Short-term effects of fertilization on loblolly pine ( Pinus taeda L.) physiology

Fertilization commonly increases biomass production in loblolly pine ( Pinus taeda L.). However, the sequence of short-term physiological adjustments allowing for the establishment of leaf area and enhanced growth is not well understood. The effects of fertilization on photosynthetic parameters, root respiration, and growth for over 200 d following the application of diammonium phosphate were intensively investigated in an effort to establish a relative sequence of events associated with improved growth. Root respiration, foliar nitrogen concentration [N] f , and lightsaturated net photosynthesis ( A sat ) temporarily increased following fertilization. A sat was correlated positively with [N] f when non-fertilized and fertilized treatments were pooled ( R 2 = 0.47). Increased photosynthetic capacity following fertilization was due to both improved photochemical efficiency and capacity and enhanced carboxylation capacity of Rubisco. Positive effects of fertilization on growth were observed shortly after A sat increased. Fertilized seedlings had 36.5% more leaf area and 36.5% greater total dry weight biomass at 211 d following fertilization. It is concluded that fertilization temporarily increased photosynthetic capacity, which resulted in a pool of photo-assimilate used to build leaf area. The N from fertilizer initially invested in photosynthetic structures and enzymes probably re-translocated to newly developing foliage, explaining the reduction in [N] f and A sat that was observed after peak levels were achieved following fertilization. Key-words : carboxylation capacity; chlorophyll fluorescence; leaf area; nitrogen; photochemical efficiency; photosynthesis; root respiration.