Green light drives CO2 fixation deep within leaves

Maximal (CO2)-C-14-fixation in spinach occurs in the middle of the palisade mesophyll [Nishio et al. (1993) Plant Cell 5: 953], however, ninety percent of the blue and red light is attenuated in the upper twenty percent of a spinach leaf [Cui et al. (1991) Plant Cell Environ. 14: 493]. In this report, we showed that green light drives (CO2)-C-14-fixation deep within spinach leaves compared to red and blue light. Blue light caused fixation mainly in the palisade mesophyll of the leaf, whereas red light drove fixation slightly deeper into the leaf than did blue light. (CO2)-C-14-fixation measured under green light resulted in less fixation in the upper epidermal layer (guard cells) and upper most palisade mesophyll compared to red and blue light, but led to more fixation deeper in the leaf than that caused by either red or blue light. Saturating white, red, or green light resulted in similar maximal (CO2)-C-14-fixation rates, whereas under the highest irradiance of blue light given, carbon fixation was not saturated, but it asymptotically approached the maximal (CO2)-C-14-fixation rates attained under the other types of light. The importance of green light in photosynthesis is discussed.