Growth and Nitrogen Status of Soilless Tomato Plants Following Nitrate Withdrawal from the Nutrient Solution

The effects of withdrawing nitrogen (N) from the nutrient solution of adult tomato plants growing in rockwool in a greenhouse were investigated over a 6 week period during fruit production. The treatment reduced total plant growth after a lag period of about 2 weeks. The commercial fruit yield after 6 weeks of N deprivation was 7.7 kg m −2 compared to 9.3 kg m −2 in control plants. During the experiment, growth of the −N plants was fuelled by N reserves contained in both the substrate (rockwool) and in plant organs. The nitrogen budget calculated for −N plants showed that only a small amount of organic-N was readily available for internal cycling from organs such as stems. It served mainly to feed growing fruits which were the main sinks in the plant. The study also established that stores of nitrate-N were fully depleted but it took 45 d for the −N plants to metabolize completely their nitrate reserves. This indicates that internal nitrate is not a readily-accessible store of labile N. An estimation of the critical N concentration (% N c ) in the aerial dry matter was made from the data. Thus, for a crop yielding about 9.9 tons DM ha −1 , % N c was close to 2.5%. This result is discussed in light of existing models that describe the ontogenic decline in % N c in dry biomass of C 3 plants. The study indicates that the current regime of N fertilization practised in soilless cultures not only leads to ineffective nitrogen use but also to large losses of N to the environment; N concentrations should be decreased in feeding recipes. The use of N-free nutrient solutions prior to the termination of plant culture may also be a means of limiting the loss of eutrophying elements, such as nitrate, to the environment. Keywords Lycopersicon esculentum , tomato, organ dry biomass, critical nitrogen concentration, compartment, rockwool, nitrate interruption, distribution, reserves References LITERATURE CITED 1 Andriolo, J, 1995, Analyse des flux de NO<$>_{3}^{\minus}<$>, H 2 O et CO 2 au cours de la culture et du nycthémère chez la tomate ( Lycopersicon esculentum Mill.) adulte en hors-sol. 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