Oecologia (2010) 164:949–958 DOI 10.1007/s00442-010-1715-0 POPULATION ECOLOGY - ORIGINAL PAPER Fruit size decline from the margin to the center of capitula is the result of resource competition and architectural constraints Rubén Torices · Marcos Méndez Received: 21 September 2009 / Accepted: 23 June 2010 / Published online: 10 July 2010 © Springer-Verlag 2010 Abstract Plants produce repeated structures, such as the size of inner fruits in comparison with outer ones. leaves, Xowers, and fruits, which diVer in size and shape. We found that both resource competition and inXorescence One example of this is fruit size, which is commonly architecture aVected the fruit size of T. porrifolius, even observed to decrease from proximal to distal positions though this species does not have linear, indeterminate within an inXorescence. The resource limitation hypothesis inXorescences. We advance the hypothesis that, when such proposes that because proximal fruits usually develop Wrst, eVects on Wtness occur, resource competition-mediated they have temporal priority on access to resources over dis- position eVects could turn, in evolutionary time, into archi- tal fruits. The non-mutually exclusive architectural eVects tectural position eVects. hypothesis suggests that these position eVects in fruit size may also be due to inherent architectural variation along Keywords Architectural constraints · Asteraceae · infructescence axes. We separated out the eVects of Flower removal, position eVects · Tragopogon porrifolius resource competition and inXorescence architecture by removing the outer or the inner Xowers within capitula of Tragopogon porrifolius. We also studied if fruit position Introduction inXuenced germination and seedling performance in order to assess Wtness consequences of position eVects. Inner The modular nature of Xowering plants has consequences fruits were signiWcantly heavier when outer Xowers were for their phenotypes because plants are constructed of iter- removed. However, outer fruits did not signiWcantly ated—but not identical—structures, such as leaves, Xower, increase when inner Xowers were removed, suggesting later and fruits. This kind of phenotypic variation at the intra- fruits were limited by the development of early fruits. Our individual scale is critical to any understanding of plant Wndings also suggest that architectural constraints restricted morphology and morphological diversiWcation, such as the great diversity of plant reproductive phenotypes (Diggle 2003). In addition, intra-individual variation may also have profound ecological implications; for example, plants Communicated by Christina Caruso. which exhibit a broad range of fruit sizes would be dis- R. Torices · M. Méndez persed by a greater diversity of fruit consumers than plants Área de Biodiversidad y Conservación, which produce a narrow variety of fruit sizes (Herrera Departamento de Biología y Geología, 2009). The intra-individual phenotypic variation sometimes Universidad Rey Juan Carlos, C/Tulipán, follows a positional pattern because each structure is pro- s/n, 28933 Móstoles, Madrid, Spain duced in a distinct positional and ontogenetic context. Present Address: An example of positional patterning is Xower and fruit size, R. Torices (&) which usually decreases from the proximal (earliest pro- Área de Botánica, Departamento de Ciencias Agroforestales, duced) to distal (latest) Xower positions within inXores- Universidad de Valladolid, Campus de los Pajaritos s/n, 42004 Soria, Spain cences (see Stephenson 1981; Wyatt 1982; Diggle 1995, e-mail: [email protected] 2003). 123 950 Oecologia (2010) 164:949–958 These so called ‘position eVects’ (Lee 1988) have been preferentially allocate more resources to early over later explained by two, non-mutually exclusive mechanisms. fruits (Brunet and Charlesworth 1995). If the disproportion- First, there can be competition for resources between Xow- ate allocation to early seeds and fruits would be advanta- ers and/or fruits. Proximal Xowers or fruits usually develop geous, we also expect that these heavier seeds will produce Wrst and thus take temporal priority on access to resources. larger seedlings. This reduces the potential for fruit and/or seed maturation In the study reported here, we combined experimental at later or distal positions within the same inXorescence and observational studies in a natural population to identify (Stephenson 1981; Lee 1988). Second, some authors have the proximate mechanisms underlying the positional proposed that proximal-to-distal declines in fruit size may decline in fruit size within the capitulum of Tragopogon also be due to inherent architectural constraints along plant porrifolius L. (Asteraceae). We separated out the eVects of axes (Wolfe 1992; see