I was at a couple of different stores recently that were selling tomato plants and saw some more from another nursery that still had green and healthy looking cotelydons (seed leaves) on them even though the plants were about 5-6" tall with a lot of true leaves. Most were in 4 packs but some were in larger pots. Same with some plants I ordered from a large seed/plant supplier online last year.
When I start my tomatoes from seed the coteledons start to yellowafter about 3 weeks when they're still only about 3" tall or so with only 2 or three sets of true leaves. I started them in 98 cell trays with an organic soil that contains fertilizer and also fertilized them with a little dilute fish fertilizer a couple of times when they were still in the 98 cell inserts.
At 4 weeks old, when I transplanted them to larger pots, the cotyledons were practically falling off the plantson most of the plants when I just touched them with the scissor to cut them off so I can plant them deeper in the larger pots. It's been about 4 weeks since potting up and the plants look great.
Why are the cotyledon's on the plants at the nursery still completely green?
Here's a picture of my seedlings at 4 weeks. Sorry for the poor quality, taken with cell phone in dark room.