The effects of free‐air CO₂ enrichment and soil water availability on spatial and seasonal patterns of wheat root growth

Abstract

Abstract Spring wheat [ Triticum aestivum (L). cv. Yecora Rojo] was grown from December 1992 to May 1993 under two atmospheric CO 2 concentrations, 550 μmol mol –1 for high‐CO 2 plots, and 370 μmol mol –1 for control plots, using a Free‐Air CO 2 Enrichment (FACE) apparatus. In addition to the two levels of atmospheric CO 2 , there were ample and limiting levels of water supply through a subsurface trip irrigation system in a strip, split‐plot design. In order to examine the temporal and spatial root distribution, root cores were extracted at six growth stages during the season at in‐row and inter‐row positions using a soil core device (86 mm ID, 1.0 m length). Such information would help determine whether and to what extent root morphology is changed by alteration of two important factors, atmospheric CO 2 and soil water, in this agricultural ecosystem. Wheat root growth increased under elevated CO 2 conditions during all observed developmental stages. A maximum of 37% increase in total root dry mass in the FACE vs. Control plots was observed during the period of stem elongation. Greater root growth rates were calculated due to CO 2 enhancement until anthesis. During the early vegetative growth, root dry mass of the inter‐row space was significantly higher for FACE than for Control treatments suggesting that elevated CO 2 promoted the production of first‐order lateral roots per main axis. Then, during the reproductive period of growth, more branching of lateral roots in the FACE treatment occurred due to water stress. Significant higher root dry mass was measured in the inter‐row space of the FACE plots where soil water supply was limiting. These sequential responses in root growth and morphology to elevated CO 2 and reduced soil water supports the hypothesis that plants grown in a high‐CO 2 environment may better compensate soil‐water‐stress conditions.