Rising Atmospheric Carbon Dioxide and Seed Yield of Soybean Genotypes

ally show less seed yield response than vegetative response with a subsequent decline in apparent harvest If intraspecific variation to rising atmospheric CO2 exists in soybean index as CO2 increases (Cure and Acock, 1986; Prior [Glycine max (L.) Merr.], such variation could be used to select for optimal, high-yielding cultivars. To quantify the range and determine and Rogers, 1995). This has been observed not only for the basis for variation in seed-yield with increasing CO2, eight ancestral soybean (‘Bragg’, Baker et al., 1989; ‘Fiskeby’, ‘Clark’, and one modern soybean cultivar differing in determinacy, maturity Ziska et al., 1998), but also for rice (Moya et al., 1998) group, and morphology were grown to reproductive maturity at two and wheat (Manderscheid and Weigel, 1997). This sugCO2 partial pressures, 40 Pa (ambient) and 71 Pa (elevated). Experigests that agronomic cultivars in current use may be illments were replicated three times in temperature controlled glasssuited to maximize the seed yield response with increashouses during 1998 and 1999. Although all cultivars showed a signifiing atmospheric CO2; consequently, evaluation of a cant increase in seed yield with elevated CO2,(|40%) Mandarin, an wider range of germplasm may be necessary to maxancestral indeterminate cultivar, showed a greater relative response imize the seed yield response to future CO2 levels. At of seed yield to increased CO2 than did all other cultivars (|80%). present, no systematic effort to select for CO2 responThe observed variation in seed yield response to CO2 was not correlated with any vegetative parameter. At maturity, significant correlasiveness for yield among soybean cultivars has been attions in the relative response of seed yield to CO2 were observed for tempted. both pod weight per plant and seed weight from branches. The later To exploit genotypic variation efficiently, it is necesobservation suggests that the sensitivity of seed yield response to CO2 sary to know which physiological or morphological traits was associated with plasticity in the ability to form new seed in axillary are associated with the maximum seed yield response branches in a high CO2 environment. Genotypic differences in the to elevated CO2. Obviously, the primary basis for the seed yield response among existing ancestral soybeans suggests that increase in growth and yield is the CO2-induced stimulasufficient germplasm is available for breeders to begin selecting lines tion of photosynthesis. However, it can be difficult to which maximize soybean yield in response to increasing atmopredict the response of seed yield from individual leaf spheric CO2. measurements since changes in the amount of acclimation or down-regulation can influence the long-term response of photosynthesis to elevated CO2 (Bunce, F studies have attempted to utilize genotypic vari1992). ation in the response to increasing atmospheric CO2 In contrast to interspecific comparisons (Bunce, as a means to maximize growth or yield within a given 1997), no data have established whether intraspecific agricultural species. Significant variation in yield by elevariation in growth stimulation by elevated CO2 is revated CO2 has been observed among cultivars of cowpea lated to differences in photosynthetic response or accli[Vigna unguiculata (L.) Walp.] (Ahmed et al., 1993), mation, although the possibility cannot be dismissed. rice (Oryza sativa L.) (Ziska et al., 1996; Moya et al., Other responses are also affected by CO2 and could alter 1998), and wheat (Triticum aestivum L.) (Manderscheid the allocation or partitioning of photosynthate among and Weigel, 1997). Genotypic variation in the response different organs with subsequent affects on photosynof early growth of tomato (Lycopersicon esculentum thetic acclimation and productivity. In rice for example, Mill.) (Lindhout and Pet, 1990) and wild radish (Raphuthe ability to respond reproductively to increased CO2 nus raphanistrum L.) (Curtis et al., 1994) to elevated in the field is associated with increased tiller formation CO2 have also been observed. Heritable variation in the (Moya et al., 1998). Newer cultivars which limit tillering response to elevated CO2 has, in fact been observed in show a poorer seed yield response to elevated CO2 wild radish with respect to stomatal response (Case et (Moya et al., 1998). al., 1998). If variation can be exploited to convert addiOne difficulty in selecting soybean genotypes which tional atmospheric CO2 into seed yield, then significant are CO2 sensitive is that space and time constraints limit increases in productivity could be achieved with relathe number of lines which can be examined concurrently tively low input and minor environmental costs. at a high CO2 environment. However, most U.S. genoIt has been argued that empirical selection for yield types were derived from a small number of ancestral will automatically select genotypes that are the most soybean cultivars brought into the USA in the early responsive to rising atmospheric CO2 (Kimball, 1985). 1900s (Carter et al., 1993). In the current experiment, That is, in the future as atmospheric CO2 continues to we utilize these genotypes (which represent an assortrise, breeders will naturally select the most CO2 sensitive ment of morphologies, determinacies, and maturity cultivar. However, it needs to be emphasized that many groups) to assess the sensitivity of seed yield to an encurrent cultivars utilized by breeders and growers generriched CO2 environment. By using ancestral lines as a starting point, we also attempted to identify characterisClimate Stress Laboratory, USDA-ARS, Bldg 046A, 10300 Baltimore tics associated with seed yield responsiveness to CO2. Avenue, Beltsville, MD 20705. Received 11 May 1999. *Corresponding author ([email protected]). Abbreviations: AHI, apparent harvest index; DAS, days after sowing; PPFD, photosynthetic photon flux density. Published in Crop Sci. 41:385–391 (2001).