Relationship between whole-plant N content (Tissue N) and normalized reflectance ratios obtained from images exposed to 450, 625, and 660 nm. A. Tissue N vs. R_870/450, B. Tissue N vs. R_870/625, and C. Tissue N vs. R_870/660. Fitted equations are Tissue N = -15.3 + 10.48· R_870/450 (r² = 0.38), Tissue N = -19.7 +13.3· R_870/625 (r² = 0.56), Tissue N = -19.9 +12.43·R₈₇₀/₆₆₀ (r² = 0.49), averaged across poinsettia cultivars.