Distribution of Dry Matter Between Shoots and Roots of Irrigated and Nonirrigated Determinate Soybeans1

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Abstract
The timing and duration of drought stress during a growing season can both limit and alter dry‐matter accumulation within plants. Distribution of dry matter between roots and aboveground parts was determined by growing soybean (Glycine max [L.] Merr., cv. ‘Bragg’ or ‘Braxton’) plants for 4 consecutive years (1980–1983) in a sieved, sandy surface soil material, uniformly packed to a bulk density of 1.52 Mg/m3 in soil compartments at the Auburn rhizotron. Three replicated compartments were irrigated by trickle irrigation (IR) whenever the soil water potential (Ψs) fell below ‐ 15 kPa at a depth of 0.4 m. Three additional compartments were not irrigated (NI). Mainstem leaf area and total cumulative root growth were determined at regular intervals. Water stress significantly reduced total shoot weight (347 g/m2) and seed weight (171.2 g/m2) but increased total root length (52 m/m2). Irrigated plants generally had fewer roots, which were generally shallower and more evenly distributed than those of NI plants. Both individual seed weight and total seed weight per treatment were significantly reduced by water stress (0.009 g/seed and 171.2 g/m2, respectively). Apparent harvest index varied between 0.36 and 0.56. A nearly linear relationship was observed between normalized final seed yield (y = 110.8 ‐ 0.738x, r2 = 0.86) or total dry matter (y = 113.3 ‐ 0.685x, r2 = 0.93) production and the percentage of time between flowering (R1) and harvest maturity (R8). Thus, aboveground dry‐matter production and seed yield are directly dependent upon availability of water during critical reproductive states.