TRANSMITTANCE AND REFLECTANCE MEASUREMENTS OF CORN LEAVES FROM PLANTSWITH DIFFERENT NITROGEN AND WATER-SUPPLY

Nitrogen is essential for crop production, but also contributes to eut rophication of surface water and degradation of drinking water quality . Modern corn production requires relatively large quantities of N, wh ich are generally supplied by fertilizers. Over-application of N ferti lizers and animal wastes frequently results in nitrate leaching. Synch ronizing N availability with crop N need offers the potential to prote ct the environment without sacrificing production. Tools are needed to rapidly and easily monitor crop N status to make timely decisions reg arding fertilizer application. Analytical and optical techniques were evaluated with greenhouse grown corn at silking to evaluate several me thods to monitor crop N status. A portable chlorophyll meter was used to measure chlorophyll content of leaves by means of transmittance mea surements. Leaf N concentration and chlorophyll meter readings were po sitively correlated, but were also affected by water stress and hybrid differences. Water stress decreased chlorophyll meter readings but in creased leaf N content and diffusive resistance. Nitrogen stress decre ased leaf N concentration, chlorophyll meter readings, and diffusive r esistance. Both water and N stresses affected crop reflectance measure ments. Reflectance values in the green and near IR portions of the spe ctrum were inversely related to crop N status. Water stress increased reflectance in red, green, and near IR wavelengths. Water stress by N status interactions were significant for chlorophyll meter readings as well as reflectance measurements. Both leaf reflectance and chlorophy ll meter measurements provided a good indication of N status for adequ ately watered plants, but the relationships were poor for plants grown under prolonged water stress.