chlorophyll, under the influence of light. over time, the term photosynthesis came into common usage. later discovery of anoxygenic photosynthetic bacteria and photophosphorylation necessitated redefinition of the term. c3 : c4 photosynthesis research in the late 1940s at the university of california, berkeley, the details of photosynthetic carbon metabolism were sorted out by the chemists melvin calvin, andrew benson, james bassham and a score of students and researchers utilizing the carbon-14 isotope and paper chromatography techniques. the pathway of co2 fixation by the algae chlorella in a fraction of a second in light resulted in a three carbon molecule called phosphoglyceric acid (pga). for that original and ground-breaking work, a nobel prize in chemistry was awarded to melvin calvin in 1961. in parallel, plant physiologists studied leaf gas exchanges using the new method of infrared gas analysis and a leaf chamber where the net photosynthetic rates ranged from 10 to 13 μmol co2·m−2·s−1, with the conclusion that all terrestrial plants have the same photosynthetic capacities, that are light saturated at less than 50% of sunlight. later in 1958–1963 at cornell university, field grown maize was reported to have much greater leaf photosynthetic rates of 40 μmol co2·m−2·s−1 and not be saturated at near full sunlight. this higher rate in maize was almost double of those observed in other species such as wheat and soybean, indicating that large differences in photosynthesis exist among higher plants. at the university of arizona, detailed gas exchange research on more than 15 species of monocot and dicot uncovered for the first time that differences in leaf anatomy are crucial factors in differentiating photosynthetic capacities among species. in tropical grasses, including maize, sorghum, sugarcane, bermuda grass and in the dicot amaranthus, leaf photosynthetic rates were around 38−40 μmol co2·m−2·s−1, and the leaves have two types of green cells, i.e. outer layer of mesophyll cells surrounding a tightly packed cholorophyllous vascular bundle sheath cells. this type of anatomy was termed kranz anatomy in the 19th century by the botanist gottlieb haberlandt while studying leaf anatomy of sugarcane. plant species with the greatest photosynthetic rates and kranz anatomy showed no apparent photorespiration, very low co2 compensation point, high optimum temperature, high stomatal resistances and lower mesophyll resistances for gas diffusion and rates never saturated at full sun light. the research at arizona was designated a citation classic in 1986. these species were later termed c4 plants as the first stable compound of co2 fixation in light has four carbons as malate and aspartate. other species that lack kranz anatomy were termed c3 type such as cotton and sunflower, as the first stable carbon compound is the three-carbon pga. at 1000 ppm co2 in measuring air, both the c3 and c4 plants had similar leaf photosynthetic rates around 60 μmol co2·m−2·s−1 indicating the suppression of photorespiration in c3 plants. factors the leaf is the primary site of photosynthesis in plants. there are four main factors influencing photosynthesis and several corollary factors. the four main are: light irradiance and wavelength water absorption carbon dioxide concentration temperature. total photosynthesis is limited by a range of environmental factors. these include the amount of light available, the amount of leaf area a plant has to capture light (shading by other plants is a major limitation of photosynthesis), the rate at which carbon dioxide can be supplied to the chloroplasts to support photosynthesis, the availability of water, and the availability of suitable temperatures for carrying out photosynthesis. light intensity (irradiance), wavelength and temperature