Characterizing Genotype Specific Differences in Survival, Growth, And

Environmental and Experimental Botany 55 (2006) 61–69

Characterizing genotype speciﬁc differences in survival, growth, and reproduction for ﬁeld grown, rapid cycling Brassica rapa Martin G. Kelly∗

Department of Biology, Buffalo State College (SUNY), 1300 Elmwood Avenue, Buffalo, NY 14222-1095, USA

Accepted 30 September 2004

Abstract

Rapid cycling Brassica rapa (RCBr) develops rapidly, and has both small adult size and a brief life cycle. Yet, in spite of many investigations using RCBr, extremely few plant ecologists have used this plant in the field. This study is the first to describe the genotype specific variation in traits describing survival, growth, and reproduction for field grown, RCBr. I also identify traits associated with fitness. Five genotypes of RCBr were used: standard, anthocyaninless, yellow-green, anthocyaninless and hairless, and anthocyaninless and yellow-green. Plants were grown outside in a “common garden”. Eight plant traits were measured: life span, height, growth rate, leaf size, number of flowers and fruits, fruit set, and fitness. All traits, except life span, differed significantly among the five plant genotypes. Correlation analysis revealed that fitness increased as each of these of seven plant traits increased. This study demonstrates that RCBr can serve as a model organism in ecological field studies. © 2004 Elsevier B.V. All rights reserved.

Keywords: Field ecology; Rapid cycling Brassica rapa; RCBr; Model organism

1. Introduction B. rapa has established weedy and naturalized populations in North and South America, Australia, and Asia One species that botanists have broadly employed in (Warwick and Francis, 1994, Part V,p. 19). Thus, many all areas of research is Brassica rapa (syn. campestris). field studies have been done with it to understand the This species is endemic throughout Europe eastward consequences of field release of genetically modified to Siberia (Warwick and Francis, 1994, Part V, pp. organisms (Jorgensen and Andersen, 1994; Hauser et 11, 19). This plant species is also widely cultivated in al., 1997, 1998a, 1998b; Snow and Palma, 1997; Snow cooler climates (Pak Choi, Turnip Rape, Choy Sum, et al., 1999; Pertl et al., 2002; Halfhill et al., 2003; Zhu Chinese Cabbage, Tendergreen, Turnip, Sarson, and et al., 2004). Broccoli Raab; Williams and Hill, 1986). Moreover, Of special importance to this investigation is rapid cycling B. rapa (RCBr). RCBr was derived using classical methods of artificial selection and breeding ∗ Tel.: +1 716 878 4608; fax: +1 716 878 4028. (Williams and Hill, 1986). Under optimal conditions E-mail address: [email protected].

