underwatering?

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i am currently growing about 70 seedlings in rockwool cubes untill i buy some pots to transplant them to, for a while they were growing nice and green, but fairly slow, at first i thought i was overwatering but now i think i might be underwatering, some of the plants around the edges are growing the same they were before but alot of the plants in the middle their leaves have shriveled up, turned brown, and are crispy, they are in a field with no trees around and they get light all day long can someone please help me as to what i should do so i dont loose these baby's?
here are some pictures of some of the healthy plants, some of the unhealthy plants, and the tray i have them in, in the field.

yeah i think they might be burnt, the summers up here are usually alot cooler, and if i just left them out in the sun they would be perfectly fine, but this is the hottest summer weve ever had.. and i didnt even think to give them some shade..

but i still got 17 plants that are deffinatly gunna make it so i still got a good harvest.. guess ill try again next year