The influence of temperature and nitrogen source on growth and nitrogen uptake of two polar-desert species, Saxifraga caespitosa and Cerastium alpinum

Polar-desert plants experience low average air temperatures during their short growing season (4-8 degreesC mean July temperature). In addition, low availability of inorganic nitrogen in the soil may also limit plant growth. Our goals were to elucidate which N sources can be acquired by polar-desert plants, and how growth and N-uptake are affected by low growth temperatures. We compared rates of N-uptake and increases in mass and leaf area of two polar-desert species (Cerastium alpinum L. and Saxifraga caespitosa L.) over a period of 3 weeks when grown at two temperatures (6 degreesC vs. 15 degreesC) and supplied with either glycine, NH4+ or NO3-. At 15 degreesC, plants at least doubled their leaf area, whereas there was no change in leaf area at 6 degreesC. Measured mean N-uptake rates varied between 0.5 nmol g(-1) root DM s(-1) on glycine at 15 degreesC and 7.5 nmol g(-1) root DM s(-1) on NH4+ at 15 degreesC. Uptake rates based upon increases in mass and tissue N concentrations showed that plants had a lower N-uptake rate at 6 degreesC, regardless of N source or species. We conclude that these polar-desert plants can use all three N sources to increase their leaf area and support flowering when grown at 15 degreesC. Based upon short-term (8 h) uptake experiments, we also conclude that the short-term capacity to take up inorganic or organic N is not reduced by low temperature (6 degreesC). However, net N-uptake integrated over a three-week period is severely reduced at 6 degreesC.