7.4.1 Carbon dioxide
Elevated [CO2] effects on soybean phenology appear to be small and not consistent. Allen et al. (1990a) summarized the effect of [CO2] on soybean developmental stages for four experiments from 1981 to 1984. They fitted vegetative stage (V) vs. DAP to a linear regression equation as follows:
where 00 = intercept of the regression on the V-stage axis and 01 = mainstem nodes per day. In general, 01 increased slightly with increasing [CO2], but the responses among the four experiments were variable.
The plastochron interval (days per mainstem node or mainstem trifoliate) tended to decrease with increasing [CO2] so that there were more mainstem nodes and trifoliate leaflets at the end of vegetative growth. We fitted the following equation of plastochron interval (1/01) versus [CO2] and obtained the following relationship for the data in Allen et al. (1990a):
Except for the slightly delayed initiation of the R1 and R2 phases of development under the severely limited [CO2] of 160 mmolmol-1, there appeared to be no effect of [CO2] on reproductive stages of development. Although the overall effects of [CO2] on phenological stages of development of soybean are minor, similar tendencies have been reported elsewhere (Hofstra and Hesketh, 1975; Rogers et al., 1984, 1986; Baker et al., 1989; also see Chapter 8, this volume).
Phenology of soybean is not likely to be impacted directly by solar radiation. However, any shift in latitude zones or planting dates will affect the day length and night length, and will likely change the phenology of the soybean plant.
Temperature exerts a major control on growth at the level of enzyme activity, protein synthesis and cell division. To a certain extent, these controls are similar at the whole organism level of the plant. Sionit et al. (1987a,b) found that the growth and development responses of soybean to CO2 enrichment increased with increasing temperature within the rather cool range of 18/12°C to 26/20°C. Vegetative processes such as rates of leaf appearance, leaf expansion and branching are enhanced by high temperatures (at least up to some threshold level). Rates of soybean leaf appearance (Hesketh et al., 1973) and leaf expansion (Hofstra and Hesketh, 1975) increase up to 30°C; relative growth rate increases up to 31 °C (Hofstra and Hesketh, 1975); leaf photosynthesis rate increases up to 35°C (Harley et al., 1985); and total biomass increases up to 28°C (Baker et al., 1989) or 32°C (Pan, 1996).
Baker et al. (1989) investigated the effect of temperature and [CO2] on rates of soybean development. Table 7.6 shows the plastochron interval and final mainstem node number for soybean as a function of temperature and [CO2]. The results show that both increasing temperature and increasing [CO2] decrease the plastochron interval. These results and those reported by Sionit et al. (1987a) and Hofstra and Hesketh (1975) indicate that temperature affects soybean developmental rate to a much greater degree than does [CO2].
The rate of soybean node addition increases as mean temperature increases to about 28-30°C, with a computed base temperature of about 8°C (Hesketh et al., 1973). Leaf photosynthesis of soybean also has a base temperature close to 8°C as computed from a linear projection of data (Harley et al., 1985). However, there are few data near the base temperature and, as in most experiments, their study did not allow for temperature acclimation.
Although soybean reproductive development shows almost no response to [CO2], it is strongly dependent on temperature (Hesketh et al., 1973; Grimm et al., 1994; Shibles et al., 1975). The R1 stage is delayed by both low (less than 23°C) and high (greater than about 35°C) temperature. Likewise, the time to reach the R7 stage is delayed by mean daily temperature above 31°C, accompanied by decreased growth rates of seed and low yield (Pan, 1996). Similar results were found in cotton (see Chapter 8, this volume).
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