New Phytol. (1991), 119, 299-305

Are seedlings from small seeds alwaysinferior to seedlings from large seeds ?Effects of seed biomass on seedling growthin Pastinaca sativa L.

BY STEPHEN D. HENDRIX\ ERIC NIELSEN^ TODD NIELSEN^ ANDMARK SCHUTT^

^Department of Botany., University of Iowa, Iowa City, IA 52242^Department of Biology, University of Iowa, Iowa City, I A 52242

{Received 16 August 1990; accepted 30 June 1991)

S U M M A R Y

Previous field studies of recruitment in Pastinaca sativa L. indicate that more seedlings from small seeds than fromlarge seeds survive short-term droughts. ' I'o explore this phenomenon, the effects of variation in seed biomass inPastinaca sativa on embryo size and seedling characteristics 10, 20, 30 and 40 days after emergence wereinvestigated. On the basis of nnost characteristics;, seedhngs from large seeds should be superior to seedlings fromsmall seeds. Embryo length and cotyledon area were positively related to seed biomass, as were above groundbiomass, total leaf area, and root biomass in all harvests. Total seedling biomass was positively related to seedbiomass in the 10, 20, and 30 day harvests, but not the 40 day har\'est. However, the ratio of maximum rootlength/total leaf area was negatively related to seed biomass in the 10 and 20 day harvests, suggesting that, underdrought conditions, seedlings from small seeds may transpire less water than those from large seeds relative to theirability to reach water supplies. Although seedhngs from larger seeds had greater root biomass, this may be of littleadvantage under drought conditions since approximately 90 '•'„ of the root biomass is in the upper 10 cm of soilwhich dries out quickly. The advantage that seedlings from small seeds have under drought conditions is short-Iiyed, lasting about 20 days in the glasshouse and an estimated 60-90 days in the field. This advantage is short-term because the relationship between seed biomass and resource allocation patterns changes during early seedlingdevelopment.

Key words: Seed biomass, seedling growth, drought tolerance, Pastinaca sativa.

germination (Black, 1959; Cidecivan & Malloch,INTRODl-CTION !^^_^ . . , . .,nci\ ' -J

1982; Weis, 1982), greater or more rapid emergenceThe importance of interspecific variation in seed from deeper depths of sowing (Black, 1956; Harperbiomass in the critical processes of dispersal and & Oheid, 1967; Haskins & Gorz, 1975 ; Wulff, 1986;population recruitment is well known (Salisbury, Weller, 1989), and less stringent requirements for1942; Harper, Lovell & Moore, 1970; Baker, 1972; emergence with respect to litter and herbaceousHarper, 1977; Fenner, 1983; Marshall, 1986). cover (Winn, 1985). As a consequence of theseIntraspecific variation in seed biomass, which com- effects, large seeds may give rise to better competitorsmonly occurs in the range of two to sixfold (Anderson, 1971 ; Wulff, 1986).(Salisbury, 1942; Twamley, 1967; Janzen, 1977, Far few er advantages of small seeds over large1978; Gross & Soule, 1981; Howe & Richter, 1982; seeds have been suggested. Small seeds may gain aWeis, 1982 ; Hendrix, 1984; Stanton, 1984; Thomp- competitive advantage over large seeds due to earlierson, 1984; Winn, 1985; Hendrix & Sun, 1989), may germination (Black & Wilkinson, 1963) and whenbe equally important. small seeds are dispersed further than large seeds

Most studies of the effects of intraspecific variation (Jackson, 1981; Howe & Richter, 1982), they mayin seed biomass on germination and early seedling have an increased probability of reaching favourablegrowth demonstrate that large seeds have many microsites.advantages over small seeds. For example, relative to In Pastinaca sativa L. (Apiaceae), field studies ofsmall seeds, large seeds usually have greater percent the effects of seed biomass on emergence and

300 S. D. Hendrix and others

1-25

1-00

075

0-50 -I

025

= 0.245, P = <0-0001

Seed biomass {mg)

Figure 1. The relationship between seed biomass andembr\'o length in Pastinaca sativa. The regression equation

survivorship indicate that seedlings from small seedscan have an additional important advantage overseedlings from large seeds (Hendrix S: Trapp, 1989;Hendrix & Trapp, 1992). These studies demonstratethat more seedlings from small seeds than from largeseeds survived short-term summer droughts of twoto four weeks. In this study, we examined the effectsof seed biomass on early seedling morphology in P.sativa to determine if developmental variation re-lated to seed biomass can account for the greaterability of seedlings from small seeds to withstandsbort-term droughts.

