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

Abstract

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 7.7kgm−2compared to 9.3kgm−2in 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 45d 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 (%Nc) in the aerial dry matter was made from the data. Thus, for a crop yielding about 9.9 tons DM ha−1, %Ncwas close to 2.5%. This result is discussed in light of existing models that describe the ontogenic decline in %Ncin dry biomass of C3plants. 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.