Atmospheric CO2 was estimated to be 280µmol mol-1 before the industrial revolution era and is currently estimated at 390µmol mol-1with predictions to increase in the future. Several vital plant functions, such as photosynthesis, transpiration, and biomass are affected by increases in atmospheric CO2 levels. For wild plants, seed quantity and quality influence their fitness and seed production, and in the presence of elevated CO2, this varies considerably among species of C3 annual plants. In this study, Hikosaka et al. perform a meta-analysis to examine whether seed production is limited by nitrogen availability or concentration. In general, studies have shown that increased ambient CO2 leads to increased N per plant and increased seed production, as seed mass per plant has increased by 28–35%. However, this study shows that various species respond differently, and understanding these differences is important to maintaining C3 plant productivity in an enriched CO2 world. The authors predict that the N contained in reproductive organs accounts for the variation in the increased CO2 response of seed production.—Taylor Jones
Hikosaka, K., Kinugasa, T., Oikawa, S., Onoda, Y., Hirose, T., 2011. Effects of elevated CO2 concentration on seed production in C3 annual plants. Experimental Botany 62, 1523-1530.
Kouki Hikosaka and colleagues performed a meta-analysis to examine the variation of seed production in annual C3 plants under increased CO2 concentrations. The enhancement ratio of seed mass per plant due to increased CO2 was 0.75–4.45 for rice, 0.93–1.87 for soybean, and 0.88–2.07 for wheat, but these differences could be attributed to different growth conditions. The authors also determined that seed production is not linked to change in total plant biomass. For example, a study cited by Jablonski et al. reported increases in fruit and seed yield of 12% and 25% respectively with a 31% increase in total plant mass. CO2 responses also differ between reproductive tissues in different species. The boll yield of cotton increased by 40% and lint increased by 54%. Studies also demonstrate an increase in pod wall mass of soybeans and greater increases in mass of reproductive structures of Xanthium canadense in low N environments compared to high N environments.
Hikosaka and colleagues also examined N use in reproductive growth and CO2 response and hypothesized that the differences in response to increased CO2 are either a result of different limiting factors (such as CO2 or N) or a constant N limitation. Compilations of studies showed that the seed mass of C3plants grown at varying levels of CO2 was not correlated with N concentration, but rather demonstrated a 1:1 correlation between seed mass per plant and N per plant. This supports the second hypothesis that seed production is only enhanced when N is more readily available.
Seed N levels experienced variation in some species more than others and may be enhanced in some plants by absorbing more during growth or retranslocating N from vegetative to reproductive organs. The N-fixing legumes showed the greatest N enhancement, but significant variation in other areas such as biomass, photosynthetic rates, and leaf-area index. In several studies, the N concentration in seeds remained the same while vegetative N concentrations decreased, showing that the vegetative organs are less conservative. The authors also examined seed N per plant and seed N concentration of three species: grass, legumes, and non-legume dicots. The studies showed increases in seed mass per plant in the presence of elevated CO2 and increases in seed N per plant for legume species.
Three quarters of variation in seed-mass enhancement was attributed to increases in seed N per plant, while one quarter was attributed to reductions in N concentration. The reduction of N concentration was noticed most in legumes, and not as much in the other species. Also, in several grass species, the presence of albumen allowed storage of high amounts of carbohydrates while N levels were low.
Overall, N limitation is key to understanding seed production and responses in elevated CO2 environments. Plants experience increased seed production when they undergo increased N acquisition or decreased N concentration. Legumes are N-fixing and grasses often experience increases in seed production through increases in N acquisition and decreases in N concentration. Decreases in N concentration may not decrease the quality of the seeds if it results from increased albumen content without reduced N in embryos.