Cotton growth rates are very sensitive to temperature and somewhat sensitive to elevated [CO2]. Growth rates of stems, leaves, bolls and roots are all responsive to both of these environmental factors. Typical growth responses (for example, stem extension rate) to temperature and [CO2] are illustrated in Fig. 8.10. Stem growth elongation rates increased by 20% due to doubling [CO2]. Leaf area was 25% greater at the end of the season on plants grown in elevated [CO2], while roots and fruits were 30% and 26% greater, respectively (K.R. Reddy et at., 1998a). The harvest index did not change because of elevated [CO2].
Seedlings grown at 28 vs. 21°C under optimum water and nutrient conditions accumulated 4 to 6 times more biomass during the first 3 weeks after emergence (K.R. Reddy et at., 1997a). Cotton plants grown in the field in elevated [CO2] environments grew faster and intercepted 15-40% more solar radiation than plants grown in similar but ambient conditions during the first
Fig. 8.10. Effect of temperature and [CO2] on cotton mainstem elongation rate.
half of the growing season (Pinter et al., 1994). This result is typical of plants grown in elevated [CO2] environments. Such plants produce more and larger leaves, resulting in accumulated growth that intercepts more radiation. Consequently, they have a production advantage when other factors are equal. However, boll retention declines sharply at canopy temperatures above 28°C (K.R. Reddy et al., 1997a). Young bolls are injured by exposure to only a few hours of such high temperatures.
Although height growth was much faster in twice-ambient [CO2] in optimum conditions during the first 3 weeks, by the end of the season plants grown in the elevated [CO2] were only 5% taller than those grown in ambient [CO2] (K.R. Reddy et al., 1997a). Obviously, during a large portion of the season, other factors determining height are more important than [CO2]. For example, leaf N may limit stem growth rates. When leaf N was about 2.5 g m-2, stem growth was 32-37 mm per day; but when the leaf N was only 1.5 g m-2, stem growth was less than 50% of the higher N treatment (K.R. Reddy et al., 1997b). Stem growth rates were about 17% faster in elevated [CO2] than in ambient [CO2] in the N-limited environment. Similarly, stem extension is limited by carbon during much of the season. Relative leaf expansion was also much slower when N was limited. Leaf expansion decreased from over 0.1 cm2 cm-2 day-1 to zero as leaf N decreased from 2.25 to 1.5 g m-2. Relative leaf expansion rates were not influenced by elevated [CO2]. Boll growth rates are sensitive to temperature but not to elevated [CO2]. At 20 and 32°C, individual boll growth was 82% and 66%, respectively, of boll growth rates at 25oC, but there were no differences in growth rate due to increased [CO2].
In the SPAR experiments (Table 8.1), there were no significant changes in the partitioning coefficients due to elevated [CO2]. Greater amounts of photo-synthate were produced in elevated [CO2], but this appeared to be used in forming additional structural mass of all kinds, and the ratios of one plant organ to another remained unchanged. Kimball and Mauney (1993) reported similar results from the open-top chamber experiments (Table 8.2). Elevated [CO2] did not affect root/shoot ratio, harvest index or lint percentage at ample or limited supplies of water and N. On the other hand, Pinter et al. (1996) showed that there were some changes in partitioning due to the 550 mmol CO2 mol-1 FACE treatment in 1991, including an increase in harvest index under well-watered conditions.
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