Table 1 rapa grown in growth chambers and the field concluded Selected ecological studies conducted with Brassica rapa presented that it was not possible to use the phenotypic outcomes in chronological order for plants grown in growth chambers as predictors for References Brassica rapa Experiments in plants grown in the field. Miller and Schemske (1990) Rapid cycling Growth chamber Though several ecological investigations have Agren and Schemske (1992) Rapid cycling Growth chamber used RCBr, ecologists—except Kelly and Terrana Agren and Schemske (1993) Rapid cycling Growth chamber (2004)—have not employed RCBr in the field (Table1). Agren and Schemske (1994) Naturalized Field and greenhouse Before RCBr can be applied as a model organism to Jorgensen and Andersen Naturalized Field more substantial ecological questions two points must (1994) be established. First, that ecologically relevant pheno- Miller (1995) Rapid cycling Greenhouse typic variation exists in field grown plants. Second, that Nakamura et al. (1995) Naturalized Field rapid cycling forms of this species grown in a natural Schmitt et al. (1995) Rapid cycling Greenhouse Davis et al. (1996) Rapid cycling Growth chamber setting can be used as model for native or naturalized B. Gurevitch et al. (1996) Rapid cycling Greenhouse rapa. Here, I characterize the growth of RCBr in a field Klaper et al. (1996) Rapid cycling Greenhouse setting. I identify some individual traits associated with Miller (1996) Naturalized Greenhouse plant survival and reproduction in a field grown popu- Mitchell-Olds (1996) Naturalized Greenhouse and lation of RCBr. I see this fundamental goal only as a growth chamber Pilson (1996) Naturalized Field first step. Ultimately, the second goal must be explicitly Hauser et al. (1997) Naturalized Field met before ecologists adopt RCBr as a model organism Hauser et al. (1998b) Naturalized Field for native or naturalized B. rapa in experiment based, Hauser et al. (1998b) Naturalized Field field research. Siemens and Mitchell-Olds Naturalized Field and growth (1998) chamber Stowe (1998) Rapid cycling Laboratory Kleier et al. (1999) Rapid cycling Growth chamber 2. Materials and methods Sleeman and Dudley (2001) Rapid cycling Greenhouse Pertl et al. (2002) Naturalized Field 2.1. Plant material Siemens et al. (2002) Naturalized Field Hauser et al. (2003) Cultivar Field Kelly and Terrana (2004) Rapid cycling Field RCBr was derived from a global collection of B. rapa (L.) varieties (Williams and Hill, 1986). Plants were selected for the following six qualities: reduced RCBr has a life cycle of 35–40 days, from parental size at maturity, minimum time from germination to seed sown to offspring seed harvest (Williams and Hill, flowering, uniformity of age at first flowering, high 1986). Compared to normal B. rapa, which can produce flower production, rapid maturation of seeds, and lack two generations in a year, under optimal conditions of seed dormancy (Tomkins and Williams, 1990). In- RCBr can produce 10 generations in a year. Thus, the dividuals that flowered fastest were used as the base potential applications of RCBr to experimental botany population. These individuals were out-crossed to gen- are diverse (Musgrave, 2000). erate seeds. In the next generation, the 10% of the off- After Williams and Hill (1986) summarized the spring population that flowered first were selected as results of their selection for rapid cycling Bras- parents. These 288, or more, plants were mass polli- sica species, plant ecologists were among the early nated to produce the next generation of seeds; artifi- adopters. Ecologists have since employed both nat- cial selection continued until the response to selection uralized and rapid cycling varieties of B. rapa in a was stabilized (Williams and Hill, 1986). Under op- wide variety of studies (Table 1). Though Miller and timal laboratory conditions, RCBr flowers within 16 Schemske (1990) were the first to publish ecological re- days of seed germination (Williams and Hill, 1986). It search using RCBr, almost all of our knowledge about is important to recognize that RCBr retains consider- the growth and development of RCBr is based on plants able isozymic variation (Williams and Hill, 1986). In raised in controlled environments (Table 1). However, addition, when inbreeding is forced, fitness is signifi- Torresen and Lotz (2000) in a direct comparison of B. cantly reduced (Evans, 1991). Both outcomes suggest M.G. Kelly / Environmental and Experimental Botany 55 (2006) 61–69 63

Table 2 Five genetically defined varieties of RCBr known to differ in phenotype Variety Genotype Phenotype Standard Rbr/Rbr Normal chlorophyll production, anthocyanin expression, and epidermal hair density Anthocyaninless anl/anl Normal chlorophyll production and epidermal hair density, lacks anthocyanin Yellow-green ygr/ygr Deficient in chlorophyll production, normal anthocyanin expression and epidermal hair density Anthocyaninless and hairless anl/anl and Hir (0–1) Normal chlorophyll production, lacks anthocyanin and has a very reduced number of epidermal hairs Anthocyaninless and yellow-green anl/anl and ygr/ygr Deficient in chlorophyll production and lacks anthocyanin, has normal epidermal hair density

