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.
Phototropism (growing towards light) occurs because the tips of growing plant shoots contain a pigment that's sensitive to light, especially blue light. When light shines on the shoot, it gets absorbed by the pigment (called phototropin), and that triggers the release of the plant hormone auxin, which makes cells grow. The direction of the light determines where the auxin acts. If light is coming from the right, auxin gets sent down the left (shady) side of the shoot. That makes the cells on the shady side grow longer, putting a kink in the shoot so it points to the light.