonine ethylene concentrations between 0.5 and 50 microlters per liter wereassessed at different temperatures and water potentials and in eithercontinuous white ight or darkness. Under all experimental treatments, theprobit-transformed percentages increased inearly with the log of theethylene concentration. In dormant seeds, the slope of the response linewas unaffected by either light or water potential but increased withdecreasing temperature. Conversely, the sope increased with increasingtemperature in a partiaBy afterripened seed lot.The ethylene response threshold for germination was little affected by
temperature or light, ranging from 0.2 to 0.7 microliter per liter, butdecreased to less than 0.1 microliter per liter at negative water potentials.Osmotic Inhibition of germination at -4 bars was largely relieved by 1mncroliter per liter ethyene. Such interactions between ethylene and otherenvironmental conditions may play an important role in the course ofgermination of soil-borne seeds.
One of the main targets in weed research is the enormousreservoir ofviable dormant weed seeds present in agricultural soils(34), which escape mechanical and chemical weed control. Becauseburied seeds are extremely hard to kill, efforts are directed mainlytoward identifying and manipulating the environmental factorsthat control their germination and emergence. Soil temperatureand water status play key roles, and brief exposure to light duringplowing can promote germination (25, 34) but the patterns ofweed emergence are poorly understood. Ethylene has receivedincreasing attention both as an endogenous or exogenous stimu-lant of seed germination, and as a potential tool in weed control.Soil injections of ethylene have successfully cleared fields ofwitchweed (Striga asiatica [L.] Kuntze 1= S. lutea L.]) seeds (5, 6),and the role ofethylene in seed germination of several other weedsis now being explored.
In unpublished work we found that dormant seeds of redrootpigweed (Amaranthus retroflexus L.) showed a roughly log-linearresponse to ethylene concentrations between 1 and 100 pl/l andthat 10, l/l enhanced germination as effectively as did continuouswhite light. Since factors such as temperature, water potential, andlight may influence the ethylene response, we investigated theinteractions of ethylene concentration with each of these factorsin the germination of redroot pigweed seeds.
'Mississippi Agriculture and Forestry Experimental Station cooperat-ing.
MATERIALS AND METHODS
Redroot' pigweed seeds were collected near the Delta BranchExperiment Station at Stoneville, Mississippi, in 1973 (seed lot A)or 1977 (seed lot C) and kept in dry storage at -20 C. Both lotsshowed 99%o viability in tetrazolium (2, 3,5-triphenyltetrazoliumchloride) tests. Seeds of lot A showed 15-20% dark germination at30 C whereas seeds from the more dormant lot C showed only 0-2%.For each replicate, 50 sound seeds were counted onto two 1- x
3-cm strips of blotter paper moistened with distilled H20. Theblotter strips were inserted into a 250-ml Erlenmeyer flask con-taining a 6-cm disc of blotter paper moistened with 5 ml distilledH20. The flask was closed with a serum cap through whichethylene was injected to give a known concentration within theflask. Immediately after injection, the flasks were placed in anincubator that maintained the desired temperature to within ± 1 C.Unless otherwise specified, seeds were incubated in darkness,receiving at most a brief exposure to a green safelight.
Preliminary work showed that in either seed lot germination(defined as protrusion of the radicle through the seed coat) oc-curred within 7 days at 15 or 20 C or at -2 to -8 bars waterpotential, and within 3 days at 0 bars and at 25 C or higher.Thereafter, little further germination took place. Because seedlingsoccasionally decayed during longer incubations at high tempera-tures, germination was recorded after 3 days unless experimentalconditions necessitated the 7-day incubation. Except where oth-erwise stated, the incubation period was the same throughout anexperiment.
Ethylene concentrations in seed flasks were measured on aShimadzu GC-3BF2 gas chromatograph to determine whetherethylene synthesis by germinating seeds and/or losses by diffusionthrough the serum cap significantly altered concentration. Wheninitially given 0.56-56 td/l and incubated at 40 C, flasks showeda 21% decrease in ethylene concentration in 24 h, and a 17% lossat lower temperatures. Because ethylene affected germinationmainly during the first 24 h of incubation (Schonbeck and Egley,unpublished) we estimated each treatment dose as the mean ofthe initial concentration and the 24-h concentration inferred fromthe above measurements. Seed flasks not injected with ethyleneaccumulated only 0.002-0.007 pd/l in 24 h, which were subthresh-old concentrations and were considered negligible in the presentstudy.