that B. rapa did not pass through a genetic bottleneck mental differences, seeds were randomly assigned to in order to establish the rapid cycling lines. one of the five plots and to a grid position in the plot. RCBr are available in a wide variety of known geno- Seed sowing positions were marked as a grid with a types with distinct phenotypes. For this experiment, I #804 SoilMasterTM dibble board (6 × 8 pegs spaced compared five self-incompatible genotypes known to 4.4 cm× 6.4 cm apart). Each 6 × 8 plot of 48 seeds was differ in plant pigmentation and trichome production 53.5 cm wide and 27 cm tall. All seeds were sown on (Table 2). I chose phenotypic traits that alone, or in August 29, 2002. combination, might affect plant survival or reproduc- Experimenter sown seeds were used for two reasons: tion. I used 48 seeds per genotype, for a total of 240 (1) the randomization of individuals across the envi- seeds. I used seeds purchased from Carolina Biolog- ronment minimizes the correlation between genotype ical Supply Company (USA). RCBr seed stocks for and environment, or the correlation of the phenotype these same five self-incompatible genotypes may also with the environment because of past environmental ef- be purchased from Blades Biological Ltd. (UK). fects (Mitchell-Olds and Shaw, 1987) and (2) Wade and Kalisz (1990) convincingly argued that the application 2.2. Field study design of quantitative methods to measure natural selection was not sufficient to determine why selection operated Plants were grown outside in a “common garden” in the manner observed; they recommended the use experiment and experienced the same general grow- of experimental manipulation to identify the agents of ing conditions. Over the course of the experiment viability and fecundity selection as a complement to mean daily high and low air temperatures were 24.1 quantitative analysis. and 14.6 ◦C, and mean daily precipitation was 0.2 cm. Six students in a Plant Ecology course collected Weather data were recorded at a station located 2.3 km data. This semester long course met one evening per from the field site. The total experimental area was week (Fall 2002). The site was surveyed every week watered and fertilized three times per week with 30 L to monitor plant presence (September 5–October 17). of water containing all-purpose fertilizer (water solu- Each seed’s place was marked with an individually ble, 20–20–20 with micronutrients) at the concentra- numbered plastic stake. Death was recorded when a tion recommended for outdoor use (4 cc/L). plant could not be located at the base of its marker, Six plots were laid out in the most compact arrange- or the plant had browned and turned brittle. Individual ment possible (2 × 3). Plots were laid out on August 29, plants were identified every census; the plant’s marker 2002. The total experimental area was 1.6 m wide and was not removed until the end of the experiment (Oc- 1.1 m tall. Five plots were used for this experiment; the tober 17th). The experiment was terminated and all sixth plot contained a different set of seeds for another plants harvested three weeks after the last fruits were project. To reduce the chance effect of local environ- initiated. 64 M.G. Kelly / Environmental and Experimental Botany 55 (2006) 61–69