M.\TER1ALS AND METHODS

Natural history, seed collection and preparation

Pastinaca sativa is an introduced, monocarpic per-ennial common in ruderal habitats in North America(Gleason, 1952). After an upright rosette stagelasting one to three years (S. D. Hendrix, personalobservation), plants produce a flowering stalk inearly spring which terminates in the primary inflore-scence (umbel). Secondary and tertiar>- umbelsmature sequentially on side shoots of the mainflowering stalk. Each flower produces tw o one-seeded mericarps commonly referred to as seeds.

Within a plant, seed biomass decreases with umbelorder and tertiary seeds weigh approximately 60 ",>of primary seeds (Hendrix, 1984). Levels of in-breeding depression in seeds from different umbelsare likely to be similar because the pattern of umbeldevelopment prevents geitonogamous pollinationsbetween umbels and temporal dioecism preventsself- and geitonogamous pollination within umbels

(Cruden & Hermann-Parker, 1977). Between plants,mean seed biomass for an umbel order is positivelyrelated to the basal diameter of the flowering stalk(Hendrix, 1984). In the midwestern United States,seeds mature in late July and are wind dispersed. P.sativa does not reproduce vegetatively.

Seeds were collected from plants in an abandonedpasture in Johnson Co., Iowa (Clear Creek Town-ship, Section 36). Seeds from the primary, sec-ondary, and tertiar\' umbels of five mature largeplants (basal stem diameter of the flowering stalk >12 mm) were collected in early August, 1988, beforedispersal, and allowed to after-ripen at room tem-perature for approximately 90 days. Seeds not filledwith endosperm or attacked by an unidentified seedpredator (Hymenoptera: Chalcidae) were removedand forty-eight seeds from each umbel order (pri-mary, secondary and tertiary) of each parent plantwere randomly selected. Each seed was weighed tothe nearest 0-1 mg and placed in a 96 cm^ potcontaining a standard glasshouse soil mixture. Seedswere allowed to imbibe water for 24 h at roomtemperature and were then dark stratified for 45 daysat 10 °C/5 °C (8/16 h). Stratification of seeds of P.sativa increases both the synchronization and rate ofgermination (Hendrix, 1984).

Experimental procedures

Following stratification, pots were transferred to theglasshouse. Seedlings were assigned to harvests at10, 20, 30, and 40 days after emergence so as toensure a full range of seed weights in each harvest.The total number of seedlings harvested at 10, 20,30, and 40 days w as 50, 50, 49, and 30, respectively.One-way ANOVA indicated that seed biomass didnot differ significantly between harvests. Fifteendays after emergence, remaining seedlings w-eretransplanted to 512 cm^ pots to prevent inhibition ofroot development. Seedlings did not become root-bound in the larger pots.

At each harvest, soil was washed from the rootsand the length of the longest root was measured tothe nearest mm. The area of cotyledons (if stillpresent) and of each mature leaf were measured tothe nearest O'Ol mm" using a portable leaf area meter(L.]-Cor Model Ll-3000). To improve the accuracyof leaf area measurements, the mean of ten replicatemeasurements was calculated for each leaf andcotyledon.

