Nitrogen and irradiance levels affecting net photosynthesis and growth of young coffee plants (Coffea arabicaL.)

Abstract

SummaryShoot growth attributes, leaf anatomical changes and net photosynthetic rates were determined in young coffee plants fertilized at high and low levels and subjected to shade or full sunlight. High nitrogen supply increased plant height irrespective of the light level imposed during growth, and promoted a larger leaf area in plants of the shade treatment. Specific leaf weight was 15% higher in plants grown in full sunlight than in shaded plants, at both nitrogen levels. The number of leaves developed on the orthotropic stem increased significantly under full irradiance and high nitrogen availability. Leaves were 11% thicker ih unshaded plants than in shaded ones, because of the increased size of the palisade and spongy parenchyma tissues. More thylakoids per granum and more grana per chloroplast were observed in shade-grown plants, mainly in those given high nitrogen treatment. Total nitrogen content expressed on a leaf-area basis was higher in sun plants supplemented with nitrogen, whereas chlorophyll a and b and protochlorophyll contents increased in shade-grown plants. Net photosynthetic rate in high nitrogen plants reached 7.19 µmol CO2 m−2 s−1 in the full irradiance treatment, and 5.46 µmol CO2 m−2 s−1 in shaded plants. Maximum net photosynthetic rates in the low nitrogen plants were 5.28 and 4.90 µmol CO2 m−2 s−1 in sun and shade plants, respectively. Increased photosynthetic light saturation was observed in high irradiance plants (628 µmol m−2 s−1) relative to shade plants (359 µmol m−2 s−1) in the high nitrogen treatment. The same pattern was observed in low nitrogen plants. The apparent quantum yield of shade acclimated plants was 14% higher than in those grown in full sunlight. Our results indicate that coffee may be classified as a facultative shade species, exhibiting features of sun adapted plants coupled with shade acclimation attributes, this fairly high adaptive capacity to shade:sun transition being strongly influenced by the level of nitrogen nutrition given to the plants.