2.3. Individual traits measured Rbr/Rbr (6.7, S.E. = 0.58, N = 42 fruits), and ygr/ygr (7.0, S.E. = 0.0, N = 2 fruits). If a plant grew but did not Eight traits were measured for each plant. Life span fruit, its fitness (seeds per plant) was set to zero. was the total days from emergence to death. As plants were measured across time, some data were treated 2.4. Statistical methods as repeated measures for analysis. For example, plant height was measured at 14, 21, 28, 35, and 42 days; The mean and its standard error, and sample size these five measures of height across time were used (N) for each measured trait was calculated. Repeated as repeated measures of height per plant (height, mm). measures analysis of variance (ANOVA) compared the Similarly, knowing plant height at 14, 21, 28, 35, and five genotypes to determine if they differed in the av- 42 days permitted me to calculate the growth rate for erage value for a trait measured across time (StatView each plant over time. Growth rates over 14, 21, 28, 35, Reference, 1999, p. 83). The null hypothesis was that and 42 days were treated as repeat measures of growth there were no differences in RCBr traits associated rate per plant (growth rate, mm/day). At 14 days, each genotype. Pearson correlation analysis of life history plant’s largest cotyledon was measured for length and variables with Fitness was performed. The statistic (r) width. Based on the cotyledon’s heart shape (Tomkins measures the proportionate linear increase or decrease and Williams, 1990), linear size was used to calculate of two variables. The null hypothesis was that there was the area of the cotyledon based on the formula for a car- no correlation between the two variables being com- dioid (Harris and Stocker, 1998, p. 323). Similarly at 21 pared (r = 0). To determine if a correlation coefficient days, each plant’s largest leaf was measured for length differed significantly from zero, the correlation statistic and width. Based on the leaf’s oval shape (Tomkins was transformed into a variable (z) with a standardized and Williams, 1990), leaf length and width were used normal distribution; this gives the probability that r =0 to calculate the area of the leaf based on the formula for (StatView Reference, 1999, p. 44). an ellipse (Harris and Stocker, 1998, p. 93). As these For fruit set (a proportion) the Kruskal–Wallis test two measures of area (cotyledon and leaf) were corre- was used as nonparametric equivalent to the one-way lated (r = 0.734, P < 0.0001, N = 111) they were used as ANOVA comparing three or more groups (StatView repeat measures of leaf size (mm2). Reference, 1999, p. 121). The null hypothesis was that If a plant flowered, the number of flowers present the distribution of fruit set was equivalent for the five on a given survey date were counted. Flowers were genotypic groups of RCBr. Both H and P as reported counted for each flowering plant on September 19th were corrected for the effect of ties. Similarly, Spear- and 26th, and October 3rd and 10th. These four mea- man correlation analysis of fruit set with fitness was sures of flower production per plant were used as repeat performed (StatView Reference, 1999, p. 121). This measures of flowering per plant (flowers). If a plant nonparametric test measures the linear increase or de- fruited, the number of fruits present on a given sur- crease of ranks for two variables. The null hypothesis vey date were counted. Fruits were counted for each was that there was no correlation between fruit set and flowering plant on September 26th, and October 3rd fitness (ρ = 0). Both ρ and P as reported were corrected and 10th. These three measures of fruit production per for the effect of tied ranks. plant were treated as repeat measures of fruiting per plant (fruits). The ability of an individual plant to con- vert its flowers into fruits was measured by its average, 3. Results proportional fruit set (fruit set). The number of seeds per mature fruit differed across genotypes (F1,3 = 44.5, The average level of germination in the field for P = 0.006). Seed set per fruiting plant was calculated RCBr was 66.7%. Germination ranged from 50.0% for by multiplying the number of fruits per plant by the av- the anthocyaninless mutant to 79.2% for the yellow- erage number of seeds per fruit on a genotype specific green mutant. Standard, RCBr seeds germinated at a basis. The average numbers of seeds per fruit for each level of 62.5%. There were no genotype specific dif- genotype were: anl/anl (3.8, S.E. = 0.58, N = 21 fruits), ferences in the level of seed germination (G = 3.86, χ2 = . anl/anl and Hir 0-1 (4.9, S.E. = 0.47, N = 29 fruits), critical 9 49). On germination, a field grown RCBr M.G. Kelly / Environmental and Experimental Botany 55 (2006) 61–69 65 plant lived an average of 22.5 days (Table 3). Though average life span ranged from 20.0 days (anthocyanin- = 32) = 126) = 126) = 18) = 32) = 59v = 92) =38 less and yellow-green mutants) to 26.0 days (antho- N N N N N N N cyaninless mutants), life span did not vary significantly N 0.1 ( 0.6 3.2 ( 2.1 ( 0.3 ( 9.2 ( 0.05 ( with genotype (F1,4 = 0.642, P = 0.6349). 0.3 ( ± ± ± ± ± ± ± ± 6 3 8 2 6 0 4 6 ...... At 14 days, the area of one cotyledon averaged . 67.8 mm2 in size. The average size for one cotyledon ranged from 51.2 mm2 (yellow-green mutants) to 99.3 mm2 (standard RCBr). Similarly at 21 days, the 2 area of the largest leaf averaged 147.8 mm . Average = 17) 13 = 104) 2 = 104) 69 = 14) 22 = 17) 0 = 50) 72 = 76) 1 =32 1 2 N N N N N N N leaf size ranged from 72.0 mm (yellow-green mutants) N 2 0.1 ( 0.4 2.7 ( 2.3 ( 0.2 ( 8.9 ( 0.07 ( to 172.0 mm (standard RCBr). Differences in average 0.2 ( ± ± ± ± ± ± ± 2 ± 4 6 7 0 5 0 5 leaf size (mm ) were found to be significantly associ- 9 ...... ated with genotype (F1,4 = 4.763, P = 0.0014). Average plant height for field grown, RCBr plants was 86.6 mm. Average plant height ranged from 58.7 mm (anthocyaninless and yellow-green mu-

