Sensing nitrogen status in mosses
The aim of this research was to assess how mosses took up nitrogen (N), assimilated this N and sensed N status. Research into the short term foliar uptake and assimilation of N investigated the rapid induction of nitrate reductase activity (NRA) in two species of moss Mnium hornum and Sphagnum fimbriatum. NRA was induced in both species in just 15 minutes. Tissue nitrate (NO3) content was measured over this period and it was shown that the species had contrasting NO3" storage capabilities. Both species also differed in their long term N use. S. fimbriatum was a less nitrophilous species and died after approximately 80 days N-deprivation but M. hornum had a tremendous capacity to sustain itself and survived for in excess of 450 days. It was suggested from electrophysiological measurements and microscopy that S. fimbriatum had a lower NO3" storage capacity due to the lack of a distinct vacuole or storage compartment for NO3". M. hornum had both cytosolic and vacuolar compartments and was more able to store NO3", even in the longer term, being more similar in structure to a higher plant. Ammonium (Nlt induced NRA in the absence of NO3" in both mosses. Electrophysiological measurements, using pH-selective microelectrodes, showed a depolarisation of the membrane potential and acidification of intracellular compartments in moss cells upon addition of NH/ an example being a pH change from pH 7.11 to 5.84 in the cytosol of M. hornum. N uptake mechanisms in these mosses supported the classical pH-stat model and current thinking on plant N transporter mechanisms in higher plants whereby the addition of NH/ caused an acidification of the cytosol, in turn inducing NRA to stabilise intracellular pH. This project brought together the biochemistry, physiology and localised changes in metabolite pools and pH to explain why these mosses behaved differently.