Why are my tomato plants beginning to yellow at the bottom of some of the plants?
Yellowing may be caused by:
1. Lack of sunlight on mature plants. When the plants become large, bushy, and full of fruit, the top portions of the plant may be blocking the lower portion from getting enough sunlight, but this is really nothing to worry too much about.
2. The plants may not be receiving enough water. Tomatoes need a lot of water when the temperature is really hot, as it is now. This is especially important when the plants are bearing fruit. Usually one good deep watering every day, early in the day, will help with this.
Hope this answer helps!