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

The response of plants to rising global CO2 concentration is of critic al research interest but one neglected aspect is its effect on roots. Root morphological changes in cotton [Gossypium hirsutum (L.) 'Delta P ine 77'] were examined in a 2-yr held study. The test crop was grown u nder 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 enr ichment [FACE] = 550 pmol mol(-1)). A FACE technique that allows for C O2 exposure under held conditions with minimal alteration of plant mic roclimate was used. Excavated root systems were partitioned into tapro ot and lateral roots at two growth phases (vegetative and reproductive ). Vertical root-pulling resistance was determined at the second sampl ing; this measure was higher because of CO2 enrichment but was unaffec ted by water stress. Water stress affected root variables only at the second sampling; water stress reduced taproot variables more than late ral variables. The larger diameter taproots seen at all sample dates u nder FACE exhibited large increases in dry weight and volume. FACE oft en increased lateral root number and lateral dry weights were higher a t all sample dates. The development of more robust taproot systems in CO2-enriched environments may allow for greater carbohydrate storage f or utilization during periods such as hop filling and to ensure root g rowth for continued exploration of the soil profile to meet nutrient a nd water demands during peak demand periods.