Diggle 1995, 2003 for reviews). This resource competition from those of inXorescence architec- ‘architectural eVect’ (Diggle 1995) might be mediated by ture by removing the outer or the inner Xowers within the developmental constraints, such as the amount of vascular capitulum. We also studied if fruit position would inXuence tissue supplying the organs (Wyatt 1982; Wolfe 1992; germination and seedling growth and survival. The speciWc Diggle 2003). The relative importance of resource competi- questions addressed are the following: (1) Is there posi- tion versus architectural eVects diVers widely among tional variation in fruit size within the capitulum? (2) Is this species and may even vary among diVerent Xoral organs eVect determined by resource competition or by architec- (Ashman and Hitchens 2000; Diggle 2003; Kliber and tural constraints? (3) Does the fruit position within the Eckert 2004). Nevertheless, few experimental studies have capitulum inXuence germination, seedling survival, and considered both mechanisms (Kliber and Eckert 2004). seedling size?. The investment decline in Xowers and fruits has most often been described for linear inXorescences, such as race- mes (Byrne and Mazer 1990; Susko and Lovett-Doust Material and methods 1999; Vallius 2000; Kudo et al. 2001; Wolfe and Denton 2001; Hiraga and Sakai 2007; Zeng et al. 2008). Although The species position eVects are also known in plants having other inXo- rescence architectures, such as umbels (Wyatt 1980) and Tragopogon porrifolius L. (Asteraceae), or common sal- capitula (Zohary 1950; Burtt 1977), these structures have sify, is a monocarpic perennial herb (Qi et al. 1996; Cle- been studied much less. The capitulum is basically a Xat- ments et al. 1999) native to Eurasia. It usually occupies tened raceme, where external Xowers are equivalent to disturbed sites in slightly moist habitats, although once proximal ones in a linear inXorescence and are the Wrst established, plants can withstand drought (Clements et al. to develop, whereas the innermost Xowers are equivalent to 1999). This species produces a rosette of long narrow distal ones and develop later (Burtt 1977). Many species grasslike leaves. InXorescences are terminal and bear pur- show a large variation in fruit size within capitula, with the ple Xowers, with an elongated ray and centripetal develop- outermost fruits usually being larger than the innermost ment (Lack 2007). Flowering in Spain occurs in April– (Rai and Tripathi 1982; Kigel 1992; Imbert et al. 1997; May. The plant is self-compatible (Cook and Soltis 1999). Ruíz de Clavijo 2000; El-Keblawy 2003; Picó and Koubek Fruits are one-seeded and indehiscent and bear a pappus 2003; but see Ruíz de Clavijo 1995; Chmielewski 1999). with many plumose bristles (Anderberg et al. 2007). Nevertheless, there is currently no experimental evidence supporting the relative importance of resource competition Positional variation in fruit mass in a natural population and/or architectural eVects on this positional variation within capitula. An examination of positional patterns in To conWrm position eVects on fruit mass within capitula in several inXorescence architectures would likely conWrm this species, we studied the variation in fruit size by collect- whether positional patterns are speciWc to linear indetermi- ing infructescences at all of the positions from the bor- nate inXorescences or whether they represent a general pat- der to the center of the capitulum. In the spring of 2007, we tern in Xowering plants irrespective of inXorescence sampled 20 infructescences from diVerent individuals arquitecture. (genets) in one population in central Spain within the campus In addition to evaluating proximate mechanisms of the University Rey Juan Carlos (Móstoles-Madrid: (resource competition and/or architectural constraints) that 40°19Ј40.7ЉN, 2°54Ј6.95ЉW; 661 m a.s.l.). Within capitula, cause this positional variation, it is also important to assess Xowers are packaged in series of long and short spirals whether these positional patterns have Wtness conse- (parastichies: Leppik 1977) intersecting each other. We quences. Early Xowers have the greatest likelihood of pro- assigned fruit positions following long spirals (Fig. 1). The ducing outcrossed seed, and it will be advantageous to outermost fruit was assigned to position 1, the next fruit in 123 Oecologia (2010) 164:949–958 951 Inner flowers removed Outer flowers removed Control Outer Inner Fig. 2 Picture representing the three treatments ‘inner Xowers re- moved’, ‘outer Xowers removed’, and ‘control’, applied to capitula of X Fig. 1 Tragopogon porrifolius receptacle. The numbers show how T. porrifolius in the experimental manipulation of ower competition. fruits were sampled to describe positional