Ethylene Dose-response Curves under Various Conditions. Foreach dose-response determination, we prepared a set of 10 flasks,one with each of the following initial ethylene dosages: 0, 0.56,
2Mention of a trademark or proprietary product does not constitute aguarantee or warranty of the product by the United States Department ofAgriculture and does not imply its approval to the exclusion of otherproducts that may also be suitable.
1.00, 1.78, 3.2, 5.6, 10.0, 17.8, 32, and 56p,ll. The effects oftemperature on the germination response to ethylene were assessedby incubating five sets of 10 flasks at each of a series of tempera-tures. To cover as broad a range as possible we used seeds of lotC for determinations at 25, 30, 35, and 40 C, and seeds from theless dormant lot A for determinations at 15, 20, 25, 30, and 35 C.Lot C was entirely ungerminable at 15-20 C, and lot A germinated85% at 40 C without ethylene; hence we did not do dose-responsedeterminations for these combinations.
Incubation period was 3 days for seed lot C. For lot A, a 7-dayincubation was necessary for the 15 and 20 C determinations, andwe planned to incubate this seed lot for 7 days at all temperaturesin order to maintain consistency of method. However, at 35 C,seedling decay rendered accurate counts difficult, and we thereforeused the 3-day incubation at this temperature.To determine whether continuous white light alters the ethylene
response, two sets of flasks were incubated side by side in anincubator illuminated by cool-white fluorescent tubes giving 30± 10,E/m2. s at seed level. One set was exposed to the light andthe other was wrapped in two layers of aluminum foil or kept ina light-tight box. Five such pairs were run for seeds from lot C at30 C and five more at 35 C. Again, we used the less dormant seedlot A to investigate interaction between ethylene and continuouslight at a lower temperature, i.e. 20 C.The effects ofwater stress on the ethylene response were assessed
by incubating five sets of flasks containing seeds from lot C ateach of five water potentials between 0 and -8 bars at 35 C in thedark. Flasks were given the series of ethylene concentrations listedearlier. Negative water potentials were obtained by incubatingseeds on blotter paper moistened with PEG 6000 solutions (20). Afew flasks of seeds were also incubated at -10 and - 12 bars, and5.1 and 51,ul/l ethylene, to determine whether the gas can causesome germination under such severe stress. We also examined theethylene-water potential interaction in continuous white light (20-30,tE/m2. s). This experiment was conducted at 30 C, as germi-nation of 35 C in light is relatively high even without ethylene,and ethylene effects would be more difficult to observe at thistemperature. Seed were incubated at 0, -3, and -9 bars in 0, 0.1,1, 10, and 100 ,ul/l ethylene, with four replicates per treatment.
Statistical Analysis. We transformed germination percentagesto probits (8), which give log-linear dose response curves for seedpopulations that are normally distributed with regard to relevantphysiological parameters (3, 4, 9, 22, 23). The slope of the line isthe reciprocal of the standard deviation of the population distri-bution, and effects of other factors on this parameter are diagnosticof factor interaction.
For the nine ethylene-treated flasks in each set, we used theprocedure of Finney (8) to obtain a regression line of the form Y= a + bX, where Y is the probit of percent germination and X isthe common logarithm of ethylene concentration. We then esti-mated the response threshold as XT = (YC- a)/b, where Yc is theprobit of percent germination in the control flask (no ethylene), aand b are the empirical intercept and slope, and XT is the commonlogarithm of the threshold ethylene concentration. When controlflasks had 0%Yo germination, which cannot be transformed to prob-its, we estimated the "germinability threshold" as the ethylenetreatment immediately above the highest concentration giving 0%Yogermination.
Because of very low germination of seeds from lot C at 25 C or-8 bars, and of seeds from lot A at 15 C, data for the five sets ineach of these conditions were averaged and a single regression wasperformed.
Factor interactions were assessed by statistical analysis of valuesfor a, b, and XT. Any factor that influenced only the intercept (a)was considered to act in conjunction with ethylene, as this indi-cated a change in seed germinability but not in ethylene sensitivity.Factors that significantly influenced either response threshold
(XT) or slope (b) were considered to interact with ethylene in thecontrol of seed germination.
