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.
Photo in phototropism means light. Light is a stimulus that plants have response to. A stimulus could be weather, touch, gravity or light. The orientation of a plant or other organism in response to light, either toward the source of light or or away from it. It is usually observed in plants, but can also take place in other organisms such as fungi. The cells on the plant that are farthest from the light have a chemical called auxin that reacts when phototropism occurs which causes the plant cells to get longer on the furthest side of the light.