BIOMASS AND GROWTH-RATE OF A SPRING WHEAT ROOT-SYSTEM GROWN IN FREE-AIR CO2 ENRICHMENT (FACE) AND AMPLE SOIL-MOISTURE

The response of a wheat crop root system to full-season CO2-enrichment was investigated using a free-air CO2 enrichment (FACE) apparatus. A spring wheat (Triticum aestivum L. cv. Yecora Rojo) crop was growing a t 0.25 m row spacing and 130 plants m(-2) on a Trix clay loam (hyperth ermic Typic Torrifluvent) under two atmospheric CO2 concentrations (FA CE: similar to 550 mu mol mol(-1); control: similar to 370 mu mol mol( -1)) and ample soil moisture (100% replacement of poten tial evapotran spiration). Irrigation was applied with a subsur face drip irrigation system. Root cores were collected at five growth stages (three-leaf, t illering, stem elongation, anthesis, dough development and final harve st, which corresponded with day of year (DOY) 16, 36, 63, 92, 113 and 159, respectively), using a soil core device (86 mm i.d.). Two cores w ere taken in-row and one in the inter-row space position to examine th e horizontal and vertical distribution of roots to a I-m depth. Root b iomass was summed over the entire root profile across all positions to obtain a total. Total root mass was higher in FACE compared to contro l for all growth stages (i.e. 34% at three-leaf, 21% at tillering, 23% at stem elongation, 28% at dough development and 19% at harvest). Roo t growth rate was significantly higher in FACE compared to control for both in-row and inter-row positions. Root senescence rate were simila r between FACE and control grown plants, but the absolute difference i n root mass during the senescence phase was greater in FACE compared t o control grown plants. Roots from FACE grown plants explored a greate r proportion of the soil profile earlier in the season than roots from control grown plants. However, there was no evidence that plants grow n in FACE had a deeper root system than plants grown in control.