Cotton rooting patterns in relation to soil temperatures and the thermal kinetic window

Throughout the USA, cotton (Gossypium hirsutum L.) is irrigated to avoid the deleterious effects of drought stress on growth and development. Irrigation, while optimizing water status, results in a concomitant cooling of soil temperatures. Previous field studies have focused on soil water status effects on root growth independent of the soil temperature changes following irrigation. This study was designed to evaluate the soil temperature characteristics of irrigated and nonirrigated field plots of cotton to determine the time that temperatures are spatially and temporally within the range for optimal cotton root metabolism. Soil temperatures in the nonirrigated treatment were within cotton's thermal kinetic window (23.5 to 32 degrees C) for 72% more time than in the irrigated treatment. Predicted rooting fronts fell between the 24 and 26 degrees C isotherms in the irrigated treatment and the 26 and 28 degrees C isotherms in the nonirrigated treatment. Greater root length density of cotton in the nonirrigated treatment, determined from soil cores, was associated with increased time at optimal temperatures. Near the soil surface, root length density decreased in the nonirrigated treatment because of reduced soil water status, not because of reduced time within the optimal temperature range. Soil temperatures cooled below the optimal range upon canopy closure in the irrigated treatment. The cooling trend was observed to a lesser extent in the nonirrigated treatments, which never realized full canopy closure. This study showed that greater cotton root development occurred across time and with depth in soils exhibiting temperatures within cotton's thermal kinetic window.