Atmospheric [CO2] is an important factor influencing cotton yields. In a controlled-environment experiment (K.R. Reddy et al., 1997a), boll and square production in five temperature conditions averaged 44% more in 720 than in the 360 |mmol CO2 mol-1 atmosphere. Excellent water and nutrient conditions were maintained throughout the season. In a series of OTC experiments under ample and limiting levels of water and N (Table 8.2), Kimball and Mauney (1993) found that [CO2] of 650 |mmol mol-1 increased the yield of seed cotton (lint plus seed) by 60% and above-ground biomass by 63% compared with yields of plants grown at ambient [CO2] (about 350 |mmol mol-1). Results from the FACE experiments (Table 8.2; Mauney et al., 1994) showed that enrichment to 550 |mmol mol-1 increased biomass production by about 35%. Whole boll yields increased by about 40% (Mauney et al., 1994) and lint yield increased by about 60% (Pinter et al., 1996).
Temperature is another important factor affecting cotton yields. The highest numbers of bolls and squares were produced in the highest temperature environments, 1995 plus 5°C and 1995 plus 7°C (K.R. Reddy et al., 1997a). However, it must be emphasized that plants grown at these high temperatures retained very few of the bolls produced - an average of only one boll per plant in the 1995 plus 5°C condition and less than that in the 1995 plus 7°C environment. The plants grown in high [CO2] retained 20% more bolls per plant than plants grown in ambient [CO2]. Average boll weights also were 20% greater in the plants grown in high [CO2].
Fruit production efficiency, defined as dry weight of boll per total dry weight, increased as temperatures increased to 29°C, then declined rapidly at temperatures above 29°C (Fig. 8.11). Since the rate of fruit retention dropped dramatically at temperatures above 29°C (K.R. Reddy et al., 1997a), the fruit production efficiency was also expected to decline at higher temperatures. The number of bolls retained was strongly influenced by temperatures higher than July average plus 2°C (29.1°C). At the two higher temperatures, the number of bolls retained or fruit production efficiency was drastically reduced.
The upper limit for cotton fruit survival is approximately 32°C, or July average plus 5°C for the Midsouth US cottonbelt. Such a statement may be misleading, because the survival and growth of bolls are not equally sensitive to high temperature throughout their development. Bolls are usually abscised within 2-4 days after flowering when exposed to high temperatures. However,
flowers which developed at lower temperatures did not abscise even when temperatures exceeded 32°C a few days after anthesis. There appears to be a short time period prior to, during and after the flowers formed when they are most vulnerable to high temperature. If the developing bolls escape high temperature during that time, many can survive unfavourable, high-temperature conditions during the rest of their growth period. However, we have yet to delineate the exact definition of the developmental stages and timing of the temperature-sensitive periods.
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