Effect of Nitrogen Supply on the Photosynthetic Performance of Leaves from 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 withoutsevere photoinhibition mainly during the period of transferfrom the nursery into the field. The present work examines someof the changes in the photosynthetic performance induced byexposure to high light and the possibility of using enhancednitrogen levels to overcome photoinhibition. For that purpose,young plants of Coffea arabica L. (cv. Catuai) grown in a shadedgreenhouse were treated with 0, 1 and 2 mmol of nitrogen and4 weeks later exposed to full solar irradiation, outside. Visible damage due to exposure to full sunlight appeared within2 d in all plants, resulting in a reduced photosynthetic leafarea and drastic shedding of leaves in the unfertilized plants.These effects were considerably less in plants with the highestN dose. After 130 d of exposure, there was 100% mortality inplants receiving no extra nitrogen, compared with 30% in theplants treated with 2 mmol nitrogen. Photosynthesis rates, leafconductance and transpiration presented minimum values after4 d of light stress. Large changes in the photosynthetic capacity(measured at high CO₂ concentration and high light intensity),quantum efficiency and fluorescence yield (F_v/F_m) indicate thatnet photosynthesis rate in the air had been reduced by bothstomatal closure and by changes at the photochemical level.All indicators show that N-fertilized plants were less affectedby photoinhibition. Key words: Coffee plant, nitrogen, photoinhibition, photosynthesis