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Free-Air Carbon Dioxide Enrichment of Cotton: Root Morphological Characteristics

Natl. Soil Dynamics Lab., USDA-Agricultural Research Service, P.O. Box 3439, Auburn, AL 36831;

G. B. Runion

School of Forestry, Auburn Univ., AL 36849;

B. A. Kimball

U.S. Water Conservation Lab., USDA-Agricultural Research Service, Phoenix, AZ 85040;

J. R. Mauney

Western Cotton Res. Lab., USDA-Agricultural Research Service, Phoenix, AZ 85040;

K. F. Lewin, J. Nagy and G. R. Hendrey

Brookhaven Natl. Lab., Upton, NY 11973.

^* Corresponding author (sprior{at}ag.auburn.edu).

ABSTRACT

The response of plants to rising global CO₂ concentration is of critical research interest but one neglected aspect is its effect on roots. Root morphological changes in cotton [Gossypium hirsutum (L.) ‘Delta Pine 77’] were examined in a 2-yr field study. The test crop was grown under two water regimes (wet, 100% of evapotranspiration [ET] replaced and dry, 75% [1990] and 67% [1991] of ET replaced) and two atmospheric CO₂ concentrations (ambient = 370 µmol mol^–1 and free-air CO₂ enrichment [FACE] = 550 µmol mol^–1). A FACE technique that allows for CO₂ exposure under field conditions with minimal alteration of plant microclimate was used. Excavated root systems were partitioned into taproot and lateral roots at two growth phases (vegetative and reproductive). Vertical root-pulling resistance was determined at the second sampling; this measure was higher because of CO₂ enrichment but was unaffected by water stress. Water stress affected root variables only at the second sampling; water stress reduced taproot variables more than lateral variables. The larger diameter taproots seen at all sample dates under FACE exhibited large increases in dry weight and volume. FACE often increased lateral root number and lateral dry weights were higher at all sample dates. The development of more robust taproot systems in CO₂-enriched environments may allow for greater carbohydrate storage for utilization during periods such as boll filling and to ensure root growth for continued exploration of the soil profile to meet nutrient and water demands during peak demand periods.

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