Photosynthetic Units

Leaf tissues of aurea mutants of tobacco and Lespedeza have been shown to have higher photosynthetic capacity per molecule of chlorophyll, a higher saturation intensity, a simpler lamellar structure, and the same quantum yield as their dark green parents. Here we report on the values of photosynthetic units for both types of plants and some algae. The unit has been assumed to be about as uniform and steady in the plant world as the quantum efficiency. The number on which all theoretical discussions have been based so far is 2400 per O₂ evolved or CO₂ reduced. With dark green plants and algae our determinations of units by means of 40 µsec flashes superimposed on a steady rate of background photosynthesis at 900 ergs cm^-2 sec^-1 of red light yielded mostly numbers between 2000 and 2700. However, the photosynthetic unit turned out to be very variable, even in these objects. In aurea mutants the unit was distinctly smaller, averaging 600 chl/CO₂. By choosing the right combination of colors for flash and background light, units as low as 300 chl/CO₂ or 40 chl/e^- could be measured consistently. We found five well-defined groups of units composed of multiples of its smallest member. These new findings are discussed in terms of structural entities that double or divide under the influence of far-red light.