The term “stress” was defined by Jackson (1986) as any disturbance that adversely influences plant growth. Various types of stress can be caused by abiotic (water deficit, nutrient deficiency, salinity, heavy metal, herbicide, air pollution, etc.) and biotic (bacteria, fungi, viruses, insects, etc.) factors. They may induce changes in leaf anatomy, chemistry, and physiology, which will result in changes in leaf optical properties (e.g., Carter, 1993).