What was done
The authors grew well-watered Epilobium angustifolium L. plants (perennial temperate clonal herbs that colonize nutrient-rich open habitats) from the seeds of five different genotypes in pots containing 12 liters of loamy soil maintained at high and low levels of nutrients by weekly supplying them with 25 ml of either 1.0 N (high level) or 0.5 N (low level) Hoagland's solution. The experiment lasted from April 1995 to July 1996 (two full growing seasons) and was conducted in naturally-lit controlled environment chambers housed within a greenhouse, half of which chambers were supplied with ambient air having a CO2 concentration of approximately 350 ppm and half of which were supplied with CO2-enriched air having a concentration of about 650 ppm CO2. Under these conditions, and in the second year of the study when most of the plants were flowering, nectar was extracted from the flowers and its volume and sugar concentration determined, along with its amino acid concentration and the total amino acid content per flower.

What was learned
Erhardt et al. report that "elevated CO2 significantly increased nectar production per day (+51%, p < 0.01), total sugar per flower (+41%, p < 0.05), amino acid concentration (+65%, p < 0.05) and total amino acids per flower (+192%, p < 0.001)," while noting that no significant genotype x CO2 interaction was found in any of the tested parameters.

What it means
What are the implications of these findings? The three researchers say that Galen and Plowright (1985) found that "increased nectar rewards led to longer bumblebee tenure on flowers and greater pollen receipt in E. angustifolium, and that bees visited more flowers per plant on plants with more nectar." In addition, they report that "in other plant species higher nectar rewards also usually led to increases in components of plant fitness (e.g., Thomson, 1986; Mitchell and Waser, 1992; Mitchell, 1993; Hodges, 1995; Irwin and Brody, 1999)."

References
Galen, C. and Plowright, R.C. 1985. The effects of nectar level and flower development on pollen carry-over in inflorescences of fireweed (Epilobium augustifolium) (Onagraceae). Canadian Journal of Botany 63: 488-491.

Hodges, S.A. 1995. The influence of nectar production on hawkmoth behavior, self pollination, and seed production in Mirabilis multiflora (Nyctaginaceae). American Journal of Botany 82: 197-204.

Irwin, R.E. and Brody, A.K. 1999. Nectar-robbing bumble bees reduce the fitness of Ipomopsis aggregata (Polemoniaceae). Ecology 80: 1703-1712.

Mitchell, R.J. 1993. Adaptive significance of Ipomopsis aggregata nectar production: observation and experiment in the field. Evolution 47: 25-35.

Mitchell, R.J. and Waser, N.M. 1992. Adaptive significance of Ipomopsis aggregata nectar production: pollination success of single flowers. Ecology 73: 633-638.

Thomson, J.D. 1986. Pollen transport and deposition by bumble bees in Erythronium: influences of floral nectar and bee grooming. Journal of Ecology 74: 329-341.