tants) to 112.5 mm (standard RCBr, Table 3). Geno- = 57) 13 = 141) 2 = 141) 58 = 10) 20 = 57) 0 = 66) 75 = 105) 0 =37 0 N N N N N N N type specific differences in average height were N 0.1 ( 1.2 4.1 ( 2.8 ( 0.2 ( 8.9 ( 0.05 ( found (F1,4 = 9.523, P < 0.0001). Similarly, growth rate 0.4 ( ± ± ± ± ± ± ± ± 6 2 0 2 0 8 6 (mm/day) for these B. rapa plants was 3.3 mm/day. 2 ...... Average growth rate ranged from 2.4 mm/day (anthocyaninless and yellow-green mutants) to 4.3 mm/day (standard RCBr, Table 3). Growth rate was also found = 39) 96 = 41) 49 = 93) 3 = 24) 6 = 93) 93 =7) 25 = 41) 2 to differ on average for these five genotypes of RCBr = 71) 3 N N N N N N N (F1,4 = 8.825, P < 0.0001). N 0.2 ( 1.8 ( 5.8 ( 5.2 ( 0.5 ( 16.5 ( 0.06 ( The average number of flowers per flowering plant 0.5 ( ± ± ± ± ± ± ± ± 8 5 4 0 2 5 5 for field grown, RCBr plants was 2.9 flowers. Aver- 9 ...... age flower number per flowering plant ranged from 0.9 flowers (anthocyaninless and yellow-green mutants) to 4.9 flowers (anthocyaninless mutants, Table 3). = 45) 112 Average flower production (per flowering plant) was = 43) 48 = 101) 3 = 30) 7 = 101) 102 = 12) 26 = 43) 3 = 77) 4 N N N N N N N significantly affected by plant genotype (F1,4 = 8.128, N 0.2 ( 3.2 ( 5.7 ( 3.3 ( 0.5 ( 30.0 ( 0.13 ( P < 0.0001). The average number of fruits per flower- 0.6 ( ± ± ± ± ± ± ± ing plant for field grown, RCBr plants was 2.1 fruits. ± 3 6 5 6 9 0 0 3 ...... Average fruit number per flowering plant ranged from . 0.5 fruits (anthocyaninless and yellow-green mutants) to 3.2 fruits (anthocyaninless mutants, Table 3). A significant genotype specific affect on the number of fruits per flowering plant was observed (F1,4 = 3.493,

P = 0.0159). The average level of fruit set in the ﬁeld = the number of measurements made). was 40.8%. Fruit set ranged from 13.4% (yellow- N green mutant) to 53.0% (standard RCBr, Table 3). The S.E. ( ± ) 172 Kruskal–Wallis test showed that fruit set (per ﬂower- 2 (mm/day) 4 †

ing plant) differed significantly among genotypes (4 (mm (%/flowering plant) 53 (number/flowering plant) 4 † (seeds/plant) 10 † (mm) 112 † (number/flowering plant) 2 d.f., H = 19.426, P = 0.0006). Average fitness for field † † †

