Narrow-band reflectance indices for mapping the combined effects of water and nitrogen stress in field grown tomato crops

Abstract

This study assessed the use of reflectance indices for detecting the combined effects of water and nitrogen stress in tomatoes (Solanum lycopersicum L.). Spectral reflectance data were acquired from tomato plants, subjected to three water and three nitrogen treatments. Irrigation water was applied in amounts of 100, 70, and 30% of full replenishment of root zone soil water to field capacity. Nitrogen application was 100, 70, and 30% of crop nutrient requirement. The treatments were replicated five times in a randomised complete block design. Plant stress indicators, including leaf temperature (Tc), relative water content (RWC), yield, and leaf chlorophyll content (LCC) were measured at the same time of leaf reflectance data, during the growing season. Reflectance indices including Normalised Difference Vegetation Index (NDVI), Renormalised Difference Vegetation Index (RDVI), Optimised Soil Adjusted Vegetation Index (OSAVI), Photochemical Reflectance Index centered at 550 nm (PRI550), normalised PRI (PRInorm), Transformed Chlorophyll Absorption in Reflectance Index (TCARI), Water Index (WI), and WI/NDVI were obtained from the reflectance data. The results showed that the PRI550, PRInorm, and WI were the most sensitive indices for distinguishing crop water stress, while RDVI, PRInorm, and TCARI had the best correlation with nitrogen stress indicators. PRInorm was the most sensitive index for detecting the combined effect of water and nitrogen stress. This study provided more insights into the usefulness of leaf spectral features for assessing crop abiotic stress. Measuring these indices with hyperspectral sensors provides a rapid, non-destructive, and reliable approach for estimating crop stress.