RESULTS
Ethylene Dose-response Relationships under Various Condi-tions. Ethylene promoted germination of dormant seeds from lotC at all four temperatures investigated, but the slope of the dose-response curve increased significantly with decreasing temperaturefrom 40 to 30 C (Fig. la, Table I). When the data were replottedto obtain temperature dose-response curves at different ethyleneconcentrations, we found that 51 ul/l ethylene lowered the mini-mum germination temperature by at least 5 C (Fig. la, inset).The ethylene response of the less dormant seeds from lot A was
relatively weak at 15 C, and b values increased significantly withtemperatures from 20 to 35 C (Fig. lb, Table I). If the data for35 C are excluded because of the shorter incubation time used atthis temperature, the trend from 20 to 30 C is still significant (P<0.05). As mentioned before, germination counts taken after 3days at 25-35 C differ little from those taken after 7 days; hencea comparison of the behavior of seed lot A with lot C over thistemperature range is possible. Dose-response slope (b) appears toincrease slightly with increasing temperature for seed lot A whileit decreases with increasing temperature in the more dormant seedlot C. Temperature dose-response curves for seed lot A wereslightly nonlinear, and became slightly steeper at 51,tl/l ethyleneconcentration (Fig. lb, inset).
Three lots of redroot pigweed seeds which had been afterripenedby prolonged dry storage at room temperature showed a greatergermination response to ethylene (10,ul/l ) at 25 C than at 15 C(data not shown). We tentatively concluded that the weak ethyleneresponse at 15 C is a general phenomenon for this species.At 30 or 35 C, continuous white light enhanced germination of
dormant seeds from lot C at all ethylene dosages without alteringthe slope for the ethylene response (Fig. 2a, Table I). Light failedto enhance germination of seeds from lot A incubated at 20 C
FIG. 1. Ethylene dose-response relationships for germination of redrootpigweed seeds at various temperatures. a: Lot C incubated in darkness for3 days at 25 (0),30 (0), 35 (A), and 40 C (A). inset: germination at 0 jil/1 (0) and 51 AIII (0) ethylene, plotted against temperature. b: Lot Aincubated in darkness for 7 days at 15 (0),20 (0), 25 (A), and 30 C (A),and for 3 days at 35 C (El). Inset: germination at 0 iLd/1 (0) and 51 AIlI(0) ethylene, plotted against temperature. Figures show mean ±f SE Ofmean, N = 5. Slopes and intercepts of regression lines are given in TableI.
Table I. Mean ± SEfor Intercept (a) and Slope (b) of Regression Linesfor Ethylene Dose-Response Relationship, and Results of Statistical Analyses ofb Values
Regression equation: Y = a + bX, where Y = probit of per cent germination and X = logiO[C2H4]. Differences in a values were highly significant ineach experiment except light for seed lot A at 20 C.
Experiment Treatment a b Statistical Analysis of b
Temperature, seed lot C 40 C 5.916 + 0.038 0.499 ± 0.023 One-factor analysis of variance, F2,12(Fig. Ia) 35 C 4.387 ± 0.065 0.691 ± 0.021 3213 P<K0005
30 C 2.600 ± 0.174 1.195 ± 0.105 _3.1,25 C 1.357a 1. 144a
Temperature, seed lot A 35 C 5.513 ± 0.072 0.705 ± 0.046'\(Fig. Ib) 30 C 4.385 ± 0.064 0.583 ± 0.039 One-factor analysis of variance, F3,16
25 C 3.591 ± 0.055 0.513 ± 0.068( = 7.88 P < 0.00520 C 3.048 ± 0.087 0.379 ± 0.03315 C 2.650a 0.153a
Light, seed lot C (Fig. 2a) 35 C, light 5.006 ± 0.089 0.680 ± 0.056 P35 C, dark 4.323 ± 0.139 0.699 ± 0.048k Paired comparson, t = 0.39, NS30 C, light 3.843 ± 0.102 0.795 ± 0.082 Paired comparison, t = 0.14, NS30 C, dark 3.050 ± 0.125 0.781 ± 0.066J
Light, seed lot A (Fig. 2b) 20 C light 3.217 ± 0.094 0.465 ± 0.098 120 C dark 3.209 ± 0.054 0.387 ± 0.036 Paired comparson, t = 0.64, NS
-3 bars 4.051l 0.668ba Experimental ethylene concentrations are as shown in the corresponding figures.b Single estimate from pooled data.
without ethylene, and caused only an insignificant increase inslope (Fig. 2b). Light was also ineffective on after-ripened seedsincubated at 20 C without ethylene, but substantially increasedgermination of seeds from the same lots incubated at 30 C (datanot shown).Dark germination of seeds from lot C at 35 C decreased with
decreasing water potential as expected, but the slope of ethylenedose-response lines remained unchanged from 0 to -8 bars (Fig.3a, Table I). At -10 bars, the seeds failed to germinate in 7 days,even at 51 ,ul/l ethylene, and only 2% germinated after a briefexposure to light and a further 7-day incubation. A similar rela-tionship between ethylene concentration and water potential wasobserved in the germination of seed lot C at 30 C in the light (Fig.3b). Germination responses to water potential were strongly non-linear at all ethylene levels, but 5.1 ,ul/l ethylene extended thelower limit for germination of seeds from lot C by about 4 bars(Fig. 3a, inset).Response Thresholds. Observed response thresholds varied con-
siderably but mean values lay mostly in the 0.1-0.7 1I/I range(Table II). With pooled data for 15 different sets of flasks wecalculated a 95% confidence interval of 0.18-0.52 ,ul/l for theresponse threshold of seeds from lot C incubated at 35 C indarkness and at 0 bars water potential.
