This is the sort of symptom you get when a tree/shrub gets dry, then (having been watered) just about recovers.
When put under stress from shortage of water, plants take action to conserve it - the most obvious of which is wilting the leaves (which are expendable to some extent). After slight wilting, leaves can recover fully (actually, with some species, it is remarkable how they can recover fully depite looking very shrivelled). In general, after extended wilting, peripheral leaf cells will die. That's what you're seeing here.
Plants in pots are particularly susceptible. That's partly just because there is not much volume of soil to provide a reservoir of water. But a black pot in the sun will dry out even quicker because the plant demands more water, and also the pot gets hot and hence the soil gets hot. Hot soil can kill roots - leaving fewer roots to such up water next time (hopefully) it is given. Also, if soil dries out, it often leaves a crack between the soil and the side of the pot - so next time it is watered, there's a tendency for the water to run out the bottom before it has soaked the soil. that's especially a problem with peaty soils.
...So, I would advise: for any pot plant intended to be planted out:
- keep it out of full sun, or at least put the pot inside another one (that keeps the inner one cooler)
- use a saucer (that helps alleviate the problem of the pot-soil crack mentioned above)
- consider using a larger pot (though there are risks in over-potting)
With trees started in pots, don't leave it too long before planting in their permanent positions. That's because it is very easy for a tree's roots to get contorted into swirls/balls, and if planted out like that, it can lead to poor stability in winds in subsequent years. Teasing the roots out doesn't help that much, because the worst-affected roots will probably be too stiff to straighten without damage.