How long have they been planted there? If it's been a while, it might be time to renew the acidity of the soil. Our water is alkaline and it slowly changes the soil back to the basic side. You can add pure sulfur or a brand name soil acidifier and it'll take a few months, but the acidity will return. You may read about other ways to quickly get it acidic again but I wouldn't risk most of the methods. Some ironite or similar fertilizer might help with it too since the actual problem with higher pH is less iron or manganese (frequently confused with magnesium) uptake.

The tips browning might be from salt burn. It's usually not very serious, but it does affect the look of the plant and it can get worse over time. You can wash out the soil by giving the plant a lot of water all at once and let it wash out the salt in the soil. (I save rainwater for this purpose)

iandyaz - The soil has been in about 6 - 8 weeks, not too long. Is that enough time to break down the acidity in soil?

It depends on a few things (initial pH of the soil, CaCO3 content of water, etc), but it's possible to change the pH of soil in just a few seconds with the right stuff, so 6-8 weeks is definitely possible. You could always run a test on the soil.. just put a handful from below the surface into a glass of distilled water and mix it up, then drop some pH tester in it. Gardenias are happiest around 5-5.5 pH

Another thing that can cause light green leaves is over-watering, but with fast draining soil and spring time in AZ, that's pretty unlikely unless it's sitting in a pool.

One of the nice things about iron fertilizers (iron chelate) is they usually have the iron in a form that can be absorbed by the plants so if the cause is too high pH, you can give the plant some of that and it'll fix itself relatively fast. Keep in mind that's just a temporary fix though.