Leaf, stem, and root tissue of each seedling wereplaced in separate vials, dried for 72 h at 60 °C, andweighed to the nearest O'l mg. To determine a depthprofile of the distribution of root biomass, the rootsystem often randomly selected seedlings from eachof the first three harvests were rehydrated, straight-ened, and cut into sections representing maximumdepths of 0-5 cm, 5-10 cm, 10-20 cm, etc. Sectionswere dried at 60 °C for 72 h, and weighed to the

Seed mass and seedling growth in Pastinaca sativa

30 10 days: r^ = 0-438, P = <0-001

301

20-

10 -

120 -

100-

80 -

60

40 ^

20

0

20 days: r = 0-201, P = <0-001

• t

1

—1 1 1 1 1—' 1 ' [ 1 1

2 3 4 5 6 7

600 n

500

400-

300

200 -

100

30 days: r = 0-24.2, P - <0-001 1400

1200 -

800 -

600-

400-

7no

40 days: r

•

• •

^ ^

•

' - 0-106,

*

•

•• •

P = <0 006

• •

Seed biomass (mg)

Figure 2. The relationships between initial seed biomass and total seedling biomass of Pastinaca sativa at 10days, 20 days, 30 days, and 40 days after emergence. Regression equations are: Y = —O'O4 + 2-39A' (10 days),Y'^ ]2-O7-HlO-61X(2Odays), Y= 111-36 + 44-84X (30 days), V = 473-89+101-31,Y (40 days).

nearest 0-1 mg. Root samples from the 40 da\' harvestwere inadvertently destroyed before these measure-ments could be taken.

An additional twenty seeds from each of the fiveparent plants, representing the range of seed bio-masses produced by that plant, were used todetermine the relationship between seed hiomassand embryo size. Seeds were allowed to imbibe waterfor 48 h. Each embryo was excised and its length wasmeasured to the nearest 001 mm using an etchedmicrometer slide. In approximately 25 "^ of theseeds, the embryo was either missing due to seedpredation or damaged during excision.

Statistical analysis

Regression analysis was used to examine the re-lationship between seed biomass and embryo length.

cotyledon area at the 20 day harvest, and growthcharacteristics of seedlings at each harvest. The 20day harvest w as chosen for analysis of cotyledon areahecause at this time cotyledons were fully expandedand true leaves were beginning to form. To examinechanges in resource partitioning between above andbelow ground biomass over time, an ANCOVA withthe ratio of root/shoot biomass as the responsevariable, seed hiomass as the covariate, and seedlingage as a blocking variable was used to test for aninteraction between seedling age and seed biomass.

RESULTS

Seed hiomass was positively correlated with embryolength (Fig. 1) and had a significant positive effect oncotyledon area 20 days after seedling emergence(y=0-31+0-44.Y, fi^ = 0-381, P = 0-0001). Seed

302 S. D. Hendrix and others

Table 1. Regression equations, associated regression coefficients, and meanvalues {±SD) for the effects of seed biomass on growth characteristics ofseedlings of Pastinaca sativa 10, 20, 30, and 40 days after emergence. Nvalues for each harvest are 50, 50, 49 and 30 seedlings, respectively

Age at harvesting Regression equation Mean + SD

{a) Above ground biomass (mg)10 days Y =20 days Y =30 days F ^ 99-62 + 20-43 A40 days F = 324-48 + 39-68A'

(6) Total leaf area (mm'')10 days F = 0-38+ 0-63A"20 days F = 3-51+2-28A'30 days Y= 30-64+ 4'93X40 days F - 52-02+11-27X

(c) Root biomass (mg)10 days F = 0-02+ 0-63A20 days F = 0-79+3-81 A'30 days F = 11-74 +24-41 A'40 days F = 149-4] +61-64A:

{d) Maximum root length (mm)10 days F— 102-04 + 4'96A'20 days F = 163-69+ 4-00A'30 days F = 223-80 + 7-25A'40 davs F = 131 -80 + 43-44A'

0-436***0-179***0-150**0-069 n.s.

0-346***0-205***0-171**0-191**

0-352***0-173***0-290***0-316***

0-000 n.s.0-000 n.s.0-000 n.s.0-232**

6-9512-7738-28 ±15-68

183-60+54-08489-66 + 138-55

2-90±l-ll12-57 + 4-965O-9O±12-3298-92 ±26-75

2-53 ±1-1015-94 ±8-88

112-07 ±47-96406-01 ±117-46

121'78±36-84179-58 + 52-84253-61+71-26312-67 + 94-87

*P<0-05; **P< 0-01; ***P<O'OOI.

