442 PLANT GROWTH AND PLANT COMMUNITIES 



central enzymatic spot. Tliis arrangement makes our plant efficient at 

 low light-intensities but less efficient at high intensities, when more 

 than ten quanta per unit of enzymatic turnover time bombard the 

 photosynthetic unit. 



The leaves and chloroplasts of a plant must function over a wide 

 variety of intensities, from null to full sunlight. A leaf may be on the 

 top or on the bottom; the sun comes up and goes down. It is my guess 

 that the size of the photosynthetic unit— the number of chlorophyll 

 molecules per reducing center— has been established during the course 

 of evolution at a figure which maximizes yield over the range of in- 

 tensities to which plants are subject. 



The analysis above is a crude and qualitative one. We know, for 

 example, that COo is not reduced by the chloroplast directly; the func- 

 tion of the absorbed light energy is to generate reducing and phos- 

 phorylating agents which then transform the initial product of CO2- 

 fixation to carbohydrate. The individual light-powered chemical acts of 

 photosynthesis may very well require fewer than ten quanta. But the 

 principle of the analysis remains unaltered: namely, that the photo- 

 synthetic act, as a multi-quantum process, cannot be simultaneously 

 efficient at both low and high intensities and some compromise must 

 be sought— that which maximizes yield. I have tried a little experiment: 

 I have tried to find out, with the aid of modern computerology, how 

 many chlorophyll molecules per photosynthetic unit a chloroplast 

 should contain in order to maximize yield over all light-intensities. I 

 have presented the facts outlined above to a computerologist and asked 

 him to please compute for me the optimum number of chlorophyll 

 molecules per reducing center. The calculation again is crude and to be 

 regarded only as a first approximation, but it agrees witli nature in 

 suggesting that the optimum number is of the order of 1,000. And so, 

 in a first sense, the answer to our question. What is the limit on the 

 productivity of plants on earth? is that it is the limit established by the 

 organization of tlie photosynthetic unit of the chloroplast. And in a 

 more limited sense, productivity is limited by the concentration of CO2 

 characteristic of air. There is little that we can do about this latter fact. 

 Plants, by the nature of their dependence on CO2 concentration, form 

 a negative feedback system controlling the COo concentration of our 

 atmosphere at the level which characterizes it today. 



Photosynthetic efficiencies of 2 to 3 per cent are by no means uni- 

 formly attained in nature or in agriculture, even for that portion of the 

 year which we know as the growing season. The over-all efficiency of 

 crop plants for the earth as a whole is probably closer to a third of 

 this value (Brown et ah, 1957; Wassink, 1959). Why is the gap between 

 the attained and the attainable so great? Undoubtedly the efficiency of 

 plants as converters of solar energy is generally limited also by the 



