Biology: What would be the limiting factor?

Well, we don't have enough information from that to say, but you should. Think about what photosynthesis entails. First, how would you measure the rate of photosynthesis? What exactly are you measuring? Something over time (to get the rate), but what is your "something"? Were the conditions under which the two plants grew the same other than light intensity? In other words, did they have enough raw materials to process in the photosynthesis reaction? If so, why might Oxalis have demonstrated that early, low-rate plateau while the bean plant kept pumping out more product as the light intensity increased? Let's go back to the early part - when you do photosynthesis, you need raw materials to make a finished product, right? But how do you convert the raw materials into the finished product? It's simple to say "raw materials plus sunlight = finished product", but how exactly does the sunlight help out here? As an aside, can you power your computer by wiring it up to a lightning rod in a thunderstorm? Do you need to do something to convert "natural" energy to "useable" energy? I bet you can work it out now!