ROOTING DEPTH AND DRY-MATTER DEVELOPMENT OF SUNFLOWER

The use of modeling and computers increases the potential to estimate crop water use and irrigation requirements. However, for that potential to be realized, data on root growth are needed for model development and/or input. Few field studies have included direct measurement of sunflower (Helianthus annuus L.) root development. Our objective was to examine plant development, with emphasis on rooting depth, of field-grown sunflower. The study was in 1985 and 1986 near Manhattan, KS on deep, barrier-free, Muir silt loam soil (fine-silty, mixed, mesic Cumulic Haplustoll). Regression analysis was applied to data on root growth and aboveground development. The resulting equations were then used to examine growth patterns and rates of development. Canopy height reached its maximum of 2.40 m during the latter part of disk flowering. The maximum rate of height increase was 72 mm d - 1 and occurred during early bud formation. Roots reached the deepest soil depth of 2.18 m late in the growing season (95% of the total of thermal units from emergence to physiological maturity). Roots reached 1.88 m at the beginning of disk flowering and 2.02 m at the completion of disk flowering. Rate of advance of the deepest observed root decreased linearly from 43 mm d - 1 during mid June (eight leaves emerged). Green leaf area index (LAI) reached its maximum immediately prior to disk flowering. Dry mass of leaves, stems, and heads reached maximums at late disk flowering, back of head yellow with bracts still green, and physiological maturity (no green on head), respectively. Total aboveground dry mass reached its maximum at 85% of the total of thermal units from emergence to physiological maturity. The rate of total aboveground dry mass yield was greatest near mid season (48% of total thermal units). The data on sunflower rooting depth reported in this article will assist in estimation of crop water use.