Reflectance indices associated with physiological changes in nitrogen- and water-limited sunflower leaves

We followed diurnal and seasonal changes in physiology and spectral reflectance of leaves throughout the canopies of sunflower plants grown in control, nitrogen (N)-limited, and water-stressed plots. Leaves from control sunflower plants had significantly higher levels of nitrogen, chlorophyll (chl), ribulose bis phosphate carboxylase / oxygenase (RuBPCase) activity and photosynthetic rates and lower starch content and leaf thickness than N-limited plants. Water-stressed plants had the highest N and chl contents (on an area basis). They also had the lowest water potential and photosynthetic rates, in spite of maintaining high RuBPCase activities. Leaves from stressed plants (especially N-limited) had significantly higher reflectances in the visible wavelengths and lower in the near IR than leaves from control plants. The only clear trend across canopy levels was the higher reflectance at all wavelengths but especially in the visible of the lower (oldest) leaves. NDVI-like parameters were useful in distinguishing stress and control leaves over the growing season. However, several narrow-band indices provided better physiological information than NDVI. The physiological reflectance index (PRI) ( R 550 − R 530 / R 550 + R 530 ) followed diurnal changes in xanthophyll pigments and photosynthetic rates of control and N-limited leaves. The maximum of the first derivative of reflectance in the green ( dG ) was correlated with diurnal photosynthetic rate, and with seasonal chl and N changes. The normalized pigment chlorophyll ratio Index (NPCI) ( R 680 − R 430 / R 680 + R 430 ) varied with total pigments / chl. The water band index (WBI) ( R 970 / R 902 ) followed water status. The normalized ratio between the maxima of the first derivatives of reflectances at the red edge and green regions (EGFN) was correlated with chl and N content. Principal components analysis yielded several indicators of physiological status. The first principal component was higher in control leaves, the second was higher in N-limited leaves, and the third was higher in water-limited leaves. Discriminant analysis based on the combination of several narrow-band spectral indices clearly separated leaves into the three treatment groups. These results illustrate the promise of narrow-band spectroradiometry for assessing the physiological state of vegetation.