grown RCBr was 4.9 seeds per plant. Average fitness The compacted variable analyzed by repeated measures ANOVA. † TraitLife span (days) 21 Rbr/Rbr anl/anl anl/anl and Hir (0–1) anl/anl and ygr/ygr ygr/ygr Table 3 Genotype specific means in survival, growth, and reproduction for field-grown RCBr Values are mean Flowers Growth rate Leaf size Fruit Set Height Fruits ranged from 0.6 seeds per plant (anthocyaninless and Fitness 66 M.G. Kelly / Environmental and Experimental Botany 55 (2006) 61–69 yellow-green mutants) to 10.6 seeds per plant (standard measured flower traits for both A. thaliana and RCBr B. rapa, Table 3). Average fitness differed significantly grown in the same controlled environments. From her across genotypes (F1,4 = 6.682, P < 0.0001). data, I calculate that flowers of RCBr are three times The correlation of measured traits with fitness re- larger than flowers from A. thaliana. This would facil- vealed that, on average, each of these traits was in- itate any hand-pollinations needed in the production of dividually and positively correlated with fitness. As progeny or families. In addition, the fruits and seeds plant life span increased, fitness increased (r = 0.454, produced from RCBr’s larger flowers are larger than N = 61, P = 0.0002). As leaf size increased, fitness the fruits and seeds produced by A. thaliana. Average increased (r = 0.597, N = 259, P < 0.0001). As plant fruit length in standard RCBr is 37.3 mm (Kelly, un- height increased, fitness increased (r = 0.557, N = 565, published data). While, fruit length across A. thaliana P < 0.0001). As plant growth rate increased, fitness in- genotypes ranges on average between 9.5 and 15.5 mm creased (r = 0.575, N = 565, P < 0.0001). For flower- (Myerscough and Marshall, 1973; Alonso-Blanco et ing plants, as flowers (r = 0.507, N = 421, P < 0.0001) al., 1999). Average RCBr seed mass (standard geno- and fruits (r = 0.789, N = 190, P < 0.0001) increased, type) is 1.9 mg (Kelly, unpublished data). Whereas, av- fitness increased. Lastly, as the proportion of flow- erage seed mass across A. thaliana genotypes ranges ers converted into fruits increased, fitness increased between 0.0196 and 0.0028 mg (Myerscough and (ρ = 0.697, N = 190, P < 0.0001). Marshall, 1973; Alonso-Blanco et al., 1999). Larger flowers, fruits, and seeds in RCBr are easier to handle, count, and measure than are the smaller flowers, fruits, 4. Discussion and seeds of A. thaliana. If RCBr is to be applied as a model organism to For field studies that require known genotypes more substantial ecological questions, it must be es- and phenotypes, there are two taxonomically related tablished that ecologically relevant phenotypic vari- plant model systems commonly used. One of these ation exists in field grown RCBr plants. All of the is Arabidopsis thaliana, a small mustard whose en- traits measured, except life span, differ significantly tire genome has been sequenced (Arabidopsis Genome among five genotypes of RCBr grown in the field. In Initiative, 2000). For research that requires Brassica addition, life span, leaf size, height, growth rate, flow- species, both cultivated and wild lines of B. oleracea, ers, fruits, and fruit set are each positively correlated B. rapa, and B. napus are available. The relatedness of with fitness for these RCBr plants. As these traits in- Brassica species to A. thaliana has permitted the iden- crease in amount, fitness increases. Murren et al. (2002) tification of consensus genetic markers (Brunel et al., while studying phenotypic integration in six species of 1999). In addition gene for gene alignment has been ob- Brassica (all rapid cycling varieties) determined that served between B. oleracea and A. thaliana (Li et al., plant height and fruit number were positively corre- 2003). Thus, the close relation of Brassica species to lated in RCBr. Byers (personal communication, 2004) A. thaliana makes the molecular genetic tools derived used exploratory path analysis to quantify phenotypic for A. thaliana transferable to RCBr. selection on life history traits in RCBr. She investi- If these model systems exist and are commonly gated the effect of different nutrient environments in used in field studies, does RCBr have anything to offer orienting the path between life history traits. At very plant ecologists? Like A. thaliana, RCBr is character- low Nitrogen (4.7 ppm), earliness in leafing, earliness ized by its rapid development, brief life cycle, small in flowering, and leaf size at flowering were positively adult size, and the wide availability of defined geno- related to fruit number (Diane Byers, personal commu- typic lines. Because RCBr has such a brief life span nication, 2004). At low Nitrogen (18.8 ppm) earliness (22.5 days), it is readily possible to study the evolving in leafing and flower number were positively linked genetic structure of RCBr populations in the field in a to fruit number (Diane Byers, personal communica- reduced amount of time. More importantly, can RCBr tion, 2004). At high Nitrogen (150 ppm) leaf size at provide any advantages over A. thaliana in ease of use? flowering and flower number were positively related to Many experimental manipulations in plant ecology rely fruit number (Diane Byers, personal communication, on breeding designs to generate seeds. Weinig (2002) 2004). M.G. Kelly / Environmental and Experimental Botany 55 (2006) 61–69 67

