NON-WATER-STRESSED BASE-LINES FOR SUNFLOWERS

Effective use of the Crop Water Stress Index (CWSI) to quantify water stress requires knowledge of a non water-stressed baseline (NWSB). This study was conducted to determine effects of plant population, plant development, leaf temperatures, canopy temperatures, and time of measurement on NWSB for sunflower (Helianthus annuus L., ''Triumph 560-A, 822B-R''). Measurements of canopy and single leaf temperatures were made with an infrared thermometer (IRT) throughout the growing season on plants in three populations (2.6, 5.3, and 7.9 plants.m-2) grow under full irrigation to provide a range of ground cover conditions. Plant population only affected NWSB based on canopy temperatures when leaf area index (LAI) was less than 2.0. Non water-stressed baselines based on single leaf temperatures were not affected by plant population. Slopes of NWSBs were similar during vegetative and flowering growth stages, but declined in absolute value during grain-filling. Non water-stressed baselines derived from midday temperature and vapor pressure deficit (VPD) measurements were not different from NWSBs derived from diurnal measurements. Measurements of single leaves of sunflower plants made with an IRT can be used to evaluate water stress early in the growing season before canopy closure occurs, or in non-irrigated production areas where canopy closure may not occur, and during grain-filling when heads become very warm and disrupt canopy temperature measurements.