What Limits the Efficiency of Photosynthesis, and Can There Be Beneficial Improvements?

Over the past 35 years a great deal has been learned about the mechanisms of photosynthesis, ranging from the ultrafast reactions involved in the initial capture of photons to the slower processes of carbon metabolism. Today our knowledge of photosynthesis and its molecular mechanisms is enormous, so much so that it is difficult for one person to absorb all the information. This is not necessarily a bad thing, since what we have achieved is sufficient information to appreciate the complexity of the “photosynthetic engine” and to identify the main factors that ultimately regulate its efficiency. In this chapter I summarize those areas of photosynthesis research with which I am reasonably familiar and, in so doing, address the question posed by the chapter title. As Blackman (1895a,b) pointed out, the rate of photosynthesis initially rises as the light intensity is increased and then levels off to a plateau. This plot is often referred to as the rate v PFD curve, where PFD stands for Photon Flux Density. Over the years rigorous analyses of the slopes of the rate v PFD curve have been made to obtain a value of the quantum yield (usually expressed as the number of quanta or photons required to produce one molecule of oxygen or to fix one molecule of carbon dioxide). With a few exceptions, the value obtained for a wide range of “non stressed” organisms and plants supplied with excess CO2 is about 9 or a little more (Björkman and Demmig, 1987; Walker, 1992). Bearing in mind that one molecule of oxygen evolved or carbon dioxide fixed is a 4e/4H+ process, then a value of 8 would he consistent with the “Z-scheme” model proposed by Hill and Bendall (1960). In this scheme, each electron is excited twice, first by photosystem two (PSII) and then by photosystem one (PSI). In this way, 8 photons are used to drive 4e/4H+ from water, through PSII and PSI to NADP.