When dealing with agricultural commodities such as grain crops, seeds comprise the yield; and in such cases, the biomass of one is the biomass of the other. Hence, when looking for effects of elevated CO2 on the seeds of such crops, one is naturally interested in something more than just their final biomass; and in this brief synopsis of pertinent papers we have reviewed, we consider a number of the sources of biomass production, as well as seed properties that go beyond biomass.
In a greenhouse study of the various components of seed biomass production, Palta and Ludwig (2000) grew narrow-leafed lupin in pots filled with soil within Mylar-film tunnels maintained at either 355 or 700 ppm CO2. They found that the extra CO2 increased (1) the final number of pods and (2) the number of pods that filled large seeds, while it (3) reduced to zero the number of pods that had small seeds, (4) reduced the number of pods with unfilled seeds from 16 to 1 pod per plant, and increased (5) pod set and (6) dry matter accumulation on the developing branches. These several CO2-induced improvements to key physiological processes resulted in 47 to 56% increases in dry matter per plant, which led to increases of 44 to 66% in seed yield per plant.
Sanhewe et al. (1996) grew winter wheat in polyethylene tunnels maintained at atmospheric CO2 concentrations of 380 and 680 ppm from the time of seed germination to the time of plant maturity, while maintaining a temperature gradient of approximately 4°C in each tunnel. In addition to the elevated CO2 increasing seed yield per unit area, they found it also increased seed weight, but not seed survival or germination. Increasing air temperature, on the other hand, increased seed longevity across the entire range of temperatures investigated (14 to 19°C).
Thomas et al. (2003) grew soybean plants to maturity in sunlit controlled-environment chambers under sinusoidally-varying day/night-max/min temperatures of 28/18, 32/22, 36/26, 40/30 and 44/34°C and two levels of atmospheric CO2 concentration (350 and 700 ppm). They determined, in their words, that the effect of temperature on seed composition and gene expression was "pronounced," but that "there was no effect of CO2." In this regard, however, they note that "Heagle et al. (1998) observed a positive significant effect of CO2 enrichment on soybean seed oil and oleic acid concentration," the latter of which parameters Thomas et al. found to increase with rising temperature all the way from 28/18 to 44/34°C. They also found that "32/22°C is optimum for producing the highest oil concentration in soybean seed," and that "the degree of fatty acid saturation in soybean oil was significantly increased by increasing temperature." In addition, they determined that crude protein concentration increased with temperature to 40/30°C.
In commenting on these findings, Thomas et al. note that "the intrinsic value of soybean seed is in its supply of essential fatty acids and amino acids in the oil and protein, respectively." This being the case, the temperature-driven changes they identified in these parameters, as well as the CO2 effect observed by Heagle et al., bode well for the future production of this important crop and its value to society in a CO2-enriched and warming world. They do note, however, that "temperatures during the soybean-growing season in the southern USA are at, or slightly higher than, 32/22°C," and that warming could negatively impact the soybean oil industry in this region. For the world as a whole, however, warming would be a positive development for soybean production; while in the southern United States, shifts in planting zones could readily accommodate changing weather patterns associated with this phenomenon.
In another soybean study, Ziska et al. (2001) grew one modern and eight ancestral genotypes in glasshouses maintained at atmospheric CO2 concentrations of 400 and 710 ppm, finding that the extra CO2 increased photosynthetic rates by an average of 75%. This enhancement in photosynthetic sugar production led to increases in seed yield that averaged 40% for all cultivars, except for one ancestral variety that exhibited an 80% increase in seed yield. Hence, if plant breeders were to utilize the highly CO2-responsive ancestral cultivar identified in this study in their breeding programs, it is possible that soybean seed yields - and all of the other good things that go with it - could be made to rise even faster and higher in the days and years ahead.
References
Heagle, A.S., Miller, J.E. and Pursley, W.A. 1998. Influence of ozone stress on soybean response to carbon dioxide enrichment: III. Yield and seed quality. Crop Science 38: 128-134.
Palta, J.A. and Ludwig, C. 2000. Elevated CO2 during pod filling increased seed yield but not harvest index in indeterminate narrow-leafed lupin. Australian Journal of Agricultural Research 51: 279-286.
Sanhewe, A.J., Ellis, R.H., Hong, T.D., Wheeler, T.R., Batts, G.R., Hadley, P. and Morison, J.I.L. 1996. The effect of temperature and CO2 on seed quality development in wheat (Triticum aestivum L.). Journal of Experimental Botany 47: 631-637.
Thomas, J.M.G., Boote, K.J., Allen Jr., L.H., Gallo-Meagher, M. and Davis, J.M. 2003. Elevated temperature and carbon dioxide effects on soybean seed composition and transcript abundance. Crop Science 43: 1548-1557.
Ziska, L.H., Bunce, J.A. and Caulfield, F.A. 2001. Rising atmospheric carbon dioxide and seed yields of soybean genotypes. Crop Science 41: 385-391.