Before ecologists adopt RCBr as a model for native relationship between the start of flowering and flower or naturalized B. rapa in field research, its ability to production; though plants that initiated flowering ear- respond comparably must be established. Torresen and lier tended to produce more flowers, they also pro- Lotz (2000) in a comparison of B. rapa grown in growth duced fewer fruits than plants that flowered later. In chambers to the field concluded “It is not possible to use addition, Nakamura et al. (1995) found that as plant directly growth reactions obtained in growth chambers size increased, so did plant fecundity. While Siemens for field conditions”. Ideally, the reliability of RCBr et al. (2002) state that growth rate, as measured by will be established by performing parallel field trials changes in total leaf area across time, was positively employing RCBr and naturalized B. rapa, and measur- correlated with plant mass. ing the same plant traits in the same way. With this type of comparison, it is possible to determine if tradeoffs in measures of plant performance (such as growth versus 5. Conclusions seed production) in RCBr represent real life history tradeoffs, or are artifacts of breeding. I welcome the Under optimal growth conditions, RCBr is char- interest and efforts of other plant ecologists in this val- acterized by its reduced size at maturity, minimal idation. At present, almost all of our knowledge about time from germination to flowering, increased unifor- the performance of RCBr—except Kelly and Terrana mity at age of first flowering, high flower production, (2004)—is based on plants grown in controlled envi- rapid maturation of seeds, and lack of seed dormancy. ronments. Thus, the discussion of these results in a Musgrave (2000) made the general case for using RCBr more general context will compare the ecology of field in areas of plant research conducted in the laboratory. I grown RCBr plants to naturalized B. rapa grown in the make the case that RCBr can valuably serve as a model field. organism for ecological studies conducted in the field. For five genotypic lines of RCBr grown in the There are at least six benefits that can be realized by field, all of the phenotypic traits measured, except life plant ecologists using RCBr in the field. (1) Because span, differ significantly. Similar, significant pheno- of its origin through artificial selection from a global typic variation between naturalized B. rapa plants from collection of B. rapa, RCBr has substantial allelic vari- different selection lines or populations, has been re- ation. (2) Genetic and genotypic variation in RCBr pro- ported for field grown plants. For example, Agren and duces significant and informative variation at the phe- Schemske (1994) after one generation of artificial se- notypic level in the field. (3) For RCBr grown in the lection for trichome number found that high trichome field, all of the phenotypic traits measured, except life plants initiated flowering slightly later than low tri- span, differed significantly between genotypic lines. It chome plants, and produced more fruits than low tri- may be that past selection for rapid cycling, has com- chome plants. Nakamura et al. (1995) found popula- pressed life span to the point where slight differences tion specific differences in plant survival to bolting, are statistically difficult to detect in small field grown fruit set, and seed set. Siemens et al. (2002) observed populations. (4) RCBr is commercially available in a significant differences in growth rate between B. rapa wide variety of known genotypes with distinct, con- lines after selection for low myrosinase and low glu- trasting phenotypes. (5) The close taxonomic relation cosinolate production versus high myrosinase and high of Brassica species to A. thaliana makes the molec- glucosinolate production. Lastly, Hauser et al. (2003) ular genetic tools derived for A. thaliana transferable found that plant density affected seed production in to RCBr. (6) The larger flowers, fruits, and seeds in pure stands of naturalized B. rapa. RCBr are easier to handle, count, and measure than For RCBr plants grown in the field life span, leaf flowers, fruits, and seeds in A. thaliana. The existing size, height, growth rate, flowers, fruits, and fruit set liability in the application of RCBr to plant field ecol- were positively correlated with fitness. Similar, sig- ogy is the lack of comparable studies. Torresen and nificant phenotypic correlations between plant traits Lotz (2000) concluded that it was not possible to use and fitness have been reported for field grown B. rapa the results from B. rapa plants grown in growth cham- from different selection lines or naturalized popula- bers as predictors for B. rapa plants grown in the field. tions. Agren and Schemske (1994) found a negative Presently, all of our knowledge about the performance 68 M.G. Kelly / Environmental and Experimental Botany 55 (2006) 61–69 of RCBr—except Kelly and Terrana (2004)—is based Evans, A.S., 1991. Leaf physiological aspects of nitrogen-use effi- on plants grown in controlled, artificial environments. ciency in Brassica campestris L.: quantitative genetic variation RCBr as a model system for plant ecology remains across nutrient treatments. Theor. Appl. Genet. 81, 64–70. Gurevitch, J.D., Taub, R., Morton, T.C., Gomez, P.L., Wang, to be established. I welcome the efforts of other plant I.N., 1996. 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