Water stress detection in field-grown maize by using spectral vegetation index

As water availability is getting more limited, practical tools are needed to make timely decisions on the need to initiate irrigation. Therefore, we investigated if the reflectance vegetation index (RVI) obtained from remote sensing of the crop during soil drying, compared with a fully watered plot as a reference could be used as such a tool for detection of water stress. RVI was measured as green crop area index (CAI) and increased from I to 4 during the early vegetative stage of growth. RVI was calculated from incoming (PARi) and reflected (PARc) photosynthetically active radiation and from incoming (NIi) and reflected (NIc) near infrared radiation using the equation RVI = (NIc/NIi)/(PARc/PARi). The effect of soil drying on leaf expansion, biomass accumulation, abscisic acid (ABA) in xylem sap, leaf water potential and leaf nitrogen content were followed as drought indicators in maize (Zea mays L.) grown in lysimeters in the field. Five to seven days after the onset of soil drying RVI started to decrease in droughted plants when xylem [ABA] increased. Leaf nitrogen content of upper leaves decreased five to ten days after the onset of soil drying. Not until 20 days after onset of soil drying when relative available soil water (RASW) was 20%, did midday leaf water potential of droughted plants significantly decrease below psi(leaf) of fully irritated plants. RVI was highly correlated with green crop area index (CAI) (R-2 = 0.84) and biomass accumulation (R-2 = 0.88). We suggest that measurements of RVI of droughted field plants related to RVI of fully irrigated reference plants within the same field can be used as an early warning system of water stress for irrigation, and hence the need during the early vegetative growth stages in maize.