Those don't look cracked, they look sliced or eaten. Not sure by which organism, but I think you have a pest in your maters.

Yes "sliced" is a good term, but there are no bugs.
Thought perhaps too fast growth resulting in less leaf thickness and perhaps being whipped by the some wind may have caused it? I repeat no chewing by critters - looks like I took my penknife & cleanly slit the leaves.
Other ideas?

I think it could be a combination of the low temps and the wind, maybe even water amounts, unless the plant is growing too fast and the leaf growth isn't consistent over the entire area.

I put many but not all of my plants out to harden in a 6ft x 4ft x 6ft tall $50 vinyl greenhouse and temp extremes have gone from (38 to 75+ outside) 46 to 98 inside the greenhouse, depending on if the zippers were open/closed and how sunny & local weather. I too noticed similar "tears" on some plants, PL's especially, and the plants still indoors have none. Maybe the hardening contracts the leaves in opposition to sunlight and watering trying to make them get bigger and something has to give. I would have sugggested leafcutter bees but they usually cut arcs & semicircles and steal the pieces for nesting, and yours are straighter cuts and not missing any pieces from what I can see.

I think it's nothing to worry about and they'll outgrow it as they get older.

Sounds like "something has to give" is what's happening with these plants. Growing too fast for their own good. I had a soil test performed & the ag center here suggested I sidedress w/ 27-4-5 at transplant as my sandy dirt had next-to-nothing N. Perhaps a bit much although no evidence of leaf burn to date.
Thank you for your advice.

Wind blowing/whipping leaves against straw blades will make slices in the leaves just like those. I have some too.

My my....I had thought that might be a possible cause as well, but having never seen it actually happen I didn't blame the straw. Last year I mulched w/ oak leaves & didn't have a problem. The plants are located away from any windbreak and it can get gusty occasionally (had a couple 12 inchers break at the stem last year because they weren't tied). I like the straw mulch & plan to use a 4" layer once the plants get a little bigger.
Thanks!

Don't give up on the straw for sure!! Good stuff. ;) Sometimes I trim off the plant's lower branches as I build up the straw or I put shredded leaves or hay on top of the straw because I have more of them available (and they're softer ;). If it gets really windy there's not alot you can do but the cuts do little or no harm to the plant.