Response thresholds of seeds from lot C declined as waterpotential decreased from 0 to -4 bars (Fig. 3a, Table II). Althoughthis trend was not quite statistically significant (P = 0.06), it wasalso noted at 30 C in the light. We repeated the experiment at alower series of ethylene levels (0.05-5 ,ul/l) to clarify the thresholdregion. At -4 bars and 35 C, the ethylene dose-response line wasmuch steeper between the threshold level, 0.1 tl/l, and about 1t,l/l (Fig. 4a) than that at higher dosages (Fig. 3a). At 40 C, thispattern was even more pronounced, with an extremely low thresh-
old of 0.026 j1/l and a distinct break in the dose-response linebetween I and 2 ,ul/ (Fig. 4b).
DISCUSSION
Redroot pigweed seed germination increased with ethyleneconcentration under all experimental conditions, but the responsewas significantly modified by temperature. In dormant seeds oflot C the slope (b) increased with decreasing temperature. Thisapparent narrowing of the population distribution with regard toethylene sensitivity may reflect changes in cellular and organellemembrane characteristics. Membrane phenomena have been im-plicated in seed germination responses to temperature (13, 28),light (14, 32) and ethylene (18), and Hendricks and Taylorson (14)reported a membrane phase shift at 32 C in dormant redrootpigweed seeds.
In direct contrast to seeds from lot C, the less dormant seedsfrom lot A showed a broadening of distribution (declining b) withdecreasing temperature. This may reflect differences between thetwo lots in membrane characteristics which influence the ethylenedose-response relationship and its interaction with temperature.Such differences could have arisen either from seed developmentand maturation in different years, or from the prolonged drystorage of seed lot A at -20 C (5.5 years compared to 1.5 years forseed lot C).
Ethylene response thresholds decreased slightly with increasingtemperature, and this trend might facilitate ethylene stimulationof soil-borne seeds when the ground is warm. No saturation levelwas observed in the ethylene response at any of the temperaturestested. Because the highest treatment (51 ,ul/l) exceeded the max-imum soil concentrations reported (20-30 ul/l) (26), saturationlevels for this regulator may not be of ecological importance.
Ethylene and light demonstrated additive coaction in the ger-
ETHYLENE CONCENTRATION, pIl/IFIG. 2. Ethylene dose-response relationships for germination ofredroot
pigweed seeds in darkness (,A) or continuous white light (0,A). a: Lot Cincubated for 3 days at 30 C (0,O) and at 35 C (A,A). b: Lot A incubatedfor 7 days at 20 C. Figure gives mean ± SE of mean, N = 5. Slopes andintercepts of regression lines are given in Table II.
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FIG. 4. Ethylene dose-response relationships of redroot pigweed seedsof lot C incubated at 0 (0) and -4 (0) bars water potential in darkness.a: 35 C. Regression equations: 0 bars, 0.91-5.1 ,l/l, Y = 4.394 + 0.591 X;-4 bars, 0.29-1.6 I/I, Y = 3.829 + 1.127 X; -4 bars, 1.6-5.1 f1/l, Y =4.053 + 0.508 X. A single regression for 0.29-5.1 ,Il/l at -4 bars gave asomewhat poorer fit. b: 40 C. Regression equations: 0 bars, 0.28-5.0 td/l,Y = 5.856 + 0.456 X; -4 bars, 0.05-1.6 ul/l. Y = 4.966 + 1.502 X; -4bars, 1.6-5.0 Ill/l, Y = 5.184 + 0.538 X. A single regression for 0.05-5 td/I for -4 bars gave a substantially poorer fit.
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ETHYLENE CONCENTRATION, pl /FIG. 5. Ethylene dose-response relationship for germination of redroot
pigweed seeds from lot C incubation in darkness at 35 C and at 0 barswater potential (pooled data from Figs. Ia, 3a, and 5a), fitted to asymptoticequation. Dotted line is the linear regression for 5.1-51 p1/1 ethylene: Y= 4.242 + 0.833X, from which Xr = (YX - a)/b = -0.19, and [C2H4JT =0.65 p1/1. Solid curve is the equation: Y = a + b loglO([C2H4JT + IC2H4lext.)= a + b logio(0.65 ud/l + treatment concentration). Points show mean +SE of per cent germination for each treatment, N = 15.
red light demonstrated additive coaction with water potential andwith duration of imbibition before light exposure in curly dockseeds (3, 4). The reportedly synergistic action of red light andtemperature shift in Rumex obtusifolius L. seeds (31) and red lightand nitrate in rough cinquefoil (Potentilla norvegica L.) seeds (29)become nearly additive when the data are transformed to probits.
