Plant leaves turn yellow for a variety of reasons, including a lack of chlorophyll caused by too much or too little water, insufficient sunlight, temperature changes and disease. Plants contain various pigments such as chlorophyll, which produces a green color, and carotene, which produces a yellow color.
Plants normally produce more chlorophyll than carotene, so the green color dominates the yellow color that is present but masked. During fall, shorter days and colder temperatures reduce chlorophyll production and causes leaves to naturally turn yellow.
Most houseplants are tropical plants which do not undergo the same seasonal transition, but are prone to inadequate levels of chlorophyll production due to other causes. In houseplants, insufficient sunlight, lack of proper nutrients and low temperatures can reduce chlorophyll production and cause leaves to yellow because they cannot produce appropriate levels of food through photosynthesis. Underwatering also prevents photosynthesis and causes leaves to turn yellow. Overwatering has a similar effect by depriving the plant of oxygen normally absorbed through the roots from the soil. Overwatering also leads to diseases from mold and fungi that cause plants to turn yellow.