Active Optical Sensors in Irrigated Durum Wheat: Nitrogen and Water Effects.

Interest in the use of active optical sensors for guiding N fertilizer management of crops like wheat has grown rapidly since the mid-1990s. Recently, active optical sensors have been used to assess water status of crops in addition to plant N status. We conducted a 2-yr study on a Casa Grande sandy loam soil in Maricopa, AZ, with durum wheat under an overhead sprinkler system. Uniquely, this study had 10 unrandomized levels of irrigation and five rates of N fertilizer. The objectives were to compare 12 vegetation indices for their ability to distinguish irrigation and N fertilizer rates and to determine how well the vegetation indices estimated biomass, plant N, grain yield, grain N, and yellow berry. The canopy chlorophyll content index, Datt, and Meris terrestrial chlorophyll index were the most consistent vegetation indices in responding well to N, with minimal water effects. No vegetation indices detected water stress with minimal N effect as well as canopy temperature measured with infrared thermometers. Interest in active optical sensors (AOS) for guiding N fertilizer management of crops like wheat (Triticum aestivum L.) has grown rapidly since their introduction in the mid-1990s. Recently, AOS have been used to assess water status of crops in addition to plant N status. Specific vegetation indices (VIs) might assess N stress while minimizing effects of water stress. A 2-yr study (2013-2014) was conducted on a Casa Grande sandy loam soil in Maricopa, AZ, with durum wheat (T. durum Desf) under an overhead sprinkler system. Uniquely, this study had 10 unrandomized levels of irrigation and five rates of N fertilizer. The objectives were to compare 12 VIs for their ability to distinguish irrigation and N fertilizer effects and to determine how well the VIs estimated biomass, plant N, grain yield, grain N, and yellow berry (opaque starchy grain). Two Crop Circle 470 AOS were passed at a fixed height, 1 m above the tallest plants in the field, every 7 to 10 d during the growing season. The normalize difference vegetation indices (NDVIs) showed highly significant response to N rate in three of four growth-stage-years, but significant water and small N effects at Zadoks 32 (early stem elongation) in 2014. The canopy chlorophyll content index (CCCI), DATT (Datt, 1999), and Meris terrestrial chlorophyll index (MTCI) were the most consistent VIs in distinguishing N rates, with minimal water effects. No VIs detected water stress with minimal N effect as well as the infrared thermometer (IRT) measurements of canopy temperature did. [ABSTRACT FROM AUTHOR]