Why Are My Grapefruit Leaf Tips Curling Back?

[DrTolhur]

I've got a small grapefruit tree that I grew from seed, and a number of its latest flush of leaves have their tips folded back on themselves. I tried digging around online, but I haven't seen anything that looks like this. Anyone have any ideas what's going on with these? They're currently sitting next to a south-facing window under grow lights, in potting soil (I know that's not ideal, but I don't know that I care enough about it to use/buy anything better at this point), and I've been using a granular citrus fertilizer on it.

 

[Firstflush]

Check for pests of course. Citrus like it dry. How much are you watering? You want the container to be much lighter than when it has been watered.

 

[DrTolhur]

It's very possible I'm overwatering. It's a small terra cotta pot, so the pot itself makes up a good heft of the weight. Since it's potting soil, is a thorough watering (until it runs out the bottom) ideal, or a lighter watering that just kind of allows the water to perfuse on its own?

 

[Stormwater]

I’ve some grapefruits, every year I start to get this leaf curling near spring. Possible not enough light?

 

[DrTolhur]

I wouldn't think light would be a particular issue. Like I said, it's a south facing window, so it gets about as much light as to be had, plus with the grow light that's on for 12 hours each day. I wonder, though, if it'd be better to offset the grow light to come on a couple hours after sunrise to stretch the total length of light longer than 12 hours.
I will say, I've also got another grapefruit tree from the same seed batch and a kumquat tree that get pretty much the same treatment, and neither has any such leaves.

 

[Wires_Guy_wires]

Is there a heater below the window sill?
Stomata are most abundant on the bottom side of the leafs, when they all open up, the bottom side of the leaf can expand a whole lot, causing the leaf to get a canoe or hotdog bun shape.
Stomata tend to open in huge numbers when a plant is trying to cool itself through respiration.

Could be what's happening here.

 

[DrTolhur]

In fact there is a heater vent below the window. I don't usually have it open (both closed and blocked by a piece of cardboard), but I do sometimes have it open. That seems entirely like it could be the culprit. I'll definitely keep that in mind going forward.