Effect of Nitrogen Supply on the Photosynthetic Performance of Leaves from Coffee Plants Exposed to Bright Light

Abstract

Although Coffea arabica L. grows naturally in shaded habitats, it can be cultivated under high light intensity, but not without severe photoinhibition mainly during the period of transfer from the nursery into the field. The present work examines some of the changes in the photosynthetic performance induced by exposure to high light and the possibility of using enhanced nitrogen levels to overcome photoinhibition. For that purpose, young plants of Coffea arabica L. (cv. Catuai) grown in a shaded greenhouse were treated with 0, 1 and 2 mmol of nitrogen and 4 weeks later exposed to full solar irradiation, outside. Visible damage due to exposure to full sunlight appeared within 2 d in all plants, resulting in a reduced photosynthetic leaf area and drastic shedding of leaves in the unfertilized plants. These effects were considerably less in plants with the highest N dose. After 130 d of exposure, there was 100% mortality in plants receiving no extra nitrogen, compared with 30% in the plants treated with 2 mmol nitrogen. Photosynthesis rates, leaf conductance and transpiration presented minimum values after 4 d of light stress. Large changes in the photosynthetic capacity (measured at high CO2 concentration and high light intensity), quantum efficiency and fluorescence yield (Fv/Fm) indicate that net photosynthesis rate in the air had been reduced by both stomatal closure and by changes at the photochemical level. All indicators show that N-fertilized plants were less affected by photoinhibition.