Q.Tomato Leaves Yellowing
I have tomato plants in my small greenhouse and they are waiting to go in the garden if it would just warm up enough. They looked beautiful and now the bottom leaves are turning yellow and we may still have a week or two before I can transplant them.

Certified GKH Gardening Expert
There are a number of reasons for leaf yellowing in tomatoes. This article should help with pinpointing and correcting the cause of yours: https://www.gardeningknowhow.com/edible/vegetables/tomato/yellow-tomato-leaves.htm