Leaf color and leaf color charts for efficient N management in rice: how do they compare?

The Leaf Color Chart (LCC) is an expensive tool used by farmers to detect the plant N demand in real-time for efficient fertilizer use and high rice yields. Several LCCs are available for irrigated rice and visual observations suggest great differences in colors and color ranges among charts. However, little information is available on the color characteristics of LCCs, and a systematic analysis whether their colors match the true color of rice leaves is lacking. The authors used a spectrophotometer (Minolta CM-3700-d) to characterize and compare the color of existing LCCs in Asia across the visible wavelength spectrum (400-700 nm). Charts differed in (i) their reflectance characterizing the range from light to dark green, and (ii) their general spectral reflectance (SR) patterns that characterize the `type of green' of each chart. The authors also detected inconsistencies in SR pattern among panels of individual charts. To identify the `true' colors of rice leaves, a two-season field experiment was conducted at IRRI's Experimental Farm in 2001 with 10 modern varieties grown at three different N levels. Regression analysis of leaf SR measurements at three growth stages of rice showed little difference in SR patterns among varieties,sampling days and seasons, so that the `type of green' is largely the same for all varieties. Color differences among varieties on plant N levels are mainly due to differences in reflectance at 550 nm. Based on the actual leaf SR measurements, the authors developed target SR patterns for an ideal LCC prototype. Recognizing the limitations of the plastic industry to match the color of rice leaves using pigments available for LCC production, the authors discussed the relevance of their observations and present a prototype of a refined LCC