FREE-AIR CARBON-DIOXIDE ENRICHMENT OF COTTON - ROOT MORPHOLOGICAL-CHARACTERISTICS

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 held 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 mu mol mol(-1) and free-air CO2 enrichment [FACE] = 550 pmol mol(-1)). A FACE technique that allows for CO2 exposure under held 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 hop filling and to ensure root growth for continued exploration of the soil profile to meet nutrient and water demands during peak demand periods.