Hi Elmo,
I got you message and I will try and help. First by answering your questions.
The temperature difference from 85 to 70, No that would not effect them.
The lower leaf turning yellow, What causes that in a seedling is the plant needs nutrients and it is drawing them from its own leaf. When it does that it takes the chloryphill as well, thats why it turns yellow. Normally the nutrient it is lacking at this stage is nitrogen because it requires quite a bit at this point.
Now, there are different ways a plant can get a nutrient deffency, First is there are no nutrients to be had in what it is growing in. like hydro grows where you supply nutrients. Second and most common, is the plant can not use the nutrients that are there. Improper pH causes this. Think of it this way, nutrients flow through the plant, improper pH clogs the lines at the roots. Another way is the plant cannot breath properly, it is unable to transfer nutrients because it is clogged at the other end. What causes this is the leafs little pours (stoma) dont open and close properly therefore your plant does not get enough co2.
By looking at what you posted it sounds like you skipped the most common mistake I havent mentioned yet, over watering. We all love them too much. But the lower leaf turned yellow, and growth has slowed, warning sign. OK what have you done, you have proper temps, lighting, water and pH right? Maybe not.
What I think is happening is that your pH is off so your plant can not absorb the nutrients in the soil and you keep misting them and clogging the leaves ability to absorb co2, this weakens the plant, it pulls on its stored nutrients (yellow leafs) but there isnt much. The stem gets weak and it folds over and dies. (damping off)
The proper way to test pH in the water is both before and after you water.
In other words Ph test the run off water after it goes through the soil. then you know whats in the dirt and can adjust accordingly.
Don't mist your seedlings. mist the area around them. You want humidity. not wet plants. Don't use a fan on them. move the air around them once they get bigger. They don't need it now.
When plants are seedlings temps are more important then once they get established. The reason being is they are so small they don't have the reserves to come back once a mistake happens. When it comes to temps whats more important then ambient temperatures is the root zone temperature. You want your root zone temps between 70 to 80d while they are seedlings The foliage can be anywhere between 55 to 90d big swing I know. Once they are established you want your root zone within 10 degrees of the foliage temp. 75d roots 85d foliage This will keep your inter node spacing (the branches) closer together.
Hope that helps.
