Using Chlorophyll Fluorescence and Vegetation Indices to Predict the Timing of Nitrogen Demand in Pentas lanceolata

The objective of this study was to predict the timing of nitrogen (N) demand through analyzing chlorophyll fluorescence (ChIF), soil-plant analysis development (SPAD), and normalized difference vegetation index (NDVI), which are positively correlated with foliar N concentration in star cluster (Pentas lanceolata). The plants were grown in potting soil under optimal conditions for 30 d, followed by weekly irrigation with five concentrations (0, 4, 8, 16, and 24 mM) of N for an additional 30 d. These five N application levels corresponded to leaf N concentrations of 2.62, 3.48, 4.00, 4.23, and 4.69%, respectively. We measured 13 morphological and physiological parameters, as well as the responses of these parameters to various N-fertilizer treatments. The general increases in Dickson's quality index (DQI), above-ground dry weight (DW), total DW, flowering rate, Delta F/Fm', and qP in response to treatment with 0 to 8 mM N were similar to those of SPAD, NDVI, and Fv/Fm. Consistent and strong correlations (R-2 = 0.60 to 0.85) were observed between leaf N concentration (%) and SPAD, NDVI, Delta F/Fm', and above-ground DW. Validation of leaf SPAD, NDVI, and Delta F/Fm' revealed that these vegetation indices are accurate predictors of leaf N concentration that can be used for non-destructive estimation of the proper timing for N-solution irrigation of P. lanceolata. Moreover, irrigation with 8 mM N-fertilizer is recommended when leaf N concentration, SPAD, NVDI, and Delta F/Fm' ratios are reduced from their saturation values of 4.00, 50.68, 0.64, and 0.137%, respectively.