Hi Cher,
This kind of looks like a nutrient deficiency in the plants. In my research I grow other brassicaceae plants in pots, and if I let them go too long in the pots they use up the nutrients in the soil and the lower leaves begin to turn a pale color like in your photo. So perhaps you need to supply more nutrients to your plants (Nitrogen, phosphorus, potassium, and trace micronutrients).
Are the plants getting enough water? over 9 years ago.