In the studies cited above, it was also evidenced that leaves at full sunlight with fast stomatal closure showed an increase in the leaf temperature, from 10 to 15°C above ambient temperature (Butler 1977; Kanechi et al. 1995). It is well known that high temperature affects the chemical reactions related to the incorporation of carbon in plants, since they are mediated by enzymes (Berry and Böjorkman 1980). Low temperatures also affect photosynthesis. In assays in which the shoot of one year-old C. arabica seedlings were exposed to low temperatures only once, photosynthe-
sis was considerably reduced, being more than 50% at 4°C (Bauer et al. 1985). At 0.5°C some injury occurred
and the tissues stopped fixing CO . When the plants were repeatedly kept during nights at 4 and 6°C, photosynthesis was progressively reduced in the following days, and after 10 days it was 10% lower than the initial rates. Keeping the plants for 15 days at 12°C did not trigger tolerance to cold, implying that the plant avoided a reduction in photosynthetic activity. It was further observed that 25% of the photosynthetic reduction was due to stomatal closure and 75% to a decrease of carboxylation reactions of the photosynthesis (Bauer et al. 1985).