EFFECTS OF FREE-AIR CO2 ENRICHMENT ON COTTON ROOT-GROWTH

The rise in atmospheric CO2 concentration is predicted to have a positive effect on agro-ecosystem productivity. However, an area which requires further investigation centers on responses of crop root systems to elevated atmospheric CO2 under field conditions. The advent of free-air CO2 enrichment (FACE) technology provides a new method of CO2 exposure with minimal alteration of plant microclimate. In 1990 and 1991, cotton (Gossypium hirsutum (L.) 'Deltapine 77') was grown under two atmospheric CO2 levels (370 and 550 mumol mol-1) and two water regimes (wet (100% of ET replaced) and dry (75% of ET replaced in 1990 and 67% in 1991)). Plant root samples were collected at early vegetative and mid-reproductive growth. Taproots of CO2-enriched plants displayed greater volume, dry weight, length, and tissue density. Water treatment effects were noted for length, volume and dry weight of roots at the second sampling in 1991. In general, whole soil profile root densities (both length and dry weight densities) and root weight per unit length at the initial sampling were increased under CO2 enrichment at each of three positions (0.00, 0.25, and 0.50 m) from row center to the middle of the inter-row space. At the second sampling, root length density and root dry weight density were generally unaffected by water stress, whereas root weight per unit length was somewhat higher. In addition, extra CO2 increased whole profile root length density only at the 0.50 m inter-row position, whereas whole profile root dry weight density and root weight per unit length were generally higher under elevated CO2 at all three positions. The results from this field experiment strongly indicated that increased atmospheric CO2 level would enhance plant root growth.