Table 2. Percentage of total root biomass {mean + SE)at different soil depths for seedlings o/Pastinaca sativagrown 10, 20, and 30 days, n — 10 for all harvests

Soil depth(cm)

0-5> 5-10> 10-20> 20-30

Seedling age

10 days

74-3 ±3-522-3 + 3-0

3-4+1-60-0 ±0-0

20 days

62'9 + 3'l28-0 + 3-08-4±M0-7+ 0-6

30 days

50-8 ±1-537-7 ±2-610-9±l-80-6 + 0-4

biomass was positively and significantly related toabove ground biomass at the 10, 20, and 30 dayharvests, but not at the 40 day harvest (Table 1 a). Atall harvests seed biomass had a significant, positiveeffect on total seedling biomass (Fig. 2) and total leafarea (Table 1 6).

Seed biomass was positively and significantlyrelated to root biomass at all harvests (Table 1 c) butto maximum root length at the 40 day harvest only(Table 1 d). Although larger seeds produce seedlingswith greater root biomass, approximately 90 "o of itoccurred within 10 cm of the soil surface for 10, 20and 30 day old seedlings (Table 2). The ratio ofmaximum root length to total leaf area, a character-istic potentially important to the ability of seedlingsto withstand drought conditions, was negativelyrelated to seed biomass 10 and 20 days afteremergence, but not 30 and 40 days after emergence(Fig. 3).

Relative allocation of biomass to roots and shootschanges substantially over time and with seedbiomass, as mdicated by the significant interactionbetween seedling age and seed biomass as predictorsofthe root/shoot biomass ratio (ANCOVA: i g ^^ =5'198, P = 0-002) (Fig. 4). At 10 and 20 days, therewas little variation in the ratio of root to shootbiomass, but at 30 and 40 days seedlings from largerseeds were allocating a substantially greater pro-portion of biomass to roots.

DISCUSSION

In Pastinaca, initial differences in seed biomass havea substantial infiuence on morphological character-istics of seedlings. In general, larger seeds producedlarger seedlings with greater total biomass (Fig. 2),above ground biomass, leaf area, and root biomass(Table 1). In other species (Harper & Obeid, 1967;Weis, 1982; Howe & Richter, 1982; Stanton, 1984;Wulff, 1986) larger seeds also produce larger seed-lings, as measured by a variety of different char-acteristics. In P. sativa, the positive relationshipbetween seed biomass and numerous size character-istics of seedlings with up to five true leaves is likelydue to the larger embryos in larger seeds (Fig. I).

Given the inherent advantages associated withlarger seeds in general and in P. sativa in particular,the superior drought resistance of seedlings fromsmall seeds in this species during short-termdroughts of about two to four weeks (Hendrix &Trapp, 1992) seems anomalous. However, the neg-

Seed mass and seedling growth in Pastinaca sativa 303

D9

oocc

120 1

100-

8 0 -

60 -

40 -

2 0 -

0

10 days: r = 0-207, P = <0-001

•

* *

* * ' • • • ^ " " " • - - . .

9 -\

8

7 •

6 -

5 -

4 -

3 -

2

1

30 days: r^ = 0-061 (n.s.)

40 1

30-

20 -

10 -

20 days: r^ = 0-220, P - <0 001

7

6-

5-

4

3 -

2-

1

1

- • .

40 days: r - 0-001 (n.s.)

Seed biomass (mg)

Figure 3. The relationships between initial seed biomass and maximum root length/total leaf area of Pastinacasativa at 10 days, 20 days, 30 days, and 40 days after emergence. Regression equations are: Y — 80-44 — 8'49,V(lOdays); F = 28-71-3'12^ (20 days); F - 6'64-0-35Jr (30 days); F = 3-22 +0-02X (40 days).

ative relationship between seed biomass and the ratioof maximum root length/total leaf area (Fig. 3) forglasshouse seedlings up to 20 days old provides apossible explanation for these results. We hypo-thesize that as the top soil dries out under droughtconditions, seedlings from small seeds lose less watervia transpiration than those from large seeds relati\ eto their ability to reach needed water supplies.

