EFFECT OF NITROGEN SUPPLY ON THE PHOTOSYNTHETIC PERFORMANCE OF LEAVESFROM COFFEE PLANTS EXPOSED TO BRIGHT LIGHT

Although Coffea arabica L. grows naturally in shaded habitats, it can be cultivated under high light intensity, but not without severe photo inhibition mainly during the period of transfer from the nursery into the field. The present work examines some of the changes in the photos ynthetic performance induced by exposure to high light and the possibi lity of using enhanced nitrogen levels to overcome photoinhibition. Fo r that purpose, young plants of Coffea arabica L. (cv. Catuai) grown i n a shaded greenhouse were treated with 0, 1 and 2 mmol of nitrogen an d 4 weeks later exposed to full solar irradiation, outside. Visible da mage due to exposure to full sunlight appeared within 2 d in all plant s, resulting in a reduced photosynthetic leaf area and drastic sheddin g of leaves in the unfertilized plants. These effects were considerabl y less in plants with the highest N dose. After 130 d of exposure, the re was 100% mortality in plants receiving no extra nitrogen, compared with 30% in the plants treated with 2 mmol nitrogen. Photosynthesis ra tes, leaf conductance and transpiration presented minimum values after 4 d of light stress. Large changes in the photosynthetic capacity (me asured at high CO2 concentration and high light intensity), quantum ef ficiency and fluorescence yield (F(v)/F(m)) indicate that net photosyn thesis rate in the air had been reduced by both stomatal closure and b y changes at the photochemical level. All indicators show that N-ferti lized plants were less affected by photoinhibition.