The Tropical Fruit Forum

As Citradia  wrote it could be many things.   I'm not surprised that you are unable to find a graft union, because I doubt that your tree is a grafted tree.  Meyer lemons root extremely easy from cuttings, therefore a huge number are raised and sold as rooted cuttings instead of a grafted tree.  Trees grown from rooted cutting do not produce long tap roots, that strongly anchor the  tree to the soil.  In areas like Florida, even in ground growing younger rooted cutting propagated trees are commonly blown over from strong winds.   This is what happened to your tree, and it more than likely has some root damage from torn roots.  Do you know exactly how old your tree is, and how long it was at Record Buck before you purchased it?  Citrus leaves have a life span of approximately 16 to 24 months.  When a citrus leaf can no longer pull its weight, the tree removes the nutrients from the old leaves, and disperses them to the rest of the tree, then discards the leaf, doing so the leaf turns a yellowish color.  These older  leaves are of course the bottom leaves on the branch.. The  pattern shown by your photo has such a look.  I have quite a few citrus trees, both in the ground and container trees.  At this time of year many of my trees have leaves showing the exact leaf pattern.   Lastly it is your tree, and you can certainly grow it as you wish.  However, citrus are HEAVY feeders and do much better where fertilizer with conventional fertilizers, than organic types.  Keep an eye on your tree, and if thing get worse we'll need a follow up.

Citradia, see there you go,  I didn't know it worked for apples and pears.

http://www.abc.net.au/news/rural/2017-07-19/sunraysia-citrus-growers-bend-tree-branches-to-meet-demands/8719810

A day time temperature of 78, and a night temperature of 60 should be sufficient for Valentine.

Any temperature  above the absolute 54.5-F would be enough to keep the tree growing.  Pummelos, such as the variety Valentine, require high heat requirements to mature. The heat units from temperatures such a 60 to 70 would take the fruit a very very long time to mature, and at those temperatures the fruit would produce very little soluble solids (sugars), and if it ever turned a mature coloration, the fruit would probably be very bitter (sour).

Is it possible - yes.  Is it probable - probably not.  If you can provide the light, and keep the temperatures around 83 or so then  maybe.  Eventually, you would need a rather large container.

More Asian Citrus Psyllid found in California.  During a routine sweep on July 6, agriculture officials in Placer County found one of the bugs linked to the mass depletion of citrus plants in Florida. The Asian Citrus Psyllid was found in a residential back yard in West Roseville.

Are those rooted cuttings, or seedlings?

http://www.msn.com/en-us/health/watch/an-orange-a-day-could-reduce-your-risk-for-dementia/vi-BBE96QJ

For the longest time I have not been able to purchase a white grapefruit at the supermarket.  All they ever have any more is red.  Good thing I have a Seedless March grapefruit planted in the ground.  Presently it is still a semi-small tree, which does not produce a large quantity of fruit.  In just a couple years the tree should really pick up .

If the new leaves are growing out without the brown center vain, but rather look normal, I would not worry about it much.

I take it you actually meant Oro Blanco (you wrote Pro Blanco).

Never seen symptoms as shown on your tree's leaves.  My impression is that it might be caused by something through root up take.  Have you applied anything different to the soil?