Although root biomass increases with initial seedsize, it is unlikely that root biomass per se benefitsseedlings from larger seeds under drought conditionsduring the early stages of development. Approxi-mately 90 % of the root hiomass occurs in the upperlOctn of soil for seedlings up to 30 days old,regardless of initial seed biomass and total rootbiomass (Table 2). Field measurements indicate thatsoil moisture to depths of 15 cm falls to 20 % of fieldcapacity (the minimum value measurable) during

brief, two week droughts (Hendrix & Trapp, 1992).Furthermore, nnaximum root length is independentof initial seed hiomass until 40 days after emergencein the glas.shouse (Table id). Thus, during earlystages of growth, seedlings from larger seeds have noadvantage in reaching water supplies at deeper soildepths and their root systems must provide water toa greater leaf surface area subject to evapotrans-piration.

The advantage that seedlings from small seedsmight have during short-term droughts is relativelyshort-lived, lasting 20 days in the glasshouse (Fig. 3),apparently hecause of temporal changes in the effectsof seed biomass on aJJocation of seedling biomass toroots and shoots. As seedlings mature, an increasingproportion of hiomass is allocated to roots relative toshoots and the greater the initial seed hiomass, thegreater the increase in allocation to roots over time

304 S. D. Hendrix and others

1-20 -,

1-00-

S 0-80 -

4DDa:

0-60 •

0-40 •

0-20

days

days

days

10 days

Seed biomass {mg)

Figure 4. Regression lines for the relationship betweeninitial seed biomass and the ratio of root to shoot biomassof Pastinaca sativa. Regression equations, regressioncoefficients, and P values are: F - O-38-O-O1 A", r ^ -O'OOO, n.s. (10 days); F = 0-28 + 0-03^, /- = 0-091, P <0-05 (20 days), F = 0-30 + 0-07A', r - 0-290, P < O'OOl {30days); F = 0-52 + 0-(.)8A', j'^ = (V125, P < 0-05 (40 days).

(Fig. 4). Furthermore, by 40 days of growth in theglasshouse, root length is significantly related toinitial seed biomass (Table \d), suggesting that bythis time seedlings from larger seeds will have anadvantage over those from smaller seeds in reachingwater supplies during droughts.

Wulffs (1986) laboratory analysis of the effects ofseed biomass on seedling growth and physiologicalperformance in Desmodium paniculatum supports theconclusion that under certain conditions seedlingsfrom small seeds may have greater drought re-sistance. When grown alone, seedlings from smallseeds maintained higher shoot-water potentials after5 days without watering. However, when seedlingsfrom large and small seeds were grown together,shoot-water potentials after a period without waterwere significantly lower in seedlings from smallseeds than in seedlings from large seeds. Theseresults suggest that in non-competitive situations,smaller seedling size may he advantagous underconditions of limited water availability (WulfT, 1986).

The advantage that small seeds have during short-term droughts may last considerably longer in thefield because 20 day old glasshouse seedlings grownunder ideal conditions without competition haveapproximately the same number of true leaves (five)as 60-90 day seedlings in the field (S. H. Hendrixpersonal observation). In the field this advantage canhave a significant impact on recruitment in P. sativa.Seedlings emerging in tbe spring account for approx-imately 80 "o of total emergence and when short-

term droughts occur during the first summer ofgrowth, the greater survivorship of seedlings fromsmall seeds relative to those from large seeds canoffset the disadvantage small seeds have in emerg-ence (Hendrix & Trapp, 1989). Because summerdroughts as short as two weeks occur two out ofevery three years (Hendrix & Trapp, 1992), re-cruitment to flowering from seeds of different sizesin P. sativa is often statistically equivalent (Hendrix& Trapp, 1989). Whether or not equivalent re-cruitment from different sized seeds in this speciesrepresents an unusual case or is typical of speciesproducing variable sized offspring remains to bemvestigated.

ACKNOW LEDGE MENT.S

We thank D. Saldana, R. Willard, M. Driscoll, and H. C.Wu for assistance in gathering data. This project waspartially supported by a Sigma Xi grant (University ofIowa chapter) to M. S. and funding from tbe University ofIowa Graduate College to S.D.H.

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