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Carbon Source-Sink Relationships within Narrow-Row Cotton Canopies¹

Cotton (Gossypium hirsutum L.) is an indeterminate crop in which vegetative and reproductive sinks compete for photosynthetic assimilates. The objective of this study was to evaluate the carbon source sink relationship in cotton canopies with differing leaf types grown under narrow-row culture and to determine if measurement of specific leaf weight (SLAV) can be used as a simplified evaluation of photosynthetic rate within cotton canopies. Three near-isogenic lines of ‘Stoneville 7A’ (normal, okra, and superokra) cotton which differed in blade area/leaf were grown in narrow-row culture at a plant density of 10 plants m_-2. During the 3rd (29 July) and 5th (13 August) weeks of flowering, whole plants were exposed to ¹⁴CO₂ and evaluated at three height intervals (0 to 30 cm, 30 to 55 cm, and above 55 cm) for leaf area index (LAI), ¹⁴CO₂ uptake, and biomass distribution. Okra and superokra leaf types generally had higher ¹⁴CO₂ uptake within the canopy on a leaf area basis, but lower LAI than normal leaf plants. Uptake of ¹⁴CO₂ among leaf types on a land area basis was different only on 13 August in the top interval where the ranking was normal > okra > superokra. By 13 August, the top interval accounted for 81, 77, and 69% of the canopy ¹⁴Cassimilate production, but contained only 29, 28, and 32% of the fruit dry weight for normal, okra, and superokra leaf plants, respectively. Mutant leaf types had a larger percentage of dry weight as fruit and less as leaves and petioles compared to normal leaf plants. This study suggests that vegetative growth is favored at the expense of reproductive growth due to high LAI at the top of normal leaf canopies when grown in narrow-row culture. However, the mutant leaf types tend to have less biomass resulting in only equivalent yields to normal leaf plants when grown under full-season culture. Significant correlation coefficients between C uptake and SLW ranged between 0.72 and 0.89 depending upon time of SLW measurement and sampling date, but were not altered by leaf type. Use of SLW is a simple method for estimating photosynthetic activity of leaves within cotton canopies.

Key Words: Gossypium hirsutum L. • Leaf type • Okra leaf • Superokra leaf • Radioactive C • Specific leaf weight

² Former graduate associate in research, (now assistant professor, New Mexico State Coop. Ext., Box 3AE, Las Cruces, NM88003), former professor, (now professor, Malheur Exp. Stn., Oregon State Univ., Ontario, OR 97914), and associate professor, Dep. of Plant Sciences, Univ. of Arizona, Tucson, AZ 85721.

Received for publication January 8, 1979.

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