Since production and the biennial cycle are strictly related to the offer of photoassimilates, several authors have tried to select productive plants based on the photosynthetic rate. Despite some differences having been observed among species and varieties from the same species, in particular in C. arabica, it was not possible to establish a significant correlation between productivity and photosynthesis (Nunes et al. 1969; Söndahl et al. 1976). Photosynthesis in productive and less productive plants is similar, although less productive plants have a biennial cycle less pronounced than productive plants (Mazzafera et al. 1995).
Photosynthesis in coffee is well studied and several studies showed that above 30°C it starts to decrease significantly (Rena and Maestri 1984). Photosynthesis in coffee begins to saturate with radiation between 600 and 1000 μmol photons m-2-1s , a light intensity usually found in sunny days during the summer, between 9 and 10 am. Thus, considering that temperatures above 30°C are usual during the hottest months of the year and that this luminosity is
easily reached in days of full sun, coffee production in tropical areas would theoretically be impractical. Production occurs because the plant canopy has to be considered as one whole object. Cannel (1985) suggested that an adult coffee plant could be divided into three photosynthetic layers, namely: a) an external one, which receives high luminosity and temperature, and as a consequence, presents inhibition of photosynthesis almost all day long; b) a second layer that receives indirect and diffuse light with lower temperature in which photosynthesis is satisfactory; c) an inner layer, which would has low rates of photosynthesis due to low light availability. In the second layer, besides diffuse light, rays of light which penetrate the canopy due to sun movement (sun flecks) would reach the leaves, permitting higher photosynthesis rates. Thus, correlating productivity to photosynthesis would only be possible if the entire plant was placed into a chamber and the CO consumption was measured. Since the accumulation of dry mass basically represents a difference between carbon input in photosynthesis and its loss in respiration, monitoring dry mass variation would be as trustworthy as sampling the whole plant; however the material would be inconveniently destroyed.