PubMed1978
Photosynthesis, the use of light energy in the conversion of CO2 and inorganic nutrients into plant material, is the ultimate source of the food protein necessary to man's existence. Given certain assumptions, the overall maximal theoretical photosynthetic efficiency of agricultural plants can be calculated. Actual measured maximal growth rates of plants are equivalent to efficiency levels well below that theoretical maximum. In air, C4 plants can some closer to the theoretical value than C3 plants, perhaps because C4 plants avoid the occurrence of measurable photorespiration and oxygen inhibition of photosynthesis. Alfalfa, a C3 legume, is an extremely productive protein source. Its protein yield per acre can surpass that of commonly grown C4 crops (corn, sorghum) and C3 seed crops (soybean, wheat, rice). Alfalfa leaf protein is of high nutritional quality and can apparently be used directly in the human diet, eliminating the protein loss involved in animal production. Plant protein productivity can be raised as part of an increase in overall crop yield. The growth of plants in atmospheres with elevated CO2 levels can result in increased yields. In C3 plants this is due, at least in part, to the suppression of photorespiration and oxygen inhibition of photosynthesis. We have investigated the effect of CO2 concentration on alfalfa photosynthetic metabolism. Our results support the contention that alfalfa productivity can be increased by an environment of elevated CO2. A second approach toward increased plant protein productivity is through regulation of carbon flow during photosynthesis so as to increase portein production relative to that of other plant constituents. In particular, we have investigated whether ammonia (the form in which plants first incorporate nitrogen) can act to regulate leaf carbon metabolism. Our results indicate that NH4+, in part through stimulation of pyruvate kinase, brings about increased production of amino acids at the expense of sucrose production in alfalfa. The effect may be of considerable importance in the regulation of green leaf protein synthesis.