Free-Air [CO.sub.2] Enrichment Effects on Apex Dimensional Growth of Spring Wheat

Although primordium initiation in wheat (Triticum aestivum L.) has been extensively researched, a complete description of the growth dynamics of the apex at elevated [CO.sub.2] concentrations is lacking. This study determined the rates of main stem and tiller apical elongation and widening in plants grown under two levels of [CO.sub.2] concentration. Spring wheat was grown at the University of Arizona's Maricopa Agricultural Center at elevated (550 µ mol [mol.sup.-1]) or ambient (370 µ mol [mol.sup.-1]) [CO.sub.2] concentrations. Individual plant samples were collected at different developmental stages and dissected. After dissection, the lengths and widths of the apices of the main stem (MS), coleoptile tiller (T0), primary tillers (T1, T2, and T3), and secondary tillers (T00, T01, T02, T10, T11, and T12) were measured with a stage micrometer. Apex dimensions were fitted to an exponential model. Elevated [CO.sub.2] increased the apex lengths of T2 at the double ridge stage, and of T3 and T10 at the double ridge and the terminal spikelet stages, and the apex widths of T2 at double ridge stage, and of T2, T3, T10, and T11 at the flag leaf appearance stage. Combining these results with a parallel study, the longer apices did not have more spikelet primordia, but wider apices had more floret primordia. Elevated [CO.sub.2] changed apex elongation or widening patterns within a plant by enhancing elongation or widening rates of the MS, and laterformed tillers. Earlier-formed tillers were less responsive to elevated [CO.sub.2] levels. This information will be used in modeling wheat apical development and grain production in the elevated atmospheric [CO.sub.2] environments of the future.
SPIKELET AND FLORET PRIMORDIA that set potential grain number on a spike reside on the apex dome. The size of the apex dome changes systematically with primordium differentiation and there [...]