You may have heard that green tea has more L-theanine than does black tea. However, as the authors indicate, the studies providing these measures extracted total theanine from the leaf—they didn't measure the amount of L-theanine that gets into the cup you actually drink.
That's why the authors of the paper measured extracted L-theanine, namely the amount in a cup of 200ml, with the tea brewed with water at 80ºC for 2 minutes.
Under these circumstances the results were reversed, as the graph below shows. For the sake of simplicity, all of the teas I included in the graph were Twinings teas—several other brands of teas were tested, with the same results.
Average theanine content of different Twining's teas.
The "Everyday" tea was a bag tea, while all the rest were loose leaf. The first difference is that the loose leaf teas released significantly less theanine than the bagged tea. The second difference is that the green and white teas released less than some, but not all, of the Twinings black teas.
When all of the teas tested (including teas produced by other companies) were taken together, however, black bagged teas released significantly more theanine than did the loose leaf black teas and the loose leaf green and white teas. It is likely that the leaf particles in bagged teas offer more surface area to the water for extraction. With respect to the difference between some black teas and other teas, the difference may come from the more extensive processing of black teas, which may release more theanine.
Incidentally, for most of these teas, the maximum theanine was released at 2 minutes of brewing; and lemon, sugar, or small amounts of milk had no effect on theanine release.
Bottom line: enjoy your specialty loose leaf teas for their exquisite flavor, but if you just need a theanine "hit," go for a bagged tea!
* Emma K. Keenan, Mike D.A. Finnie, Paul S. Jones, Peter J. Rogers, Caroline M. Priestley. How much theanine in a cup of tea? Effects of tea type and method of preparation. Food Chemistry 125 (2011) 588–594.