I had to leave my home for 2 days. I watered the plants and opened the window (for bugs and air circulation). I thought there won't be any problem. However, when I got home (just now) I noticed that the new, growing traps (which looked totally healthy when I left) started to turn black. Why is this happening? Could this be sunburn? Late afternoon (~6 pm) the sun shines on the VFT, but since a few days it's really hot here. I keep them in a florarium, lit by artificial light. I am attaching some pictures.
No, the top of the aquarium isn't covered, there's at least 1,5 meters space from the top to the ceiling (I mean it's not between two shelves). Also it is exposed to direct (late afternoon) sunlight for ~1,5-2 hours.
The in your photos new emerging leaves/traps look OK. Did you feed the traps that later turned black? Had the plant experienced full sun before? Any change in more sun exposure could cause this. It could be sunburn, heat burn, or an overfed trap.
I am giving them ionized water. I read that it's ok too, also on the label it is stated that it's suitable for plant caring (and aquaristics). The pot stays in a tray which always has water in it. The water level reaches ~1/3 of the pot.
Ionized water? Do you mean de-ionized water? If you are really using ionized water, this could be your problem. You might be purposefully killing your plants by using mineral-rich water.
A water ionizer separates water into alkaline and acid fractions using electrolysis. Ionized alkaline water is also known as Electrolyzed Reduced Water. Ionized acidic water is also known as Electrolyzed Oxidative Water.
De-ionized water has no minerals in it. It is made by distilling water or through reverse osmosis.
sorry, I am from Hungary, I'm not always sure if I translated the words well. The water is de-ionized. Ok, I'm not sure how it is made, but I know that on a hungarian forum many of use use the same water (called "Old Boy", bought in a hypermarket) and none of us experienced problems with it.
1) I like Bob's suggestion about the sunlight, having never experienced it before. That is very plausible. 2) Given that you have only had it a week, was it in a humidity dome when you got it (or when the person you got it from got it)? If so, I would also like to offer up the suggestion that it is it is experiencing shock from the rapid change in environment. VFTs tend to be very visible about this. Leaves go black, etc...
Both of these situations really follow a common theme and that is that VFTs tend to like more graduate changes in environmental conditions(incidentally, I think we have all done this- I know I have!). This can include humidity, light levels, etc... Are you familiar with "hardening off" plants? If you think it may be related to humidity, I would "harden it off" which basically just means keep it in high humidity for a little while (a week should do it) then gradually expose it to more ambient air. If you google search this you can find examples of how to do this (I may have posted a thread about last time I hardened off a Pinguicula from a hardware store, dont remember if I did but that plant is doing great today).
If you think it may be related to light, put it under artificial lights for a little bit to recover, then gradually expose it to more and more direct sun. Say, go up by a half hour to an hour a day until you reach your goal.
Thank you all for the help. AFAIK where I ordered it from, they were grown in high humidity. As I keep them in a terrarium, I don't think that the sudden change would be the problem, or if it is, just partially. However I am beginning to think that the main problem is sunburn. A sudden increase in temperature started in Hungary when I left home, and unfortunately I left the window open, so the strong sunlight did reach them.
I live in Nebraska (middle of the United States). It gets HOT here, and when I say hot, I mean hot. 110 F (43 C) streaks for a week at a time here isn't uncommon in the dead of summer. I have had great luck with my flytraps being outside, but only during the morning and evening hours when it gets like this - otherwise I bring them inside or put them into the shade during the hot afternoons.
When it gets incredibly hot, I'll sometimes poor refrigerated water into their trays (and on them) to keep the root system cool. I don't know if others have done this before with success, but for me it really helps them out. The sarracenia enjoy this method too, it seems.
...the point is, that in the rare occasion where I miss an afternoon and they bake a bit too much, I'll sometimes see blackening of young traps like you have here. Even though they're accustomed to the intense light around here, on those days they get heat burn as described above if I'm not incredibly careful. My vote would be just that.
But there are people on here far more experienced than I.
For what it's worth - I learned about the cooling of the water by visiting South Carolina and seeing a bunch of these little dudes in their natural habitat. It was INCREDIBLY hot that day, a sweltering day. I began to wonder why I had lost flytraps in the heat of Nebraska summers like this whereas they were doing fine here. I stuck my finger in the peaty/marshy soil and it was actually rather cool. And there was my answer.
Addendum: note I said refrigerated water. Not ice water.