I'm going to note here that this is pure speculation; more of a creative answer based on science than the creative application of science.
That said; plants generally want all the heat they can get, within reason. That's because photosynthesis is an endothermic reaction, requiring energy to convert CO2 and Water to CHO (carbohydrate) compounds and O2. Initially, plants do this for themselves but as we know, they produce more carbohydrates and oxygen gas than they need. The O2 is 'corrosive' to the plant so instead of trying to contain it, they just release it into the atmosphere. This is the very reason that animals can exist; all animal life is effectively parasitic in that it relies on plants for both O2 and food. Even Carnivores have herbivores down at the bottom of their food chains.
So; imagine that instead of releasing excess O2, plants 'decide' to release the excess energy instead. What we're talking about here is the ability to suspend photosynthesis when the O2 and CHO compound needs are met.
The problem with this scenario is that it's not sunny all the time, and temperatures change through the year, etc. Plants with non-continuous photosynthesis may develop limited capacity to store O2 (carbs are less of a problem for a plant, just take a look at fruit) so that they can metabolise energy when neeeded, but ultimately plants would still want as large a surface area as possible facing the sun so that during winter (for example) they can still survive. During summer, they have to dissipate the excess energy so as not to overproduce their O2 and carbohydrate holdings.
In such a case, it seems like a possible solution that they could radiate. Why the tight beam? Ultimately, like any living organism, they don't want their entire body to get hot, especially during summer when the ambient heat is only going to exacerbate their problem. It seems logical that emitting a tight beam of radiation would allow them to dissipate the energy they're absorbing from the environment in an efficient way that gets the heat away from them as much as possible.
Now for the caveats;
If the entire planet of plants has developed this method of controlling photosynthesis, you don't have animals. This is because you don't have O2 in the atmosphere. Sure, some traces would be released if the plant dies for instance, but those amounts could easily be bound to iron in the rocks, etc.
This is a more complex form of plant life with several more regulatory systems than Earth plants have. Plants are unlikely to go to this trouble (evolutionary speaking) unless there's a reason to do so, and off the top of my head I can't think of one. Certainly storing O2 for overnight and cold / cloudy days seems like a lot of trouble when you have a massive container around you called the atmosphere.
Forest canopies may look very different. If the plants on the forest or jungle floor start emitting upwards for instance, I don't know how much of an impact that may have on the tree canopies above them. Forest and Jungle ecosystems would probably look very different in such a world.
Finally, the radiation burst would only happen in specific conditions; it's not a constant stream of energy as it's only happening when there is excess energy to dissipate. You'd get all your radiation bursts during hot days, probably when you least want them. Also, given the amount of energy we're talking about, this is probably not a very dangerous burst to (say) human life.
Ultimately, this is a feature that is unlikely to evolve alongside conventional plants because of the evolutionary complexity, meaning that you're looking at a very alien world to develop such a feature. One that's not very friendly to animal life.