Light was ineffective on redroot pigweed seeds at 20 C, andethylene was less effective at 15-20 C than at higher temperatures.Seed germination of some species is actually inhibited by light atlow temperature, and Vidaver (33) suggested that this mechanism
Table II. Mean ± SE of Threshold Ethylene ConcentrationsFor treatments giving 0%o germination in two or more controls, table gives germinability threshold, defined as
the ethylene dose immediately above the highest dose giving 0% germination. For other treatments, table listsresponse threshold: XT = (YC- a)/b where Y, = probit of per cent germination in control flask, and XT = logio(threshold C2H4 concentration).
-3 bars -2.06b 0.009a Two of the 5 1-1l/l flasks showed no germination.b Single estimate from pooled data.
prevents germination when cool daytime temperatures foretell a
possible nighttime frost.Water stress of -4 bars severely inhibited germination of dor-
mant redroot pigweed seeds at 35 C but enhanced their ethylenesensitivity (Fig. 4a). Ethylene at only 0.9 ,ul/l considerably relievedosmotic inhibition at 40 C, an effect also observed in lettuce seeds(21). The abrupt decrease in slope around 1-2 AI/l (Fig. 4b)suggests a change in seed germination kinetics at this level. Abovethis break, the effects of ethylene concentration and water statusbecame additive. Ethylene elicited little response at -10 bars, a
water potential which even inhibits germination of nondormantseeds (Schonbeck and Egley, unpublished) and may represent thelower limit for cellular expansion.The conspicuously low ethylene response threshold at -4 bars
might indicate that water stress blocks the seeds' capacity tosynthesize ethylene. Endogenous ethylene levels regulate seeddormancy and germination in peanut (Arachis hypogaea L.) (19)and common cocklebur (Xanthium strumarium L.) (16, 17). Ketr-ing (18) stated that some hormones regulate germination via theethylene synthesizing system ofthe seeds. Burdett (1, 2) found thatimbibition at 30 C curtails ethylene synthesis in lettuce seeds as
well as inducing thermodormancy which can be relieved by ex-
ogenous ethylene. Esashi et al. (7) suggested a similar relationshipbetween ethylene synthesis and thermodormancy in cocklebur. Inboth cases, thermodormant seeds showed an enhanced sensitivityto low concentrations of ethylene, as did water stressed redrootpigweed seeds.A seed that synthesizes ethylene would probably maintain a
steady-state tissue concentration, and lesser environmental con-centrations would have little influence on the seed. The responsethreshold, [C2H4JT, observed in a dose-response determinationwould approximately equal this steady state concentration, andthe ethylene dose-response relationship would be asymptotic:
Y = a + b loglO([C2H4]T + [C2H4Jext.)where [C2H4]ezt. is the environmental ethylene concentration.Many of our results show a slight deviation from linearity whichseems to fit this equation (Fig. 5).
According to this hypothesis, any factor that influences the rateof ethylene synthesis would alter the response threshold to exog-enous ethylene. Such effects would be manifested as "synergistic"or "antagonistic" interactions in factorial experiments that includea treatment level of 0 tsl/I ethylene, but would be missed if thelowest treatment is, for example, I !d/l.
Regardless of whether ethylene synthesis is involved, the ob-served influences of water potential and possibly temperature onresponse thresholds have important ecological and agronomicconsequences. In many plants, seed germination is inhibited bywater potentials considerably higher (less negative) than the lowerlimit for seedling growth (11, 12, 24). This physiological blockmight serve to ensure that seeds germinate only when water supplyis ample for growth (11, 15) unless soil ethylene levels are sufficientto reverse the inhibition. Although soil ethylene concentrationsare usually less than 1 .l/I (10, 27), such levels could be critical iftemperature or moisture conditions have depressed the seeds'response threshold. The high soil temperatures and water deficits
characteristic of the Mississippi Delta during many summer daysmight have this effect on seeds of redroot pigweed and possiblyother weeds. Application of ethylene to the soil at such timesmight cause large numbers ofseeds to germinate and subsequentlydie for lack of moisture, thus reducing weed seed populations.
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