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I grew 2 avocado plants from the pit, and they've been growing well for quite some time. They are now about 2 ft tall. A few weeks ago, I noticed some slight yellowing on the leaves and browning at the leaf tips. I checked the roots and noticed that they were pretty root bound and needed repotting, so I repotted them. But the yellowing and browning has gotten worse. Here is a picture of how it looks now. https://www.flickr.com/photos/17398687@N02/5301425262/
As you can see in the bottom right picture, I have the plants under fluorescent light in the daytime. Also I've noticed that new leaf growth is turning white, drying, and falls off to the touch (Upper left picture). I thought maybe the yellowing could be N deficiency, so I fertilized with a houseplant fertilizer and this hasn't seemed to help, so I'm thinking maybe chlorosis? How can I treat chlorosis in a potted houseplant? I don't really want to fertilize again with it being wintertime. And I still don't know why the leaf tips are browning? I have also noticed that the lower leaves (which are very large) are very stiff and drooping. Is this from inadequate light? Because the fluorescent lights don't really reach the lower leaves well. Do you think the fluorescent lights are too intense on the upper leaves? Desperately need advice on this ??

It's a bit hard to tell due to some connection glitch I'm experiencing, but it looks like your light is the skinny extra bright T-5 and it's very likely too close to the top leaves. The leaves look on the verge of sunburn. the already damaged leaves might have been in actual contact with the tube -- or close enough to burn.

I kind of think it wasn't a good time to repot even if they were pot bound, but no use crying over spilt milk. Water well when soil surface is dry, make sure the soil is well draining and don't let the pot sit in excess water after watering. Depending on what kind of potting soil mix you used, you shouldn't need o fertilize or worry about lack of chelated iron for the time being -- not the best time to fertilize anyway as it may promote growth that will not grow well indoors in the winter. With the forced air heat on, I mist them thoroughly every morning with filtered water.

Avocado is sensitive to salt so don't overdo blue fertilizer if you use them, and water only with de-chlorinated water. I use CuZn filter, but you can leave an open container of tap water for 24 hrs or use boiled or rain (or rm temp snowmelt) water. I occasionally add used coffee grounds to the
Watering can.

I posted photos of my avocados in [url=https://www.helpfulgardener.com/forum/viewtopic.php?p=173115#173115]this thread[/url]. The mango has been shedding some leaves but not the avocados. As you can see I've shredded the mango leaves for surface mulch. They have a pleasantly spicy kind of scent.

This was years ago, but mine did the same thing... Bounced back in spring though...

But yeah, I'd move the light up a few inches and check the humidity, like AS said

Thanks for all the information. The leaves with the yellowing and browning tips are actually mostly the lower ones. The Newly emerged leaves closest to the light are actually looking pretty good. I've made sure that the lights are not touching the leaves also.
I do know that the humidity in the room is probably not ideal for the avocados, but I'm pretty leery about misting, because of potential fungus problems. Hopefully they will bounce back in the spring.

Ah, in that case, I'm more inclined to believe the brown tips are due to damage to older roots -- most likely too heavy soil and perhaps the root bound conditions as you thought (did you notice any older blackened roots when you repotted?) -- or salt build up in the soil or accidentally leaving in excess water. They can be indicators of problems so it's a good idea to review your care regimen.

Older leaves do eventually get shed however, and as you saw in my photos, the newer leaves will keep getting bigger with each new flush of growth.

Good luck with your avocados! While in college, my brother rented a room from someone who had a HUGE avocado tree scraping the high ceiling of an older NYC apartment with spreading branches dominating the living room.