Some tomato plant leaves yellow due to a lack of sunlight on mature plants. When the plants become large, bushy, and heavy with fruit, the top portions of the plant can block the lower portions from getting sunlight.This can cause the leaves to yellow, and is really nothing to worry about.ou may need to also check your soil for a lack of nitrogen. Nitrogen will give a plant (everything from grass to vegetable plants) its dark green color, and good growth. A lack of nitrogen in the soil can cause the yellow leaves on tomatoes as well.
One example of phototropism is if you place a plant with sunlight to the right, than it will grow to the right where the sunlight is. Another example of phototropism is if you place a plant under something such as a table with sunlight shining above it, it will grow under and out in the sunlight.
In positive phototropism, the stem bends toward the light source. In plagiotropism, or diatropism, the leaf blades move at an angle toward the falling light. In negative phototropism the plant organs bend in a direction opposite to the light source (for example, ivy stems and the apices of certain roots). The same organ may be positively phototropic to weak light, negatively phototropic to intense light, and not at all phototropic to light of medium intensity.