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? almost 10 years ago.