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How plant reproduction responds to rising CO2 has important implications for the performance of both natural (Körner et al., 1996) and agro-ecosystems (Wittwer, 1995; Bazzaz & Sombroek, 1996; Rosenzweig & Hillel, 1998). For crops, changes in the mass, number, and nutrient concentration of fruits and seeds with elevated CO2 could affect agricultural production practices and food quality (Conroy et al., 1994; Murray, 1995; Idso & Idso, 2001). Changes in reproductive success of undomesticated species could alter the composition and hence the functioning of unmanaged plant communities (Bazzaz, 1996; Grunzweig & Körner, 2001). Since vegetative phase responses to elevated CO2 are not always well correlated with those of reproductive traits (e.g., Ackerly & Bazzaz, 1995; Farnsworth & Bazzaz, 1995; Jablonski, 1997), predicting the response of plants to future atmospheric CO2 conditions requires investigation of the effects of CO2 enrichment throughout a plant's life cycle (Norby et al., 2001).
Domesticated crops and undomesticated species (henceforth referred to as wild species) often differ markedly in their patterns of carbon and nitrogen allocation, and might also be expected to differ in their reproductive responses to CO2 enrichment. Crop species have been bred to maximize yield and produce harvestable products of consistent size and quality. Indeed, intense selection by plant breeders for increased harvest index (proportion of biomass invested in the harvested organ) over the past century may have provided a mechanism for some crops to take advantage of rising atmospheric CO2 in boosting fruit and grain yield (Hall & Ziska, 2000). Most modern crop varieties also have been bred to perform best under relatively predictable and benign environmental conditions, or to show a broad range of adaptability to stress (Evans, 1993). However, wild species are subject to natural selection from pollinators and dispersal agents and are subject to more diverse allocational trade-offs to sustain growth, resource acquisition, reproduction and defense. They are therefore likely to be more plastic than crop plants in a greater number of vegetative and reproductive traits (Waller, 1988) and less likely to allocate as great a fraction of available resources to reproductive structures.
Other functional groupings of plants that have shown consistent and distinctive vegetative responses to elevated CO2 are based on photosynthetic physiology or ability to fix atmospheric nitrogen. Among herbaceous species, N2-fixing legumes are typically the most responsive to CO2, followed by nonlegume C3 species, with C4 species being the least responsive (Poorter, 1993; Wand et al., 1999). Since vegetative- and reproductive-phase physiology are often well coupled (Egli, 1998), this rank order in vegetative responsiveness to CO2 may generally hold true for other life history traits. However, physiological changes that can occur under high CO2 such as reductions in leaf nitrogen concentration ([N]), accumulation of leaf starch, and downregulation of photosynthesis (Stitt & Krapp, 1999; Körner, 2000), may affect the carbon and nitrogen supply available to reproductive organs (Lawlor, 2002) and in turn constrain the magnitude of potential responses to high CO2 during this phase of the life cycle.
Kimball (1983 ) conducted the first quantitative review of crop yield responses to elevated CO 2 and reported an average increase of 33% across 37 agricultural species, which included fiber, root/tuber, and leaf crops. Increases at high CO 2 were comparable, or in some cases less, for crops whose yield was as reproductive organs per se : +23% for fruit crops, +31% for C 3 grains, +31% for legume seed, and +12% for flower crops ( Kimball, 1986 ). Cure (1985 ) and Cure and Acock (1986 ) assembled yield data for 10 major crops (grain, leaf, tuber and fiber) and found an average increase with CO 2 doubling of 41%. Soybean (28 studies) and wheat (17 studies) were the most common species examined, and yield responses were similar to those reported for mass accumulation, with little change in harvest index. Amthor (2001 ), in a semiquantitative review of wheat yield from 50 publications, found that CO 2 enrichment increased grain mass 31%. Ainsworth et al. (2002 ) used meta-analysis on soybean experiments published from 1980 to 2000 and found that yield (pod number) was stimulated 24%, total biomass increased 37%, and harvest index declined 11% in elevated compared to ambient CO 2 -grown plants.
Reviews of elevated CO2 responses by wild species have focused primarily on the vegetative phase (Poorter, 1993, 1998; Poorter & Nagel, 2000; Poorter & Perez-Soba, 2001). Wild species appear to show less stimulation by high CO2 than do crops, and fast-growing wild species respond more to CO2 than do slow-growing wild species (Poorter, 1993, 1998). For plants grown to reproductive maturity, reviews have been largely qualitative (Murray, 1995) or have targeted particular plant groups. For example, Ackerly and Bazzaz (1995) reported a 30% enhancement in reproductive mass across six old field annuals, with the magnitude of the response depending on life-cycle stage and planting density. They found that the C3 species were stimulated more by elevated CO2 (+50%) than were the C4 species (+10%). Newton (1991) observed a trend towards higher seed weight per plant and higher individual seed weight, but a variable response in seed numbers, among nine pasture and old-field species at elevated compared with ambient CO2. Elevated CO2 effects on seed nutrients have been the least studied of seed quality parameters, and no effect was seen in initial studies of wheat, maize, soybean or Scirpus (reviewed in Newton, 1991), although Conroy et al. (1994) and Murray (1995) reported a decline in wheat ear and grain [N] under high CO2.
Our objectives in this meta-analysis were twofold: to provide a comprehensive, quantitative synthesis of published reports on plant reproductive responses to elevated CO2, and to test for differences among plant functional groups or among growth conditions in affecting the magnitude of these responses. We considered CO2 effects on the number and mass of reproductive parts, which are key measures of reproductive effort, and on individual seed mass and seed [N], which reflect seed quality. We hypothesized that the effects of elevated CO2 on reproductive effort would be quantitatively similar to CO2 effects on vegetative mass, with a similar rank order in response among legumes, nonlegume C3, and C4 plants. However, because of artificial selection for increased carbon allocation to fruits and seeds, we expected a greater response to CO2 enrichment by crops than by wild species. We also hypothesized that because of decreased leaf [N] under high CO2 (Cotrufo et al., 1998; Stitt & Krapp, 1999), which is a substantial contributor to seed protein (Murray, 1995), seed [N] would be similarly reduced. Our results provide the first comprehensive and statistically robust summary of plant reproduction under CO2 enrichment and demonstrate important differences between the likely behavior of natural vs agro-ecosystems in a higher CO2 environment.